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This webpage explains all aspects of the Fukushima accident.

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A major earthquake on 11 March 2011 caused a 15-metre tsunami to strike the Fukushima Daiichi nuclear power plant on Japan's Tohoku coast, disabling the power supply and heat sinks, thereby triggering a nuclear accident. The reactors involved were boiling water units of a 1960s design owned and operated by Tokyo Electric Power Company and supplied by GE, Toshiba and Hitachi. Reactors 1-4 came into commercial operation 1971-78.

Without cooling water, the cores of units 1, 2 and 3 overheated and largely melted in the first three days. Hydrogen generated by this high-temperature process caused explosions in the upper service floors of reactor buildings at units 1 and 3. Unit 4 had not been operating, but was affected by a hydrogen explosion due to gas back-flow from unit 3. All four reactors are written off. Two other reactors at the plant were not involved in the accident.

The major accident was rated at Level 7 on the International Nuclear Event Scale due to high radioactive releases to air in the first few days. The bulk of releases occurred with the explosions, while a leak of contaminated water to sea continued for two months. Further releases of radioactivity to the air were brought to insignificant levels before the end of 2011, although much radioactivity remains dispersed on the ground in the surrounding area.
Effects on People
Significant amounts of radioactivity were released, but prompt evacuation from the immediate area made sure that no member of the public received enough exposure to cause harm. Some 160,000 people were evacuated from their homes and only in 2012 were some allowed limited return. Certain areas are still off limits but the Japanese government has announced it is ready to lift the evacuation order on the first nearby town in April.

Radiation was never expected to have any measureable effect on the health of the population and this was confirmed in 2013 by an estimation from the UN Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) that no person in Fukushima prefecture would be exposed, through the environment or their food, to more than 10 mSv in their entire lifetime. This is one tenth of the level at which health effects are known to become more likely, and therefore no measureable increase in cancer rates is expected. The government continues to monitor the health of all Fukushima residents. Stress, worry and the social problems of relocation have been repeatedly identified as the only likely causes of ill health.
Effects on the Sea, Fishing and Food
Groundwater travels naturally from the land to the sea and, in doing so is believed to mingle with heavily contaminated water in the basements of the power plant buildings. This continues to sea and a major effort is underway to identify the routes it is taking and manage groundwater to reduce this to the maximum extent possible. A silt fence has long been in place to prevent contamination reaching the open sea and the diluting effects of ocean currents mean that radioactivity cannot be detected in seawater beyond the plant harbour. Radioactive material continues to run off from the land through rivers to the sea and can be found in certain species of fish. However, all food from affected areas has been strictly monitored since the accident and prevented from sale if in excess of highly conservative standards.
Current Situation
It is presumed that the remains of the reactor cores (molten corium or fuel debris) are within the buildings and stably cooled by water circulation. A large water treatment plant was built to cope with the fact that this water becomes contaminated by the core materials in the destroyed reactors. Also there is considerable storage capacity built at the site to hold decontaminated water and avoid its release. Management of extensive water storage at various levels of radioactivity is becoming a challenge that has been given much media attention.

Nitrogen is being injected into all three reactors to ensure inert atmosphere there and prevent any chance of further hydrogen explosions. Nuclear fuel in storage pools is being stably cooled and is believed not to have been significantly damaged. Removal of fuel from the storage pool in unit 4 began in November 2013, and is well underway.

In about ten years Japanese technicians expect to be ready to begin removal of the melted core material from inside units 1-3. The four reactors will be decommissioned in 30-40 years, which is typical for any nuclear facility.
Actions to Improve Safety
The industry has taken several steps to enhance safety at nuclear energy facilities based on lessons from the accident at Fukushima. These actions include:

Acquiring or ordering backup safety equipment, including diesel generators, pumps and emergency response vehicles.

Enhancing the ability of nuclear energy facilities to remain safe even if there is an extended loss of electric power. The loss of all power sources at Fukushima Daiichi prevented the use of backup cooling systems for the reactors.

Developing strategies to mitigate external events beyond the design envelope for nuclear energy facilities.

Improving plants' ability to monitor water level and temperature in storage pools for used nuclear fuel during an extended loss of electric power.

Assessing the staff needed to respond to a large-scale natural event at multiple reactors at a site and to implement strategies contained in the emergency plan.

Assessing communications and equipment used during an emergency to ensure that power is maintained during a large-scale natural event.

Sources:
World Nuclear Association, http://www.world-nuclear.org/Features/Fukushima/Situation-at-Fukushima/.
Nuclear Energy Institute, http://www.nei.org/Master-Document-Folder/Backgrounders/Fact-Sheets/Safety-Nuclear-Industry-Responds-to-Fukushima-Acci.

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A major earthquake on 11 March 2011 caused a 15-metre tsunami to strike the Fukushima Daiichi nuclear power plant on Japan's Tohoku coast, disabling the power supply and heat sinks, thereby triggering a nuclear accident. The reactors involved were boiling water units of a 1960s design owned and operated by Tokyo Electric Power Company and supplied by GE, Toshiba and Hitachi.

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This webpage explains all aspects of the Chernobyl accident.

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The accident, which occurred in the early morning of April 26, 1986, resulted when operators took actions in violation of the plant's technical specifications. Operators ran the plant at very low power, without adequate safety precautions and without properly coordinating or communicating the procedure with safety personnel.

The four Chernobyl reactors were pressurized water reactors of the Soviet RBMK design, or Reactor BolshoMoshchnosty Kanalny, meaning "high-power channel reactor." Designed to produce both plutonium and electric power, they were very different from standard commercial designs, employing a unique combination of a graphite moderator and water coolant.

The reactors also were highly unstable at low power, primarily owing to control rod design and "positive void coefficient," factors that accelerated nuclear chain reaction and power output if the reactors lost cooling water.

These factors all contributed to an uncontrollable power surge that led to Chernobyl 4's destruction. The power surge caused a sudden increase in heat, which ruptured some of the pressure tubes containing fuel.

The hot fuel particles reacted with water and caused a steam explosion, which lifted the 1,000-metric-ton cover off the top of the reactor, rupturing the rest of the 1,660 pressure tubes, causing a second explosion and exposing the reactor core to the environment. The fire burned for 10 days, releasing a large amount of radiation into the atmosphere.

The Chernobyl plant did not have the massive containment structure common to most nuclear power plants elsewhere in the world. Without this protection, radioactive material escaped into the environment.

The crippled Chernobyl 4 reactor now is enclosed in a concrete structure that is growing weaker over time. Ukraine and the Group of Eight industrialized nations have agreed on a plan to stabilize the existing structure by constructing an enormous new sarcophagus around it, which is expected to last more than 100 years.

Officials shut down reactor 2 after a building fire in 1991 and closed Chernobyl 1 and 3 in 1996 and 2000, respectively.
Effects
Soviet scientists reported that the Chernobyl 4 reactor contained about 190 metric tons of uranium dioxide fuel and fission products. Estimates are that 13 percent to 30 percent of this escaped into the atmosphere.

Contamination from the accident did not spread evenly across the surrounding countryside but scattered irregularly, depending on weather conditions. Reports from Soviet and western scientists indicate that Belarus received about 60 percent of the contamination that fell on the former Soviet Union. A large area in the Russian Federation south of Bryansk also was contaminated, as were parts of northwestern Ukraine.
Short-Term Effects
Workers involved in the recovery and cleanup after the accident received high doses of radiation. In most cases, these workers were not equipped with individual dosimeters to measure the amount of radiation received. Further, dosimetric procedures varied, so experts could only estimate their doses.

According to Soviet estimates, between 300,000 and 600,000 people participated in the cleanup of the 30-kilometer evacuation zone around the reactor, but many did not enter the zone until two years after the accident.

Soviet officials estimated that 211,000 workers participated in cleanup activities in the first year after the accident and received an average dose of 16.5 rem.

Some children in contaminated areas received high thyroid doses because of an intake of radioiodine, a relatively short-lived isotope, from contaminated local milk. Several studies have found that the incidence of thyroid cancer among children under the age of 15 in Belarus, Russia and Ukraine has risen sharply.

The childhood thyroid cancers that have appeared are treatable, if detected early, with surgery followed by iodine 131 therapy for any metastases and then thyroid hormone replacement.
Long-Term Effects
Immediately following the accident, the main health concern involved radioiodine, which has a half-life of eight days. For the longer term, there is concern about contamination of the soil with cesium-137, which has a half-life of about 30 years.

