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Acidente de Chernobyl

Data: 26 de abril de 1986

Localização: Usina Nuclear de Chernobyl, Pripyat, Ucrânia (então parte da União Soviética)

Causa: Erro humano durante um teste de segurança, combinado com falhas no projeto do reator.

Consequências: Liberação massiva de material radioativo, evacuação de áreas vizinhas, aumento de casos de câncer e impacto ambiental significativo.

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The Chernobyl disaster began on 26 April 1986 with the explosion of the No. 4 reactor of the Chernobyl Nuclear Power Plant, near the city of Pripyat in the north of the Ukrainian SSR, close to the border with the Byelorussian SSR, in the Soviet Union. It is one of only two nuclear energy accidents rated at seven-the maximum severity-on the International Nuclear Event Scale, the other being the 2011 Fukushima nuclear accident in Japan. The initial emergency response and subsequent mitigation efforts involved more than 500,000 personnel and cost an estimated 18billion roubles -roughly US$68billion in 2019, adjusted for inflation. It is considered the worst nuclear disaster in history.

The accident occurred during a test of the steam turbine's ability to power the emergency feedwater pumps in the event of a simultaneous loss of external power and coolant pipe rupture. Following an accidental drop in reactor power to near-zero, the operators restarted the reactor in preparation for the turbine test with a prohibited control rod configuration. Upon successful completion of the test, the reactor was then shut down for maintenance. Due to a variety of factors, this action resulted in a power surge at the base of the reactor which brought about the rupture of reactor components and the loss of coolant. This process led to steam explosions and a meltdown, which destroyed the containment building. This was followed by a reactor core fire which lasted until 4 May 1986, during which airborne radioactive contaminants were spread throughout the USSR and Europe. In response to the initial accident, a 10-kilometre (6.2 mi) radius exclusion zone was created 36 hours after the accident, from which approximately 49,000 people were evacuated, primarily from Pripyat. The exclusion zone was later increased to a radius of 30 kilometres (19 mi), from which an additional ~68,000 people were evacuated.

Following the reactor explosion, which killed two engineers and severely burned two more, an emergency operation to put out the fires and stabilize the surviving reactor began, during which 237 workers were hospitalized, of which 134 exhibited symptoms of acute radiation syndrome (ARS). Among those hospitalized, 28 died within the following three months. In the following 10 years, 14 more workers (9 of whom had been hospitalized with ARS) died of various causes mostly unrelated to radiation exposure. 15 childhood thyroid cancer deaths were attributed to the disaster as of 2011. A United Nations committee found that to date fewer than 100 deaths have resulted from the fallout. Model predictions of the eventual total death toll in the coming decades vary. The most widely cited study conducted by the World Health Organization in 2006 predicted 9,000 cancer-related fatalities in Ukraine, Belarus and Russia.

Background

Reactor cooling after shutdown

Reactor decay heat shown as % of thermal power from time of sustained fission shutdown using two different correlations. Due to decay heat, solid fuel power reactors need high flows of coolant after a fission shutdown for a considerable time to prevent fuel cladding damage, or in the worst case, a full core meltdown.

In power-generating operation, most of the heat generated in a nuclear reactor by its fuel rods is derived from nuclear fission, but a significant fraction (over 6%) is derived from the radioactive decay of the accumulated fission products, a process known as decay heat. This decay heat continues for some time after the fission chain reaction has been stopped, such as following a reactor shutdown, either emergency or planned, and continued pumped circulation of coolant is essential to prevent core overheating, or in the worst case, core meltdown. RBMK reactors, like those at Chernobyl, use water as a coolant, circulated by electrically driven pumps. The coolant flow rate is considerable - Reactor No. 4 had 1661 individual fuel channels, each requiring a coolant flow of 28 m ³/h (990 cu ft/h) at full reactor power, for a total of over 45 million litres per hour (12 million gallons per hour) for the entire reactor.

In case of a total power loss at the station, each of Chernobyl's reactors had three backup diesel generators, but they took 60-75 seconds to attain full load and generate the 5.5‑ megawatt output required to run one main pump. In the interim, special counterweights on each pump would enable them to provide coolant via inertia, thereby bridging the gap to generator startup. However, a potential safety risk existed in the event that a station blackout occurred simultaneously with the rupture of a 600-millimetre (24 in) coolant pipe (the so-called Design Basis Accident). In this scenario the emergency core cooling system (ECCS) needed to pump additional water into the core, replacing coolant lost to evaporation.

It had been theorized that the rotational momentum of the reactor's steam turbine could be used to generate the required electrical power to operate the ECCS via the feedwater pumps. The turbine's speed would run down as energy was taken from it, but analysis indicated that there might be sufficient energy to provide electrical power to run the coolant pumps for 45 seconds. This would not quite bridge the gap between an external power failure and the full availability of the emergency generators, but would alleviate the situation.

Safety test

The turbine run-down energy capability still needed to be confirmed experimentally, and previous tests had ended unsuccessfully. An initial test carried out in 1982 indicated that the excitation voltage of the turbine-generator was insufficient; it did not maintain the desired magnetic field after the turbine trip. The electrical system was modified, and the test was repeated in 1984 but again proved unsuccessful. In 1985, the test was conducted a third time but also yielded no results due to a problem with the recording equipment. The test procedure was to be run again in 1986 and was scheduled to take place during a controlled power-down of reactor No. 4, which was preparatory to a planned maintenance outage.

A test procedure had been written, but the authors were not aware of the unusual RBMK-1000 reactor behaviour under the planned operating conditions. It was regarded as purely an electrical test of the generator, not a complex unit test, even though it involved critical unit systems. According to the regulations in place at the time, such a test did not require approval by either the chief design authority for the reactor (NIKIET) or the Soviet nuclear safety regulator. The test program called for disabling the emergency core cooling system, a passive/active system of core cooling intended to provide water to the core in a loss-of-coolant accident, and approval from the Chernobyl site chief engineer had been obtained according to regulations.

The test procedure was intended to run as follows:

Test preparation

1. The test would take place prior to a scheduled reactor shutdown

2. The reactor thermal power was to be reduced to between 700 MW and 1,000 MW (to allow for adequate cooling, as the turbine would be spun at operating speed whilst disconnected from the power grid)

3. The steam-turbine generator was to be run at normal operating speed

4. Four out of eight main circulating pumps were to be supplied with off-site power, while the other four would be powered by the turbine

Electrical test

1. When the correct conditions were achieved, the steam supply to the turbine generator would be closed off, and the reactor would be shut down

2. The voltage provided by the coasting turbine would be measured, along with the voltage and RPMs of the four main circulating pumps being powered by the turbine

3. When the emergency generators supplied full electrical power, the turbine generator would be allowed to continue free-wheeling down

Test delay and shift change

The test was to be conducted during the day-shift of 25 April 1986 as part of a scheduled reactor shut down. The day shift crew had been instructed in advance on the reactor operating conditions to run the test, and, in addition, a special team of electrical engineers was present to conduct the one-minute test of the new voltage regulating system once the correct conditions had been reached. As planned, a gradual reduction in the output of the power unit began at 01:06 on 25 April, and the power level had reached 50% of its nominal 3,200 MW thermal level by the beginning of the day shift.

The day shift performed many unrelated maintenance tasks, and was scheduled to perform the test at 14:15. Preparations for the test were carried out, including the disabling of the emergency core cooling system. Meanwhile, another regional power station unexpectedly went offline. At 14:00, the Kiev electrical grid controller requested that the further reduction of Chernobyl's output be postponed, as power was needed to satisfy the peak evening demand, so the test was postponed.

Soon, the day shift was replaced by the evening shift. Despite the delay, the emergency core cooling system was left disabled. This system had to be disconnected via a manual isolating slide valve, which in practice meant that two or three people spent the whole shift manually turning sailboat-helm-sized valve wheels. The system would have no influence on the events that unfolded next, but allowing the reactor to run for 11 hours outside of the test without emergency protection was indicative of a general lack of safety culture.

The night shift had very limited time to prepare for and carry out the experiment. Anatoly Dyatlov, deputy chief-engineer of the Chernobyl Nuclear Power Plant, was present to supervise and direct the test. He was one of the test's chief authors and he was the highest-ranking individual present. Unit Shift Supervisor Aleksandr Akimov was in charge of the Unit 4 night shift, and Leonid Toptunov was the Senior Reactor Control Engineer responsible for the reactor's operational regimen, including the movement of the control rods. 25-year-old Toptunov had worked independently as a senior engineer for approximately three months.

Unexpected drop of the reactor power

The test plan called for a gradual decrease in reactor power to a thermal level of 700-1000 MW, and an output of 720 MW was reached at 00:05 on 26 April. However, due to the reactor's production of a fission byproduct, xenon-135, which is a reaction-inhibiting neutron absorber, power continued to decrease in the absence of further operator action, a process known as reactor poisoning. In steady-state operation, this is avoided because xenon-135 is "burned off" as quickly as it is created from decaying iodine-135 by the absorption of neutrons from the ongoing chain reaction, becoming highly stable xenon-136. With the reactor power reduced, high quantities of previously produced iodine-135 were decaying into the neutron-absorbing xenon-135 faster than the reduced neutron flux could "burn it off". Xenon poisoning in this context made reactor control more difficult, but was a predictable and well-understood phenomenon during such a power reduction.

When the reactor power had decreased to approximately 500 MW, the reactor power control was switched from LAR (Local Automatic Regulator) to the Automatic Regulators, in order to manually maintain the required power level. AR-1 then activated, removing all four of AR-1's control rods automatically, but AR-2 failed to activate due to an imbalance in its ionization chambers. In response, Toptunov reduced power to stabilize the Automatic Regulators' ionization sensors. The result was a sudden power drop to an unintended near- shutdown state, with a power output of 30 MW thermal or less. The exact circumstances that caused the power drop are unknown. Most reports attribute the power drop to Toptunov's error, but Dyatlov reported that it was due to a fault in the AR-2 system.

The reactor was now producing only 5% of the minimum initial power level prescribed for the test. This low reactivity inhibited the burn-off of xenon-135 within the reactor core and hindered the rise of reactor power. To increase power, control-room personnel removed numerous control rods from the reactor. Several minutes elapsed before the reactor was restored to 160 MW at 00:39, at which point most control rods were at their upper limits, but the rod configuration was still within its normal operating limit, with Operational Reactivity Margin (ORM) equivalent to having more than 15 rods inserted. Over the next twenty minutes, reactor power would be increased further to 200 MW.

The operation of the reactor at the low power level (and high poisoning level) was accompanied by unstable core temperatures and coolant flow, and, possibly, by instability of neutron flux. The control room received repeated emergency signals regarding the low levels in one half of the steam/water separator drums, with accompanying drum separator pressure warnings. In response, personnel triggered several rapid influxes of feedwater. Relief valves opened to relieve excess steam into a turbine condenser.

Reactor conditions priming the accident

The combined effect of these various actions was an extremely unstable reactor configuration. Nearly all of the 211 control rods had been extracted manually, and excessively high coolant flow rates through the core meant that the coolant was entering the reactor very close to the boiling point. Unlike other light-water reactor designs, the RBMK design at that time had a positive void coefficient of reactivity at low power levels. This meant that the formation of steam bubbles (voids) from boiling cooling water intensified the nuclear chain reaction owing to voids having lower neutron absorption than water. Unbeknownst to the operators, the void coefficient was not counterbalanced by other reactivity effects in the given operating regime, meaning that any increase in boiling would produce more steam voids which further intensified the chain reaction, leading to a positive feedback loop. Given this characteristic, reactor No. 4 was now at risk of a runaway increase in its core power with nothing to restrain it. The reactor was now very sensitive to the regenerative effect of steam voids on reactor power.

Test execution

Plan view of reactor No. 4 core. The number on each control rod indicates the insertion depth in centimeters one minute prior to the disaster.
neutron detectors (12)
control rods (167)
short control rods from below reactor (32)
automatic control rods (12)
pressure tubes with fuel rods (1661)

At 01:23:04, the test began. Four of the eight main circulating pumps (MCP) were to be powered by voltage from the coasting turbine, while the remaining four pumps received electrical power from the grid as normal. The steam to the turbines was shut off, beginning a run-down of the turbine generator. The diesel generators started and sequentially picked up loads; the generators were to have completely picked up the MCPs' power needs by 01:23:43. As the momentum of the turbine generator decreased, so did the power it produced for the pumps. The water flow rate decreased, leading to increased formation of steam voids in the coolant flowing up through the fuel pressure tubes.

Reactor shutdown and power excursion

At 01:23:40, as recorded by the SKALA centralized control system, a scram (emergency shutdown) of the reactor was initiated as the experiment was wrapping up. The scram was started when the AZ-5 button (also known as the EPS-5 button) of the reactor emergency protection system was pressed: this engaged the drive mechanism on all control rods to fully insert them, including the manual control rods that had been withdrawn earlier.

The personnel had already intended to shut down using the AZ-5 button in preparation for scheduled maintenance and the scram likely preceded the sharp increase in power. However, the precise reason why the button was pressed when it was is not certain, as only the deceased Akimov and Toptunov partook in that decision, though the atmosphere in the control room was calm at that moment. Meanwhile, the RBMK designers claim that the button had to have been pressed only after the reactor already began to self-destruct.

Steam plumes continued to be generated days after the initial explosion[37]

Consequently, injecting a control rod downward into the reactor in a scram initially displaced neutron-absorbing water in the lower portion of the reactor with neutron-moderating graphite. Thus, an emergency scram could initially increase the reaction rate in the lower part of the core. This behaviour was discovered when the initial insertion of control rods in another RBMK reactor at Ignalina Nuclear Power Plant in 1983 induced a power spike. Procedural countermeasures were not implemented in response to Ignalina. The IAEA investigative report INSAG-7 later stated, "Apparently, there was a widespread view that the conditions under which the positive scram effect would be important would never occur. However, they did appear in almost every detail in the course of the actions leading to the Chernobyl accident."

A few seconds into the scram, a power spike did occur, and the core overheated, causing some of the fuel rods to fracture. Some have speculated that this also blocked the control rod columns, jamming them at one-third insertion. Within three seconds the reactor output rose above 530 MW.

Instruments did not register the subsequent course of events; it was reconstructed through mathematical simulation. Per the simulation, the power spike would have caused an increase in fuel temperature and steam buildup, leading to a rapid increase in steam pressure. This caused the fuel cladding to fail, releasing the fuel elements into the coolant and rupturing the channels in which these elements were located.

Steam explosions

The reactor lid (upper biological shield)[39] nicknamed "Elena"[40] with torn off fuel channel piping is shown lying on its side where it came to rest in the explosion crater. The view transitions to showing the relative position of the paired steam tanks, reactor hall floor and roof trusses overlaid on the explosion crater. Source animation

As the scram continued, the reactor output jumped to around 30,000 MW thermal, 10 times its normal operational output, the indicated last reading on the power meter on the control panel. Some estimate the power spike may have gone 10 times higher than that. It was not possible to reconstruct the precise sequence of the processes that led to the destruction of the reactor and the power unit building, but a steam explosion-like the explosion of a steam boiler from excess vapour pressure-appears to have been the next event. There is a general understanding that it was explosive steam pressure from the damaged fuel channels escaping into the reactor's exterior cooling structure that caused the explosion that destroyed the reactor casing, tearing off and blasting the upper plate called the upper biological shield, to which the entire reactor assembly is fastened, through the roof of the reactor building. This is believed to be the first explosion that many heard.

This explosion ruptured further fuel channels, as well as severing most of the coolant lines feeding the reactor chamber, and as a result, the remaining coolant flashed to steam and escaped the reactor core. The total water loss combined with a high positive void coefficient further increased the reactor's thermal power.

A second, more powerful explosion occurred about two or three seconds after the first; this explosion dispersed the damaged core and effectively terminated the nuclear chain reaction. This explosion also compromised more of the reactor containment vessel and ejected hot lumps of graphite moderator. The ejected graphite and the demolished channels still in the remains of the reactor vessel caught fire on exposure to air, significantly contributing to the spread of radioactive fallout and the contamination of outlying areas. The explosion is estimated to have had the power equivalent of 225 tons of TNT.

After the larger explosion, several employees at the power station went outside to get a clearer view of the extent of the damage. One such survivor, Alexander Yuvchenko, recounts that once he stepped out and looked up towards the reactor hall, he saw a "very beautiful" laser-like beam of blue light caused by the ionized-air glow that appeared to be "flooding up into infinity".

Crisis management

Fire containment

Contrary to safety regulations, bitumen, a combustible material, had been used in the construction of the roof of the reactor building and the turbine hall. Ejected material ignited at least five fires on the roof of the adjacent reactor No. 3, which was still operating. It was imperative to put those fires out and protect the cooling systems of reactor No. 3. Inside reactor No. 3, the chief of the night shift, Yuri Bagdasarov, wanted to shut down the reactor immediately, but chief engineer Nikolai Fomin would not allow this. The operators were given respirators and potassium iodide tablets and told to continue working. At 05:00, Bagdasarov made his own decision to shut down the reactor, which was confirmed in writing by Dyatlov and Station Shift Supervisor Rogozhkin.

Shortly after the accident, firefighters arrived to try to extinguish the fires. First on the scene was a Chernobyl Power Station firefighter brigade under the command of Lieutenant Volodymyr Pravyk, who died on 11 May 1986 of acute radiation sickness. They were not told how dangerously radioactive the smoke and the debris were, and may not even have known that the accident was anything more than a regular electrical fire: "We didn't know it was the reactor. No one had told us." Grigorii Khmel, the driver of one of the fire engines, later described what happened:

We arrived there at 10 or 15 minutes to two in the morning ... We saw graphite scattered about. Misha asked: "Is that graphite?" I kicked it away. But one of the fighters on the other truck picked it up. "It's hot," he said. The pieces of graphite were of different sizes, some big, some small enough to pick them up [...] We didn't know much about radiation. Even those who worked there had no idea. There was no water left in the trucks. Misha filled a cistern and we aimed the water at the top. Then those boys who died went up to the roof-Vashchik, Kolya and others, and Volodya Pravik ... They went up the ladder ... and I never saw them again.

Anatoli Zakharov, a fireman stationed in Chernobyl since 1980, offered a different description in 2008: "I remember joking to the others, 'There must be an incredible amount of radiation here. We'll be lucky if we're all still alive in the morning.'" He also stated, "Of course we knew! If we'd followed regulations, we would never have gone near the reactor. But it was a moral obligation-our duty. We were like kamikaze."

It was thought by some that the core fire was extinguished by a combined effort of helicopters dropping more than 5,000 tonnes (11 million pounds) of sand, lead, clay, and neutron-absorbing boron onto the burning reactor. It is now known that virtually none of these materials reached the core. Historians estimate that about 600 Soviet pilots risked dangerous levels of radiation to fly the thousands of flights needed to cover reactor No. 4 in this attempt to seal off radiation.

From eyewitness accounts of the firefighters involved before they died (as reported on the CBC television series ), one described his experience of the radiation as "tasting like metal", and feeling a sensation similar to that of pins and needles all over his face. This is consistent with the description given by Louis Slotin, a Manhattan Project physicist who died days after a fatal radiation overdose from a criticality accident.

The explosion and fire threw hot particles of the nuclear fuel and also far more dangerous fission products (radioactive isotopes such as caesium-137, iodine-131, strontium-90, and other radionuclides) into the air. The residents of the surrounding area observed the radioactive cloud on the night of the explosion.

Radiation levels

The ionizing radiation levels in the worst-hit areas of the reactor building have been estimated to be 5.6 roentgens per second (R/s), equivalent to more than 20,000 roentgens per hour. A lethal dose is around 500 roentgens (~5 Gray (Gy) in modern radiation units) over five hours, so in some areas, unprotected workers received fatal doses in less than a minute. Unfortunately, a dosimeter capable of measuring up to 1,000 R/s was buried in the rubble of a collapsed part of the building, and another one failed when turned on. Most remaining dosimeters had limits of 0.001 R/s and therefore read "off scale". Thus, the reactor crew could ascertain only that the radiation levels were somewhere above 0.001 R/s (3.6 R/h), while the true levels were much higher in some areas.

Because of the inaccurate low readings, the reactor crew chief Aleksandr Akimov assumed that the reactor was intact. The evidence of pieces of graphite and reactor fuel lying around the building was ignored, and the readings of another dosimeter brought in by 04:30 were dismissed under the assumption that the new dosimeter must have been defective. Akimov stayed with his crew in the reactor building until morning, sending members of his crew to try to pump water into the reactor. None of them wore any protective gear. Most, including Akimov, died from radiation exposure within three weeks.

Evacuation

The nearby city of Pripyat was not immediately evacuated. The townspeople, in the early hours of the morning, at 01:23 local time, went about their usual business, completely oblivious to what had just happened. However, within a few hours of the explosion, dozens of people fell ill. Later, they reported severe headaches and metallic tastes in their mouths, along with uncontrollable fits of coughing and vomiting. As the plant was run by authorities in Moscow, the government of Ukraine did not receive prompt information on the accident.

Shevchenko then spoke over the phone to Volodymyr Shcherbytsky, general secretary of the Communist Party of Ukraine and de facto head of state, who said he anticipated a delegation of the state commission headed by Boris Shcherbina, the deputy chairman of the Council of Ministers of the USSR.

A commission was established later in the day to investigate the accident. It was headed by Valery Legasov, First Deputy Director of the Kurchatov Institute of Atomic Energy, and included leading nuclear specialist Evgeny Velikhov, hydro-meteorologist Yuri Izrael, radiologist Leonid Ilyin, and others. They flew to Boryspil International Airport and arrived at the power plant in the evening of 26 April. By that time two people had already died and 52 were hospitalized. The delegation soon had ample evidence that the reactor was destroyed and extremely high levels of radiation had caused a number of cases of radiation exposure. In the early daylight hours of 27 April, approximately 36 hours after the initial blast, they ordered the evacuation of Pripyat. Initially it was decided to evacuate the population for three days; later this was made permanent.

By 11:00 on 27 April, buses had arrived in Pripyat to start the evacuation. The evacuation began at 14:00. A translated excerpt of the evacuation announcement follows:

For the attention of the residents of Pripyat! The City Council informs you that due to the accident at Chernobyl Power Station in the city of Pripyat the radioactive conditions in the vicinity are deteriorating. The Communist Party, its officials and the armed forces are taking necessary steps to combat this. Nevertheless, with the view to keep people as safe and healthy as possible, the children being top priority, we need to temporarily evacuate the citizens in the nearest towns of Kiev region. For these reasons, starting from 27 April 1986, 14:00 each apartment block will be able to have a bus at its disposal, supervised by the police and the city officials. It is highly advisable to take your documents, some vital personal belongings and a certain amount of food, just in case, with you. The senior executives of public and industrial facilities of the city has decided on the list of employees needed to stay in Pripyat to maintain these facilities in a good working order. All the houses will be guarded by the police during the evacuation period. Comrades, leaving your residences temporarily please make sure you have turned off the lights, electrical equipment and water and shut the windows. Please keep calm and orderly in the process of this short-term evacuation.

To expedite the evacuation, residents were told to bring only what was necessary, and that they would remain evacuated for approximately three days. As a result, most personal belongings were left behind, and residents were only allowed to recover certain items after months had passed. By 15:00, 53,000 people were evacuated to various villages of the Kiev region. The next day, talks began for evacuating people from the 10-kilometre (6.2 mi) zone. Ten days after the accident, the evacuation area was expanded to 30 kilometres (19 mi).: 115, 120-121  The Chernobyl Nuclear Power Plant Exclusion Zone has remained ever since, although its shape has changed and its size has been expanded.

Official announcement

Evacuation began one and a half days before the accident was publicly acknowledged by the Soviet Union. In the morning of 28 April, radiation levels set off alarms at the Forsmark Nuclear Power Plant in Sweden, over 1,000 kilometres (620 mi) from the Chernobyl Plant. Workers at Forsmark reported the case to the Swedish Radiation Safety Authority, which determined that the radiation had originated elsewhere. That day, the Swedish government contacted the Soviet government to inquire about whether there had been a nuclear accident in the Soviet Union. The Soviets initially denied it, and it was only after the Swedish government suggested they were about to file an official alert with the International Atomic Energy Agency, that the Soviet government admitted that an accident had taken place at Chernobyl.

At first, the Soviets only conceded that a minor accident had occurred, but once they began evacuating more than 100,000 people, the full scale of the situation was realized by the global community. At 21:02 the evening of 28 April, a 20-second announcement was read in the TV news programme : "There has been an accident at the Chernobyl Nuclear Power Plant. One of the nuclear reactors was damaged. The effects of the accident are being remedied. Assistance has been provided for any affected people. An investigative commission has been set up."

This was the entire announcement, and the first time the Soviet Union officially announced a nuclear accident. The Telegraph Agency of the Soviet Union (TASS) then discussed the Three Mile Island accident and other American nuclear accidents, which Serge Schmemann of The New York Times wrote was an example of the common Soviet tactic of whataboutism. The mention of a commission also indicated to observers the seriousness of the incident, and subsequent state radio broadcasts were replaced with classical music, which was a common method of preparing the public for an announcement of a tragedy in the USSR.

Around the same time, ABC News released its report about the disaster. Shevchenko was the first of the Ukrainian state top officials to arrive at the disaster site early on 28 April. There she spoke with members of medical staff and people, who were calm and hopeful that they could soon return to their homes. Shevchenko returned home near midnight, stopping at a radiological checkpoint in Vilcha, one of the first that were set up soon after the accident.

There was a notification from Moscow that there was no reason to postpone the 1 May International Workers' Day celebrations in Kiev (including the annual parade), but on 30 April a meeting of the Political bureau of the Central Committee of the CPSU took place to discuss the plan for the upcoming celebration. Scientists were reporting that the radiological background level in Kiev was normal. At the meeting, which was finished at 18:00, it was decided to shorten celebrations from the regular three and a half to four hours to under two hours.

Several buildings in Pripyat were officially kept open after the disaster to be used by workers still involved with the plant. These included the Jupiter factory (which closed in 1996) and the Azure Swimming Pool, used by the Chernobyl liquidators for recreation during the clean-up (which closed in 1998).

Core meltdown risk mitigation

Chernobyl lava-like corium, formed by fuel-containing mass, flowed into the basement of the plant.[69]

Bubbler pools

Two floors of bubbler pools beneath the reactor served as a large water reservoir for the emergency cooling pumps and as a pressure suppression system capable of condensing steam in case of a small broken steam pipe; the third floor above them, below the reactor, served as a steam tunnel. The steam released by a broken pipe was supposed to enter the steam tunnel and be led into the pools to bubble through a layer of water. After the disaster, the pools and the basement were flooded because of ruptured cooling water pipes and accumulated firefighting water.

The smoldering graphite, fuel and other material above, at more than 1,200 °C (2,190 °F), started to burn through the reactor floor and mixed with molten concrete from the reactor lining, creating corium, a radioactive semi-liquid material comparable to lava. It was feared that if this mixture melted through the floor into the pool of water, the resulting steam production would further contaminate the area or even cause a steam explosion, ejecting more radioactive material from the reactor. It became necessary to drain the pool. These fears ultimately proved unfounded, since corium began dripping harmlessly into the flooded bubbler pools before the water could be removed. The molten fuel hit the water and cooled into a light-brown ceramic pumice, whose low density allowed the substance to float on the water's surface.

Unaware of this fact, the government commission directed that the bubbler pools be drained by opening its sluice gates. The valves controlling it, however, were located in a flooded corridor in a subterranean annex adjacent to the reactor building. Volunteers in diving suits and respirators (for protection against radioactive aerosols), and equipped with dosimeters, entered the knee-deep radioactive water and managed to open the valves. These were the engineers Alexei Ananenko and Valeri Bezpalov (who knew where the valves were), accompanied by the shift supervisor Boris Baranov. All three men were awarded the Order For Courage by Ukrainian President Petro Poroshenko in May 2018.

Foundation protection measures

The government commission was concerned that the molten core would burn into the earth and contaminate groundwater below the reactor. To reduce the likelihood of this, it was decided to freeze the earth beneath the reactor, which would also stabilize the foundations. Using oil well drilling equipment, the injection of liquid nitrogen began on 4 May. It was estimated that 25 tonnes (55 thousand pounds) of liquid nitrogen per day would be required to keep the soil frozen at −100 °C (−148 °F). This idea was quickly scrapped.

Bolshov's graphite cooling plate, alongside the prior nitrogen injection proposal, were not used following the drop in aerial temperatures and indicative reports that the fuel melt had stopped. It was later determined that the fuel had flowed three floors, with a few cubic meters coming to rest at ground level. The precautionary underground channel with its active cooling was therefore deemed redundant, as the fuel was self-cooling. The excavation was then simply filled with concrete to strengthen the foundation below the reactor.

Immediate site and area remediation

Debris removal

In the months after the explosion, attention turned to removing the radioactive debris from the roof. While the worst of the radioactive debris had remained inside what was left of the reactor, it was estimated that there was approximately 100 tons of debris on that roof which had to be removed to enable the safe construction of the "sarcophagus"-a concrete structure that would entomb the reactor and reduce radioactive dust being released into the atmosphere. The initial plan was to use robots to clear the debris off the roof. The Soviets used approximately 60 remote-controlled robots, most of them built in the Soviet Union itself. Many failed due to the difficult terrain, combined with the effect of high radiation fields on their batteries and electronic controls. In 1987, Valery Legasov, first deputy director of the Kurchatov Institute of Atomic Energy in Moscow, said: "We learned that robots are not the great remedy for everything. Where there was very high radiation, the robot ceased to be a robot-the electronics quit working."

Consequently, the most highly radioactive materials were shoveled by Chernobyl liquidators from the military wearing heavy protective gear (dubbed "bio-robots"). These soldiers could only spend a maximum of 40-90 seconds working on the rooftops of the surrounding buildings because of the extremely high doses of radiation given off by the blocks of graphite and other debris. Though the soldiers were only supposed to perform the role of the "bio-robot" a maximum of once, some soldiers reported having done this task five or six times. Only 10% of the debris cleared from the roof was performed by robots; the other 90% was removed by 3,828 men who absorbed, on average, an estimated dose of 25 rem (250 mSv) of radiation each.

Construction of the sarcophagus

With the extinguishing of the open air reactor fire, the next step was to prevent the spread of contamination. This could be due to wind action which could carry away loose contamination, and by birds which could land within the wreckage and then carry contamination elsewhere. In addition, rainwater could wash contamination away from the reactor area and into the sub-surface water table, where it could migrate outside the site area. Rainwater falling on the wreckage could also weaken the remaining reactor structure by accelerating corrosion of steelwork. A further challenge was to reduce the large amount of emitted gamma radiation, which was a hazard to the workforce operating the adjacent reactor No. 3.

The solution chosen was to enclose the wrecked reactor by the construction of a huge composite steel and concrete shelter, which became known as the "Sarcophagus". It had to be erected quickly and within the constraints of high levels of ambient gamma radiation. The design started on 20 May 1986, 24 days after the disaster, and construction was from June to late November. This major construction project was carried out under the very difficult circumstances of high levels of radiation both from the core remnants and the deposited radioactive contamination around it.

The construction workers had to be protected from radiation, and techniques such as crane drivers working from lead-lined control cabins were employed. The construction work included erecting walls around the perimeter, clearing and surface concreting the surrounding ground to remove sources of radiation and to allow access for large construction machinery, constructing a thick radiation shielding wall to protect the workers in reactor No. 3, fabricating a high-rise buttress to strengthen weak parts of the old structure, constructing an overall roof, and provisioning a ventilation extract system to capture any airborne contamination arising within the shelter.

Investigations of the reactor condition

During the construction of the sarcophagus, a scientific team, as part of an investigation dubbed "Complex Expedition", re-entered the reactor to locate and contain nuclear fuel to prevent another explosion. These scientists manually collected cold fuel rods, but great heat was still emanating from the core. Rates of radiation in different parts of the building were monitored by drilling holes into the reactor and inserting long metal detector tubes. The scientists were exposed to high levels of radiation and radioactive dust.

Area cleanup

The official contaminated zones saw a massive clean-up effort lasting seven months. The official reason for such early (and dangerous) decontamination efforts, rather than allowing time for natural decay, was that the land must be repopulated and brought back into cultivation. Within fifteen months 75% of the land was under cultivation, even though only a third of the evacuated villages were resettled. Defence forces must have done much of the work. Yet this land was of marginal agricultural value. According to historian David Marples, the administration had a psychological purpose for the clean-up: they wished to forestall panic regarding nuclear energy, and even to restart the Chernobyl power station.: 78-79, 87, 192-193

Although a number of radioactive emergency vehicles were buried in trenches, many of the vehicles used by the liquidators, including the helicopters, still remained, as of 2018, parked in a field in the Chernobyl area. Scavengers have since removed many functioning, but highly radioactive, parts. Liquidators worked under deplorable conditions, poorly informed and with poor protection. Many, if not most of them, exceeded radiation safety limits.

The urban decontamination liquidators first washed buildings and roads with "Barda", a sticky polymerizing fluid, designed to entrap radioactive dust.[dubious ]

A unique "clean up" medal was given to the clean-up workers, known as "liquidators".

Investigations and the evolution of identified causes

To investigate the causes of the accident the IAEA used the International Nuclear Safety Advisory Group (INSAG), which had been created by the IAEA in 1985. It produced two significant reports on Chernobyl; INSAG-1 in 1986, and a revised report, INSAG-7 in 1992. In summary, according to INSAG-1, the main cause of the accident was the operators' actions, but according to INSAG-7, the main cause was the reactor's design. Both IAEA reports identified an inadequate "safety culture" (INSAG-1 coined the term) at all managerial and operational levels as a major underlying factor of different aspects of the accident. This was stated to be inherent not only in operations but also during design, engineering, construction, manufacture and regulation.

The INSAG-7 conclusion of major factors contributory to the accident was:

"The Accident is now seen to have been the result of concurrence of the following major factors: specific physical characteristics of the reactor; specific design features of the reactor control elements; and the fact that the reactor was brought to a state not specified by procedures or investigated by an independent safety body. Most importantly, the physical characteristics of the reactor made possible its unstable behaviour."

INSAG-1 report (1986)

The first official Soviet explanation of the accident was given by Soviet scientists and engineers to representatives of IAEA member states and other international organisations at the first Post-Accident Review Meeting, held at the IAEA in Vienna 25-29 August 1986. This explanation effectively placed the blame on the power plant operators. The IAEA INSAG-1 report followed shortly afterwards in September 1986, and on the whole also supported this view, based also on the information provided in discussions with the Soviet experts at the Vienna review meeting. In this view, the catastrophic accident was caused by gross violations of operating rules and regulations. For instance; "During preparation and testing of the turbine generator under run-down conditions using the auxiliary load, personnel disconnected a series of technical protection systems and breached the most important operational safety provisions for conducting a technical exercise."

It was stated that at the time of the accident the reactor was being operated with many key safety systems turned off, most notably the emergency core cooling system (ECCS), LAR (Local Automatic control system), and AZ (emergency power reduction system). Personnel had an insufficient understanding of technical procedures involved with the nuclear reactor, and knowingly ignored regulations to expedite the electrical test completion. Several procedural irregularities also helped to make the accident possible, one of which was insufficient communication between the safety officers and the operators in charge of the test.

Soviet criminal trial (1987)

The trial took place from 7 to 30 July 1987 in a temporary courtroom set up in the House of Culture in the city of Chernobyl, Ukraine. Five plant employees ( Anatoly S. Dyatlov, the former deputy chief engineer; Viktor P. Bryukhanov, the former plant director; Nikolai M. Fomin, the former chief engineer; Boris V. Rogozhin, the shift director of Reactor 4; and Aleksandr P. Kovalenko, the chief of Reactor 4); and Yuri A. Laushkin (Gosatomenergonadzor [USSR State Committee on Supervision of Safe Conduct of Work in Atomic Energy] inspector) were sentenced to ten, ten, ten, five, three, and two years respectively in labor camps. The families of Aleksandr Akimov, Leonid Toptunov and Valery Perevozchenko had received official letters, but prosecution against the employees had been terminated at their deaths.

Anatoly Dyatlov was found guilty "of criminal mismanagement of potentially explosive enterprises" and sentenced to ten years imprisonment-of which he would serve three-for the role that his oversight of the experiment played in the ensuing accident.

INSAG-7 report (1992)

A simplified diagram comparing the Chernobyl RBMK and the most common nuclear reactor design, the Light water reactor. RBMK issues: 1. Using a graphite moderator in a water-cooled reactor, permitting criticality in a total loss of coolant accident. 2. A positive steam void coefficient that made the destructive power excursion possible. 3. Control rods design; taking 18-20 seconds to be fully inserted, and with graphite tips that increased reactivity initially. 4. No reinforced containment building.[21][28][101]

In the INSAG-7 report, most of the earlier accusations against staff for breach of regulations were acknowledged to be either erroneous, being based on incorrect information obtained in August 1986, or were judged less relevant. The INSAG-7 report also reflected the view of the 1991 USSR State Commission account which held that the operators' actions in turning off the emergency core cooling system, interfering with the settings on the protection equipment, and blocking the level and pressure in the separator drum did not contribute to the original cause of the accident and its magnitude, although they may have been a breach of regulations. In fact, turning off the emergency system designed to prevent the two turbine generators from stopping was not a violation of regulations. Soviet authorities had identified a multitude of operator actions as regulation violations in the original 1986 report while no such regulations were in fact in place.

The primary design cause of the accident, as determined by INSAG-7, was a major deficiency in safety features, in particular the "positive scram" effect due to the control rods' graphite tips that actually initially increased reactivity when control rods entered the core to reduce reactivity. There was also an overly positive void coefficient of the reactor, whereby steam-generated voids in the fuel cooling channels would increase reactivity because neutron absorption was reduced, resulting in more steam generation, and thereby more voids; a regenerative process.

To avoid such conditions, it was necessary for the operators to track the value of the reactor operational reactivity margin (ORM) but this value was not readily available to the operators and they were not aware of the safety significance of ORM on void and power coefficients. However, regulations did forbid operating the reactor with a small margin of reactivity. Yet "post-accident studies have shown that the way in which the real role of the ORM is reflected in the Operating Procedures and design documentation for the RBMK-1000 is extremely contradictory", and furthermore, "ORM was not treated as an operational safety limit, violation of which could lead to an accident".

Even in this revised analysis, the human factor remained identified as a major factor in causing the accident; particularly the operating crew's deviation from the test programme. "Most reprehensibly, unapproved changes in the test procedure were deliberately made on the spot, although the plant was known to be in a very different condition from that intended for the test." This included operating the reactor at a lower power level than the prescribed 700 MW before starting the electrical test. The 1986 assertions of Soviet experts notwithstanding, regulations did not prohibit operating the reactor at this low power level.

Positive void coefficient

Control rod design

There was a significant flaw in the design of the control rods. The reactor core was 7 metres (23 feet) high. The upper half of the rod 7 metres (23 feet) was boron carbide, which absorbs neutrons and thereby slows the reaction. The bottom section of each control rod was a 4.5 meter graphite displacer, which prevented the channels from filling with water when rods were withdrawn. The flaw lay in the 1.25-metre (4.1-foot) gap between the bottom of the graphite displacer and the bottom of the reactor, meaning that the lowest portion of control rod channel was filled with water and not graphite. See page 123. Fig 11-10. With this design, when the rods were inserted from the fully retracted position to stop the reaction on the AZ-5 signal, the graphite displaced neutron-absorbing water, causing fewer neutrons to be absorbed and increasing reactivity. For the first 11 to 14 seconds of rod deployment until the boron was in position, reactor power across the floor of the reactor could increase, rather than decrease. This feature of control rod operation was counter-intuitive and not known to the reactor operators.

Management and operational deficiencies

Other deficiencies were noted in the RBMK-1000 reactor design, as were its non-compliance with accepted standards and with the requirements of nuclear reactor safety. While INSAG-1 and INSAG-7 reports both identified operator error as an issue of concern, the INSAG-7 identified that there were numerous other issues that were contributing factors that led to the incident. These contributing factors include:

1. The plant was not designed to safety standards in effect and incorporated unsafe features

2. "Inadequate safety analysis" was performed

3. There was "insufficient attention to independent safety review"

4. "Operating procedures not founded satisfactorily in safety analysis"

5. Safety information not adequately and effectively communicated between operators, and between operators and designers

6. The operators did not adequately understand safety aspects of the plant

7. Operators did not sufficiently respect formal requirements of operational and test procedures

8. The regulatory regime was insufficient to effectively counter pressures for production

9. There was a "general lack of safety culture in nuclear matters at the national level as well as locally"

Fizzled nuclear explosion hypothesis

Their evidence came from Cherepovets, a city 1,000 kilometres (620 mi) northeast of Chernobyl, where physicists from the V.G. Khlopin Radium Institute measured anomalous high levels of xenon-135 -a short half-life isotope-four days after the explosion. This meant that a nuclear event in the reactor may have ejected xenon to higher altitudes in the atmosphere than the later fire did, allowing widespread movement of xenon to remote locations. This was an alternative to the more accepted explanation of a positive-feedback power excursion where the reactor disassembled itself by steam explosion.

Pakhomov and Dubasov's nuclear fizzle hypothesis was examined in 2017 by Lars-Erik De Geer, Christer Persson and Henning Rodhe, who put the hypothesized fizzle event as the more probable cause of the first explosion. Both analyses argue that the nuclear fizzle event, whether producing the second or first explosion, consisted of a prompt chain reaction that was limited to a small portion of the reactor core, since self-disassembly occurs rapidly in fizzle events.

De Geer, Persson and Rodhe commented:

"We believe that thermal neutron mediated nuclear explosions at the bottom of a number of fuel channels in the reactor caused a jet of debris to shoot upwards through the refuelling tubes. This jet then rammed the tubes' 350kg plugs, continued through the roof and travelled into the atmosphere to altitudes of 2.5-3km where the weather conditions provided a route to Cherepovets. The steam explosion which ruptured the reactor vessel occurred some 2.7 seconds later."

This second explosion was estimated by the authors to have had the power equivalent of 225 tons of TNT.

Release and spread of radioactive materials

At Chernobyl, approximately 100,000 square kilometres (39,000 sq mi) of land was significantly contaminated with fallout, with the worst hit regions being in Belarus, Ukraine and Russia. Lower levels of contamination were detected over all of Europe except for the Iberian Peninsula. Most of the fallout with radioactive dust particles was released during the first ten days after the accident. By around May 2, a radioactive cloud had reached the Netherlands and Belgium.

It was Sweden's elevated radioactivity level, detected at noon on 28 April that, having been determined as not caused by a leak at the Swedish plant, first suggested a serious nuclear problem originating from the western Soviet Union. The evacuation of Pripyat on 27 April had been silently completed without the disaster being declared outside the Soviet Union. A rise in radiation levels in the subsequent days had also been measured in Finland, but a civil service strike delayed the response and publication.

Contamination from the Chernobyl accident was scattered irregularly depending on weather conditions, much of it deposited on mountainous regions such as the Alps, the Welsh mountains and the Scottish Highlands, where adiabatic cooling caused radioactive rainfall. The resulting patches of contamination were often highly localized, and localized water-flows contributed to large variations in radioactivity over small areas. Sweden and Norway also received heavy fallout when the contaminated air collided with a cold front, bringing rain. There was also groundwater contamination.

Rain was deliberately seeded over 10,000 square kilometres (3,900 sq mi) Belarus by the Soviet Air Force to remove radioactive particles from clouds heading toward highly populated areas. Heavy, black-coloured rain fell on the city of Gomel. Reports from Soviet and Western scientists indicate that the Belarusian SSR received about 60% of the contamination that fell on the former Soviet Union. However, the 2006 TORCH report stated that up to half of the volatile particles had actually landed outside the former USSR area currently making up of Ukraine, Belarus, and Russia. An unconnected large area in Russian SFSR south of Bryansk was also contaminated, as were parts of northwestern Ukrainian SSR. Studies in surrounding countries indicate that more than one million people could have been affected by radiation.

2016 data from a long-term monitoring program (The Korma Report II) shows a decrease in internal radiation exposure of the inhabitants of a region in Belarus close to Gomel. Resettlement may even be possible in prohibited areas provided that people comply with appropriate dietary rules.

In Western Europe, precautionary measures taken in response to the radiation included banning the importation of certain foods. A 2006 study by the French society for nuclear energy found that contamination was "relatively limited, diminishing from west to east", such that a hunter consuming 40 kilograms of contaminated wild boar in 1997 would be exposed to about one millisievert.

Relative isotopic abundances

At different times after the accident, different isotopes were responsible for the majority of the external dose. The remaining quantity of any radioisotope, and therefore the activity of that isotope, after 7 decay half-lives have passed, is less than 1% of its initial magnitude, and it continues to reduce beyond 0.78% after 7 half-lives to 0.10% remaining after 10 half-lives have passed and so on. Some radionuclides have decay products that are likewise radioactive, which is not accounted for here. The release of radioisotopes from the nuclear fuel was largely controlled by their boiling points, and the majority of the radioactivity present in the core was retained in the reactor.

All of the noble gases, including krypton and xenon, contained within the reactor were released immediately into the atmosphere by the first steam explosion. The atmospheric release of xenon-133, with a half-life of 5 days, is estimated at 5200 PBq.

50 to 60% of all core 1760×10radioiodine in the reactor, about 1760 PBq (¹⁵ becquerels), or about 0.4 kilograms (0.88 lb), was released, as a mixture of sublimed vapour, solid particles, and organic iodine compounds. Iodine-131 has a half-life of 8 days.

20 to 40% of all core caesium-137 was released, 85 PBq in all. Caesium was released in aerosol form; caesium-137, along with isotopes of strontium, are the two primary elements preventing the Chernobyl exclusion zone being re-inhabited. equals 24 kilograms of caesium-137. Cs-137 has a half-life of 30 years.

Tellurium-132, half-life 78 hours, an estimated 1150 PBq was released.

An early estimate for total nuclear fuel material released to the environment was %; this was later revised to %. This corresponds to the atmospheric emission of 6 tonnes (5.9 long tons; 6.6 short tons) of fragmented fuel.

Two sizes of particles were released: small particles of 0.3 to 1.5 micrometres, each an individually unrecognizable small dust or smog sized particulate matter and larger settling dust sized particles that therefore were quicker to fall-out of the air, of 10 micrometres in diameter. These larger particles contained about 80% to 90% of the released high boiling point or non-volatile radioisotopes; zirconium-95, niobium-95, lanthanum-140, cerium-144 and the transuranic elements, including neptunium, plutonium and the minor actinides, embedded in a uranium oxide matrix.