Soviet authorities started evacuating people from the area around Chernobyl within 36 hours of the accident. By May 1986, about a month later, authorities had relocated all those living within a 30-kilometer (18-mile) radius of the plant-about 116,000 people.
Epidemiological Studies
The International Chernobyl Project conducted the first major assessment of the radiological consequences of the Chernobyl accident. Led by an advisory group of international experts, the project included the Commission of the European Communities, United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), World Health Organization, Food and Agriculture Organization, International Labor Organization, and International Atomic Energy Agency (IAEA).

In a 1991 study, the project's scientists noted that, as expected, official Soviet data did not indicate a marked increase in the incidence of leukemia or other cancers. However, several researchers pointed out that the project's sample size was too small and the study's time frame too short to identify an increase in the incidence of cancers with long latent periods. The project's scientists noted that "reported absorbed thyroid dose estimates in children are such that there may be a statistically detectable increase in the incidence of thyroid tumors in the future."

Institutions including WHO, the European Union, IAEA and UNSCEAR conducted exhaustive subsequent health studies. In general, these studies found that, apart from the "substantial increase" in thyroid cancer after childhood exposure observed in Belarus, Russia and Ukraine, there was very little evidence of "a major public health impact" related to ionizing radiation.

In addition, there was a consensus that the accident took a tremendous toll on the mental health of those living in affected areas, leading to an increase in anxiety, depression and stress-related disorders.
The Chernobyl Forum
A landmark study published in September 2005 by the Chernobyl Forum introduced new findings.

The report, "Chernobyl's Legacy: Health, Environment and Socio-Economic Impacts," incorporates the work of hundreds of scientists, economists and health experts. They studied the latest epidemiological data "to settle the outstanding questions about how much death, disease and economic fallout really resulted from the Chernobyl accident," said Burton Bennett, chairman of the forum, which is comprised of eight specialized U.N. agencies, as well as the governments of Belarus, Russia and Ukraine.

"The health effects of the accident were potentially horrific, but when you add them up using validated conclusions from good science, the public health effects were not nearly as substantial as had at first been feared," said Michael Repacholi, manager of WHO's radiation program.

Among the study's findings:

Most emergency workers and people living in contaminated areas received relatively low whole-body radiation doses, comparable to natural background levels.

About 4,000 individuals, most of whom were children or adolescents at the time of the accident, were stricken with thyroid cancer as a result of the contamination, and nine of them have died from the disease. However, the survival rate among such cancer victims has been almost 99 percent.

The study theorizes that some 4,000 people eventually could die from cancer caused by radiation exposure. However, the international team of experts has found no evidence of any increases in the incidence of leukemia and other cancers among affected residents.

The experts found no evidence or likelihood of decreased fertility or of increases in congenital malformations that could be attributed to radiation exposure.

Poverty, mental health problems and "lifestyle" diseases, such as alcoholism and tobacco dependency, pose a far greater threat to local communities than does radiation exposure. Relocation proved a "deeply traumatic experience" for some 350,000 people moved out of the affected areas, the study noted, while persistent myths and misperceptions about the threat of radiation have resulted in a "paralyzing fatalism" among residents of affected areas. Seeing themselves as "victims" rather than "survivors" has led to overcautious and exaggerated health concerns.
Source:
Nuclear Energy Institute, July 2011. http://www.nei.org/Master-Document-Folder/Backgrounders/Fact-Sheets/Chernobyl-Accident-and-Its-Consequences.

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The accident, which occurred in the early morning of April 26, 1986, resulted when operators took actions in violation of the plant's technical specifications. Operators ran the plant at very low power, without adequate safety precautions and without properly coordinating or communicating the procedure with safety personnel.

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The World Nuclear Association's report on the Fukushima Accident 

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The World Nuclear Association's report on the Fukushima Accident 

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Following a major earthquake, a 15-metre tsunami disabled the power supply and cooling of three Fukushima Daiichi reactors, causing a nuclear accident on 11 March 2011. All three cores largely melted in the first three days.

The accident was rated 7 on the INES scale, due to high radioactive releases over days 4 to 6, eventually a total of some 940 PBq (I-131 eq).

Four reactors were written off due to damage in the accident - 2719 MWe net.

After two weeks, the three reactors (units 1-3) were stable with water addition and by July they were being cooled with recycled water from the new treatment plant. Official 'cold shutdown condition' was announced in mid-December.

Apart from cooling, the basic ongoing task was to prevent release of radioactive materials, particularly in contaminated water leaked from the three units. This task became newsworthy in August 2013.

There have been no deaths or cases of radiation sickness from the nuclear accident, but over 100,000 people were evacuated from their homes to ensure this. Government nervousness delays the return of many.

Official figures show that there have been well over 1000 deaths from maintaining the evacuation, in contrast to little risk from radiation if early return had been allowed.

The Great East Japan Earthquake of magnitude 9.0 at 2.46 pm on Friday 11 March 2011 did considerable damage in the region, and the large tsunami it created caused very much more. The earthquake was centred 130 km offshore the city of Sendai in Miyagi prefecture on the eastern cost of Honshu Island (the main part of Japan), and was a rare and complex double quake giving a severe duration of about 3 minutes. An area of the seafloor extending 650 km north-south moved typically 10-20 metres horizontally. Japan moved a few metres east and the local coastline subsided half a metre. The tsunami inundated about 560 sq km and resulted in a human death toll of about 19,000 and much damage to coastal ports and towns, with over a million buildings destroyed or partly collapsed.

Eleven reactors at four nuclear power plants in the region were operating at the time and all shut down automatically when the quake hit. Subsequent inspection showed no significant damage to any from the earthquake. The operating units which shut down were Tokyo Electric Power Company's (Tepco) Fukushima Daiichi 1, 2, 3, and Fukushima Daini 1, 2, 3, 4, Tohoku's Onagawa 1, 2, 3, and Japco's Tokai, total 9377 MWe net. Fukushima Daiichi units 4, 5&6 were not operating at the time, but were affected. The main problem initially centred on Fukushima Daiichi units 1-3. Unit 4 became a problem on day five.

The reactors proved robust seismically, but vulnerable to the tsunami. Power, from grid or backup generators, was available to run the residual heat removal (RHR) system cooling pumps at eight of the eleven units, and despite some problems they achieved 'cold shutdown' within about four days. The other three, at Fukushima Daiichi, lost power at 3.42 pm, almost an hour after the quake, when the entire site was flooded by the 15-metre tsunami. This disabled 12 of 13 back-up generators on site and also the heat exchangers for dumping reactor waste heat and decay heat to the sea. The three units lost the ability to maintain proper reactor cooling and water circulation functions. Electrical switchgear was also disabled. Thereafter, many weeks of focused work centred on restoring heat removal from the reactors and coping with overheated spent fuel ponds. This was undertaken by hundreds of Tepco employees as well as some contractors, supported by firefighting and military personnel. Some of the Tepco staff had lost homes, and even families, in the tsunami, and were initially living in temporary accommodation under great difficulties and privation, with some personal risk. A hardened emergency response centre on site was unable to be used in grappling with the situation, due to radioactive contamination.

Three Tepco employees at the Daiichi and Daini plants were killed directly by the earthquake and tsunami, but there have been no fatalities from the nuclear accident.

Among hundreds of aftershocks, an earthquake with magnitude 7.1, closer to Fukushima than the 11 March one, was experienced on 7 April, but without further damage to the plant. On 11 April a magnitude 7.1 earthquake and on 12 April a magnitude 6.3 earthquake, both with epicenter at Fukushima-Hamadori, caused no further problems.

The two Fukushima plants and their siting

The Daiichi (first) and Daini (second) Fukushima plants are sited about 11 km apart on the coast, Daini to the south.

The recorded seismic data for both plants - some 180 km from the epicentre - shows that 550 Gal (0.56 g) was the maximum ground acceleration for Daiichi, and 254 Gal was maximum for Daini. Daiichi units 2, 3 and 5 exceeded their maximum response acceleration design basis in E-W direction by about 20%. The recording was over 130-150 seconds. (All nuclear plants in Japan are built on rock - ground acceleration was around 2000 Gal a few kilometres north, on sediments).