Environmental impact

Water bodies

The Chernobyl nuclear power plant is located next to the Pripyat River, which feeds into the Dnieper reservoir system, one of the largest surface water systems in Europe, which at the time supplied water to Kiev's 2.4 million residents, and was still in spring flood when the accident occurred. The radioactive contamination of aquatic systems therefore became a major problem in the immediate aftermath of the accident.

In the most affected areas of Ukraine, levels of radioactivity (particularly from radionuclides ¹³¹I, ¹³⁷Cs and ⁹⁰Sr) in drinking water caused concern during the weeks and months after the accident. Guidelines for levels of radioiodine in drinking water were temporarily raised to 3,700 Bq/L, allowing most water to be reported as safe. Officially it was stated that all contaminants had settled to the bottom "in an insoluble phase" and would not dissolve for 800-1000 years. A year after the accident it was announced that even the water of the Chernobyl plant's cooling pond was within acceptable norms. Despite this, two months after the disaster the Kiev water supply was switched from the Dnieper to the Desna River. Meanwhile, massive silt traps were constructed, along with an enormous 30-metre (98 ft) deep underground barrier to prevent groundwater from the destroyed reactor entering the Pripyat River.

Groundwater was not badly affected by the Chernobyl accident since radionuclides with short half-lives decayed away long before they could affect groundwater supplies, and longer-lived radionuclides such as radiocaesium and radiostrontium were adsorbed to surface soils before they could transfer to groundwater. However, significant transfers of radionuclides to groundwater have occurred from waste disposal sites in the 30 km (19 mi) exclusion zone around Chernobyl. Although there is a potential for transfer of radionuclides from these disposal sites off-site (i.e. out of the 30 km (19 mi) exclusion zone), the IAEA Chernobyl Report argues that this is not significant in comparison to current levels of washout of surface-deposited radioactivity.

Bio-accumulation of radioactivity in fish resulted in concentrations (both in western Europe and in the former Soviet Union) that in many cases were significantly above guideline maximum levels for consumption. Guideline maximum levels for radiocaesium in fish vary from country to country but are approximately 1000 Bq/kg in the European Union. In the Kiev Reservoir in Ukraine, concentrations in fish were in the range of 3000 Bq/kg during the first few years after the accident.

Chernobyl's radiocaesium deposits were used to calibrate sedimentation samples from Lake Qattinah in Syria. The provides a sharp, maximal, data point in radioactivity of the core sample at the 1986 depth, and acts as a date check on the depth of the in the core sample.

Flora, fauna, and funga

After the disaster, four square kilometres (1.5 sq mi) of pine forest directly downwind of the reactor turned reddish-brown and died, earning the name of the " Red Forest". Some animals in the worst-hit areas also died or stopped reproducing. Most domestic animals were removed from the exclusion zone, but horses left on an island in the Pripyat River 6 km (4 mi) from the power plant died when their thyroid glands were destroyed by radiation doses of 150-200 Sv. Some cattle on the same island died and those that survived were stunted because of thyroid damage. The next generation appeared to be normal. The mutation rates for plants and animals have increased by a factor of 20 because of the release of radionuclides from Chernobyl. There is evidence for elevated mortality rates and increased rates of reproductive failure in contaminated areas, consistent with the expected frequency of deaths due to mutations.

On farms in Narodychi Raion of Ukraine it is claimed that from 1986 to 1990 nearly 350 animals were born with gross deformities such as missing or extra limbs, missing eyes, heads or ribs, or deformed skulls; in comparison, only three abnormal births had been registered in the five years prior.

Subsequent research on microorganisms, while limited, suggests that in the aftermath of the disaster, bacterial and viral specimens exposed to the radiation (including Mycobacterium tuberculosis, herpesvirus, cytomegalovirus, hepatitis-causing viruses, and tobacco mosaic virus) underwent rapid changes. Activations of soil micromycetes have been reported. It is currently unclear how these changes in species with rapid reproductive turnover (which were not destroyed by the radiation but instead survived) will manifest in terms of virulence, drug resistance, immune evasion, and so on; a paper in 1998 reported the discovery of an Escherichia coli mutant that was hyper-resistant to a variety of DNA-damaging elements, including x-ray radiation, UV-C, and 4-nitroquinoline 1-oxide (4NQO). Cladosporium sphaerospermum, a species of fungus that has thrived in the Chernobyl contaminated area, has been investigated for the purpose of using the fungus' particular melanin to protect against high-radiation environments, such as space travel. The disaster has been described by lawyers, academics and journalists as an example of ecocide.

Human food chain

With radiocaesium binding less with humic acid, peaty soils than the known binding "fixation" that occurs on kaolinite rich clay soils, many marshy areas of Ukraine had the highest soil to dairy-milk transfer coefficients, of soil activity in ~ 200 kBq/m ² to dairy milk activity in Bq/L, that had ever been reported, with the transfer, from initial land activity into milk activity, ranging from 0.3 −2 to 20 −2 times that which was on the soil, a variance depending on the natural acidity-conditioning of the pasture.

In 1987, Soviet medical teams conducted some 16,000 whole-body count examinations on inhabitants in otherwise comparatively lightly contaminated regions with good prospects for recovery. This was to determine the effect of banning local food and using only food imports on the internal body burden of radionuclides in inhabitants. Concurrent agricultural countermeasures were used when cultivation did occur, to further reduce the soil to human transfer as much as possible. The expected highest body activity was in the first few years, where the unabated ingestion of local food, primarily milk consumption, resulted in the transfer of activity from soil to body; after the dissolution of the USSR, the now-reduced scale initiative to monitor the human body activity in these regions of Ukraine, recorded a small and gradual half-decadal-long rise, in internal committed dose, before returning to the previous trend of observing ever lower body counts each year.

This momentary rise is hypothesized to be due to the cessation of the Soviet food imports together with many villagers returning to older dairy food cultivation practices and large increases in wild berry and mushroom foraging, the latter of which have similar peaty soil to fruiting body, radiocaesium transfer coefficients.

After the disaster, four square kilometres (1.5 sq mi) of pine forest directly downwind of the reactor turned reddish-brown and died, earning the name of the "Red Forest", though it soon recovered.[135] This photograph was taken years later, in March 2009,[146] after the forest began to grow again, with the lack of foliage at the time of the photograph merely due to the local winter at the time.[147]

Of the 440,350 wild boar killed in the 2010 hunting season in Germany, approximately one thousand were contaminated with levels of radiation above the permitted limit of 600 becquerels of caesium per kilogram, of dry weight, due to residual radioactivity from Chernobyl. While all animal meat contains a natural level of potassium-40 at a similar level of activity, with both wild and farm animals in Italy containing "415 ± 56 becquerels kg−1 dw" of that naturally occurring gamma emitter.

Because Elaphomyces fungal species bioaccumulate radiocaesium, boars of the Bavarian Forest that consume these "deer truffles" are contaminated at higher levels than their environment's soil. Given that nuclear weapons release a higher ¹³⁵C/ ¹³⁷C ratio than nuclear reactors, the high ¹³⁵ C content in these boars suggests that their radiological contamination can be largely attributed to the Soviet Union's nuclear weapons testing in Ukraine, which peaked during the late 1950s and early 1960s.

In 2015, long-term empirical data showed no evidence of a negative influence of radiation on mammal abundance.

Precipitation on distant high ground

On high ground, such as mountain ranges, there is increased precipitation due to adiabatic cooling. This resulted in localized concentrations of contaminants on distant areas; higher in Bq/m ² values to many lowland areas much closer to the source of the plume. This effect occurred on high ground in Norway and the UK.

The Norwegian Agricultural Authority reported that in 2009 a total of 18,000 livestock in Norway required uncontaminated feed for a period before slaughter, to ensure that their meat had an activity below the government permitted value of caesium per kilogram deemed suitable for human consumption. This contamination was due to residual radioactivity from Chernobyl in the mountain plants they graze on in the wild during the summer. 1,914 sheep required uncontaminated feed for a time before slaughter during 2012, with these sheep located in only 18 of Norway's municipalities, a decrease from the 35 municipalities in 2011 and the 117 municipalities affected during 1986. The after-effects of Chernobyl on the mountain lamb industry in Norway were expected to be seen for a further 100 years, although the severity of the effects would decline over that period. Scientists report this is due to radioactive caesium-137 isotopes being taken up by fungi such as Cortinarius caperatus which is in turn eaten by sheep while grazing.

United Kingdom

The United Kingdom restricted the movement of sheep from upland areas when radioactive caesium-137 fell across parts of Northern Ireland, Wales, Scotland, and northern England. In the immediate aftermath of the disaster in 1986, the movement of a total of 4,225,000 sheep was restricted across a total of 9,700 farms, to prevent contaminated meat entering the human food chain. The number of sheep and the number of farms affected has decreased since 1986. Northern Ireland was released from all restrictions in 2000, and by 2009, 369 farms containing around 190,000 sheep remained under the restrictions in Wales, Cumbria, and northern Scotland. The restrictions applying in Scotland were lifted in 2010, while those applying to Wales and Cumbria were lifted during 2012, meaning no farms in the UK remain restricted because of Chernobyl fallout.

The legislation used to control sheep movement and compensate farmers (farmers were latterly compensated per animal to cover additional costs in holding animals prior to radiation monitoring) was revoked during October and November 2012, by the relevant authorities in the UK. Had restrictions in the UK not occurred, a heavy consumer of lamb meat would likely have received a dose of 4.1 mSv over a lifetime.

Human impact

Acute radiation effects and immediate aftermath

The only known, causal deaths from the accident involved workers in the plant and firefighters. The reactor explosion killed two engineers and severely burned two others who were among the 237 workers hospitalized in the immediate aftermath. Of the hospitalized workers, 134 exhibited symptoms of acute radiation syndrome, including one disputed case. 28 of the hospitalized workers died within the following three months, all of whom were hospitalized for ARS and 26 were among the 56 patients hospitalized for burns. Among the fatalities in the acute phase (approximately three months), all but one patient (with grade 2 ARS) were hospitalized for grade 3 or 4 ARS. Fifteen out of 22 patients with grade 3 ARS survived. Only one patient out of 21 with grade 4 ARS survived.

There were a number of fishermen on the reservoir a half-kilometer from the reactor to the east. Of these, two shore fishermen, Protosov and Pustavoit, are said to have sustained doses estimated at 400 roentgens and vomited, but survived. The vast majority of Pripyat residents slept through the distant sound of the explosion, including station engineer Breus, who only became aware at 6am, the beginning of his next work shift. He was later taken to hospital and, while there, made the acquaintance of one teen who had ventured out alone by bicycle to watch the roof fires during the night, stopping for a time and viewing the scene at the "Bridge of Death" . Contrary to this sensationalist label, the youthful night biker was treated and released from hospital, remaining in touch with Breus as of 2019.

Most serious cases of ARS were treated with the assistance of American specialist Robert Peter Gale, who documented a first of its kind treatment and supervized a number of bone marrow transplant procedures which were not successful. In 2019, Gale wrote a letter to correct the popularized, though egregious, portrayal of his patients as dangerous to visitors. All those who died were station operators and firefighters, over half of which from the continued wearing of dusty soaked uniforms, causing beta burns to cover large areas of skin. In the first few minutes to days, (largely due to Np-239, a 2.4-day half-life) the beta-to-gamma energy ratio is some 30:1. Owing to the large area of burned skin and sensitivity of the GI tract, bacterial infection was and remains the overarching concern to those affected with ARS, as a leading cause of death, quarantine from the outside environment is a part of the normal treatment protocol. Many of the surviving firefighters, continue to have skin that is atrophied, spider veined with underlying fibrosis due to experiencing extensive beta burns.

Long-term impact

In the 10 years following the accident, 14 more people who had been initially hospitalized (9 who had been hospitalized with ARS) died of various causes mostly unrelated to radiation exposure. Only two of these deaths were the result of myelodysplastic syndrome. Scientific consensus, in the form of the Chernobyl Forum, suggests that, although unexpected, there has been no statistically significant increase in the incidence rate of solid cancers among rescue workers. Follow-up studies have also found this to be the case, with apparent increases in thyroid cancer simply attributed to more meticulous cancer screening for rescue workers.

Childhood thyroid cancer is an exception, with approximately 4,000 new incidents in the general population by 2002 within contaminated regions of Belarus, Russia, and Ukraine, most of which are attributed to high environmental levels of radioactive iodine shortly after the accident. The recovery rate is ~99%, with only 15 terminal cases (9 deaths) at the time of the report. There has been no increase in mutation rate among the children of the liquidators or general population living in the contaminated areas.

From this same report is also a commonly cited estimate for potential future cancer fatalities in the form of an increase in cancer mortality (i.e. lethality) which speculated that, at worst, ~4,000 additional cancer-related fatalities were to be expected. Although it is reasonable and forward-thinking to assume that an increase in mortality has occurred among the affected population, studies have yet to confirm such an increase with meaningful statistical certainty.

Whether the area was publicly announced as a contaminated area is a better predictor of general health than the contamination itself. "Resettlement status" is an even stronger predictor: the residents of contaminated regions who were evacuated and resettled into uncontaminated regions can be compared with the residents who remained in the contaminated regions. Resettled citizens erroneously believed they had an illness related to radiation exposure more often than citizens who remained in the contaminated regions. This brings into question the effectiveness of resettlement.

By 2000, the number of Ukrainians claiming to be radiation "sufferers" ( poterpili) and receiving state benefits had jumped to 3.5 million, or 5% of the population. Many of these are populations resettled from contaminated zones or former or current Chernobyl plant workers. There was and remains a motivated "push" to achieve "sufferer" status as it gives access to state benefits and medical services that would otherwise not be made available. The apparent increases of ill health in this large group result partly from increased medical vigilance following the accident; many benign cases that would previously have gone unnoticed and untreated (especially of cancer) are now being registered.

In September 1987, the IAEA held an Advisory Group Meeting at the Curie Institute in Paris on the medical handling of the skin lesions relating to the acute deaths.

Effects of main harmful radionuclides

The four most harmful radionuclides spread from Chernobyl were iodine-131, caesium-134, caesium-137 and strontium-90, with half-lives of 8.02 days, 2.07 years, 30.2 years and 28.8 years respectively. The iodine was initially viewed with less alarm than the other isotopes, because of its short half-life, but it is highly volatile and now appears to have travelled furthest and caused the most severe health problems. Strontium is the least volatile of the four and is of main concern in the areas near Chernobyl itself.

Long term hazards such as caesium tends to accumulate in vital organs such as the heart, while strontium accumulates in bones and may thus be a risk to bone-marrow and lymphocytes. Radiation is most damaging to cells that are actively dividing. In adult mammals cell division is slow, except in hair follicles, skin, bone marrow and the gastrointestinal tract, which is why vomiting and hair loss are common symptoms of acute radiation sickness.

Disputed investigation

The two primary individuals involved with the attempt to suggest that the mutation rate among animals was, and continues to be, higher in the Chernobyl zone, are the Anders Moller and Timothy Mousseau group. Apart from continuing to publish experimentally unrepeatable and discredited papers, Mousseau routinely gives talks at the Helen Caldicott organized symposiums for " Physicians for Social Responsibility", an anti-nuclear advocacy group devoted to bring about a "nuclear free planet". In years past, Moller was previously caught and reprimanded for publishing papers that crossed the scientific "misconduct"/"fraud" line. The duo have more recently attempted to publish meta-analyses, in which the primary references they weigh-up, analyse and draw their conclusions from is their own prior papers along with the discredited book Chernobyl: Consequences of the Catastrophe for People and the Environment.

Withdrawn investigation

In 1996, geneticist colleagues Ronald Chesser and Robert Baker published a paper on the thriving vole population within the exclusion zone, in which the central conclusion of their work was essentially that "The mutation rate in these animals is hundreds and probably thousands of times greater than normal". This claim occurred after they had done a comparison of the mitochondrial DNA of the "Chernobyl voles" with that of a control group of voles from outside the region. The paper appeared on the front cover of the journal Nature. Not long after publication, the authors discovered they had incorrectly classified the species of vole and therefore were genetically comparing two entirely different vole species. They issued a retraction in 1997.

Following the accident, journalists mistrusted many medical professionals (such as the spokesman from the UK National Radiological Protection Board), and in turn encouraged the public to mistrust them. Throughout the European continent, due to this media-driven framing of the contamination, many requests for induced abortions of otherwise normal pregnancies were obtained out of fears of radiation from Chernobyl.

Worldwide, an estimated excess of about 150,000 elective abortions may have been performed on otherwise healthy pregnancies out of fears of radiation from Chernobyl, according to Robert Baker and ultimately a 1987 article published by Linda E. Ketchum in the Journal of Nuclear Medicine which mentions but does not reference an IAEA source on the matter.

The available statistical data excludes the Soviet-Ukraine-Belarus abortion rates, as they are presently unavailable. From the available data, an increase in the number of abortions in what were healthy developing human offspring in Denmark occurred in the months following the accident, at a rate of about 400 cases. In Italy, a "slightly" above the expected number of induced abortions occurred, approximately 100. In Greece, following the accident, many obstetricians were unable to resist requests from worried pregnant mothers over fears of radiation. Although it was determined that the effective dose to Greeks would not exceed one mSv (100 mrem), a dose much lower than that which it was determined would induce embryonic abnormalities or other non- stochastic effects, there was an observed 2,500 increase of otherwise wanted pregnancies being terminated.

No evidence of changes in the prevalence of human deformities/birth congenital anomalies that might be associated with the accident are apparent in Belarus or Ukraine, the two republics that had the highest exposure to fallout. In Sweden and in Finland where no increase in abortion rates occurred, it was likewise determined that "no association between the temporal and spatial variations in radioactivity and variable incidence of congenital malformations [was found]." A similar null increase in the abortion rate and a healthy baseline situation of no increase in birth defects was determined by assessing the Hungarian Congenital Abnormality Registry. Findings were also mirrored in Austria.

In very high doses, it was known at the time that radiation could cause a physiological increase in the rate of pregnancy anomalies, but unlike the dominant linear no-threshold model of radiation and cancer rate increases, it was known, by researchers familiar with both the prior human exposure data and animal testing, that the "Malformation of organs appears to be a deterministic effect with a threshold dose" below which, no rate increase is observed. This teratology (birth defects) issue was discussed by Frank Castronovo of the Harvard Medical School in 1999, publishing a detailed review of dose reconstructions and the available pregnancy data following the Chernobyl accident, inclusive of data from Kiev's two largest obstetrics hospitals.

The Chernobyl liquidators, essentially an all-male civil defense emergency workforce, would go on to father normal children, without an increase in developmental anomalies or a statistically significant increase in the frequencies of germline mutations in their progeny. This normality is similarly seen in the children of the survivors of the Goiânia accident.

A 2021 study based on whole-genome sequencing of children of parents employed as liquidators indicated no trans-generational genetic effects of exposure of parents to ionizing radiation.

Cancer assessments

A report by the International Atomic Energy Agency examines the environmental consequences of the accident. The United Nations Scientific Committee on the Effects of Atomic Radiation has estimated a global collective dose of radiation exposure from the accident "equivalent on average to 21 additional days of world exposure to natural background radiation"; individual doses were far higher than the global mean among those most exposed, including 530,000 primarily male recovery workers (the Chernobyl liquidators) who averaged an effective dose equivalent to an extra 50 years of typical natural background radiation exposure each.

Estimates of the number of deaths that will eventually result from the accident vary enormously; disparities reflect both the lack of solid scientific data and the different methodologies used to quantify mortality-whether the discussion is confined to specific geographical areas or extends worldwide, and whether the deaths are immediate, short term, or long term. In 1994, thirty-one deaths were directly attributed to the accident, all among the reactor staff and emergency workers.

In a peer-reviewed paper in the International Journal of Cancer in 2006, the authors expanded the discussion on those exposed to all of Europe (but following a different conclusion methodology to the Chernobyl Forum study, which arrived at the total predicted death toll of 4,000 after cancer survival rates were factored in) they stated, without entering into a discussion on deaths, that in terms of total excess cancers attributed to the accident:

The risk projections suggest that by now [2006] Chernobyl may have caused about 1000 cases of thyroid cancer and 4000 cases of other cancers in Europe, representing about 0.01% of all incident cancers since the accident. Models predict that by 2065 about 16,000 cases of thyroid cancer and 25,000 cases of other cancers may be expected due to radiation from the accident, whereas several hundred million cancer cases are expected from other causes.

Two anti-nuclear advocacy groups have publicized non-peer-reviewed estimates that include mortality estimates for those who were exposed to even smaller amounts of radiation. The Union of Concerned Scientists (UCS) calculated that, among the hundreds of millions of people exposed worldwide, there will be an eventual 50,000 excess cancer cases, resulting in 25,000 excess cancer deaths, excluding thyroid cancer. However, these calculations are based on a simple linear no-threshold model multiplication and the misapplication of the collective dose, which the International Commission on Radiological Protection (ICRP) states "should not be done" as using the collective dose is "inappropriate to use in risk projections".

Thyroid cancer incidence in children and adolescents in Belarus

Adolescents, ages 15 to 18

While widely regarded as having a cause and effect relationship, the causality of Chernobyl with the increases in recorded rates of thyroid cancer is disputed,[221] as in both the US and South Korea, upon the advent of ultrasonography and widespread medical screening, the latter recorded an almost identical epidemic in thyroid cancer rates, with South Korea reporting a 15 fold increase upon the switch of diagnostic tool, the highest thyroid cancer rate in the world.[222]

Yet the death rate from thyroid cancer has remained the same as prior to the technology. For these and other reasons, it is suggested that no reliable increase has been detected in the environs of Chernobyl, that cannot otherwise be explained as an artifact of the globally well documented Screening effect. In 2004, the UN collaborative, Chernobyl Forum, revealed thyroid cancer among children to be one of the main health impacts from the Chernobyl accident. This is due to the ingestion of contaminated dairy products, along with the inhalation of the short-lived, highly radioactive isotope, Iodine-131. In that publication, more than 4,000 cases of childhood thyroid cancer were reported. It is important to note that there was no evidence of an increase in solid cancers or leukemia. It said that there was an increase in psychological problems among the affected population. The WHO's Radiation Program reported that the 4,000 cases of thyroid cancer resulted in nine deaths.

According to the United Nations Scientific Committee on the Effects of Atomic Radiation, up to the year 2005, an excess of more than 6,000 cases of thyroid cancer had been reported. That is, over the estimated pre-accident baseline thyroid cancer rate, more than 6,000 casual cases of thyroid cancer have been reported in children and adolescents exposed at the time of the accident, a number that is expected to increase. They concluded that there is no other evidence of major health impacts from the radiation exposure.

Well-differentiated thyroid cancers are generally treatable, and when treated the five-year survival rate of thyroid cancer is 96%, and 92% after 30 years. the United Nations Scientific Committee on the Effects of Atomic Radiation had reported 15 deaths from thyroid cancer in 2011. The International Atomic Energy Agency (IAEA) also states that there has been no increase in the rate of birth defects or abnormalities, or solid cancers-such as lung cancer-corroborating the assessments by the UN committee. UNSCEAR raised the possibility of long term genetic defects, pointing to a doubling of radiation-induced minisatellite mutations among children born in 1994. However, the risk of thyroid cancer associated with the Chernobyl accident is still high according to published studies.

The German affiliate of the anti-nuclear energy organization, the International Physicians for the Prevention of Nuclear War suggest that 10,000 people are affected by thyroid cancer as of 2006, and that 50,000 cases are expected in the future.

Other disorders

The report went into depth about the risks to mental health of exaggerated fears about the effects of radiation. According to the IAEA the "designation of the affected population as 'victims' rather than 'survivors' has led them to perceive themselves as helpless, weak and lacking control over their future". The IAEA says that this may have led to behaviour that has caused further health effects.

Long-term radiation deaths

The number of potential deaths arising from the Chernobyl disaster is heavily debated. The World Health Organization's prediction of 4,000 future cancer deaths in surrounding countries is based on the Linear no-threshold model (LNT), which assumes that the damage inflicted by radiation at low doses is directly proportional to the dose. Radiation epidemiologist Roy Shore contends that estimating health effects in a population from the LNT model "is not wise because of the uncertainties".

According to the Union of Concerned Scientists the number of excess cancer deaths worldwide (including all contaminated areas) is approximately 27,000 based on the same LNT.

Chernobyl: Consequences of the Catastrophe for People and the Environment is a 2007 Russian publication that concludes that there were 985,000 premature deaths as a consequence of the radioactivity released. The results were criticized by M. I. Balonov from the Institute of Radiation Hygiene in St. Petersburg, who described them as biased, drawing from sources that were difficult to independently verify and lacking a proper scientific base. Balanov expressed his opinion that "the authors unfortunately did not appropriately analyse the content of the Russian-language publications, for example, to separate them into those that contain scientific evidence and those based on hasty impressions and ignorant conclusions".

According to U.S. Nuclear Regulatory Commission member and Professor of Health Physics Kenneth Mossman, the "LNT philosophy is overly conservative, and low-level radiation may be less dangerous than commonly believed." Yoshihisa Matsumoto, a radiation biologist at the Tokyo Institute of Technology, cites laboratory experiments on animals to suggest there must be a threshold dose below which DNA repair mechanisms can completely repair any radiation damage. Mossman suggests that the proponents of the current model believe that being conservative is justified due to the uncertainties surrounding low level doses and it is better to have a "prudent public health policy".

Another significant issue is establishing consistent data on which to base the analysis of the impact of the Chernobyl accident. Since 1991, large social and political changes have occurred within the affected regions and these changes have had significant impact on the administration of health care, on socio-economic stability, and the manner in which statistical data is collected. Ronald Chesser, a radiation biologist at Texas Tech University, says that "the subsequent Soviet collapse, scarce funding, imprecise dosimetry, and difficulties tracking people over the years have limited the number of studies and their reliability".

Socio-economic impact

It is difficult to establish the total economic cost of the disaster. According to Mikhail Gorbachev, the Soviet Union spent 18 billion Rbls (the equivalent of US$2.5 billion at that time, or $5.32 billion in today's dollars) on containment and decontamination, virtually bankrupting itself. In 2005, the total cost over 30 years for Belarus which includes the monthly payments to liquidators, was estimated at US$235 billion; about $318 billion in today's dollars given inflation rates. Gorbachev in April 2006 wrote "The nuclear meltdown at Chernobyl 20 years ago this month, even more than my launch of perestroika, was perhaps the real cause of the collapse of the Soviet Union."

Ongoing costs are well known; in their 2003-2005 report, The Chernobyl Forum stated that between five and seven percent of government spending in Ukraine is still related to Chernobyl, while in Belarus more than $13 billion is thought to have been spent between 1991 and 2003, with 22% of national budget having been Chernobyl-related in 1991, falling to six percent by 2002. In 2018, Ukraine spent five to seven percent of its national budget on recovery activities related to the Chernobyl disaster. Overall economic loss is estimated at $235 billion in Belarus. Much of the current cost relates to the payment of Chernobyl-related social benefits to some seven million people across the three countries.

A significant economic impact at the time was the removal of 784,320 ha (1,938,100 acres) of agricultural land and 694,200 ha (1,715,000 acres) of forest from production. While much of this has been returned to use, agricultural production costs have risen due to the need for special cultivation techniques, fertilizers and additives. Politically, the accident gave great significance to the new Soviet policy of glasnost, and helped forge closer Soviet-US relations at the end of the Cold War, through bioscientific cooperation. The disaster also became a key factor in the dissolution of the Soviet Union in 1991, and a major influence in shaping the new Eastern Europe.

Ukrainians viewed the Chernobyl disaster as another attempt by Russians to destroy them, comparable to the Holodomor. Meanwhile, commentators have argued that the events of the Chernobyl disaster were uniquely inclined to occur in a communist country versus a capitalist country. It has been argued that Soviet power plant administrators were not empowered to make crucial decisions when time was of the essence.

Long term site remediation

Portraits of deceased Chernobyl liquidators used for an anti-nuclear power protest in Geneva

Following the accident, questions arose about the future of the plant and its eventual fate. All work on the unfinished reactors No. 5 and No. 6 was halted three years later. However, the trouble at the Chernobyl plant did not end with the disaster in reactor No. 4. The damaged reactor was sealed off and 200 cubic meters (260 cu yd) of concrete was placed between the disaster site and the operational buildings. The work was managed by Grigoriy Mihaylovich Naginskiy, the deputy chief engineer of Installation and Construction Directorate - 90. The Ukrainian government allowed the three remaining reactors to continue operating because of an energy shortage in the country.

Decommissioning of other reactors

In October 1991, a fire broke out in the turbine building of reactor No. 2; the authorities subsequently declared the reactor damaged beyond repair, and it was taken offline. Reactor No. 1 was decommissioned in November 1996 as part of a deal between the Ukrainian government and international organizations such as the IAEA to end operations at the plant. On 15 December 2000, then-President Leonid Kuchma personally turned off reactor No. 3 in an official ceremony, shutting down the entire site.

No. 4 reactor confinement

Soon after the accident, the reactor building was quickly encased by a mammoth concrete sarcophagus in a notable feat of construction under severe conditions. Crane operators worked blindly from inside lead-lined cabins taking instructions from distant radio observers, while gargantuan-sized pieces of concrete were moved to the site on custom-made vehicles. The purpose of the sarcophagus was to stop any further release of radioactive particles into the atmosphere, isolate the exposed core from the weather and provide safety for the continued operations of adjacent reactors one through three.

The concrete sarcophagus was never intended to last very long, with a lifespan of only 30 years. On 12 February 2013, a 600 m ² (6,500 sq ft) section of the roof of the turbine-building collapsed, adjacent to the sarcophagus, causing a new release of radioactivity and temporary evacuation of the area. At first it was assumed that the roof collapsed because of the weight of snow, however the amount of snow was not exceptional, and the report of a Ukrainian fact-finding panel concluded that the collapse was the result of sloppy repair work and aging of the structure. Experts warned the sarcophagus itself was on the verge of collapse.

In 1997, the international Chernobyl Shelter Fund was founded to design and build a more permanent cover for the unstable and short-lived sarcophagus. It received €864 million from international donors in 2011 and was managed by the European Bank for Reconstruction and Development (EBRD). The new shelter was named the New Safe Confinement and construction began in 2010. It is a metal arch 105 metres (344 ft) high and spanning 257 metres (843 ft) built on rails adjacent to the reactor No. 4 building so that it could be slid over the top of the existing sarcophagus. The New Safe Confinement was completed in 2016 and slid into place over top the sarcophagus on 29 November. The huge steel arch was moved into place over several weeks. Unlike the original sarcophagus, the New Safe Confinement is designed to allow the reactor to be safely dismantled using remotely operated equipment.

Waste management

Used fuel from units 1-3 was stored in the units' cooling ponds, and in an interim spent fuel storage facility pond, ISF-1, which now holds most of the spent fuel from units 1-3, allowing those reactors to be decommissioned under less restrictive conditions. Approximately 50 of the fuel assemblies from units 1 and 2 were damaged and required special handling. Moving fuel to ISF-1 was thus carried out in three stages: fuel from unit 3 was moved first, then all undamaged fuel from units 1 and 2, and finally the damaged fuel from units 1 and 2. Fuel transfers to ISF-1 were completed in June 2016.

A need for larger, longer-term radioactive waste management at the Chernobyl site is to be fulfilled by a new facility designated ISF-2. This facility is to serve as dry storage for used fuel assemblies from units 1-3 and other operational wastes, as well as material from decommissioning units 1-3 (which will be the first RBMK units decommissioned anywhere).

Fuel-containing materials

According to official estimates, about 95% of the fuel in reactor No. 4 at the time of the accident (about 180 tonnes (180 long tons; 200 short tons)) remains inside the shelter, with a total radioactivity of nearly 18 million curies (670 PBq). The radioactive material consists of core fragments, dust, and lava-like "fuel containing materials" (FCM)-also called " corium"-that flowed through the wrecked reactor building before hardening into a ceramic form.

It has been reported that the degradation of the lava is likely to be a slow, gradual process, rather than sudden and rapid. The same paper states that the loss of uranium from the wrecked reactor is only 10 kg (22 lb) per year; this low rate of uranium leaching suggests that the lava is resisting its environment. The paper also states that when the shelter is improved, the leaching rate of the lava will decrease. As of 2021, some fuel had already degraded significantly. The famous elephant's foot, which originally was so hard that it required the use of an armor piercing AK-47 round to remove a chunk, had softened to a texture similar to sand.

Exclusion zone

Mass media sources have provided a variety of generalized estimates for when the Zone could be considered habitable again. These informal estimates have ranged from approximately 300 years (corresponding to 10 half-lives of the widespread radionuclides Cesium-137 and Strontium-90), to multiples of 20,000 years, referring to the half-life of Plutonium-239 which contaminates the central portion of the Zone.

In the years following the disaster, residents known as illegally returned to their abandoned homes to regain their lives. Most people are retired and survive mainly from farming and packages delivered by visitors. As of 2016, 187 locals had returned to the zone and were living permanently there.

In 2011, Ukraine opened up the sealed zone around the Chernobyl reactor to tourists wishing to learn more about the 1986 tragedy. Sergii Mirnyi, a radiation reconnaissance officer at the time of the accident, and now an academic at National University of Kyiv-Mohyla Academy, has written about the psychological and physical effects on survivors and visitors, and worked as an advisor to Chernobyl tourism groups.

Forest fire concerns

During the dry season, forest fires are a perennial concern in areas contaminated by radioactive material. Dry conditions and build-up of debris make the forests a ripe breeding ground for wildfires. Depending on prevailing atmospheric conditions, smoke from wildfires could potentially spread more radioactive material outside the exclusion zone. In Belarus, the Bellesrad organization is tasked with overseeing food cultivation and forestry management in the area.

Recovery projects

The Chernobyl Trust Fund was created in 1991 by the United Nations to help victims of the Chernobyl accident. It is administered by the United Nations Office for the Coordination of Humanitarian Affairs, which also manages strategy formulation, resource mobilization, and advocacy efforts. Beginning in 2002, under the United Nations Development Programme, the fund shifted its focus from emergency assistance to long-term development.

The Chernobyl Shelter Fund was established in 1997 at the G8 summit in Denver to finance the Shelter Implementation Plan (SIP). The plan called for transforming the site into an ecologically safe condition through stabilization of the sarcophagus and construction of a New Safe Confinement (NSC) structure. While the original cost estimate for the SIP was US$768 million, the 2006 estimate was $1.2 billion. The SIP is being managed by a consortium of Bechtel, Battelle, and , and conceptual design for the NSC consisted of a movable arch, constructed away from the shelter to avoid high radiation, then slid over the sarcophagus. The NSC was moved into position in November 2016 and was expected to be completed by late 2017.

In 2003, the United Nations Development Programme launched the Chernobyl Recovery and Development Programme (CRDP) for the recovery of affected areas. The programme was initiated in February 2002 based on the recommendations in the report on Human Consequences of the Chernobyl Nuclear Accident. The main goal of the CRDP was supporting the Government of Ukraine in mitigating long-term social, economic, and ecological consequences of the Chernobyl catastrophe. CRDP works in the four most affected Ukrainian areas: Kyivska, Zhytomyrska, Chernihivska and Rivnenska.

The International Project on the Health Effects of the Chernobyl Accident was created and received US$20 million, mainly from Japan, in hopes of discovering the main cause of health problems due to iodine-131 radiation. These funds were divided among Ukraine, Belarus, and Russia, the three main affected countries, for further investigation of health effects. As there was significant corruption in former Soviet countries, most of the foreign aid was given to Russia, and no results from the funding were demonstrated.

In 2019, it became known that the Ukrainian government in power at the time aimed to make Chernobyl a tourist attraction.

Nuclear debate

The Chernobyl accident attracted a great deal of interest. Because of the distrust that many people had in the Soviet authorities, which engaged in a major cover-up of the disaster, a great deal of debate about the situation at the site occurred in the First World during the early days of the event. Because of defective intelligence based on satellite imagery, it was thought that unit number three had also had a dire accident. Journalists mistrusted many professionals, and they in turn encouraged the public to mistrust them.

The accident also raised concerns about the cavalier safety culture in the Soviet nuclear power industry, slowing industry growth and forcing the Soviet government to become less secretive about its operating procedures. The government coverup of the Chernobyl disaster was a catalyst for glasnost, which "paved the way for reforms leading to the Soviet collapse." Numerous structural and construction quality issues, as well as deviations from the original plant design, had been known to KGB since at least 1973 and passed on to the Central Committee, which took no action and classified the information.

In Italy, the Chernobyl accident was reflected in the outcome of the 1987 referendum. As a result of that referendum, Italy began phasing out its nuclear power plants in 1988, a decision that was effectively reversed in 2008. A 2011 referendum reiterated Italians' strong objections to nuclear power, thus abrogating the government's 2008 decision.

In Germany, the Chernobyl accident led to the creation of a federal environment ministry, after several states had already created such a post. The post has been held, among others, by Angela Merkel who would later become leader of the opposition and then chancellor. The German environmental minister was given the authority over reactor safety as well, a responsibility the current minister still holds today. The Chernobyl disaster is also credited with strengthening the anti-nuclear movement in Germany, which culminated in the decision to end the use of nuclear power made by the 1998-2005 Schröder government. A temporary reversal of this policy was in turn reverted after the Fukushima nuclear disaster.

In direct response to the Chernobyl disaster, a conference to create a Convention on Early Notification of a Nuclear Accident was called in 1986 by the International Atomic Energy Agency. The resulting treaty has bound signatory member states to provide notification of any nuclear and radiation accidents that occur within its jurisdiction that could affect other states, along with the Convention on Assistance in the Case of a Nuclear Accident or Radiological Emergency.

The Chernobyl disaster, along with the space shuttle Challenger disaster, the Three Mile Island accident, and the Bhopal disaster have been used together as case studies, both by the US government and by third parties, in research concerning the root causes of such disasters, such as sleep deprivation and mismanagement.

Cultural impact

The Chernobyl tragedy has inspired many artists across the world to create works of art, animation, video games, theatre and cinema about the disaster. The HBO series and the book Voices from Chernobyl by the Ukrainian-Belarusian writer Svetlana Alexievich are two well-known works that talk about the catastrophe that destroyed millions of lives. The Ukrainian artist Roman Gumanyuk created a series of artworks called "Pripyat Lights, or Chernobyl shadows" that includes 30 oil paintings about the Chernobyl accident. The series of artwork was exhibited at the National Fine Art Museum of Kyrgyzstan in Bishkek, the Kasteev State Museum of Arts of Kazakhstan in Almaty, the Vashchenko Art Gallery of Gomel in Belarus, and at the Museum of Chernobyl in Kharkiv in Ukraine in the years 2012-2013.

The video game S.T.A.L.K.E.R.: Shadows of Chernobyl released by THQ in 2007, is a first-person shooter set in the Exclusion Zone. A prequel called S.T.A.L.K.E.R.: Clear Sky was released in 2008 following with a sequel S.T.A.L.K.E.R.: Call of Pripyat released in 2010. Finally, the horror film Chernobyl Diaries released in 2012 is about six tourists that hire a tour guide to take them to the abandoned city of Pripyat where they discover they are not alone.

Filmmakers have created documentaries that examine the aftermath of the disaster over the years. Documentaries like the Oscar-winning Chernobyl Heart released in 2003, explore how radiation affected people living in the area and information about the long-term side effects of radiation exposure over the years that include mental disabilities, physical disabilities, and genetic mutations after the disaster. The Babushkas of Chernobyl released in 2015, is a documentary that explores the story of the three women who decided to return to the exclusion zone after the disaster. In the documentary, the Babushkas show the polluted water, their food from radioactive gardens, and explain how they manage to survive in this exclusion zone despite the radioactive levels of it. The documentary The Battle of Chernobyl, released in 2006, shows a rare original footage a day before the disaster in the city of Pripyat, then through different methods the documentary goes in depth on the chronological events that led to the explosion of the reactor No. 4 and the disaster response in which 50,000 men from Soviet Union engaged to liquidate the radioactivity of the damaged reactor.

In July 2019, Ukrainian president Volodymyr Zelenskyy announced that the Chernobyl site would become an official tourist attraction. Zelenskyy said, "We must give this territory of Ukraine a new life," after Chernobyl saw an increase in visitors since the HBO mini-series. Dr. T. Steen, a microbiology and immunology teacher at Georgetown's School of Medicine, recommends tourists to wear clothes and shoes they are comfortable with throwing away. Most importantly, Steen suggests to avoid plant life, especially the depths of the forest due to the high levels of radiation. Because the areas were not cleaned in the aftermath of the disaster, they remain highly contaminated. Research showed that fungus, moss, and mushrooms are radioactive. Drinking or eating from there could be dangerous. Generally speaking, Chernobyl can be a safe place, Dr. Steen said, "but it depends on how people behave."

References

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The Chernobyl disaster began on 26 April 1986 with the explosion of the No. 4 reactor of the Chernobyl Nuclear Power Plant, near the city of Pripyat in the north of the Ukrainian SSR, close to the border with the Byelorussian SSR, in the Soviet Union.

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Desastre Nuclear de Fukushima

Data: 11 de março de 2011

Localização: Usina Nuclear de Fukushima Daiichi, Japão

Causa: Tsunami resultante de um terremoto de magnitude 9.0, levando a falhas nos sistemas de refrigeração dos reatores.

Consequências: Liberação de material radioativo, evacuação de uma grande área, preocupações com a segurança alimentar e impacto na indústria pesqueira.

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Cross-section of a typical BWR Mark I containment as used in units 1 to 5.
RPV: reactor pressure vessel
DW: drywell enclosing reactor pressure vessel
WW: wetwell - torus-shaped all around the base enclosing steam suppression pool. Excess steam from the drywell enters the wetwell water pool via downcomer pipes.
SFP: spent fuel pool area
SCSW: secondary concrete shield wall

The Fukushima nuclear accident was a major nuclear accident at the Fukushima Daiichi nuclear power plant in Ōkuma, Fukushima, Japan which began on March 11, 2011. The proximate cause of the accident was the 2011 Tōhoku earthquake and tsunami, which resulted in electrical grid failure and damaged nearly all of the power plant's backup energy sources. The subsequent inability to sufficiently cool reactors after shutdown compromised containment and resulted in the release of radioactive contaminants into the surrounding environment. The accident was rated seven (the maximum severity) on the INES by NISA, following a report by the JNES.

No adverse health effects among Fukushima residents or power station workers have been documented that are directly attributable to radiation exposure from the accident. Criticisms have been made about the public perception of radiological hazards resulting from accidents and the implementation of evacuations (similar to the Chernobyl nuclear accident), as they caused much more harm than they prevented. Following the accident, at least 164,000 residents of the surrounding area were permanently or temporarily displaced (either voluntarily or by evacuation order). This response resulted in at least 51 fatalities, with more attributed to subsequent stress or fear of radiological hazards.

Investigations faulted lapses in safety and oversight, namely failures in risk assessment and evacuation planning. Controversy surrounds the disposal of treated wastewater once used to cool the reactor, resulting in numerous protests in neighboring countries.

At the time of the Tōhoku earthquake on 11 March 2011, units 1-3 were operating. However, the spent fuel pools of all units still required cooling.

Many of the internal components and fuel assembly cladding are made from a zirconium alloy (Zircaloy) for its low neutron cross-section. At normal operating temperatures (~300 °C (572 °F), it is inert. However, above 1,200 degrees Celsius (2,190 °F), Zircaloy can be oxidized by steam to form hydrogen gas or by uranium dioxide to form uranium metal. Both of these reactions are exothermic. In combination with the exothermic reaction of boron carbide with stainless steel, these reactions can contribute to the overheating of a reactor.

In the event of an emergency situation, reactor pressure vessels (RPV) are automatically isolated from the turbines and main condenser and are instead switched to a secondary condenser system which is designed to cool the reactor without the need for pumps powered by external power or generators. The isolation condenser (IC) system involved a closed coolant loop from the pressure vessel with a heat exchanger in a dedicated condenser tank. Steam would be forced into the heat exchanger by the reactor pressure, and the condensed coolant would be fed back into the vessel by gravity. Each reactor was initially designed to be equipped with two redundant ICs which were each capable of cooling the reactor for at least 8 hours (at which point, the condenser tank would have to be refilled). However, it was possible for the IC system to cool the reactor too rapidly shortly after shutdown which could result in undesirable thermal stress on the containment structures. To avoid this, protocol called for reactor operators to manually open and close the condenser loop using electrically operated control valves.

After the construction of unit 1, the following units were designed with new open-cycle reactor core isolation cooling (RCIC) systems. This new system utilized the steam from the reactor vessel to drive a turbine which would power a pump to inject water into the pressure vessel from an external storage tank to maintain the water level in the reactor vessel and was designed to operate for at least 4 hours (until the depletion of coolant or mechanical failure). Additionally, this system could be converted into a closed-loop system which draws coolant from the suppression chamber (SC) instead of the storage tank, should the storage tank be depleted. Although this system could function autonomously without an external energy source (besides the steam from the reactor), DC power was needed to remotely control it and receive parameters and indications and AC power was required to power the isolation valves.

In an emergency situation where backup on-site power was partially damaged or insufficient or to last until a grid connection to off-site power could be restored, these cooling systems could no longer be relied upon to reliably cool the reactor. In such a case, the expected procedure was to vent both the reactor vessel and primary containment using electrically or pneumatically operated valves using the remaining electricity on site. This would lower the reactor pressure sufficiently to allow for low-pressure injection of water into the reactor using the fire protection system in order to replenish water lost to evaporation.

In the event of a loss of off-site power, emergency diesel generators (EDG) would automatically start in order to provide AC power. Two EDGs were available for each of units 1-5 and three for unit 6. Of the 13 EDGs, 10 were water-cooled and placed in the basements about 7-8 m below the ground level. The coolant water for the EDGs was carried by a number of seawater pumps placed on the shoreline which also provide water for the main condenser. These components were unhoused and only protected by the seawall. The other three EDGs were air-cooled and were connected to units 2, 4, and 6. The air-cooled EDGs for units 2 and 4 were placed on the ground floor of the spent fuel building, but the switches and various other components were located below, in the basement. The third air-cooled EDG was in a separate building placed inland and at higher elevation. Although these EDGs are intended to be used with their respective reactors, switchable interconnections between unit pairs (1 and 2, 3 and 4, and 5 and 6) allowed reactors to share EDGs should the need arise.

The power station was also equipped with backup DC batteries kept charged by AC power at all times, designed to be able to power the station for approximately 8 hours without EDGs. In units 1, 2, and 4, the batteries were located in the basements alongside the EDGs. In units 3, 5, and 6, the batteries were located in the turbine building where they were raised above ground level.