The original design basis tsunami height was 3.1 m for Daiichi based on assessment of the 1960 Chile tsunami and so the plant had been built about 10 metres above sea level with the seawater pumps 4 m above sea level. The Daini plant was built 13 metres above sea level. In 2002 the design basis was revised to 5.7 metres above, and the seawater pumps were sealed. In the event, tsunami heights coming ashore were about 15 metres, and the Daiichi turbine halls were under some 5 metres of seawater until levels subsided. Daini was less affected. The maximum amplitude of this tsunami was 23 metres at point of origin, about 180 km from Fukushima.

In the last century there have been eight tsunamis in the region with maximum amplitudes at origin above 10 metres (some much more), these having arisen from earthquakes of magnitude 7.7 to 8.4, on average one every 12 years. Those in 1983 and in 1993 were the most recent affecting Japan, with maximum heights at origin of 14.5 metres and 31 metres respectively, both induced by magnitude 7.7 earthquakes. The June 1896 earthquake of estimated magnitude 8.3 produced a tsunami with run-up height of 38 metres in Tohoku region, killing more than 27,000 people.

The tsunami countermeasures taken when Fukushima Daiichi was designed and sited in the 1960s were considered acceptable in relation to the scientific knowledge then, with low recorded run-up heights for that particular coastline. But some 18 years before the 2011 disaster, new scientific knowledge had emerged about the likelihood of a large earthquake and resulting major tsunami of some 15.7 metres at the Daiichi site. However, this had not yet led to any major action by either the plant operator, Tepco, or government regulators, notably the Nuclear & Industrial Safety Agency (NISA). Discussion was ongoing, but action minimal. The tsunami countermeasures could also have been reviewed in accordance with IAEA guidelines which required taking into account high tsunami levels, but NISA continued to allow the Fukushima plant to operate without sufficient countermeasures such as moving the backup generators up the hill, sealing the lower part of the buildings, and having some back-up for seawater pumps, despite clear warnings.

A report from the Japanese government's Earthquake Research Committee on earthquakes and tsunamis off the Pacific coastline of northeastern Japan in February 2011 was due for release in April, and might finally have brought about changes. The document includes analysis of a magnitude 8.3 earthquake that is known to have struck the region more than 1140 years ago, triggering enormous tsunamis that flooded vast areas of Miyagi and Fukushima prefectures. The report concludes that the region should be alerted of the risk of a similar disaster striking again. The 11 March earthquake measured magnitude 9.0 and involved substantial shifting of multiple sections of seabed over a source area of 200 x 400 km. Tsunami waves devastated wide areas of Miyagi, Iwate and Fukushima prefectures.
(See also background on Earthquakes and Seismic Protection for Nuclear Power Plants in Japan)
Events at Fukushima Daiichi 1-3 & 4

It appears that no serious damage was done to the reactors by the earthquake, and the operating units 1-3 were automatically shut down in response to it, as designed. At the same time all six external power supply sources were lost due to earthquake damage, so the emergency diesel generators located in the basements of the turbine buildings started up. Initially cooling would have been maintained through the main steam circuit bypassing the turbine and going through the condensers.

Then 41 minutes later, at 3:42 pm, the first tsunami wave hit, followed by a second 8 minutes later. These submerged and damaged the seawater pumps for both the main condenser circuits and the auxiliary cooling circuits, notably the Residual Heat Removal (RHR) cooling system. They also drowned the diesel generators and inundated the electrical switchgear and batteries, all located in the basements of the turbine buildings (the one surviving air-cooled generator was serving units 5 & 6). So there was a station blackout, and the reactors were isolated from their ultimate heat sink. The tsunamis also damaged and obstructed roads, making outside access difficult.

All this put those reactors 1-3 in a dire situation and led the authorities to order, and subsequently extend, an evacuation while engineers worked to restore power and cooling. The 125-volt DC back-up batteries for units 1 & 2 were flooded and failed, leaving them without instrumentation, control or lighting. Unit 3 had battery power for about 30 hours.

At 7.03 pm Friday 11 March a Nuclear Emergency was declared, and at 8.50pm the Fukushima Prefecture issued an evacuation order for people within 2 km of the plant. At 9.23 pm the Prime Minister extended this to 3 km, and at 5.44 am on 12th he extended it to 10 km. He visited the plant soon after. On Saturday 12th he extended the evacuation zone to 20 km.

Inside the Fukushima Daiichi reactors

The Fukushima Daiichi reactors are GE boiling water reactors (BWR) of an early (1960s) design supplied by GE, Toshiba and Hitachi, with what is known as a Mark I containment. Reactors 1-3 came into commercial operation 1971-75. Reactor capacity is 460 MWe for unit 1, 784 MWe for units 2-5, and 1100 MWe for unit 6.

When the power failed at 3.42 pm, about one hour after shutdown of the fission reactions, the reactor cores would still be producing about 1.5% of their nominal thermal power, from fission product decay - about 22 MW in unit 1 and 33 MW in units 2&3. Without heat removal by circulation to an outside heat exchanger, this produced a lot of steam in the reactor pressure vessels housing the cores, and this was released into the dry primary containment (PCV) through safety valves. Later this was accompanied by hydrogen, produced by the interaction of the fuel's very hot zirconium cladding with steam after the water level dropped.

As pressure started to rise here, the steam was directed into the suppression chamber/ wetwell under the reactor, within the containment, but the internal temperature and pressure nevertheless rose quite rapidly. Water injection commenced, using the various systems provide for this and finally the Emergency Core Cooling System (ECCS). These systems progressively failed over three days, so from early Saturday water injection to the reactor pressure vessel (RPV) was with fire pumps, but this required the internal pressures to be relieved initially by venting into the suppression chamber/ wetwell. Seawater injection into unit 1 began at 7pm on Saturday 12th, into unit 3 on 13th and unit 2 on 14th. Tepco management ignored an instruction from the prime minister to cease the seawater injection into unit 1, and this instruction was withdrawn shortly afterwards.

Inside unit 1, it is understood that the water level dropped to the top of the fuel about three hours after the scram (about 6 pm) and the bottom of the fuel 1.5 hours later (7.30 pm). The temperature of the exposed fuel rose to some 2800°C so that the central part started to melt after a few hours and by 16 hours after the scram (7 am Saturday) most of it had fallen into the water at the bottom of the RPV. After that, RPV temperatures decreased steadily.

As pressure rose, attempts were made to vent the containment, and when external power and compressed air sources were harnessed this was successful, by about 2.30 pm Saturday, though some manual venting was apparently achieved at about 10.17 am. The venting was designed to be through an external stack, but in the absence of power much of it apparently backflowed to the service floor at the top of the reactor building, representing a serious failure of this system (though another possibility is leakage from the drywell). The vented steam, noble gases and aerosols were accompanied by hydrogen. At 3.36 pm on Saturday 12th, there was a hydrogen explosion on the service floor of the building above unit 1 reactor containment, blowing off the roof and cladding on the top part of the building, after the hydrogen mixed with air and ignited. (Oxidation of the zirconium cladding at high temperatures in the presence of steam produces hydrogen exothermically, with this exacerbating the fuel decay heat problem.)

In unit 1 most of the core - as corium comprised of melted fuel and control rods - was assumed to be in the bottom of the RPV, but later it appeared that it had mostly gone through the bottom of the RPV and eroded about 65 cm into the drywell concrete below (which is 2.6 m thick). This reduced the intensity of the heat and enabled the mass to solidify.

Much of the fuel in units 2&3 also apparently melted to some degree, but to a lesser extent than in unit 1, and a day or two later. In mid-May 2011 the unit 1 core would still be producing 1.8 MW of heat, and units 2&3 would be producing about 3.0 MW each.

In mid-2013 the Nuclear Regulation Authority (NRA) confirmed that the earthquake itself had caused no damage to unit 1.

In unit 2, water injection using the steam-driven back-up water injection system failed on Monday 14th, and it was about six hours before a fire pump started injecting seawater into the RPV. Before the fire pump could be used RPV pressure had to be relieved via the wetwell, which required power and nitrogen, hence the delay. Meanwhile the reactor water level dropped rapidly after back-up cooling was lost, so that core damage started about 8 pm, and it is now provisionally understood that much of the fuel then melted and probably fell into the water at the bottom of the RPV about 100 hours after the scram. Pressure was vented on 13th and again on 15th, and meanwhile the blowout panel near the top of the building was opened to avoid a repetition of unit 1 hydrogen explosion. Early on Tuesday 15th, the pressure suppression chamber under the actual reactor seemed to rupture, possibly due to a hydrogen explosion there, and the drywell containment pressure inside dropped. However, subsequent inspection of the suppression chamber did not support the rupture interpretation. Later analysis suggested that a leak of the primary containment developed on Tuesday 15th. Most of the radioactive releases from the site appeared to come from unit 2.