In the late 1990s, three additional EDGs were placed in new buildings located inland and at higher elevation to comply with new regulatory requirements, but the switching stations that connected the EDGs to units 1-5 were located in the turbine buildings. Only the switching station for unit 6 was inside of the reactor building.[citation needed ]

The units and central storage facility contained the following numbers of fuel assemblies:

The original design basis was a zero-point ground acceleration of 250 Gal and a static acceleration of 470 Gal, based on the 1952 Kern County earthquake (0.18 g, 1.4 m/s ², 4.6 ft/s ²). After the 1978 Miyagi earthquake, when the ground acceleration reached 0.125 g (1.22 m/s ², 4.0 ft/s ²) for 30 seconds, no damage to the critical parts of the reactor was found. In 2006, the design of the reactors were reevaluated with new standards (which included vertical acceleration and differentiated E/W and N/S motion) which found the reactors would withstand accelerations ranging from 412 Gal to 489 Gal.

The height of the tsunami that struck the station approximately 50 minutes after the earthquake.
A: Power station buildings
B: Peak height of tsunami
C: Ground level of site
D: Average sea level
E: Seawall to block waves

The 9.0 earthquake occurred at 14:46 on Friday, 11 March 2011, with the epicenter off of the east coast of the Tōhoku region. It produced maximum ground g-forces of 0.56, 0.52, 0.56 at units 2, 3, and 5 respectively. This exceeded the seismic reactor design tolerances of 0.45, 0.45, and 0.46 g for continued operation, but the seismic values were within the design tolerances at units 1, 4, and 6.

Upon detecting the earthquake, all three operating reactors (units 1, 2, and 3) automatically shut down. Due to expected grid failure and damage to the switch station as a result of the earthquake, the power station automatically started up the EDGs, isolated the reactor from the primary coolant loops, and activated the emergency shutdown cooling systems.

The waves first damaged the seawater pumps along the shoreline, disabling the 10 water cooled EDGs. The waves then flooded all turbine and reactor buildings, damaging EDGs and other electrical components and connections located on the ground or basement levels at approximately 15:41. The switching stations that provided power from the three EDGs located higher on the hillside also failed when the building that housed them flooded. One air cooled EDG, that of unit 6, was unaffected by the flooding and continued to operate. The DC batteries for units 1, 2, and 4 were also inoperable shortly after flooding.

Two workers were killed by the impact of the tsunami.

The dry cask storage building was also flooded, causing some concerns about possible damage.

The isolation condenser (IC) was functioning prior to the tsunami, but the DC-operated control valve outside of the primary containment had been in the closed position at the time to prevent thermal stresses on the reactor components. This status was uncertain at the time due to a loss of indications in the control room, who had correctly assumed loss of coolant (LOC). 3 hours later, the plant operators attempted to manually open the control valve, but the IC failed to function, suggesting that the isolation valves were closed. Although they were kept open during IC operation, the loss of DC power in unit 1 (which occurred shortly prior to the loss of AC power) automatically closed the AC-powered isolation valves in order to prevent uncontrolled cooling or a potential LOC. Although this status was unknown to the plant operators, they correctly interpreted the loss of function in the IC system and manually closed the control valves. The plant operators would continue to periodically attempt to restart the IC in the following hours and days, but it did not function.

The plant operators then attempted to utilize the building's fire protection (FP) equipment, operated by a diesel-driven fire pump (DDFP), in order to inject water into the reactor vessel. A team was dispatched to the reactor building (RB) in order to carry out this task, but the team found that the reactor pressure had already increased significantly to 7 MPa, which was many times greater than the limit of the DDFP which could only operate below 0.8 MPa. Additionally, the team detected high levels of radiation within the RB, indicating damage to the reactor core, and found that the primary containment vessel (PCV) pressure (0.6 MPa) exceeded design specifications (0.528 MPa). In response to this new information, the reactor operators began planning to lower the PCV pressure by venting. The PCV reached its maximum pressure of 0.84 MPa at 02:30, after which it stabilized around 0.8 MPa. Venting of the PCV was completed later that afternoon at 14:00.

At the same time, pressure in the reactor vessel had been decreasing to equalize with the PCV, and the workers prepared to inject water into the reactor vessel using the DDFP once the pressure had decreased below the 0.8 MPa limit. Unfortunately, the DDFP was found to be inoperable and a fire truck had to be hooked up to the FP system. This process took about 4 hours, as the FP injection port was hidden under debris. The next morning (March 12, 04:00), approximately 12 hours after loss of power, freshwater injection into the reactor vessel began, later replaced by a water line at 09:15 leading directly from the water storage tank to the injection port to allow for continuous operation (the fire engine had to be periodically refilled). This continued into the afternoon until the freshwater tank was nearly depleted. In response, injection stopped at 14:53 and the injection of seawater, which had collected in a nearby valve pit (the only other source of water), began.

In November 2013, Mari Yamaguchi reported for Associated Press that there are computer simulations that suggest that "the melted fuel in Unit 1, whose core damage was the most extensive, has breached the bottom of the primary containment vessel and even partially eaten into its concrete foundation, coming within about 30 cm (1 ft) of leaking into the ground" - a Kyoto University nuclear engineer said with regard to these estimates: "We just can't be sure until we actually see the inside of the reactors."

TEPCO estimated for Unit 1 that "the decay heat must have decreased enough, the molten fuel can be assumed to remain in PCV (primary containment vessel)".

In February 2015, TEPCO started the muon scanning process for Units 1, 2, and 3. With this scanning setup it was possible to determine the approximate amount and location of the remaining nuclear fuel within the RPV, but not the amount and resting place of the corium in the PCV. In March 2015 TEPCO released the result of the muon scan for Unit 1 which showed that no fuel was visible in the RPV, which would suggest that most if not all of the molten fuel had dropped onto the bottom of the PCV - this will change the plan for the removal of the fuel from Unit 1.

Unit 2 was the only other operating reactor which experienced total loss of AC and DC power. Prior to blackout, the RCIC was functioning as designed without the need for operator intervention. The safety relief valve (SRV) would intermittently release steam directly into the PCV suppression torus at its design pressure and the RCIC properly replenished lost coolant. However, following the total blackout of unit 2, the plant operators (similar to unit 1) assumed the worst case scenario and prepared for a LOC incident. However, when a team was sent to investigate the status of the RCIC of unit 2 the following morning (02:55), they confirmed that the RCIC was operating with the PCV pressure well below design limits. Based on this information, efforts were focused onto unit 1. However, the condensate storage tank from which the RCIC draws water from was nearly depleted by the early morning, and so the RCIC was manually reconfigured at 05:00 to recirculate water from the suppression chamber instead.

On the 13th, unit 2 was configured to vent the PCV automatically (manually opening all valves, leaving only the rupture disk) and preparations were made to inject seawater from the valve pit via the FP system should the need arise. However, as a result of the explosion in unit 3 the following day, the seawater injection setup was damaged and the isolation valve for the PCV vent was found to be closed and inoperable.

At 13:00 on the 14th, the RCIC pump for unit 2 failed after 68 hours of continuous operation. With no way to vent the PCV, in response, a plan was devised to delay containment failure by venting the reactor vessel into the PCV using the SRV in order to allow for seawater injection into the reactor vessel.

In February 2017, TEPCO released images taken inside unit 2 by a remote-controlled camera that show a 2 m (6.5 ft) wide hole in the metal grating under the pressure vessel in the reactor's primary containment vessel, which could have been caused by fuel escaping the pressure vessel, indicating a meltdown/melt-through had occurred, through this layer of containment. Ionizing radiation levels of about 210 sieverts (Sv) per hour were subsequently detected inside the Unit 2 containment vessel. Undamaged spent fuel typically has values of 270 Sv/h, after ten years of cold shutdown with no shielding.

Although AC power was lost, some DC power was still available in unit 3 and the workers were able to remotely confirm that the RCIC system was continuing to cool the reactor. However, knowing that their DC supply was limited, the workers managed to extend the backup DC supply to about 2 days by disconnecting nonessential equipment, until replacement batteries were brought from a neighboring power station on the morning of the 13th (with 7 hours between loss and restoration of DC power). At 11:36 the next day, after 20.5 hours of operation, the RCIC system failed. In response, the high pressure coolant injection (HPCI) system was activated to alleviate the lack of cooling while workers continued to attempt to restart the RCIC. Additionally, the FP system was utilized to spray the PCV (mainly the SC) with water in order to slow the climbing temperatures and pressures of the PCV.

Later that morning (9:08), workers were able to depressurize the reactor by operating the safety relief valve using batteries collected from nearby automobiles. This was shortly followed by the bursting of the venting line rupture disk and the depressurization of the PCV. Unfortunately, venting was quickly stopped by a pneumatic isolation valve which closed on the vent path due to a lack of compressed air, and venting was not resumed until over 6 hours later once an external air compressor could be installed. Despite this, the reactor pressure was immediately low enough to allow for water injection (borated freshwater, as ordered by TEPCO) using the FP system until the freshwater FP tanks were depleted, at which point the injected coolant was switched to seawater from the valve pit.

Cooling was lost once the valve pit was depleted, but was quickly resumed two hours later (unit 1 cooling was postponed until the valve pit was filled). However, despite being cooled, PCV pressure continued to rise and the RPV water level continued to drop until the fuel became uncovered on the morning of the 14th (6:20), as indicated by a water level gauge, which was followed by workers evacuating the area out of concerns about a possible second hydrogen explosion similar to unit 1.

Shortly after work resumed to reestablish coolant lines, an explosion occurred in the unit 3 RB at 11:01 on March 14, which further delayed unit 1 cooling and damaged unit 3's coolant lines. Work to reestablish seawater cooling directly from the ocean began two hours later, and cooling of unit 3 resumed in the afternoon (approximately 16:00) and continued until cooling was lost once more as a result of site evacuation on the 15th.

In August 2014, TEPCO released a new revised estimate that reactor 3 had a complete melt through in the initial phase of the accident. According to this new estimate within the first three days of the accident the entire core content of reactor 3 had melted through the RPV and fallen to the bottom of the PCV. These estimates were based on a simulation, which indicated that reactor 3's melted core penetrated through 1.2 m (3 ft 11 in) of the PCV's concrete base, and came close to 26-68 cm (10-27 in) of the PCV's steel wall.

Unit 4 was not fueled at the time, but the unit 4 spent fuel pool (SFP) contained a number of fuel rods.

Unit 5 was fueled and was undergoing a RPV pressure test at the time of the accident, but the pressure was maintained by an external air compressor and the reactor was not otherwise operating. Removal of decay heat using the RCIC was not possible, as the reactor was not producing sufficient steam. However, the water within the RPV proved sufficient to cool the fuel, with the SRV venting into the PCV, until AC power was restored on March 13 using the unit 6 interconnection, allowing the use of the low-pressure pumps of the residual heat removal (RHR) system.

Cold shutdown was achieved in the afternoon on the 20th.

Unit 6 was not operating, and its decay heat was low because it had been in an outage since August 2010.

All but one EDG was disabled by the tsunami, allowing unit 6 to retain AC-powered safety functions throughout the incident. However, because the RHR was damaged, workers decided to activate the make-up water condensate system to maintain the reactor water level until the RHR was restored on the 20th.

Cold shutdown was achieved on the 20th, less than an hour after unit 5.

In the initial hours of the accident, in response to station blackout and uncertainty regarding the cooling status of units 1 and 2, a 2 km radius evacuation of 1,900 residents was ordered at 20:50. However, due to difficulty coordinating with the national government, a 3 km evacuation order of ~6,000 residents and a 10 km shelter-in-place order for 45,000 residents was established nearly simultaneously at 21:23. The following morning (05:44), this evacuation radius was expanded to 10 km by local authorities in response to the unit 1 core damage and plans to vent the PCV later that day. The evacuation radius was further revised at 18:25 to 20 km, involving a total of 78,000 residents, in response to the hydrogen explosion at unit 1. However, miscommunication of this final evacuation order resulted in those within 20 km to shelter in place. Additionally, many municipalities independently ordered evacuations ahead of orders from the national government due to loss of communication with authorities; at the time of the 3 km evacuation order, the majority of residents within the zone had already evacuated.

Due to the multiple overlapping evacuation orders, many residents had evacuated to areas which would shortly be designated as evacuation areas. This resulted in many residents having to move multiple times until they reached an area outside of the final 20 km evacuation zone. 20% of residents who were within the initial 2 km radius had to evacuate more than six times.

Additionally, a 30 km shelter in place order was communicated on the 15th, although some municipalities within this zone had already decided to evacuate their residents. This order was followed by a voluntary evacuation recommendation on the 25th, although the majority of residents had evacuated from the 30 km zone by then. The shelter in place order was lifted on April 22, but the evacuation recommendation remained.

The predominant mechanism by which fission products can leave the core during core melt is through vaporization, thus only relatively volatile nuclides mix with the vaporized coolant and can be transported by the flow of gas. This gas can then exit the RPV and into the PCV through small leak paths in imperfections in the RPV, but in a situation in which the RCIC is used, this gas flows through the RCIC system and into the suppression pool, where some of the vaporized or suspended fission products are condensed or captured (scrubbed) by the SC, although some remainder (notably, radioactive noble gasses) will remain vaporized or suspended inside of the PCV. From the PCV, similar to the RPV, some small quantity inevitably leaks through small imperfections in the structure, but the predominant designed path for the escape of suspended radionuclides is through venting of the PCV where they are dispersed by the vent stack. However, if the PCV is compromised, the gas will be released directly into the secondary containment, and the potential loss of the SC function would also increase the concentration of unwanted fission products in the gas.[citation needed ]

The fraction of releases associated to certain events is debated, as some of the detected fluctuations in the environment do not strongly correlate with events at the power station.

Once released into the atmosphere, those which remain in a gaseous phase will simply be diluted by the atmosphere, but some which precipitate will eventually settle on land or in the ocean. Thus, the majority (90~99%) of the radionuclides which are deposited are isotopes of iodine and caesium, with a small portion of tellurium, which are almost fully vaporized out of the core due to their low vapor pressure. The remaining fraction of deposited radionuclides are of less volatile elements such as barium, antimony, and niobium, of which less than a percent is evaporated from the fuel.

In addition to atmospheric deposition, there was also a significant quantity of direct releases into groundwater (and eventually the ocean) through leaks of coolant which had been in direct contact with the fuel. Estimates for this release vary from 1 to 5.5 PBq. Although the majority had entered the ocean shortly following the accident, a significant fraction remains in the groundwater and continues to mix with coastal waters.

According to the French Institute for Radiological Protection and Nuclear Safety, the release from the accident represents the most important individual oceanic emissions of artificial radioactivity ever observed. The Fukushima coast has one of the world's strongest currents ( Kuroshio Current). It transported the contaminated waters far into the Pacific Ocean, dispersing the radioactivity. As of late 2011 measurements of both the seawater and the coastal sediments suggested that the consequences for marine life would be minor. Significant pollution along the coast near the plant might persist, because of the continuing arrival of radioactive material transported to the sea by surface water crossing contaminated soil. The possible presence of other radioactive substances, such as strontium-90 or plutonium, has not been sufficiently studied. Recent measurements show persistent contamination of some marine species (mostly fish) caught along the Fukushima coast.

In January 2015, the number of residents displaced due to the accident was around 119,000, peaking at 164,000 in June 2012. In terms of months of life lost, the loss of life would have been far smaller if all residents had done nothing at all, or were sheltered in place, instead of evacuated.

In the former Soviet Union, many patients with negligible radioactive exposure after the Chernobyl accident displayed extreme anxiety about radiation exposure. They developed many psychosomatic problems, including radiophobia along with an increase in fatalistic alcoholism. As Japanese health and radiation specialist Shunichi Yamashita noted:

We know from Chernobyl that the psychological consequences are enormous. Life expectancy of the evacuees dropped from 65 to 58 years - not because of cancer, but because of depression, alcoholism, and suicide. Relocation is not easy, the stress is very big. We must not only track those problems, but also treat them. Otherwise people will feel they are just guinea pigs in our research.

A survey[when? ] by the Iitate local government obtained responses from approximately 1,743 evacuees within the evacuation zone. The survey showed that many residents are experiencing growing frustration, instability, and an inability to return to their earlier lives. Sixty percent of respondents stated that their health and the health of their families had deteriorated after evacuating, while 39.9% reported feeling more irritated compared to before the accident.

Summarizing all responses to questions related to evacuees' current family status, one-third of all surveyed families live apart from their children, while 50.1% live away from other family members (including elderly parents) with whom they lived before the disaster. The survey also showed that 34.7% of the evacuees have suffered salary cuts of 50% or more since the outbreak of the nuclear disaster. A total of 36.8% reported a lack of sleep, while 17.9% reported smoking or drinking more than before they evacuated.

A 2014 metareview of 48 articles indexed by PubMed, PsycINFO, and EMBASE, highlighted several psychophysical consequences among the residents in Miyagi, Iwate, Ibaraki, Tochigi and Tokyo. The resulting outcomes included depressive symptoms, anxiety, sleep disturbance, social functioning, social isolation, admission rates, suicide rates and cerebral structure changes, radiation impacting food safety, maternal anxiety and lowered maternal confidence. The rates of psychological distress among evacuated people rose fivefold compared to the Japanese average due to the experience of the accident and evacuation. An increase in childhood obesity in the area after the accident has also been attributed to recommendations that children stay indoors instead of going outside to play.

Worldwide media coverage of the incident has been described as "ten years of disinformation", with media and environmental organizations routinely conflating the casualties of the earthquake and tsunami, with casualties of the nuclear incident. The incident dominated media coverage while the victims of the natural disasters were "ignored", and a number of media reports incorrectly describing thousands of victims of tsunami as if they were victims of the "nuclear disaster".

Electricity generation by source in Japan (month-level data). Nuclear energy's contribution declined steadily throughout 2011 due to shutdowns and has been mainly replaced with thermal power stations such as fossil gas and coal power plants.

Part of the Seto Hill Windfarm in Japan, one of several windfarms that continued generating without interruption after the 2011 earthquake and tsunami and the Fukushima nuclear accident

Prior to the accident, over 25% of domestic electricity generation in Japan utilized nuclear power and Japan had set a fairly ambitious GHG reduction target of 25% below 1990 levels by 2020, which involved increasing the share of nuclear power in electricity generation from 30% to 50%. However, this plan was abandoned and target was quickly revised to a 3% emissions increase by 2020 following the accident, alongside a focus on reducing dependence on nuclear power in favor of improved thermal efficiency in fossil fuel energy use and increasing the share of "renewables". The contribution of nuclear energy dropped to less than a percent following the accident and all nuclear reactors in the country were shut down by 2013. This resulted in an increase in the share of fossil fuel energy use, which had increased to ~94% by 2015 (the highest of any IEA member state, with the remaining ~6% produced by renewables, an increase from 4% in 2010). The required fossil fuel imports in 2011 resulted in a trade deficit for the first time in decades which would continue in the following decade.

In the immediate aftermath, nine prefectures served by TEPCO experienced power rationing. The government asked major companies to reduce power consumption by 15%, and some shifted their weekends to weekdays to smooth power demand. As of 2013, TEPCO and eight other Japanese power companies were paying approximately 3.6 trillion JPY (37 billion USD) more in combined imported fossil fuel costs compared to 2010 to make up for the missing power.

On 16 December 2012, Japan held its general election. The Liberal Democratic Party (LDP) had a clear victory, with Shinzō Abe as the new Prime Minister. Abe supported nuclear power, saying that leaving the plants closed was costing the country 4 trillion yen per year in higher costs. The comment came after Junichiro Koizumi, who chose Abe to succeed him as premier, made a statement to urge the government to take a stance against using nuclear power. A survey on local mayors by the Yomiuri Shimbun newspaper in 2013 found that most of them from cities hosting nuclear plants would agree to restarting the reactors, provided the government could guarantee their safety. More than 30,000 people marched on 2 June 2013, in Tokyo against restarting nuclear power plants. Marchers had gathered more than 8 million petition signatures opposing nuclear power.

In the aftermath, Germany accelerated plans to close its nuclear power reactors and decided to phase the rest out by 2022 (see also Nuclear power in Germany). Belgium and Switzerland have also changed their nuclear policies to phase-out all nuclear energy operations. Italy held a national referendum, in which 94 percent voted against the government's plan to build new nuclear power plants. In France, President Hollande announced the intention of the government to reduce nuclear usage by one third. However, the government earmarked only one power station for closure - the aging Fessenheim Nuclear Power Plant on the German border - which prompted some to question the government's commitment to Hollande's promise. Industry Minister Arnaud Montebourg is on record as saying that Fessenheim will be the only nuclear power station to close. On a visit to China in December 2014 he reassured his audience that nuclear energy was a "sector of the future" and would continue to contribute "at least 50%" of France's electricity output. Another member of Hollande's Socialist Party, the MP Christian Bataille, said that Hollande announced the nuclear curb to secure the backing of his Green coalition partners in parliament.

China suspended its nuclear development program briefly, but restarted it shortly afterwards. The initial plan had been to increase the nuclear contribution from 2 to 4 percent of electricity by 2020, with an escalating program after that. Renewable energy supplies 17 percent of China's electricity, 16% of which is hydroelectricity. China plans to triple its nuclear energy output to 2020, and triple it again between 2020 and 2030.

New nuclear projects were proceeding in some countries. KPMG reports 653 new nuclear facilities planned or proposed for completion by 2030. By 2050, China hopes to have 400-500 gigawatts of nuclear capacity - 100 times more than it has now. The Conservative Government of the United Kingdom is planning a major nuclear expansion despite some public objection.[citation needed ] So is Russia. India is also pressing ahead with a large nuclear program, as is South Korea. Indian Vice President M Hamid Ansari said in 2012 that "nuclear energy is the only option" for expanding India's energy supplies, and Prime Minister Modi announced in 2014 that India intended to build 10 more nuclear reactors in a collaboration with Russia.

In the wake of the accident, the Senate Appropriations Committee requested the United States Department of Energy "to give priority to developing enhanced fuels and cladding for light water reactors to improve safety in the event of accidents in the reactor or spent fuel pools". This brief has led to ongoing research and development of Accident Tolerant Fuels, which are specifically designed to withstand the loss of cooling for an extended period, increase time to failure, and increase fuel efficiency. This is accomplished by incorporating specially designed additives to standard fuel pellets and replacing or altering the fuel cladding in order to reduce corrosion, decrease wear, and reduce hydrogen generation during accident conditions. While research is still ongoing, on 4 March 2018, the Edwin I. Hatch Nuclear Power Plant near Baxley, Georgia has implemented "IronClad" and "ARMOR" (Fe-Cr-Al and coated Zr claddings, respectively) for testing.

Radiation exposure of those living in proximity to the accident site is expected to be below 10 mSv, over the course of a lifetime. In comparison, the dosage of background radiation received over a lifetime is 170 mSv. Very few cancers are expected as a result of accumulated radiation exposures and residents who were evacuated were exposed to so little radiation that radiation-induced health effects were likely to be below detectable levels. There is no increase in miscarriages, stillbirths or physical and mental disorders in babies born after the accident.

Outside the geographical areas most affected by radiation, even in locations within Fukushima prefecture, the predicted risks remain low, and no observable increases in cancer above natural variation in baseline rates are anticipated.

-  World Health Organization, 2013

Estimated effective doses outside Japan are considered to be below (or far below) the levels regarded as very small by the international radiological protection community. The Integrated Fukushima Ocean Radionuclide Monitoring project (InFORM) failed to show any significant amount of radiation and as a result its authors received death threats from supporters of a "wave of cancer deaths across North America" theory.

The overwhelming majority of thyroid growths are benign growths that will never cause symptoms, illness, or death, even if nothing is ever done about the growth. Autopsy studies on people who died from other causes show that more than one third of adults technically have a thyroid growth/cancer. As a precedent, in 1999 in South Korea, the introduction of advanced ultrasound thyroid examinations resulted in an explosion in the rate of benign thyroid cancers being detected and needless surgeries occurring. Despite this, the death rate from thyroid cancer has remained the same.

There is a statistically significant correlation between external radiation dose and thyroid cancer in those under the age of 18. However, this is mostly attributable to the early detection of non- symptomatic disease cases by the screening-effect. Rates of thyroid cancer in children controlled for examination frequency showed no association between the nuclear accident/radiation exposure and thyroid cancer.

As of 2020, research into the correlation between air-dose and internal-dose and thyroid cancers remains ongoing. Further research is necessary in understanding the dose-response relationship and the prevalence of incident cancers.

Thyroid cancer is one of the most survivable cancers, with an approximate 94% survival rate after first diagnosis. That rate increases to a nearly 100% survival rate if caught early. Cancer may spread to another part of the body, however, and in cases where the thyroid must be removed, the resulting hormonal deficiencies are terminal. In January 2022, six such patients who were children at the time of the accident sued TEPCO for 616 million yen after developing thyroid cancer.

Infant/fetal cancer risk

Evacuated infant girls, the most radiation-sensitive demographic, have an estimated increased lifetime risk of developing thyroid cancer of 1.25% (compared to 0.75% background risk), with the increase being slightly less for males. The risks from a number of additional radiation-induced cancers are also expected to be elevated. There is an estimated 7% higher relative risk of leukemia in males exposed as infants and a 6% higher relative risk of breast cancer in females exposed as infants. In total, an overall 1% higher lifetime risk of developing cancers of all types is predicted for infant females, with the risk slightly lower for males. The fetuses, depending on their sex, would have the same elevations in risk as the infant groups.

Linear no-threshold models (LNT)

LNT models estimate that the accident would most likely cause 130 cancer deaths. However, LNT models have large uncertainties and are not useful for estimating health effects from radiation, especially when the effects of radiation on the human body are not linear, and with obvious thresholds. Producing a statistically useful estimate would require an impractically large number of patients, and LNT models have been described as "junk science". In September 2018, one cancer fatality was the subject of a financial settlement, to the family of a former nuclear station workman.

As of March 2012, no cases of radiation-related ailments had been reported.

In April 2014, studies confirmed the presence of radioactive tuna off the coasts of the Pacific U.S. Researchers carried out tests on 26 albacore tuna caught prior to the 2011 power plant accident and those caught after. However, the amount of radioactivity is less than that found naturally in a single banana. Cs-137 and Cs-134 have been noted in Japanese whiting in Tokyo Bay as of 2016. "Concentration of radiocesium in the Japanese whiting was one or two orders of magnitude higher than that in the sea water, and an order of magnitude lower than that in the sediment." They were still within food safety limits.

In June 2016, the political advocacy group " International Physicians for the Prevention of Nuclear War", asserted that 174,000 people have been unable to return to their homes and ecological diversity has decreased and malformations have been found in trees, birds, and mammals. Although physiological abnormalities have been reported within the vicinity of the accident zone, the scientific community has largely rejected any such findings of genetic or mutagenic damage caused by radiation, instead showing it can be attributed either to experimental error or other toxic effects.

To assuage fears, the government enacted an order to decontaminate over a hundred areas where the level of additional radiation was greater than one millisievert per year. This is a much lower threshold than is necessary for protecting health. The government also sought to address the lack of education on the effects of radiation and the extent to which the average person was exposed.

Tokyo Electric Power Company (TEPCO) is going to remove the remaining nuclear fuel material from the plants. TEPCO completed the removal of 1535 fuel assemblies from the Unit 4 spent fuel pool in December 2014 and 566 fuel assemblies from the Unit 3 spent fuel pool in February 2021. TEPCO plans to remove all fuel rods from the spent fuel pools of Units 1, 2, 5, and 6 by 2031 and to remove the remaining molten fuel debris from the reactor containments of Units 1, 2, and 3 by 2040 or 2050. An ongoing intensive cleanup program to both decontaminate affected areas and decommission the plant will take 30 to 40 years from the accident, plant management estimated.

As of 2013, about 400 metric tons (390 long tons; 440 short tons) of cooling water per day was being pumped into the reactors. Another 400 metric tons (390 long tons; 440 short tons) of groundwater was seeping into the structure. Some 800 metric tons (790 long tons; 880 short tons) of water per day was removed for treatment, half of which was reused for cooling and half diverted to storage tanks. Ultimately the contaminated water, after treatment to remove radionuclides other than tritium, has to be discharged into the Pacific. TEPCO decided to create an underground ice wall to block the flow of groundwater into the reactor buildings. A $300 million 7.8 MW cooling facility freezes the ground to a depth of 30 meters. As of 2019, the contaminated water generation had been reduced to 170 metric tons (170 long tons; 190 short tons) per day.

In February 2014, NHK reported that TEPCO was reviewing its radioactivity data, after finding much higher levels of radioactivity than was reported earlier. TEPCO now says that levels of 5 MBq (0.12 milli curies) of strontium per liter (23 MBq/ imp gal; 19 MBq/ U.S. gal; 610 μCi/imp gal; 510 μCi/U.S. gal) were detected in groundwater collected in July 2013 and not the 900 kBq (0.02 milli curies) (4.1 MBq/ imp gal; 3.4 MBq/ U.S. gal; 110 μCi/imp gal; 92 μCi/U.S. gal) that were initially reported.

A committee set up by the Japanese Government concluded that the purified water should be released to the sea or evaporated to the atmosphere. The committee calculated that discharging all the water to the sea in one year would cause a radiation dose of 0.81 microsieverts to the local people, whereas evaporation would cause 1.2 microsieverts. For comparison, Japanese people get 2100 microsieverts per year from natural radiation. IAEA considers that the dose calculation method is appropriate. Further, the IAEA recommends that a decision on the water disposal must be made urgently.

Despite the negligible doses, the Japanese committee is concerned that the water disposal may cause reputational damage to the prefecture, especially to the fishing industry and tourism.

Tanks used to store the water are expected to be filled in 2023. In July 2022, Japan's Nuclear Regulation Authority approved discharging the treated water into the sea. A US State Department spokesperson supported the decision. South Korea's foreign minister and activists from Japan and South Korea protested the announcement. In April 2023, fishers and activists held protests in front of the Japanese embassy in the Philippines in opposition to the planned release of 1.3 million tons of treated water into the Pacific Ocean. On 22 August, Japan announced that it would start releasing treated radioactive water from the tsunami-hit Fukushima nuclear plant into the Pacific Ocean in 48 hours, despite opposition from its neighbours. Japan says the water is safe, many scientists agree, and the decision comes weeks after the UN's nuclear watchdog approved the plan; but critics say more studies need to be done and the release should be halted. On 24 August, Japan begun the discharge of treated waste water into the Pacific Ocean, sparking protests in the region and retaliation from China, who said it would block all imports of seafood from Japan.

Initial estimates of costs to Japanese taxpayers were in excess of 12 trillion yen ($100 billion). In December 2016 the government estimated decontamination, compensation, decommissioning, and radioactive waste storage costs at 21.5 trillion yen ($187 billion), nearly double the 2013 estimate. By 2021 12.1 trillion yen had already been spent, with 7 trillion yen on compensation, 3 trillion yen on decontamination, and 2 trillion yen on decommissioning and storage. Despite concerns, the government expected total costs to remain under budget.

The amount of compensation to be paid by TEPCO is expected to reach 7 trillion yen.

In March 2017, a Japanese court ruled that negligence by the Japanese government had led to the Fukushima accident by failing to use its regulatory powers to force TEPCO to take preventive measures. The Maebashi district court near Tokyo awarded ¥39 million (US$345,000) to 137 people who were forced to flee their homes following the accident. On 30 September 2020, the Sendai High Court ruled that the Japanese government and TEPCO are responsible for the accident, ordering them to pay $9.5 million in damages to residents for their lost livelihoods. In March 2022, Japan's Supreme Court rejected an appeal from TEPCO and upheld the order for it to pay damages 1.4 billion yen ($12 million) to about 3,700 people whose lives were harmed by the accident. Its decision covered three class-action lawsuits, among more than 30 filed against the utility.

On 17 June 2022, the Supreme Court acquitted the government of any wrongdoing regarding potential compensation to over 3,700 people affected by the accident.

On 13 July 2022, four former TEPCO executives were ordered to pay 13 trillion yen ($95 billion) in damages to the operator of power plant, in the civil case brought by Tepco shareholders.

Equipment, facility, and operational changes

A number of nuclear reactor safety system lessons emerged from the incident. The most obvious was that in tsunami-prone areas, a power station's sea wall must be adequately tall and robust. At the Onagawa Nuclear Power Plant, closer to the epicenter of 11 March earthquake and tsunami, the sea wall was 14 meters (46 ft) tall and successfully withstood the tsunami, preventing serious damage and radioactivity releases.

Nuclear power station operators around the world began to install passive autocatalytic recombiners ("PARs"), which do not require electricity to operate. PARs work much like the catalytic converter on the exhaust of a car to turn potentially explosive gases such as hydrogen into water. Had such devices been positioned at the top of the reactor buildings, where hydrogen gas collected, the explosions would not have occurred and the releases of radioactive isotopes would arguably have been much less.

Unpowered filtering systems on containment building vent lines, known as Filtered Containment Venting Systems (FCVS), can safely catch radioactive materials and thereby allow reactor core depressurization, with steam and hydrogen venting with minimal radioactivity emissions. Filtration using an external water tank system is the most common established system in European countries, with the water tank positioned outside the containment building. In October 2013, the owners of Kashiwazaki-Kariwa nuclear power station began installing wet filters and other safety systems, with completion anticipated in 2014.

For generation II reactors located in flood or tsunami prone areas, a 3+ day supply of back-up batteries has become an informal industry standard. Another change is to harden the location of back-up diesel generator rooms with water-tight, blast-resistant doors and heat sinks, similar to those used by nuclear submarines. The oldest operating nuclear power station in the world, Beznau, which has been operating since 1969, has a 'Notstand' hardened building designed to support all of its systems independently for 72 hours in the event of an earthquake or severe flooding. This system was built prior to Fukushima Daiichi.

Upon a station blackout, similar to the one that occurred after back-up battery supply was exhausted, many constructed Generation III reactors adopt the principle of passive nuclear safety. They take advantage of convection (hot water tends to rise) and gravity (water tends to fall) to ensure an adequate supply of cooling water to handle the decay heat, without the use of pumps.

As the crisis unfolded, the Japanese government sent a request for robots developed by the U.S. military. The robots went into the plants and took pictures to help assess the situation, but they couldn't perform the full range of tasks usually carried out by human workers. The accident illustrated that robots lacked sufficient dexterity and robustness to perform critical tasks. In response to this shortcoming, a series of competitions were hosted by DARPA to accelerate the development of humanoid robots that could supplement relief efforts. Eventually a wide variety of specially designed robots were employed (leading to a robotics boom in the region), but as of early 2016 three of them had promptly become non-functional due to the intensity of the radioactivity; one was destroyed within a day.[citation needed ]

On 5 July 2012, the NAIIC found that the causes of the accident had been foreseeable, and that the plant operator (TEPCO) had failed to meet basic safety requirements such as risk assessment, preparing for containing collateral damage, and developing evacuation plans. At a meeting in Vienna three months after the accident, the IAEA faulted lax oversight by the Japanese Ministry of Economy, Trade and Industry, saying the ministry faced an inherent conflict of interest as the government agency in charge of both regulating and promoting the nuclear power industry. On 12 October 2012, TEPCO admitted that it had failed to take necessary measures for fear of inviting lawsuits or protests against its nuclear plants.

On 30 October 1991, one of unit 1's EDGs failed as a result of a condensate coolant leak in the turbine building, as reported by former employees in December 2011. A TEPCO report in 2011 detailed that the room was flooded through a door and some holes for cables, but the power supply was not cut off by the flooding. An engineer was quoted as saying that he informed his superiors of the possibility that a tsunami could damage the generators.

In 2000, an in-house TEPCO report recommended safety measures against seawater flooding, based on the potential of a 50 foot (15 m) tsunami. TEPCO did not act due to concerns about creating anxieties over the safety of the nuclear power plant.

in 2002, the government earthquake research headquarters estimated that a tsunami up to 15.7 meters (52 ft) could hit the power station.

In 2004, the cabinet office warned that tsunamis taller than the maximum of 5.6 meters (18 ft) forecast by TEPCO and government officials were possible.

In 2008, another in-house study identified an immediate need to better protect the facility from flooding by seawater which cited the 15.7 meters (52 ft) estimate from the 2002 study.

Many criticisms have been made which claim that the evacuation zone should have been further expanded, namely regarding the limited proliferation of data which governments of less affected areas may have acted upon. The national government only sent data from the SPEEDI network to the Fukushima prefectural government and was later criticized for delaying the communication of data to the U.S. military. Additionally, the U.S. military produced a detailed map using aircraft and provided it to the Ministry of Economy, Trade and Industry (METI) on 18 March and to the Ministry of Education, Culture, Sports, Science and Technology (MEXT) two days later, but no new evacuation plans were made a week after the accident. The data was not sent to the prime minister or the Nuclear Safety Commission, for which the government was criticized, but was made accessible to the public on the 23rd.

On 14 March 2011 TEPCO officials were instructed not to use the phrase "core meltdown" at press conferences.

Japan towns, villages, and cities in and around the Daiichi nuclear plant exclusion zone. The 20 and 30 km (12 and 19 mi) areas had evacuation and shelter in place orders, and additional administrative districts that had an evacuation order are highlighted. However, the above map's factual accuracy is called into question as only the southern portion of Kawamata district had evacuation orders. More accurate maps are available.[232][233]

The international reaction to the accident was diverse and widespread. Many inter-governmental agencies immediately offered help, often on an ad hoc basis. Responders included IAEA, World Meteorological Organization and the Preparatory Commission for the Comprehensive Nuclear Test Ban Treaty Organization.

In May 2011, UK chief inspector of nuclear installations Mike Weightman traveled to Japan as the lead of an International Atomic Energy Agency (IAEA) expert mission. The main finding of this mission, as reported to the IAEA ministerial conference that month, was that risks associated with tsunamis in several sites in Japan had been underestimated.

In September 2011, IAEA Director General Yukiya Amano said the Japanese nuclear disaster "caused deep public anxiety throughout the world and damaged confidence in nuclear power". Following the accident, it was reported in The Economist that the IAEA halved its estimate of additional nuclear generating capacity to be built by 2035.

Three investigations into the accident showed the man-made nature of the catastrophe and its roots in regulatory capture associated with a "network of corruption, collusion, and nepotism." A New York Times report found that the Japanese nuclear regulatory system consistently sided with, and promoted, the nuclear industry based on the concept of amakudari ('descent from heaven'), in which senior regulators accepted high paying jobs at companies they once oversaw.

In 2016 three former TEPCO executives, chairman Tsunehisa Katsumata and two vice presidents, were indicted for negligence resulting in death and injury. In June 2017 the first hearing took place, in which the three pleaded not guilty to professional negligence resulting in death and injury. In September 2019 the court found all three men not guilty.

The Fukushima Nuclear Accident Independent Investigation Commission (NAIIC) was the first independent investigation commission by the National Diet in the 66-year history of Japan's constitutional government.

"Its fundamental causes are to be found in the ingrained conventions of Japanese culture: our reflexive obedience; our reluctance to question authority; our devotion to 'sticking with the program'; our groupism; and our insularity."

The Commission recognized that the affected residents were still struggling and facing grave concerns, including the "health effects of radiation exposure, displacement, the dissolution of families, disruption of their lives and lifestyles and the contamination of vast areas of the environment".

The purpose of the Investigation Committee on the Accident at the Fukushima Nuclear Power Stations (ICANPS) was to identify the accident's causes and propose policies designed to minimize the damage and prevent the recurrence of similar incidents. The 10 member, government-appointed panel included scholars, journalists, lawyers, and engineers. It was supported by public prosecutors and government experts and released its final 448-page investigation report on 23 July 2012.

Cited

Others

Caldicott, Helen [ed.]: Crisis Without End: The Medical and Ecological Consequences of the Fukushima Nuclear Catastrophe. [From the "Symposium at the New York Academy of Medicine, March 11-12, 2013"]. The New Press, 2014. ISBN 978-1-59558-970-5 (eBook)

Nadesan, Majia (2013). Fukushima and the Privatization of Risk. London, Palgrave. ISBN 978-1-13734311-6

Cleveland, Kyle; Knowles, Scott Gabriel; Shineha, Ryuma, eds. (2021). Legacies of Fukushima. University of Pennsylvania Press. ISBN 978-0-8122-9800-0.

Videos, films, drawings, and images

"Inside Japan's Nuclear Meltdown", Season 2012, Episode 4, PBS Frontline

Video of the Unit 1 explosion

Video of the Unit 3 explosion

Webcam Fukushima nuclear power plant I, Unit 1 through Unit 4

Inside the slow and dangerous clean up of the Fukushima nuclear crisis

TerraFly Timeline Aerial Imagery of Fukushima Nuclear Reactor after 2011 Tsunami and Earthquake

In graphics: Fukushima nuclear alert, as provided by the BBC, 9 July 2012

Analysis by IRSN of the Fukushima Daiichi accident

Kumamoto, Murata & Nakate: "Fukushima Evacuees Face New Hardship Six Years On", provided by the Foreign Correspondents' Club of Japan, 9 March 2017

Video from the Unit 2 containment below the reactor in February 2019

The Days, docudrama, 2023, 8 one hour parts, based in part on Masao Yoshida's testimony in The Fukushima Nuclear Accident Report, 2012 - https://www.nirs.org/wp-content/uploads/fukushima/naiic_report.pdf This series "depicts the Fukushima Daiichi Nuclear Power Plant accident that occurred in 2011 over a period of 7 days." - https://www.imdb.com/title/tt22074484/]

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The Fukushima nuclear accident was a major nuclear accident at the Fukushima Daiichi nuclear power plant in Ōkuma, Fukushima, Japan which began on March 11, 2011. The proximate cause of the accident was the 2011 Tōhoku earthquake and tsunami, which resulted in electrical grid failure and damaged nearly all of the power plant's backup energy sources.

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Acidente de Kyshtym

Data: 29 de setembro de 1957

Localização: Complexo Mayak Production Association, União Soviética (atual Rússia)

Causa: Explosão em um tanque de resíduos radioativos de alta atividade.

Consequências: Liberação significativa de materiais radioativos, contaminação da área circundante, evacuações e impactos na saúde da população local.

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Lucía Blasco

BBC News Mundo

Cuando el reactor número 4 de la planta nuclear de Chernóbil explotó en abril de 1986 causando la mayor catástrofe nuclear de la historia, muchos lo identificaron erróneamente como un desastre "sin precedentes".

Sin embargo, hubo un accidente anterior que permaneció en secreto durante dos décadas, gracias al estricto protocolo soviético de ocultar la información perjudicial para el régimen.

Ocurrió en septiembre de 1957, en una planta secreta de reprocesamiento de combustible nuclear llamada Mayak (Mayak Production Association, MPA), que la Unión Soviética tenía cerca de Kyshtym, en los montes Urales, a unos 2.000 km al este de Moscú.

Una enorme nube radioactiva se extendió cientos de kilómetros sobre el país, afectando a unas 250.000 personas y provocando evacuaciones a gran escala.

Aunque docenas de trabajadores murieron y al menos dos centenares de personas fallecieron por síndrome de irradiación aguda (una reacción a las radiaciones ionizantes), el mundo permaneció ajeno a aquella catástrofe durante años.

"La Unión Soviética había vivido en 1957 el -hasta entonces- mayor desastre nuclear que, a diferencia de Chernóbil, sí lograron silenciar", le contó a BBC Mundo el periodista estadounidense Adam Higginbotham, autor de Midnight in Chernobyl ("Medianoche en Chernóbil", 2019), un libro que investiga los mitos y secretismos de Chernóbil.

Más sobre Chernóbil

"Adoptaron el mismo enfoque en Chernóbil. Pero en ese caso, a diferencia de en Mayak, la planta estaba muy cerca de las fronteras de la URSS con Occidente. Además, la contaminación que emanó de la explosión del reactor número 4 fue mucho mayor".

El historiador Serhii Plokhii, director del Instituto de Investigación Ucraniano de la Universidad de Harvard, EE.UU., le dijo a BBC Mundo que, efectivamente, el desastre de Kyshtym fue "mucho más pequeño" que Chernóbil, pero causó "gran contaminación".

Se liberaron 20 millones de curies de material radiactivo a la atmósfera, causando el tercer peor desastre nuclear de la historia después de Chernóbil y Fukushima, en 2011 en Japón.

Aquel 29 de septiembre, los técnicos que trabajaban en Mayak comenzaron su turno como de costumbre. Parecía un día normal.

Anna Sharova, ingeniera química, recordó el día del accidente en un documental que se publicaría años más tarde. "Teníamos un turno de 13:00 a 19:00. Trabajábamos en el laboratorio y, como era domingo, había menos trabajo", declaró.

"De repente, escuchamos una especie de trueno. Las ventanas estallaron, las puertas de cristal se quebraron. Miramos hacia afuera y vimos una nube enorme".

Lo que Anna escuchó fue la explosión causada por el sobrecalentamiento de uno de los tanques, que contenía residuos nucleares altamente concentrados. Pero el personal de la planta tenía prohibido hacer preguntas impertinentes. "No podíamos mencionar a nadie el accidente. Ni una sola palabra", dijo un colega de Sharova.

Pocos escucharon la noticia fuera de aquellas paredes.

"Secretismo total"

Según Serhii Plokhii, "no había información por ningún lado sobre lo ocurrido", pues guardar silencio era "un protocolo normalizado en la Unión Soviética"

Además, Mayak formaba parte del programa militar soviético: "Había una conexión muy cercana entre la producción de las bombas atómicas y la construcción de reactores para uso civil, fabricados por los militares". De hecho, fue en Mayak donde se produjo la primera bomba nuclear soviética.

"Y era la Guerra Fría, por eso había un secretismo total".

La planta de Mayak había comenzado a construirse en 1946 bajo un estricto secretismo. Paralelamente, se crearía una ciudad para albergar a sus trabajadores, que sería conocida como Chelyabinsk-40 (ciudad-40) y después recibiría el nombre de Ozersk, pero no aparecía en los mapas de la URSS.

Cuando ocurrió el accidente en 1957, la mayoría de la población soviética nunca había oído hablar de Mayak ni de Ozersk, y a la URSS no le interesaba que eso cambiara, especialmente estando a las puertas del 40 aniversario de la Revolución de Octubre y a cinco días del lanzamiento del Sputnik, el primer satélite artificial de la historia.

Era mejor mantenerlo en secreto.

Fuente de la imagen, Getty Images

Pie de foto,
El accidente ocurrió cinco días antes del lanzamiento del Sputnik 1.

Pero los soviéticos no fueron los únicos en guardar silencio.

"Los estadounidenses encontraron algunas señales de que había habido una explosión y contaminación en aquel primer desastre, pero no dijeron nada porque ellos mismos estaban en el proceso de desarrollar grandes planes nucleares, y no quisieron crear alarma", agregó Plokhii.