In Unit 3, the main back-up water injection system failed at about 11 am on Saturday 12 th and early on Sunday 13 th, water injection using the high pressure system failedalsoand water levels dropped dramatically. RPV pressure was reduced by venting steam into the wetwell, allowing injection of seawater using a fire pump from just before noon. Early on Sunday venting the suppression chamber and containment was successfully undertaken. It is now understood that core damage started about 5:30 am and much or all of the fuel melted on the morning of Sunday 13 th and fell into the bottom of the RPV, with some probably going through the bottom of the reactor pressure vessel and onto the concrete below.

Early on Monday 14 th PCV venting was repeated, and this evidently backflowed to the service floor of the building, so that at 11 am a very large hydrogen explosion here above unit 3 reactor containment blew off much of the roof and walls and demolished the top part of the building. This explosion created a lot of debris, and some of that on the ground near unit 3 was very radioactive.

In defuelled unit 4, at about 6 am on Tuesday 15 March, there was an explosion which destroyed the top of the building and damaged unit 3's superstructure further. This was apparently from hydrogen arising in unit 3 and reaching unit 4 by backflow in shared ducts when vented from unit 3.

Units 1-3: Water has been injected into each of the three reactor units more or less continuously, and in the absence of normal heat removal via external heat exchanger this water was boiling off for some months. In the government report to IAEA in June it was estimated that to the end of May about 40% of the injected water boiled off, and 60% leaked out the bottom. In June 2011 this was adding to the contaminated water on site by about 500 m ³ per day. In January 2013 4.5 to 5.5 m ³/hr was being added to each RPV via core spray and feed water systems, hence 370 m3 per day, and temperatures at the bottom of RPVs were 19°C in unit 1 and 32°C in units 2&3, at little above atmospheric pressure.

There was a peak of radioactive release on 15th, apparently mostly from unit 2, but the precise source remains uncertain. Due to volatile and easily-airborne fission products being carried with the hydrogen and steam, the venting and hydrogen explosions discharged a lot of radioactive material into the atmosphere, notably iodine and caesium. NISA said in June that it estimated that 800-1000 kg of hydrogen had been produced in each of the units.

Nitrogen is being injected into the containment vessels (PCVs) of all three reactors to remove concerns about further hydrogen explosions, and in December this was started also for the pressure vessels. Gas control systems which extract and clean the gas from the PCV to avoid leakage of caesium have been commissioned for all three units.

Throughout 2011 injection into the RPVs of water circulated through the new water treatment plant achieved relatively effective cooling, and temperatures at the bottom of the RPVs were stable in the range 60-76°C at the end of October, and 27-54°C in mid-January 2012. RPV pressures ranged from atmospheric to slightly above (102-109 kPa) in January, due to water and nitrogen injection. However, since they are leaking, the normal definition of "cold shutdown" does not apply, and Tepco waited to bring radioactive releases under control before declaring "cold shutdown condition" in mid-December, with NISA's approval. This, with the prime minister's announcement of it, formally brought to a close the 'accident' phase of events.

The AC electricity supply from external source was connected to all units by 22 March. Power was restored to instrumentation in all units except unit 3 by 25 March. However, radiation levels inside the plant were so high that normal access was impossible until June.

Event sequence following earthquake (timing from it: 14:46, 11 March)

Tepco has written off the four reactors damaged by the accident, and is decommissioning them.

By March 2016 total decay heat in units 1-3 had dropped to 1 MW for all three, about 1% of the original level, meaning that cooling water injection - then 100 m ³/d - could be interrupted for up to two days.

Results of muon measurements in unit 2 in 2016 indicate that most of the fuel debris in unit 2 is in the bottom of the reactor vessel.

Summary: Major fuel melting occurred early on in all three units, though the fuel remains essentially contained except for some volatile fission products vented early on, or released from unit 2 in mid-March, and some soluble ones which were leaking with the water, especially from unit 2, where the containment is evidently breached. Cooling is provided from external sources, using treated recycled water, with a stable heat removal path from the actual reactors to external heat sinks. Temperatures at the bottom of the reactor pressure vessels have decreased to well below boiling point and are stable. Access has been gained to all three reactor buildings, but dose rates remain high inside. Nitrogen is being injected into all three containment vessels and pressure vessels. Tepco declared "cold shutdown condition" in mid-December 2011 when radioactive releases had reduced to minimal levels.

(See also background on nuclear reactors at Fukushima Daiichi.)

Fuel ponds: developing problems

Used fuel needs to be cooled and shielded. This is initially by water, in ponds. After about three years under water, used fuel can be transferred to dry storage, with air ventilation simply by convection. Used fuel generates heat, so the water is circulated by electric pumps through external heat exchangers, so that the heat is dumped and a low temperature maintained. There are fuel ponds near the top of all six reactor buildings at the Daiichi plant, adjacent to the top of each reactor so that the fuel can be unloaded under water when the top is off the reactor pressure vessel and it is flooded. The ponds hold some fresh fuel and some used fuel, the latter pending its transfer to the central used/spent fuel storage on site. (There is some dry storage on site to extend the plant's capacity.)

At the time of the accident, in addition to a large number of used fuel assemblies, unit 4's pond also held a full core load of 548 fuel assemblies while the reactor was undergoing maintenance, these having been removed at the end of November, and were to be repplaced in the core.

A separate set of problems arose as the fuel ponds, holding fresh and used fuel in the upper part of the reactor structures, were found to be depleted in water. The primary cause of the low water levels was loss of cooling circulation to external heat exchangers, leading to elevated temperatures and probably boiling, especially in heavily-loaded unit 4. Here the fuel would have been uncovered in about 7 days due to water boiling off. However, the fact that unit 4 was unloaded meant that there was a large inventory of water at the top of the structure, and enough of this replenished the fuel pond to prevent the fuel becoming uncovered - the minimum level reached was about 1.2 m above the fuel on about 22 April.

After the hydrogen explosion in unit 4 early on Tuesday 15 March, Tepco was told to implement injection of water to unit 4 pond which had a particularly high heat load (3 MW) from 1331 used fuel assemblies in it, so it was the main focus of concern. It needed the addition of about 100 m ³/day to replenish it after circulation ceased.

From Tuesday 15 March attention was given to replenishing the water in the ponds of units 1, 2, 3 as well. Initially this was attempted with fire pumps but from 22 March a concrete pump with 58-metre boom enabled more precise targeting of water through the damaged walls of the service floors. There was some use of built-in plumbing for unit 2. Analysis of radionuclides in water from the used fuel ponds suggested that some of the fuel assemblies might be damaged, but the majority were intact.

There was concern about structural strength of unit 4 building, so support for the pond was reinforced by the end of July.

New cooling circuits with heat exchangers adjacent to the reactor buildings for all four ponds were commissioned after a few months, and each reduced the pool temperature from 70°C to normal in a few days. Each has a primary circuit within the reactor and waste treatment buildings and a secondary circuit dumping heat through a small dry cooling tower outside the building.

The next task was to remove the salt from those ponds which had seawater added, to reduce the potential for corrosion.

In July 2012 two of the 204 fresh fuel assemblies were removed from the unit 4 pool and transferred to the central spent fuel pool for detailed inspection to check damage, particularly corrosion. They were found to have no deformation or corrosion. Unloading the 1331 spent fuel assemblies in pond 4 and transferring them to the central spent fuel storage commenced in mid-November 2013 and was completed 13 months later. These comprised 783 spent fuel plus the full fuel load of 548.

The next focus of attention was the unit 3 pool. In 2015 the damaged fuel handling equipment and other wreckage was removed from the destroyed upper level of the reactor building. Toshiba has built a 74-tonne fuel handling machine for transferring the 566 fuel assemblies into casks and to remove debris in the pool, and a crane for lifting the fuel transfer casks. Installation of a cover over the fuel handling machine was completed in February 2018.

The onsite central spent fuel pool in 2011 held about 60% of the Daiichi used fuel, and is immediately west (inland) of unit 4. It lost circulation with the power outage, and temperature increased to 73°C by the time mains power and cooling were restored after two weeks. In late 2013 this pond, with capacity for 6840*, held 6375 fuel assemblies, the same as at the time of the accident. The older ones will be transferred to 65 casks in dry storage, with total capacity of at least 2930 assemblies - each dry cask holds 50 fuel assemblies. Eventually these will be shipped to JNFL's Rokkasho reprocessing plant or to Recyclable Fuel Storage Company's new Mutsu facility. The dry storage area held 408 fuel assemblies at the time of the accident, and 1004 have been transferred there since (to mid-2014).

Summary: The spent fuel storage pools survived the earthquake, tsunami and hydrogen explosions without significant damage to the fuel or significant radiological release, or threat to public safety. The new cooling circuits with external heat exchangers for the four ponds are working well. Temperatures are normal. Analysis of water has confirmed that most fuel rods are intact. All fuel assemblies have been removed from unit 4 pool. Those at unit 3 will be removed next.

(See also background on Fukushima Fuel Ponds and Decommissioning section below.)

Radioactive releases to air

Regarding releases to air and also water leakage from Fukushima Daiichi, the main radionuclide from among the many kinds of fission products in the fuel was volatile iodine-131, which has a half-life of 8 days. The other main radionuclide is caesium-137, which has a 30-year half-life, is easily carried in a plume, and when it lands it may contaminate land for some time. It is a strong gamma-emitter in its decay. Cs-134 is also produced and dispersed, it has a two-year half-life. Caesium is soluble and can be taken into the body, but does not concentrate in any particular organs, and has a biological half-life of about 70 days. In assessing the significance of atmospheric releases, the Cs-137 figure is multiplied by 40 and added to the I-131 number to give an "iodine-131 equivalent" figure.

As cooling failed on the first day, evacuations were progressively ordered, due to uncertainty about what was happening inside the reactors and the possible effects. By the evening of Saturday 12 March the evacuation zone had been extended to 20 km from the plant. From 20 to 30 km from the plant, the criterion of 20 mSv/yr dose rate was applied to determine evacuation, and is now the criterion for return being allowed. 20 mSv/yr was also the general limit set for children's dose rate related to outdoor activities, but there were calls to reduce this.In areas with 20-50 mSv/yr from April 2012 residency is restricted, with remediation action taken. See later section on Public health and return of evacuees.

A significant problem in tracking radioactive release was that 23 out of the 24 radiation monitoring stations on the plant site were disabled by the tsunami.

There is some uncertainty about the amount and exact sources of radioactive releases to air. (See also background on Radiation Exposure.)

Japan's regulator, the Nuclear & Industrial Safety Agency (NISA), estimated in June 2011 that 770 PBq (iodine-131 equivalent) of radioactivity had been released, but the Nuclear Safety Commission (NSC, a policy body) in August lowered this estimate to 570 PBq. The 770 PBq figure is about 15% of the Chernobyl release of 5200 PBq iodine-131 equivalent. Most of the release was by the end of March 2011.

Tepco sprayed a dust-suppressing polymer resin around the plant to ensure that fallout from mid-March was not mobilized by wind or rain. In addition it removed a lot of rubble with remote control front-end loaders, and this further reduced ambient radiation levels, halving them near unit 1. The highest radiation levels on site came from debris left on the ground after the explosions at units 3&4.

Reactor covers

In mid-May 2011 work started towards constructing a cover over unit 1 to reduce airborne radioactive releases from the site, to keep out the rain, and to enable measurement of radioactive releases within the structure through its ventilation system. The frame was assembled over the reactor, enclosing an area 42 x 47 m, and 54 m high. The sections of the steel frame fitted together remotely without the use of screws and bolts. All the wall panels had a flameproof coating, and the structure had a filtered ventilation system capable of handling 40,000 cubic metres of air per hour through six lines, including two backup lines. The cover structure was fitted with internal monitoring cameras, radiation and hydrogen detectors, thermometers and a pipe for water injection. The cover was completed with ventilation systems working by the end of October 2011. It was expected to be needed for two years. In May 2013 Tepco announced its more permanent replacement, to be built over four years. It started demolishing the 2011 cover in 2014 and finished in 2016. It then plans to remove concrete and other rubble on the top floor of the building. A crane and other equipment for fuel removal would then be installed in a new cover over the building, similar to that over unit 4.

More substantial covers were designed to fit around units 3&4 reactor buildings after the top floors were cleared up in 2012.

A cantilevered structure was built over unit 4 from April 2012 to July 2013 to enable recovery of the contents of the spent fuel pond. This is 69 x 31 m cover (53 m high) and it was fully equipped by the end of 2013 to enable unloading of used fuel from the storage pond into casks, each holding 22 fuel assemblies, and removal of the casks. This operation was accomplished under water, using the new fuel handling machine (replacing the one destroyed by the hydrogen explosion) so that the used fuel could be transferred to the central storage on site. Transfer was completed in December 2014. Animation video of the process here.

A different design of cover is being built over unit 3, and foundation work had begun in 2012. Large rubble removal took place from 2013 to 2015, including the damaged fuel handling machine. An arched cover has been prefabricated, 57 m long and 19 m wide, to be supported by the turbine building on one side and the ground on the other. A crane to remove the 566 fuel assemblies from the pool will be fitted, with its operation removing some remaining rubble. Toshiba's 74-tonne fuel handling machine for transferring the fuel assemblies into casks is expected be installed in 2017, and the fuel is to be removed from mid-2018. Fuel debris retrieval from within the reactor is scheduled from about 2021, after that from units 1&2 is started.

Spent fuel removal from units 1&2 pools is scheduled in 2018, and fuel debris retrieval from within the reactors from 2020.

Tests on radioactivity in rice have been made and caesium was found in a few of them. The highest levels were about one quarter of the allowable limit of 500 Bq/kg, so shipments to market are permitted.

Maps from MEXT aerial surveys carried out approximately one year apart show the reduction in contamination from late 2011 to late 2012. Areas with colour changes in 2012 showed approximately half the contamination as surveyed in 2011, the difference coming from decay of caesium-134 (two year half-life) and natural processes like wind and rain. In blue areas, ambient radiation is very similar to global background levels at <0.5uSv/h which is equal to <4.38 mSv/y.

Summary: Major releases of radionuclides, including long-lived caesium, occurred to air, mainly in mid-March. The population within a 20km radius had been evacuated three days earlier. Considerable work was done to reduce the amount of radioactive debris on site and to stabilise dust. The main source of radioactive releases was the apparent hydrogen explosion in the suppression chamber of unit 2 on 15 March. A cover building for unit 1 reactor was built and is now being dismantled, a more substantial one for unit 4 was built to enable fuel removal during 2014. Radioactive releases in mid-August 2011 had reduced to 5 GBq/hr, and dose rate from these at the plant boundary was 1.7 mSv/yr, less than natural background.

Sequence of evacuation orders based on the report by The National Diet of Japan Fukushima Nuclear Accident Independent Investigation Commission:

11 March
14:46 JST The earthquake occurred.
15:42 TEPCO made the first emergency report to the government.
19:03 The government announced nuclear emergency.
20:50 The Fukushima Prefecture Office ordered 2km radius evacuation.
21:23 The government ordered 3km evacuation and to keep staying inside buildings in the area of 3-10km radius.

12 March
05:44 The government ordered 10km radius evacuation.
18:25 The government ordered 20km evacuation.

15 March
11:01 The government ordered to keep staying inside buildings in the area of 20-30km from the plant.

25 March The government requested voluntary evacuation in the area of 20-30km.

21 April The government set the 20km radius no-go area.

Radiation exposure on the plant site

By the end of 2011, Tepco had checked the radiation exposure of 19,594 people who had worked on the site since 11 March. For many of these both external dose and internal doses (measured with whole-body counters) were considered. It reported that 167 workers had received doses over 100 mSv. Of these 135 had received 100 to 150 mSv, 23 150-200 mSv, three more 200-250 mSv, and six had received over 250 mSv (309 to 678 mSv) apparently due to inhaling iodine-131 fume early on. The latter included the two unit 3-4 control room operators in the first two days who had not been wearing breathing apparatus. There were up to 200 workers on site each day. Recovery workers are wearing personal monitors, with breathing apparatus and protective clothing which protect against alpha and beta radiation. So far over 3500 of some 3700 workers at the damaged Daiichi plant have received internal check-ups for radiation exposure, giving whole body count estimates. The level of 250 mSv was the allowable maximum short-term dose for Fukushima Daiichi accident clean-up workers through to December 2011, 500 mSv is the international allowable short-term dose "for emergency workers taking life-saving actions". Since January 2012 the allowable maximum has reverted to 50 mSv/yr.