Hasta que un disidente ruso decidió hablar.

El científico que rompió el silencio

Zhores Medvedev fue el primero en revelar oficialmente a la comunidad internacional aquel grave accidente nuclear ocurrido en los Urales a finales de los 50. Había estudiado con detalle las consecuencias de aquella explosión.

Medvedev habló con la BBC en septiembre de 2016.

El bioquímico e historiador le contó a la periodista del programa Witness de la BBC Dina Newman su versión sobre los hechos.

"No existían suficientes regulaciones sobre la temperatura y, en un momento dado, el sistema de regulación de uno de los tanques dejó de operar. Cuando la temperatura llegó a ciertos niveles, el tanque explotó", dijo sobre el accidente.

Se calcula que al menos unas 200 personas murieron en 10 días y cientos de miles fueron seriamente afectadas, explicó Newman. Pero no hay cifras oficiales.

"No sabemos cuánta gente sufrió, cuántos murieron, cuántos eran soldados o criminales . Esas cifras no están disponibles", le contó Medvedev.

El científico supo de la historia porque trabajaba en un laboratorio de investigación en Moscú, y fue uno de los pocos expertos soviéticos encargado de monitorear la contaminación nuclear en la región. Era una de las pocas personas en el mundo que sabía realmente lo que ocurrió allí.

Medvedev se convirtió en disidente, exiliándose en Londres, Reino Unido. Casi 20 años más tarde, en 1976 publicó un artículo en el que mencionaba el desastre de Kyshtym.

"El elemento sorpresivo es que nadie en Occidente sabía sobre ello, excepto algunas agencias de inteligencia, que tenían información poco precisa", le contó el científico a la BBC.

Accidentes nucleares: cronología

Planta nuclear de Mayak, URSS, 29 de septiembre de 1957

Incendio de Windscale, Reino Unido, 7 de octubre de 1957

Laboratorio Nacional de Idaho, EE.UU., 3 de enero de 1961

Central nuclear Three Mile Island, EE.UU., 29 de marzo de 1979

Central nuclear de Chernóbil, URSS, 26 de abril de 1986

Tomsk-7 (actualmente Seversk), Rusia, 6 de abril de 1993

Accidente de Tokaimura, Japón, 30 de septiembre de 1999

Central nuclear de Mihama, Japón, 9 de agosto de 2004

Central nuclear de Fukushima, Japón, 11 de marzo de 2011

Complejo nuclear de Marcoule, Francia, 12 de septiembre de 2011

John Hill, presidente de la Autoridad para la Energía Atómica del Reino Unido, negó la historia. Al país europeo no le interesaba mala prensa sobre la energía nuclear.

Pero Medvedev siguió adelante con su plan: "Decidí revelar información más concreta sobre lo ocurrido y cómo había afectado a la vida de animales y plantas en la zona, y lo que pasó con la gente. Publiqué más documentos".

Después, reuniría sus descubrimientos en un libro, Nuclear disaster in the Urals ("Desastre nuclear en los Urales, 1980).

Hill nunca aceptó que estaba equivocado. Pero, tras el desastre de Chernóbil, la industria nuclear se abrió más a las críticas.

Medvedev murió en noviembre de 2018. Opinaba que hasta que no se solucionaran los problemas con los residuos nucleares, el mundo no estaría preparado para seguir desarrollando este tipo de energía.

Hoy, Mayak sigue siendo una de las principales centrales nucleares en Rusia.

Ahora puedes recibir notificaciones de BBC News Mundo. Descarga la nueva versión de nuestra app y actívala para no perderte nuestro mejor contenido.

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El desastre de Kyshtym, el accidente nuclear previo a Chernobyl que la URSS mantuvo en secreto durante dos décadas - BBC News Mundo

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Casi tres décadas antes de Chernóbil, hubo otro accidente nuclear en la Unión Soviética que los líderes del Partido Comunista lograron mantener en secreto durante años, hasta que un científico disidente rompió el silencio.

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Incidente de Windscale Piles

Data: 10 de outubro de 1957

Localização: Planta de reprocessamento de Windscale, Reino Unido

Causa: Incêndio em um reator de plutônio.

Consequências: Liberação de iodo radioativo na atmosfera, contaminação local, mas com impactos limitados em comparação com outros desastres

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Los reactores de Windscale (centro y a la derecha) en 1985.

El incendio de Windscale fue un incendio ocurrido el 10 de octubre de 1957 en las instalaciones de Windscale, en el condado de Cumbria (extremo norte de Inglaterra). Fue el peor accidente nuclear en la historia de Reino Unido, de una magnitud de nivel 5 de un máximo de 7 en la Escala Internacional de Accidentes Nucleares.​

Los dos reactores habían sido construidos de forma apresurada como parte del proyecto de la bomba atómica británica.​ El reactor número 1 de Windscale se declaró en estado de funcionamiento en octubre de 1950, seguido por el reactor número 2 en junio de 1951.​ El accidente ocurrió cuando el núcleo de la unidad 1 del reactor nuclear en Windscale, Cumberland (ahora Sellafield, Cumbria) se incendió, liberando sustanciales cantidades de contaminación radiactiva en la zona que lo rodeaba. Especialmente preocupante en ese momento resultaba el isótopo radiactivo yodo-131, que puede causar cáncer de tiroides. Con posterioridad, se estimó que el incidente causó 240 muertes adicionales debido al cáncer.​ No fue evacuado nadie en la zona, pero se temía que la leche pudiera estar peligrosamente contaminada. La leche producida en la zona de aproximadamente 500 km² alrededor del accidente fue diluida y destruida durante aproximadamente un mes. Un estudio del año 2010 sobre trabajadores que participaron directamente en las tareas de limpieza no encontró efectos a largo plazo sobre su salud provocados por su participación en dichas tareas.​​

El diseño del reactor número 1 de Windscale. Leyenda: Filter: Filtro, Channel: Canal, Fuel rods: varillas de combustible, Graphite core: Núcleo de grafito, Air flow: Flujo de aire.

Diagrama esquemático del reactor de Windscale. Leyenda: Filters: Filtros, Biological Cover: Cubierta biológica, Graphite Core: Núcleo de grafito, Coolant Air Intake: Ingreso de aire de refrigeración, Fuel Output: Salida del combustible.

Después de la Segunda Guerra Mundial, el Gobierno británico no quería quedarse atrás como potencia mundial en la emergente carrera armamentística, por lo que se embarcó en un programa para construir su propia bomba atómica tan rápidamente como fuera posible.

Los reactores fueron construidos en poco tiempo cerca de la pequeña aldea de Seascale, Cumberland, y fueron conocidos como la Pila 1 y la Pila 2 de Windscale. Se habían encerrado en grandes edificios de hormigón a pocos centenares de metros uno del otro. Los reactores eran del tipo moderado por grafito y refrigerados por aire. Debido a que la fisión nuclear produce grandes cantidades de calor, era necesario enfriar los núcleos de los reactores soplando aire a través de unos canales en el grafito. El aire frío era introducido por una batería de grandes ventiladores, y el aire caliente posteriormente era extraído por la parte trasera del núcleo hasta una chimenea. Se agregaron filtros en las etapas finales de la construcción en la parte más alta del cañón de las chimeneas, a solicitud de John Cockcroft y estos eran almacenados en galerías en la parte más alta de los cañones de descarga de las chimeneas. Se consideraban innecesarios, un desperdicio de tiempo y dinero y representaron un problema de ingeniería, al ser agregados tardíamente en el proceso de construcción en grandes bodegas de hormigón en la parte superior de unas chimeneas de 120 metros de alto. Debido a esto, fueron conocidos como la Locura de Cockcroft por los trabajadores e ingenieros. Como resultaron finalmente las cosas, la Locura de Cockcroft probablemente previno que un desastre se convirtiera en una catástrofe.

Los reactores fueron construidos a partir de un núcleo de grafito sólido, con canales horizontales a través de los cuales se podían pasar cartuchos de uranio y de isótopos, para poder exponer a los cartuchos de isótopos a la radiación neutrónica producida por el uranio y fabricar de esa forma plutonio y radioisótopos, respectivamente. El combustible y los isótopos eran colocados en el interior de los canales en la parte frontal del reactor, la cara de carga, y el combustible nuclear gastado era empujado a través del núcleo y sacado por la parte trasera, la cara de descarga, hacia un conducto de agua para un enfriamiento inicial previo a su recuperación y procesamiento para extraer el plutonio.

Combustible

Se usaba metal de uranio no enriquecido en cartuchos de aluminio con aletas para mejorar su refrigeración para la producción de plutonio. Como el propósito de este plutonio era ser usado en armas, el grado de combustión del combustible habría sido mantenido bajo para reducir la producción de isótopos más pesados del plutonio (, ²⁴¹ Pu, etc.).

Cartuchos de isótopos

Las siguientes sustancias eran colocadas al interior de cartuchos metálicos y eran sometidas a la irradiación de neutrones para crear radioisótopos. Tanto el material blanco y algunos de los isótopos producidos se indican más abajo. De estos, la liberación de polonio 210 produjo la contribución más significativa a la dosis de contaminación radiactiva sufrida por la población general.​

En la época en que los reactores se estaban construyendo, los británicos, a diferencia de los estadounidenses y los soviéticos, tenían poca experiencia con el comportamiento del grafito cuando este era expuesto a neutrones. El físico estadounidense de ascendencia húngara Eugene Wigner había descubierto que el grafito, cuando era bombardeado por neutrones, sufría de dislocaciones en su estructura cristalina que causaban la acumulación de energía potencial. Esta energía, si se permitía su acumulación, podía escapar espontáneamente en un poderoso golpe de calor. Una vez comisionado y en operaciones, la Pila 2 de Windscale, experimentó un misterioso aumento en la temperatura del núcleo, lo que fue atribuido a un súbita liberación de energía de Wigner. Esto preocupó a los científicos británicos, así que se buscó una forma segura de liberar la energía almacenada. La única solución viable fue un proceso de recocido, en el que el núcleo de grafito era calentado a una temperatura de 250 °C por medio del combustible nuclear para permitir que los átomos de carbono desplazados regresaran a su lugar en la estructura cristalina, liberando gradualmente su energía almacenada como calor, y causando una entrega uniforme dispersada a través de todo el núcleo.​ El recocido tuvo éxito en prevenir la acumulación de energía de Wigner, pero el equipo de monitorización, el reactor en sí mismo y todo el resto de las instalaciones tales como el sistema de refrigeración nunca fueron diseñados para esto. Cada ciclo de recocido era ligeramente diferente y progresivamente más difícil a medida que el tiempo pasaba; muchos de los últimos ciclos tuvieron que ser repetidos, y cada vez se requerían temperaturas más y más altas para poder iniciar el proceso de recocido. También se pudo determinar que permanecían algunos paquetes de energía de Wigner que no pudieron ser liberados en las ocasiones previas.

Durante el accidente, el combustible de uranio se incendió, no el moderador de grafito como se había pensado mayoritariamente. Una inspección hecha en 2005 mostró que el grafito dañado estaba localizado alrededor de los elementos de combustible.​ Las fases de recocido no fueron parte del plan original, así que se colocaron termopares en diferente posiciones en el reactor para monitorizar las operaciones normales, pero no para controlar el proceso de recocido. Esto permitió que se formaran puntos de calor desconocidos. El combustible de uranio metálico del reactor podía incendiarse fácilmente en presencia de oxígeno, a diferencia del dióxido de uranio usado en los reactores modernos. El vaciado directo del aire de refrigeración hacia la atmósfera significaba que cualquier escape de material radiactivo desde el núcleo que pudiera pasar a través de los filtros sería liberado hacia el ambiente externo.

Con el propósito de acordar un tratado de armas nucleares con Estados Unidos, los británicos tenían que demostrar que eran tecnológicamente sus iguales. Las instalaciones de Windscale fueron construidas para producir plutonio para la primera bomba atómica británica. Después de la exitosa explosión de la bomba atómica, Estados Unidos diseñó y probó una bomba termonuclear que requería tritio. Los británicos no tenían ninguna instalación para producir tritio y decidieron usar los reactores de Windscale. El tritio puede ser producido en reactores nucleares usando la activación por neutrones del litio-6. Se necesitaban temperaturas más altas para este proceso que las necesarias para producir plutonio y se decidió reducir el tamaño de las aletas de refrigeración (quedando un total aproximado de 500.000 aletas individuales) de los cartuchos de aluminio del combustible. Finalmente se pudo producir tritio al forzar el diseño de primera generación de las instalaciones de Windscale más allá de sus límites y con reducido margen de seguridad en la operación. Después de producir con éxito un primer lote de tritio en la Pila 1, se asumió que el problema del calor era despreciable y se comenzó la producción a plena escala, pero al elevar la temperatura del reactor más allá de las especificaciones de diseño, los científicos alteraron la distribución normal del calor en el núcleo, causando que se desarrollaran puntos de calor en la Pila 1. Estos picos de calor no fueron apreciados por los científicos ya que los termopares usados para medir las temperaturas del núcleo estaban localizados según el diseño de distribución de calor original y no estaban midiendo las partes más calientes del reactor, llevando a lecturas falsamente optimistas.

El 7 de octubre de 1957, los operadores comenzaron un ciclo de recocido para la Pila 1 de Windscale poniendo los ventiladores de refrigeración a un nivel de energía bajo y estabilizando el reactor a baja potencia. Al día siguiente, para ejecutar el proceso de recocido, los operadores incrementaron la potencia del reactor. Cuando parecía que el proceso de recocido estaba llevándose a cabo, se introdujeron las varillas de control en el núcleo para apagar el reactor, pero se pudo apreciar que la liberación de la energía de Wigner no estaba ocurriendo a través de todo el núcleo, sino que se estaba acabando prematuramente. Los operadores retiraron nuevamente las varillas de control para aplicar un segundo recalentamiento nuclear y completar el proceso de recocido. Debido a que las termocuplas no estaban localizadas en las partes más calientes del núcleo, los operadores no eran conscientes de que algunas áreas estaban considerablemente más calientes que otras. Se sospecha que esto, y el segundo recalentamiento, fueron los factores decisivos detrás del incendio, aunque la causa precisa permanece desconocida. El informe oficial sugiere que un cartucho de uranio se rompió y se oxidó causando un mayor recalentamiento y el incendio, pero un informe más reciente sugiere que puede haber sido un cartucho de isótopo de magnesio/litio. Todo lo que se pudo ver en los instrumentos fue un suave aumento de la temperatura, justo lo que era de esperarse durante la liberación de la energía de Wigner.

A principios de la mañana del 10 de octubre, se sospechó que algo inusual estaba ocurriendo. La temperatura en el núcleo, que se suponía debería estar cayendo gradualmente a medida que finalizaba la liberación de la energía de Wigner, comenzó a comportarse de forma ambigua tal como lo mostraba el equipo de control, pero además un termopar mostraba que la temperatura en el núcleo estaba aumentando en vez de disminuir. En un esfuerzo para ayudar a enfriar el reactor, se incrementó el flujo del aire. Esto alimentó al incendio con más oxígeno y llevó material radiactivo hacia la chimenea y las galerías de los filtros. Fue entonces cuando los trabajadores en la sala de control se dieron cuenta de que los dispositivos de control de radiación que vigilaban la radiación existente en la parte superior de los cañones de las chimeneas estaban dando lecturas en el tope de la escala. De acuerdo con los procedimientos escritos, el capataz declaró una emergencia.

Los operadores trataron de examinar el reactor con un escáner remoto, pero se había bloqueado. Tom Hughes, segundo en el mando de Organismo Administrador del Reactor, sugirió examinar personalmente el reactor, así que él y otro operador se dirigieron a la cara de carga del reactor, vestidos con ropa protectora. Se sacó un tapón de inspección del canal de combustible cercano a un termopar que estaba registrando altas temperaturas y fue entonces cuando los operadores vieron que el combustible estaba al rojo vivo por el calor.

Se sacó un tapón de inspección, dijo Tom Hughes en una entrevista posterior, y vimos, para nuestro completo horror, cuatro canales de combustible brillando con un color rojo cereza intenso.

No había duda de que el reactor estaba incendiándose, y de que lo había estado haciendo durante casi 48 horas. El administrador del reactor Tom Tuohy​ se puso un equipo protector completo y un aparato de respiración autónoma y escaló los 24 metros hasta la parte superior del edificio del reactor, donde se detuvo por encima de la tapa del reactor para examinar la parte trasera de este, la cara de descarga. Allí informó de una luminiscencia roja amortiguada, iluminando la brecha entre la parte trasera del reactor y la parte trasera del edificio de contención. Los cartuchos de combustible al rojo vivo brillaban en los canales de combustible de la cara de descarga. Regresó varias veces a la parte superior del edificio de contención del reactor durante la duración del incidente, en cuyo punto álgido ocurría un feroz incendio en la cara de descarga y en la parte trasera de la contención de hormigón reforzado-estructura de hormigón cuyas especificaciones requerían que la temperatura estuviera bajo cierto nivel para prevenir su desintegración y colapso.​

Los operadores no estaban seguros de cómo tratar el incendio. Primero procuraron apagar las llamas poniendo los ventiladores a toda potencia e incrementar la refrigeración, pero esto avivó el incendio. Tom Hughes y su colega ya habían tratado de crear un cortafuegos sacando algunos de los cartuchos de combustible no dañados durante el incendio, y Tom Tuohy sugirió tratar de sacar algunos directamente desde el centro del fuego, golpeando los cartuchos derretidos con los postes de los andamios desde el reactor hacia la piscina de refrigeración que había detrás. Esto resultó imposible de realizar ya que las barras de combustible no se pudieron mover, sin importar la fuerza realizada. Los postes se retiraron con sus extremos al rojo vivo y, uno de ellos, fue sacado al rojo vivo y goteando metal derretido. Hughes sabía que esto tenía que ser uranio irradiado derretido, lo que causó serios problemas de radiación en el elevador de carga.

Brillaba [el canal de combustible expuesto] al blanco, dijo el colega de Hughes a cargo del elevador de carga que se encontraba con él, estaba simplemente blanco por el calor. Nadie, absolutamente nadie, creería lo caliente que podría estar.

A continuación los operadores trataron de extinguir el fuego usando dióxido de carbono. Los nuevos reactores refrigerados por gas de Calder Hall acababan de recibir una entrega de 25 toneladas de dióxido de carbono líquido, que fue lanzado en la cara de carga de la Pila 1 de Windscale, pero hubo problemas para lograr que llegara al fuego en cantidades efectivas. El fuego era tan caliente que agotaba el oxígeno del dióxido de carbono que pudo ser aplicado.

Habíamos organizado esto, recuerda Hughes, y teníamos un pobre tubo de dióxido de carbono y no tenía ninguna esperanza de que fuera a funcionar.

En la mañana del viernes 11 de octubre, cuando el fuego estaba en su peor momento, once toneladas de uranio estaban incendiadas. Las temperaturas eran extremas (una termocupla registró 1.300 °C) y la contención biológica alrededor del reactor siniestrado estaba en peligro de colapso. Para afrontar la crisis, los operadores decidieron usar agua. Esto era arriesgado, ya que el metal derretido se oxida en contacto con el agua, sacando el oxígeno de las moléculas de agua y dejando hidrógeno libre, el cual puede mezclarse con el aire que llega y explotar, destruyendo la ya debilitada contención. En ausencia de alternativas, los operadores decidieron continuar con el plan. Aproximadamente una docena de mangueras contraincendios fueron llevadas a la cara de carga del reactor; se cortaron las bocas de las mangueras y estas se ataron a postes de andamios e introducidas en los canales de combustible a aproximadamente un metro sobre el núcleo del incendio. Tuohy nuevamente se arrastró a la parte superior de la protección del reactor y ordenó que se conectase el agua, escuchando cuidadosamente en los agujeros de inspección ante cualquier signo de una reacción de hidrógeno a medida que la presión aumentaba. El agua no tuvo éxito en extinguir el incendio, por lo que hubo que tomar nuevas medidas.

A continuación Tom Tuohy ordenó a todos que salieran del edificio del reactor excepto él mismo y el jefe de bomberos, con la idea de apagar toda la refrigeración y el aire de ventilación que entraba al reactor. Luego Tuohy subió varias veces e informó de que las llamas que saltaban desde la cara de descarga estaban lentamente extinguiéndose. Durante una de las inspecciones encontró que las tapas de inspección-que se quitaban con un gancho metálico para facilitar la visión de la cara de descarga del núcleo-estaban pegadas por succión. Esto, según informó, era debido a que el fuego estaba absorbiendo el aire desde cualquier lugar que pudiera.

No tengo duda de que en ese momento incluso estaba aspirando aire a través de la chimenea para tratar de mantenerse, recuerda Tuohy en una entrevista.

Finalmente logró sacar una placa de inspección y pudo ver cómo el incendio estaba apagándose.

Primero las llamas se apagaron, luego se redujeron y el brillo comenzó a morir, describe Tuohy, Subí varias veces para comprobar hasta que estuve satisfecho de que el fuego se había extinguido. Estaba de pie de costado, con algo de esperanza, relata, pero si miras directamente al núcleo de un reactor apagado vas a recibir una buena dosis de radiación.

El agua se mantuvo fluyendo a través del reactor durante otras 24 horas hasta que el reactor quedó completamente frío.

El tanque del reactor en sí mismo ha permanecido sellado desde el accidente y aún contiene aproximadamente unas 15 toneladas de combustible de uranio. Se pensó que el combustible restante podría volver a incendiarse nuevamente si se manipulaba, debido a la presencia de hidruro de uranio pirofórico formado a partir de la inundación original con agua.​ Investigaciones subsecuentes, llevadas a cabo como parte del proceso de descontaminación, descartaron esta posibilidad.​ El desmantelamiento final del reactor está programado para el año 2037.