Tepco figures submitted to NRA for the period to end January 2014 showed 173 workers had received more than 100 mSv (six more than two years earlier) and 1578 had received 50 to 100 mSv. This was among a total of 32,024, 64% more than had worked there two years earlier. Since April 2013 none of the 13,154 who had worked on site had received more than 50 mSv, and 96% of these had less than 20 mSv dose. Early in 2014 there were about 4000 on site each weekday.

No radiation casualties (acute radiation syndrome) occurred, and few other injuries, though higher than normal doses were being accumulated by several hundred workers on site. High radiation levels in the three reactor buildings has hindered access there.

Monitoring of seawater, soil and atmosphere is at 25 locations on the plant site, 12 locations on the boundary, and others further afield. Government and IAEA monitoring of air and seawater is ongoing. Some high but not health-threatening levels of iodine-131 were found in March, but with an eight-day half-life, most I-131 had gone by the end of April 2011.

A radiation survey map of the site made in March 2013 revealed substantial progress: the highest dose rate anywhere on the site was 0.15 mSv/h near units 3 and 4. (Soon after the accident a similar survey put the highest dose rate at 300 mSv/h near rubble lying alongside unit 3.) The majority of the power plant area was at less than 0.01 mSv/h. These reduced levels are reflected in worker doses: during January 2013, the 5702 workers at the site received an average of 0.86 mSv, with 75% of workers recorded as receiving less than 1 mSv. In total, only about 2% of workers received over 5 mSv and the highest dose in January was 12.65 mSv for one worker.

Media reports have referred to "nuclear gypsies" - casual workers employed by subcontractors on a short-term basis, and allegedly prone to receiving higher and unsupervised radiation doses. This transient workforce has been part of the nuclear scene for at least four decades, and at Fukushima their doses are very rigorously monitored. If they reach certain levels, e.g. 30 mSv but varying according to circumstance, they are reassigned to lower-exposure areas.

Summary: Six workers received radiation doses apparently over the 250 mSv level set by NISA, but at levels below those which would cause radiation sickness.

Radiation exposure and fallout beyond the plant site

On 4 April 2011, radiation levels of 0.06 mSv/day were recorded in Fukushima city, 65 km northwest of the plant, about 60 times higher than normal but posing no health risk according to authorities. Monitoring beyond the 20 km evacuation radius to 13 April showed one location - around Iitate - with up to 0.266 mSv/day dose rate, but elsewhere no more than one-tenth of this. At the end of July the highest level measured within 30km radius was 0.84 mSv/day in Namie town, 24 km away. The safety limit set by the central government in mid-April for public recreation areas was 3.8 microsieverts per hour (0.09 mSv/day).

In June 2013, analysis from Japan's Nuclear Regulation Authority (NRA) showed that the most contaminated areas in the Fukushima evacuation zone had reduced in size by three-quarters over the previous two years. The area subject to high dose rates (over 166 mSv/yr) diminished from 27% of the 1117 km ² zone to 6% over 15 months to March 2013, and in the 'no residence' portion (originally 83-166 mSv/yr) no areas remained at this level and 70% was below 33 mSv/yr. The least-contaminated area is now entirely below 33 mSv/yr.

In August 2011 The Act on Special Measures Concerning the Handling of Radioactive Pollution was enacted and it took full effect from January 2012 as the main legal instrument to deal with all remediation activities in the affected areas, as well as the management of materials removed as a result of those activities. It specified two categories of land:
- Special Decontamination Areas consisting of the "restricted areas" located within a 20 km radius from the Fukushima Daiichi plant, and "deliberate evacuation areas" where the annual cumulative dose for individuals was anticipated to exceed 20 mSv. The national government promotes decontamination in these areas. These areas are subdivided into three: dose 1- 20 mSv/yr (green) dose 20-50 mSv/yr (yellow) and dose over 50 mSv/yr and over 20 mSv/yr average over 5 years (red).
- Intensive Contamination Survey Areas including the so-called Decontamination Implementation Areas, where an additional annual cumulative dose between 1mSv and 20mSv was estimated for individuals. Municipalities implement decontamination activities in these areas.

In May 2013, the UN Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) reported, following a detailed study by 80 international experts. It concluded that "Radiation exposure following the nuclear accident at Fukushima Daiichi did not cause any immediate health effects. It is unlikely to be able to attribute any health effects in the future among the general public and the vast majority of workers." The only exception are the 146 emergency workers that received radiation doses of over 100 mSv during the crisis. They will be monitored closely for "potential late radiation-related health effects at an individual level." UNSCEAR's follow-up white paper in October 2015 said that none of the new information appraised after the 2013 report "materially affected the main findings in, or challenged the major assumptions of, the 2013 Fukushima report."

By contrast, the public was exposed to 10-50 times less radiation. Most Japanese people were exposed to additional radiation amounting to less than the typical natural background level of 2.1 mSv per year.

People living in Fukushima prefecture are expected to be exposed to around 10 mSv over their entire lifetimes, while for those living further away the dose would be 0.2 mSv per year. The UNSCEAR conclusion reinforces the findings of several international reports to date, including one from the World Health Organisation (WHO) that considered the health risk to the most exposed people possible: a postulated girl under one year of age living in Iitate or Namie that did not evacuate and continued life as normal for four months after the accident. Such a child's theoretical risk of developing any cancer would be increased only marginally, according to WHO's analysis.

Eleven municipalities in the former restricted zone or planned evacuation area, within 20 km of the plant or where annual cumulative radiation dose is greater than 20 mSv, are designated Special Decontamination Areas, where decontamination work is being implemented by the government. A further 100 municipalities in eight prefectures, where dose rates are equivalent to over 1 mSv per year are classed as Intensive Decontamination Survey Areas, where decontamination is being implemented by each municipality with funding and technical support from the national government. In the Special Decontamination Areas, decontamination is proceeding and was complete to target levels in one municipality by June 2013.

In October 2013 a 16-member IAEA mission reported on remediation and decontamination in the Special Decontamination Areas. Its preliminary report said that decontamination efforts were commendable but driven by unrealistic targets. If annual radiation dose was below 20 mSv, such as generally in Intensive Decontamination Survey Areas, this level was "acceptable and in line with the international standards and with the recommendations from the relevant international organisations, e.g. ICRP, IAEA, UNSCEAR and WHO." The clear implication is that people in such areas should be allowed to return home. Furthermore the government should increase efforts to communicate this to the public, and should explain that its long-term goal of achieving an additional individual dose of 1 mSv/yr is unrealistic and unnecessary in the short term. Also, there is potential to produce more food safely in contaminated areas.

Radioactivity, primarily from caesium-137, in the evacuation zone and other areas beyond it has been reported in terms of kBq/kg (compared with kBq/m ² around Chernobyl. A total of 3,000 km was contaminated above 180 kBq/m ², compared with 29,400 km from Chernobyl). However the main measure has been presumed doses in mSv/yr. The government has adopted 20 mSv/yr as its goal for the evacuation zone and more contaminated areas outside it, and supports municipal government work to reduce levels below that. The total area under consideration for attention is 13,000 km ². In 2016 the Ministry of Environment announced that material with less than 8 kBq/kg caesium would no longer be specified as waste, and subject to restrictions on disposal. It allowed use of contaminated soil for embankments, where the activity was less then 8 kBq/kg, and unrestricted use if less than 100 Bq/kg. Most of the stored wastes have decayed to below the 8 kBq/kg level.

Summary: There have been no harmful effects from radiation on local people, nor any doses approaching harmful levels. However, some 160,000 people were evacuated from their homes and only from 2012 were allowed limited return. In October 2013, 81,000 evacuees remained displaced due to government concern about radiological effects from the accident.

Public health and return of evacuees

Permanent return remains a high priority, and the evacuation zone is being decontaminated where required and possible, so that evacuees (81,000 from this accident according to METI) can return without undue delay. There are many cases of evacuation stress including transfer trauma among evacuees, and once the situation had stabilised at the plant these outweighed the radiological hazards of returning, with over 1000 deaths reported (see below). There were also 267,000 tsunami survivor refugees remaining displaced in February 2014.

In December 2011 the government said that where annual radiation dose would be below 20 mSv/yr, the government would help residents return home as soon as possible and assist local municipalities with decontamination and repair of infrastructure. In areas where radiation levels are over 20 mSv/yr evacuees will be asked to continue living elsewhere for "a few years" until the government completes decontamination and recovery work. The government said it would consider purchasing land and houses from residents of these areas if the evacuees wish to sell them.

In November 2013 the NRA decided to change the way radiation exposure was estimated. Instead of airborne surveys being the basis, personal dosimeters would be used, giving very much more accurate figures, often much less than airborne estimates. The same criteria would be used, as above, with 20 mSv/yr being the threshold of concern to authorities.

In February 2014 the results of a study were published showing that 458 residents of two study areas 20 to 30 km from the plant and a third one 50 km northwest received radiation doses from the contaminated ground similar to the country's natural background levels. Measurement was by personal dosimeters over August-September 2012.

By October 2012, over 1000 disaster-related deaths that were not due to radiation-induced damage or to the earthquake or to the tsunami had been identified by the Reconstruction Agency < http://www.reconstruction.go.jp/english/>, based on data for areas evacuated for no other reason than the nuclear accident. About 90% of deaths were for persons above 66 years of age. Of these, about 70% occurred within the first three months of the evacuations. (A similar number of deaths occurred among evacuees from tsunami- and earthquake-affected prefectures. These figures are additional to the 19,000 that died in the actual tsunami.)

The premature deaths reported in 2012 were mainly related to the following: (1) somatic effects and spiritual fatigue brought on by having to reside in shelters; (2) Transfer trauma - the mental or physical burden of the forced move from their homes for fragile individuals; and (3) delays in obtaining needed medical support because of the enormous destruction caused by the earthquake and tsunami. However, the radiation levels in most of the evacuated areas were not greater than the natural radiation levels in high background areas elsewhere in the world where no adverse health effect is evident, so maintaining the evacuation beyond a precautionary few days was evidently the main disaster in relation to human fatalities.
http://www.reconstruction.go.jp/topics/20121102_sinsaikanrensi.pdf
http://www.reconstruction.go.jp/topics/240821_higashinihondaishinsainiokerushinsaikanrenshinikansuruhoukoku.pdf

Fukushima prefecture provided a further report early in 2014 which said that the 'indirect' deaths in the prefecture were greater than the number (1607) killed in the quake and tsunami. It put the figure at 1656 as determined by municipal panels that examine links between the disaster's aftermath and death. The figure is greater than for Iwate and Miyagi prefectures, with 434 and 879 respectively, though they had much higher loss of life in the quake and tsunami - about 14,200. The disparity is attributed to the older age group involved among Fukushima's evacuated quake/tsunami survivors, about 90% of indirect deaths being of people over 66. Causes of indirect deaths include physical and mental stress stemming from long stays at shelters, a lack of initial care as a result of hospitals being disabled by the disaster, and suicides. The high rate of these deaths continues three years later as the evacuation is maintained for about 135,000 people - apparently some 75,000 from the nuclear accident and 60,000 from the natural disaster itself. Evaluation of 'indirect deaths' is according to a model developed by Niigata prefecture after the 2004 earthquake there. ( Japan Times 20/2/14)

Evacuees receive JPY 100,000 ($1,030) per month in psychological suffering compensation. The money is tax-exempt and paid unconditionally. In October 2013, about 84,000 evacuees received the payments. Statistics indicate that an average family of four has received about JPY 90 million ($900,000) in compensation from Tepco. The average compensation for real estate was JPY 49.1 million ($490,000), JPY 10.9 million ($110,000) for lost wages, and JPY 30 million ($300,000) as "consolation money" for pain and suffering. (Asahi Shimbun 26/10/13)

The Fukushima prefecture has 17,000 government-financed temporary housing units for some 29,500 evacuees from the accident. The prefectural government said residents could continue to use these until March 2015. The number compares with very few built in Miyagi, Iwate and Aomori prefectures for the 222,700 tsunami survivor refugees there. (Japan Times 17/11/13) Another reported contrast from the Reconstruction Agency is that some $30 billion had been paid to 84,000 nuclear accident refugees but only some $20 billion to 300,000 tsunami survivors in the Tohoku region.

Evacuation orders have been progressively lifted, apart from some 300 km designated areas with annual dose levels above 20 mSv with continuous occupation.

According to a survey released by the prefectural government in April 2017, the majority of people who voluntarily evacuated their homes after the accident and who are now living outside of Fukushima prefecture do not intend to return. A Mainichi report said that 78.2% of respondents to the survey preferred to continue living in the area to which they had moved, while only 18.3% intended to move back to the prefecture. Of the voluntary evacuees still living in Fukushima prefecture, 23.6% planned to stay where they were, while 66.6% hoped to return to their original homes.

An August 2012 Reconstruction Agency report also considered workers at Fukushima power plant. Of almost 1500 surveyed, many were stressed, due to evacuating their homes (70%), believing they had come close to death (53%), the loss of homes in the tsunami (32%), deaths of colleagues (20%) and of family members (6%) mostly in the tsunami. The death toll directly due to the nuclear accident or radiation exposure remained zero, but stress and disruption due to the continuing evacuation remains high.

Tokyo's Board of Audit reported in October 2013 that 23% of recovery funding - about JPY 1.45 trillion ($14.5 billion) - had been misappropriated. Some 326 out of about 1400 projects funded had no direct relevance to the natural disaster or Fukushima Daiichi accident. (Mainichi 1/11/13)

Summary: Many evacuated people remain unable to fully return home due to government-mandated restrictions based on conservative radiation exposure criteria. However, over 1000 premature deaths have been caused by maintaining the evacuation beyond a prudent week or so. Decontamination work is proceeding while radiation levels decline naturally. The October 2013 IAEA report makes it clear that many evacuees should be allowed to return home.

A journalist article is in line with the World Nuclear Association's understanding of the situation in 2015.

Managing contaminated water

Removing contaminated water from the reactor and turbine buildings had become the main challenge in week 3, along with contaminated water in trenches carrying cabling and pipework. This was both from the tsunami inundation and leakage from reactors. Run-off from the site into the sea was also carrying radionuclides well in excess of allowable levels. By the end of March all storages around the four units - basically the main condenser units and condensate tanks - were largely full of contaminated water pumped from the buildings. Some 1000 storage tanks were set up progressively, including initially 350 steel tanks with rubber seams, each holding 1200 m ³. A few of these developed leaks in 2013.

Accordingly, with government approval, Tepco over 4-10 April released to the sea about 10,400 cubic metres of slightly contaminated water (0.15 TBq total) in order to free up storage for more highly-contaminated water from unit 2 reactor and turbine buildings which needed to be removed to make safe working conditions. Unit 2 is the main source of contaminated water, though some of it comes from drainage pits. NISA confirmed that there was no significant change in radioactivity levels in the sea as a result of the 0.15 TBq discharge.

Tepco built a new wastewater treatment facility to treat contaminated water. The company used both US proprietary adsorbtion and French conventional technologies in the new 1200 m ³/day treatment plant. A supplementary and simpler SARRY plant to remove caesium using Japanese technology and made by Toshiba and Shaw Group was installed and commissioned in August 2011. These plants reduce caesium from about 55 MBq/L to 5.5 kBq/L - about ten times better than designed. Desalination is necessary on account of the seawater earlier used for cooling, and the 1200 m ³/day desalination plant produces 480 m ³ of clean water while 720 m ³ goes to storage. A steady increase in volume of the stored water (about 400 m ³/d net) is due to groundwater finding its way into parts of the plant and needing removal and treatment.

Early in 2013 Tepco started to test and commission this Advanced Liquid Processing System ( ALPS), developed by EnergySolutions and Toshiba. Each of six trains is capable of processing 250 m ³/day to remove 62 remaining radioisotopes. By the end of 2014, an Advanced ALPS of 500 m ³/d had been added, making total capacity 2000 m ³/d. NRA approved the extra capacity in August 2014.