Hubo una fuga de material radiactivo que se dispersó a través del Reino Unido y el Continente.​ Se estima que el incendio liberó 740 terabecquereles (20.000 curies) de yodo-131, así como 22 TBq (594 curies) de cesio-137 y 12.000 TBq (324.000 curies) de xenón-133, entre otros radioisótopos.​ El análisis posterior de los datos de contaminación ha mostrado un nivel de contaminación nacional e internacional más alto de lo previamente estimado.​ En comparación, la explosión de 1986 en Chernobyl liberó aproximadamente 1.760.000 TBq de yodo-131; 79.500 TBq de cesio-137; 6.500.000 TBq de xenón-133; 80.000 TBq de estroncio-90; y 6100 TBq de plutonio, junto con aproximadamente una docena de otros radioisótopos en grandes cantidades.​ El accidente de Three Mile Island en 1979 liberó 25 veces más de xenón-135 que Windscale, pero mucho menos yodo, cesio y estroncio.​

La presencia de los depuradores en las chimeneas en Windscale logró una contención parcial y de esa forma minimizó el contenido radiactivo del humo que escapaba por la chimenea durante el incendio. Estos depuradores supusieron un gran gasto y se instalaron tras la insistencia de John Cockcroft. Hasta el incendio de 1957 eran conocidos como "la locura de Cockcroft".​

Especialmente preocupante en ese momento era el isótopo radiactivo yodo-131, que tiene una vida media de solo 8 días pero es absorbido por el cuerpo humano y almacenado en la glándula tiroides. Como un resultado, el consumo de yodo-131 a menudo provoca cáncer de tiroides. Previamente había sido estimado que el incidente causó unos 200 casos adicionales de cáncer, aunque recientemente esta cifra ha sido revisado al aumento hasta 240 casos.​

No hubo evacuaciones en la zona adyacente, pero se temía que la leche podría estar peligrosamente contaminada. Esto hizo que la leche de aproximadamente 500 km² de la campiña cercana fuera destruida (diluida mil veces y vaciada al Mar de Irlanda) durante aproximadamente un mes. Un estudio del año 2010 de los trabajadores involucrados directamente en las tareas de limpieza -y por lo tanto con una tasa mayor de exposición a la radiación- no encontró efectos significativos de largo término sobre la salud.​​

El reactor era irrecuperable. Algunas de las varas de combustible se pudieron recuperar, y el escudo biológico del reactor se selló y se dejó intacto. Aproximadamente 6.700 elementos de combustible dañados y 1.700 cartuchos de isótopos dañados por el fuego permanecen en el reactor. El núcleo del reactor dañado estaba aún ligeramente caliente como resultado de las reacciones nucleares que continuaban en él.​ La Pila 2 de Windscale, aunque no fue dañada en el incendio, fue considerada demasiado insegura para continuar su uso. Fue clausurada poco después. Ningún reactor refrigerado por aire ha sido construido desde entonces. Se programó la recuperación final del combustible del reactor dañado para comienzos de 2008, con una duración de cuatro años.​

La Junta de Investigación del Gobierno británico se reunió bajo la presidencia de William Penney entre el 17 y el 25 de octubre de 1957. Su informe (el "Informe Penney") fue remitido al presidente de la Autoridad para la Energía Atómica del Reino Unido y formó la base del libro blanco gubernamental remitido al Parlamento en noviembre de 1957. El informe en sí mismo fue entregado a la Oficina de Registros Públicos en enero de 1988. En 1989 fue entregada una transcripción revisada, continuando el trabajo para mejorar la transcripción de las grabaciones originales.​​

Penney informó el 26 de octubre de 1957, 16 días después de apagar el incendio​ y alcanzó cuatro conclusiones:

La causa primaria del accidente había sido el segundo recalentamiento nuclear del 8 de octubre, aplicado demasiado pronto y demasiado rápido.

Los pasos tomados para enfrentar el accidente, una vez descubierto, fueron rápidos y eficientes y mostraron considerable devoción al deber por para parte de todos.

Las medidas tomadas para enfrentar las consecuencias del accidente fueron adecuadas y no hubo daño inmediato a la salud de ningún civil o de los trabajadores de Windscale. Era improbable que cualquier efecto dañino se pudiera dar. Pero el informe fue muy crítico de las deficiencias técnicas y organizativas.

Era necesaria una evaluación técnica más detallada, que llevara a cambios organizativos, responsabilidades por la salud y seguridad más claras y mejores definiciones de los límites de las dosis de radiación.

Aquellos que estuvieron involucrados directamente en los hechos se sintieron aliviados por la conclusión de Penney de que los pasos tomados habían sido rápidos y eficientes y que habían mostrado considerable abnegación". Algunos pensaban que a pesar de todo, la determinación y valentía mostrada por Thomas Touhy, así como el papel esencial que él jugó en conjurar un completo desastre, no habían sido totalmente reconocidos. Touhy murió el 12 de marzo de 2008 sin haber recibido nunca ninguna clase de reconocimiento público por sus esfuerzos.​

El sito de Windscale fue descontaminado y aún está en uso. Parte del sitio fue posteriormente renombrado Sellafield después de ser transferido a BNFL. En la actualidad todo el sitio es de propiedad de la Nuclear Decommissioning Authority.

Comparación con otros accidentes

La fuga de radiación por el incendio de Windscale fue excedida por mucho por el desastre de Chernóbil en 1986, pero el incendio fue descrito como el peor accidente de un reactor nuclear hasta el de Three Mile Island en 1979. Estimaciones epidemiológicas ponen el número de casos de cánceres adicionales causados por el accidente de Three Mile Island accidente en no más de uno. Solo Chernóbil produjo víctimas inmediatas.​

Three Mile Island era un reactor civil, y Chernóbil principalmente también lo era, ambos eran usados para la producción de energía eléctrica. En contraste Windscale era usado exclusivamente con propósitos militares.

Los reactores en Three Mile Island, a diferencia de aquellos en Windscale y Chernóbil, estaban en edificios diseñados para contener los materiales radiactivos generados por un accidente del reactor.

Otros reactores militares han producido víctimas inmediatas y conocidas tales como el incidente de 1961 en la planta SL-1 en Idaho que mató tres operadores, o el accidente de criticidad que mató a Louis Slotin en el Laboratorio Nacional de Los Álamos en 1946.

El accidente de Windscale fue contemporáneo al desastre de Kyshtym, un accidente más serio que ocurrió el 29 de septiembre de 1957 en la planta Mayak en la Unión Soviética, cuando la falla del sistema de refrigeración para un estanque que almacenaba decenas de miles de toneladas de desecho nuclear resultó en una explosión no nuclear.

En 1999, la BBC produjo un documental educativo acerca del incendio como un episodio de 30 minutos de "Disaster" (Serie 3) titulado El Incendio Windscale. Subsecuentemente fue publicado en DVD.​

En 2007, la BBC produjo otro documental acerca del accidente titulado "Windscale: Britain's Biggest Nuclear Disaster"​ (en castellano: Windscale: el desastre nuclear más grande de Gran Bretaña), en el que se investiga la historia de la primera instalación nuclear británica y su rol en el desarrollo de las armas nucleares. El documental presenta entrevistas con científicos y operadores de la planta claves, tales como Tom Tuohy, quien era el administrador general de Windscale. El documental sugiere que el incendio en Windscale en 1957 -el primer incendio en cualquier instalación nuclear- fue causado por la relajación en las medidas de seguridad, como resultado de la presión por el Gobierno británico para producir rápidamente material para fabricar bombas.​

Véase también

1. Richard Black (18 de marzo de 2011). "Fukushima - disaster or distraction?". BBC.

2. Gowing, M, Independence and Deterrence, Vol 2, p 386 ff.

3. Editorial, J. Radiol. Prot. 27 (2007) 211-215

4. ↑ Morelle, Rebecca (6 de octubre de 2007). "Windscale fallout underestimated". BBC News.

5. McGeoghegan, D.; Whaley, S.; Binks, K.; Gillies, M.; Thompson, K.; McElvenny, D. M. (2010). "Mortality and cancer registration experience of the Sellafield workers known to have been involved in the 1957 Windscale accident: 50 year follow-up". Journal of Radiological Protection 30 (3): 407-431.

6. McGeoghegan, D.; Binks, K. (2000). "Mortality and cancer registration experience of the Sellafield employees known to have been involved in the 1957 Windscale accident". Journal of Radiological Protection 20 (3): 261-274.

7. Crick, MJ; Linsley GS (noviembre de 1984). "An assessment of the radiological impact of the Windscale reactor fire, October 1957". Int J Radiat Biol Relat Stud Phys Chem Med 46 (5): 479-506. . doi:10.1080/09553008414551711.

8. W. BOTZEM, J. WÖRNER (NUKEM Nuklear GmbH, Alzenau, Germany) (14 de junio de 2001). "INERT ANNEALING OF IRRADIATED GRAPHITE BY INDUCTIVE HEATING".

9. ↑ "Meeting of RG2 with Windscale Pile 1 Decommissioning Project Team" (PDF). Nuclear Safety Advisory Committee. 29 de septiembre de 2005. NuSAC(2005)P 18. Archivado desde el original el 19 de octubre de 2020.

10. "Windscale Manager who doused the flames of 1957 fire - Obituary in The Independent 2008-03-26". London. 26 de marzo de 2008.

11. "Getting to the core issue Archivado el 17 de octubre de 2015 en Wayback Machine.", The Engineer, 14 May 2004.

12. ↑ John R. Cooper, Keith Randle, Ranjeet S. Sokhi (2003). M. J. Crick, G. S. Linsley (1984). Radioactive releases in the environment: impact and assessment. Wiley. p. 150. .. Citing: An assessment of the radiological impact of the Windscale reactor fire, October 1957. National Emergency Training Center. .

13. Los detalles de los niveles y la naturaleza de la radiactividad restante en el núcleo pueden ser vistos en http://www.irpa.net/irpa10/cdrom/00322.pdf .

14. Paul Dwyer (5 de octubre de 2007). "Windscale: A nuclear disaster". BBC News.

15. "Proceedings into the fire at Windscale Pile Number One (1989 revised transcript of the "Penney Report")" (PDF). UKAEA. 18 de abril de 1989.

16. "Disaster - Series 3". bbcactivevideoforlearning.com. 1999. Archivado desde el original el 9 de marzo de 2012.

17. "BBC documentary reveals government reckless in drive for nuclear weapons". WSWS. 29 de abril de 2008.

Bibliografía

'The Windscale reactor accident--50 years on' R. Wakeford. Journal of Radiological Protection 2007 Sep vol. 27(3) pp211-5. Epub Aug 29,2007

"Windscale fallout blew right across Europe", Rob Edwards. New Scientist, October 6, 2007.

Windscale, 1957: Anatomy of a Nuclear Accident, Lorna Arnold. New York : St. Martin's Press 1992

"Chernobyl: worst but not first", Walter C. Patterson. Bulletin of the Atomic Scientists, August/September 1986.

'Secrets of the Windscale fire revealed', F. Pearce. New Scientist vol 99 29 September 1983 p. 911

'Windscale; increased cáncer incidence alleged', T. Beardsley. Nature vol 306 Issue 5938 Nov 3 1983 p.5

"Accident at Windscale No.1 Pile on 10 October 1957". Cmnd. 302. (H.M.S.O., 1958).

"Accident at Windscale: World's First Atomic Alarm", Hartley Howe. Popular Science, October 1958, Vol. 173, No. 4.

"An Assessment of the Radiological Impact of the Windscale Reactor Fire", M.J. Crick, G.S. Linsley. NRPB Reports, Oct. 1957, Nov. 1982.

An airborne radiometric survey of the Windscale area, October 19-22nd, 1957. A.E.R.E. reports, no. R2890. (Atomic Energy Research Establishment).

The deposition of strontium 89 and strontium 90 on agricultural land and their entry into milk after the reactor accident at Windscale in October, 1957. A.H.S.B. (United Kingdom Atomic Energy Authority).

'Accident at Windscale' British Medical Journal 16 Nov 1957;2 (5054) pp 1166-8.

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Incendio de Windscale - Wikipedia, la enciclopedia libre

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El incendio de Windscale fue un incendio ocurrido el 10 de octubre de 1957 en las instalaciones de Windscale, en el condado de Cumbria (extremo norte de Inglaterra).

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Acidente nuclear de SL-1

Data: 3 de janeiro de 1961

Localização: Estação de Geração de Energia Nuclear SL-1, Idaho, EUA

Causa: Manipulação inadequada durante a manutenção levou a uma reação em cadeia não controlada.

Consequências: Três operadores mortos, contaminação local, e a destruição do reator.

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De acordo com investigações, a explosão do reator nuclear SL-1, que resultou no pior desastre nuclear dos Estados Unidos no início dos anos 1960, foi causada por uma "pegadinha".

O reator, que ficava em Idaho, era destinado a fornecer energia a pequenas instalações militares remotas naquela época e, durante a manutenção, as hastes de controle (que absorvem nêutrons no núcleo do reator) precisavam ser puxadas manualmente apenas alguns centímetros antes de serem reconectadas. Mas, se forem retirados demais, uma explosão é possível.

E isso aconteceu em 3 de janeiro de 1961, quando um dos operadores puxou a haste muito longe, fazendo com que o SL-1 ficasse em estado crítico. Três pessoas morreram no desastre nuclear, o pior que já aconteceu em solo americano, com um homem sendo empalado na virilha pela haste, que saiu por seu ombro e o prendeu no teto.

Uma investigação foi ordenada sobre o evento para evitar que ele ocorresse novamente. Eles descobriram que a haste havia sido retirada 50 centímetros (20 polegadas), em vez dos poucos centímetros que deveria ter sido. No entanto, a investigação descobriu como o acidente aconteceu do ponto de vista mecânico, mas nada além disso.

Perguntas começaram a circular: o que poderia ter feito o operador balançar o suficiente para remover a haste até agora? Foi de propósito? Foi suicídio ou assassinato? Uma das teorias, se provada correta, tornaria este um dos mais estúpidos incidentes nucleares já registrados.

Richard Legg (o homem que mais tarde foi empalado até o teto) tinha um histórico de ser um pouco brincalhão. Se Legg tivesse limitado suas pegadinhas fora do horário de trabalho, talvez ele não fosse listado como suspeito. Porém, em certa ocasião, ele desligou um ventilador que servia para resfriar parte do reator, a fim de disparar um alarme e assustar seus colegas de propósito.

Uma de suas pegadinhas favoritas era "brincar" com os colegas, que é, essencialmente, beliscá-los na bunda quando não estão olhando. Isso levou à teoria de que Legg viu seu colega Byrnes ficar em pé sobre a vara, sacudindo a bunda, e a tentação fora demais.

Ele aproveitou a oportunidade, diz a teoria, e o choque subsequente fez com que Byrnes levantasse a vara longe demais, matando todos com um único beliscão de bunda. Absurdamente, a teoria foi testada por investigadores.

"Nós olhamos para o ganso", disse C. Wayne Bills, um dos investigadores, ao autor de "Idaho Falls: A História Não Contada do Primeiro Acidente Nuclear da América". "Você sabe, alguém agarrando o cara no traseiro e fazendo-o pular."

Em um experimento, os voluntários foram convidados a operar uma haste em uma réplica do reator. Sem que eles soubessem, enquanto puxavam a vara, alguém se aproximava e os beliscava no traseiro. Isso aconteceu com muitos voluntários até que os experimentadores estivessem satisfeitos. Mas, nenhum deles deslocou a haste o suficiente para fazer o reator ficar crítico.

No entanto, a verdade é, provavelmente, muito mais simples: se você tiver um maquinário defeituoso, mais cedo ou mais tarde acidentes acontecerão. E o SL-1 estava com defeito, o que é perigoso mesmo antes de você levar em conta que os operadores foram treinados apenas por seis semanas antes de começarem seu trabalho.

"Os caras da Comissão de Energia Atômica estavam cobrindo suas bundas. Eles não queriam que seu programa nuclear fosse contestado. Eu não acho que eles acreditaram muito se alguma coisa que escreveram sobre eles. Mas o que eles disseram irritou muitos dos velhos caras da bomba nuclear. Eles sentiram que esses homens estavam trabalhando em um reator mal construído e perderam suas vidas por causa disso e então eles foram atingidos por esse legado de merda feito pelos patrões", disse William McKeown ao The New York Post.

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Pior desastre nuclear dos EUA pode ter sido causado por uma 'pegadinha', mostra investigação - Planeta

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De acordo com investigações, a explosão do reator nuclear SL-1, que resultou no pior desastre nuclear dos Estados Unidos no início dos anos 1960, foi causada por uma "pegadinha".

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Desastre de Kashiwazaki-Kariwa

Data: 16 de julho de 2007

Localização: Usina Nuclear de Kashiwazaki-Kariwa, Japão

Causa: Terremoto de magnitude 6.6, além de falhas nas medidas de segurança.

Consequências: Vazamento de água contaminada, danos estruturais, e paralisação temporária das operações.

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Acidente de Three Mile Island

Data: 28 de março de 1979

Localização: Central Nuclear de Three Mile Island, Pensilvânia, EUA

Causa: Falha em um sistema de refrigeração, seguida por erro humano.

Consequências: Liberação limitada de gases radioativos, sem mortes diretas, mas com impacto na confiança pública na energia nuclear.

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Foto da central nuclear de Three Mile Island. Os reatores encontram-se nas cúpulas mais pequenas de topos arredondados (as grandes chaminés são apenas torres de arrefecimento).

O acidente de Three Mile Island foi um derretimento nuclear parcial da Unidade 2 da central nuclear de Three Mile Island, no condado de Dauphin, perto de Harrisburg , em 28 de Março de 1979.

Foi o mais significativo acidente na história da indústria de geração comercial de energia nuclear americana, resultando na libertação de até 481 P Bq de gases radioactivos e menos de 740 G Bq do particularmente perigoso iodo-131.

O acidente nuclear de Three Mile Island foi considerado o mais grave, até ser superado pelo Acidente nuclear de Chernobil em abril de 1986 e o Acidente nuclear de Fukushima I em março de 2011

Three Mile Island em 2014. Usam só uma central nuclear, a TMI-1 à direita. TMI-2 à esquerda, não foi usada desde o acidente.

O acidente começou às 4 da manhã da quarta-feira, dia 28 de Março de 1979, com falhas no sistema secundário não-nuclear, seguido por uma válvula de alívio operada por piloto do sistema primário que tinha ficado aberta, permitindo que grandes quantidades de líquido de arrefecimento escapassem. As falhas mecânicas foram criadas pela falha inicial dos operadores do reator em reconhecer a situação como um acidente de perda de líquido refrigerante devido a treino inadequado e factores humanos como erros de desenho industrial relacionados com a presença de indicadores ambíguos na sala de controlo no interface do utilizador da central. O âmbito e complexidade do acidente tornaram-se claros no decurso de cinco dias, quando empregados da Metropolitan Edison, oficiais do estado da Pensilvânia e membros da Comissão Nuclear Reguladora dos Estados Unidos tentavam compreender o problema, comunicavam a situação para a imprensa e comunidade local, decidiam se o acidente requeria uma evacuação de emergência e por fim davam conta do fim da crise.

No final, o reator foi de novo controlado, apesar de maiores detalhes do acidente só terem sido descobertos mais tarde, seguindo extensas investigações por uma comissão presidencial e pela Comissão Nuclear Reguladora. O Relatório da Comissão Kemeny concluiu que "ou não haverá casos de câncer ou o número será tão pequeno que nunca será possível detectá-los. A mesma conclusão é aplicável para outros efeitos de saúde." Alguns estudos epidemiológicos posteriores ao acidente suportaram a conclusão de que a libertação de radiação no acidente não teve efeitos perceptíveis na incidência de cancro nos residentes perto do reator, apesar de estas descobertas terem sido contestadas por uma equipa de pesquisadores.

A reação pública relativa ao evento foi provavelmente influenciada pela saída, 12 dias antes do acidente, de um filme chamado The China Syndrome, que descrevia um acidente num reator nuclear. Comunicações efetuadas por oficiais durante as fases iniciais do acidente foram sentidas como confusas. O acidente foi seguido por uma cessação de construção de novas centrais nucleares nos Estados Unidos da América.

Falhas humanas e de equipamento

Equipe faz limpeza em prédio auxiliar do reator danificado, 1979

Uma bolha de gás altamente radioativa havia se instalado na parte de cima do reator, uma parte do urânio derreteu e o material fisionazion entrou em contato com a água, impedindo o acesso da água de refrigeração. Somente no dia 2 de abril, os técnicos conseguiram reduzir a bolha de gás em volta do reator de 50 metros cúbicos para cerca de um metro cúbico. Enquanto isso aumentavam nos Estados Unidos as críticas às medidas de segurança.

Em contrapartida, a empresa que administrava a usina acusou as autoridades de exagero ao comentarem o incidente. Algum tempo depois, os elementos combustíveis resfriaram e o perigo de explosão estava afastado.

No dia 1 de novembro de 1979, uma comissão nomeada pelo então presidente dos Estados Unidos, Jimmy Carter, a Kemeny Commission Investigation, chegou à conclusão de que o acidente fora causado por uma combinação de diversos factores: falhas várias do equipamento, erros humanos, procedimentos administrativos inadequados e deficiências do projeto. A comissão acusou severamente a Babcock & Wilcox, a Met Ed (Metropolitan Edison Company) , GPU (General Public Utilities), e a NRC (Nuclear Regulatory Commission) , mas esclareceu que não tinha mandato para desenhar conclusões sobre o futuro da indústria nuclear ou para fazer comparações da energia nuclear com outras fontes de energia. Acrescentou contudo que, sem as falhas humanas, o acidente teria sido provavelmente bem menor.

A princípio, a direção da usina pretendia reparar o reator danificado. Os técnicos constataram, no entanto, que os danos haviam sido maiores do que se suspeitava. Setenta por cento do núcleo do reator fora destruído pelo calor.

Vinte e oito horas após o acidente ter começado, o então vice-governador da Pensilvânia, William Scranton III, apareceu em um boletim de notícias para dizer que a Metropolitan Edison, dona da fábrica, assegurou ao estado que "tudo estava sob controle". Mais tarde naquele dia, o vice-governador mudou sua afirmação, dizendo que a situação era "mais complexa do que a empresa antes nos levou a acreditar". Havia declarações conflitantes sobre lançamentos de radioatividade. As escolas foram fechadas e os moradores foram instados a permanecer em casa. Os agricultores foram informados para manter seus animais em lugares cobertos, com alimentos.

O então governador Dick Thornburgh, sob o conselho do presidente da Comissão Reguladora Nuclear Joseph Hendrie, aconselhou a evacuação "de mulheres grávidas e crianças de idade pré-escolar (...) dentro de um raio de 5 milhas (8 km) da instalação de Three Mile Island". A zona de evacuação foi ampliada para um raio de 20 milhas (32 km) na sexta-feira, 30 de março. Dentro de dias, 140.000 pessoas deixaram a área. Mais da metade da população de 663.500 pessoas no raio de 20 milhas (32 km) permaneceu naquela área. De acordo com uma pesquisa realizada em abril de 1979, 98% dos evacuados voltaram para suas casas dentro de três semanas.

Um dia depois do acidente, um grupo de ecologistas mediu a radioatividade em volta da usina, a intensidade era oito vezes maior que a letal. Uma área de até 16 quilômetros em volta de Three Mile Island estava contaminada. Apesar de ter sido declarado o estado de emergência, nenhum dos 15 mil habitantes que moravam numa área até dois quilômetros da área contaminada foi evacuado. O governador Dick Thornburgh, iniciou a retirada dos habitantes só dois dias depois, começando com as gestantes e crianças.

Referências

Bibliografia

Walker, J. Samuel (2004) - Three Mile Island : a nuclear crisis in historical perspective - University of California Press.

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Acidente de Three Mile Island - Wikipédia, a enciclopédia livre

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O acidente de Three Mile Island foi um derretimento nuclear parcial da Unidade 2 da central nuclear de Three Mile Island, no condado de Dauphin, perto de Harrisburg , em 28 de Março de 1979.

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Incidente radiológico de Goiânia

Data: Setembro de 1987

Localização: Goiânia, Brasil

Causa: Abandono de um equipamento médico contendo césio-137, que foi posteriormente desmontado por pessoas não treinadas.

Consequências: Quatro mortes, várias pessoas contaminadas, evacuações e deslocamento de resíduos radioativos.

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Desastre nuclear de SL-2

Data: 13 de janeiro de 1961

Localização: Estação de Geração de Energia Nuclear SL-2, Idaho, EUA

Causa: Falha em um experimento de resfriamento.

Consequências: Um operador morto, contaminação local, e a destruição do reator.

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Incidente de contaminação de Chelyabinsk

Data: 1950s-1960s (período estendido)

Localização: Complexo Mayak Production Association, União Soviética (atual Rússia)

Causa: Liberação não autorizada de resíduos radioativos diretamente nos rios locais.

Consequências: Contaminação crônica da água e solo, impactando a saúde da população local ao longo de vários anos.