The ALPS is a chemical system which will remove radionuclides to below legal limits for release. However, because tritium is contained in water molecules, ALPS cannot remove it, which gives rise to questions about the discharge of treated water to the sea. Tritium is a weak beta-emitter which does not bio-accumulate (half-life 12 years), and its concentration has levelled off at about 1 MBq/L in the stored water, with dilution from groundwater balancing further release from the fuel debris.

The clean tritiated water was the focus of attention in 2014. A September 2013 report from the Atomic Energy Society of Japan recommended diluting the ALPS-treated water with seawater and releasing it to the sea at the legal discharge concentration of 0.06 MBq/L, with monitoring to ensure that normal background tritium levels of 10 Bq/L are not exceeded. (WHO drinking water guideline is 0.01 MBq/L tritium) The IAEA is reported to support release of tritiated water to the ocean, as does Dr Dale Klein, chairman of Tepco's nuclear reform monitoring committee (NRMC) and former chairman of US Nuclear Regulatory Commission. The government had an expert Task Force considering the options.

In 2016 Kurion (now owned by Veolia) completed a demonstration project for tritium removal at low concentrations, with its new Modular Detritiation System (MDS),* in response to a JPY 1 billion commission from METI.
* In this, an electrolyser produces hydrogen and oxygen, with the tritium reporting in the hydrogen. This is fed through a catalytic exchange column with a little water which preferentially takes up the tritium. The concentrated tritiated water is fed through a 'getter bed' of dry metal hydrate, where the tritium replaces hydrogen, and the material is stored, being stable up to 500°C. It can be incorporated into concrete and disposed as low-level waste. The tritium is concentrated 1000 to 20,000 times. The MDS is the first system to be able economically to treat large volumes of water with low tritium concentrations, and builds on existing heavy water tritium removal systems. Each module treats up to 7200 litres per day.
In 2014 a new Kurion (now owned by Veolia) strontium removal system was commissioned. This is mobile and can be moved around the tank groups to further clean up water which has been treated by ALPS.

In June 2015, 108 m ³/day of clean water was being circulated through each reactor (1-3). Collected water from them, with high radioactivity levels, was being treated for caesium removal and re-used. Apart from this recirculating loop, the cumulative treated volume was then 1.232 million cubic metres. In storage onsite was 459,000 m ³ of fully treated water (by ALPS), and 190,000 m ³ of partially-treated water (strontium removed), which was being added to at 400 m ³/day due to groundwater inflow. Almost 600 m ³ of sludge from the water treatment was stored in shielded containers.

In October 2018 Tepco announced that some 80% of its 900,000 m ³ of stored water from reactor buildings remained above legal limits for discharge to the sea. Treatment using ALPS is ongoing at about 210 m ³/day.

By the end of June 2011, Tepco had installed 109 concrete panels to seal the water intakes of units 1-4, preventing contaminated water leaking to the harbour. From mid-June some treatment with zeolite of seawater at 30 m ³/hr was being undertaken near the water intakes for units 2&3, inside submerged barriers installed in April. From October, a steel water shield wall was built on the sea frontage of units 1-4. It extends about one kilometre, and down to an impermeable layer beneath two permeable strata which potentially leak contaminated groundwater to the sea. The inner harbour area which has some contamination is about 30 ha in area. The government in September 2013 said that "At present, statistically-significant increase of radioactive concentration in the sea outside the port of the TEPCO's Fukushima Daiichi NPS has not been detected." And also that "The results of monitoring of sea water in Japan are constantly below the standard of 10 Bq/L" (the WHO standard for Cs-137 in drinking water). In 2012 the Japanese standard for caesium in food supply was dropped from 500 to 100 Bq/kg. In July-August 2014 only 0.6% of fish caught offshore from the plant exceeded this lower level, compared with 53% in the months immediately following the accident.

Apart from the above-ground water treatment activity, there is now a groundwater bypass to reduce the groundwater level above the reactors by about 1.5 metres, pumping from 12 wells and from May 2014, discharging the uncontaminated water into the sea. This prevents some of it flowing into the reactor basements and becoming contaminated. In addition, an impermeable wall is being constructed on the sea-side of the reactors, and inside this a frozen soil wall will further block water flow into the reactor buildings.

In October 2013 guidelines for rainwater release from the site allowed Tepco to release water to the sea without specific NRA approval as long as it conformed to activity limits. Tepco has been working to 25 Bq/L caesium and 10 Bq/L strontium-90.

Summary: A large amount of contaminated water has accumulated on site and has been treated to remove all but traces of tritium, which limits the potential to release treated water to the sea. Some radioactivity has been released to the sea, but this has mostly been low-level and it has not had any significant impact beyond the immediate plant structures. Concentrations outside these structures have been below regulatory levels since April 2011.

IRID and NDF involvement

The International Research Institute for Nuclear Decommissioning ( IRID) was set up in August 2013 Japan by JAEA, Japanese utilities and reactor vendors, with a focus on Fukushima 1-4.

In September 2013 IRID called for submissions on the management of contaminated water at Fukushima. In particular, proposals were sought for dealing with: the accumulation of contaminated water (in storage tanks, etc); the treatment of contaminated water including tritium removal; the removal of radioactive materials from the seawater in the plant's 30 ha harbour; the management of contaminated water inside the buildings; measures to block groundwater from flowing into the site; and, understanding the flow of groundwater. Responses were submitted to the government in November.

In December 2013 IRID called for innovative proposals for removing fuel debris from units 1-3 about 2020.

In August 2014 the Nuclear Damage Compensation and Decommissioning Facilitation Corporation (NDF) was set up by government as a planning body with management support for R&D projects, taking over IRID's planning role. It will work closely with IRID, whose focus now is on developing mid- and long-term decommissioning technologies. NDF will also work closely with Tepco Fukushima Daiichi D&D Engineering Co. which has responsibility for operating the actual decommissioning work there. The NDF will be the main body interacting with government (METI) to implement policy.

Fukushima Daiichi 5&6

Units 5&6, in a separate building, also lost power on 11 March due to the tsunami. They were in 'cold shutdown' at the time, but still requiring pumped cooling. One air-cooled diesel generator at Daiichi 6 was located higher and so survived the tsunami and enabled repairs on Saturday 19th, allowing full restoration of cooling for units 5 and 6. While the power was off their core temperature had risen to over 100°C (128°C in unit 5) under pressure, and they had been cooled with normal water injection. They were restored to cold shutdown by the normal recirculating system on 20th, and mains power was restored on 21-22nd.

In September 2013 Tepco commenced work to remove the fuel from unit 6. Prime Minister Abe then called for Tepco to decommission both units. Tepco announced in December 2013 that it would decommission both units from the end of january 2014. Unit 5 is a 760 MWe BWR the same as units 2-4, and unit 6 is larger - 1067 MWe. They entered commercial operation in 1978 and 1979 respectively. It is proposed that they will be used for training.

Meanwhile Tepco and Mitsubishi plan to build and operate two new 500 MWe coal-burning power plants near Fukushima Daiichi at Hirono Town and Iwaki City. This will partly compensate for the decommissioning of the Fukushima Daiichi units. The investment is expected to be JPY 300 billion ($3 billion). The plants will be state-of-the-art integrated combined gasification cycle with less atmospheric pollution than the coal plants now operating in Japan. The companies planned to apply for a government subsidy to help defray costs. (Mainichi 23/11/13)

Remediation on site and decommissioning units 1-4

Tepco published a six- to nine-month plan in April 2011 for dealing with the disabled Fukushima reactors, and updated this several times subsequently. Remediation over the next couple of years proceeded approximately as planned. in August 2011 Tepco announced its general plan for proceeding with removing fuel from the four units, initially from the spent fuel ponds and then from the actual reactors. At the end of 2013 Tepco announced the establishment of an internal entity to focus on measures for decommissioning units 1-6 and dealing with contaminated water. The name of the new company is Fukushima Daiichi Decontamination & Decommissioning Engineering Company, and it commenced operations in April 2014.

In June 2015 the government revised the decommissioning plan or roadmap for the second time, though without major change. It clarified milestones to accomplish preventive and multilayered measures, involving the three principles of removing the source of the contamination, isolating groundwater from the contamination source, and preventing leakage of the contaminated water. It included a new goal of cutting the amount of groundwater flowing into the buildings to less than 100 m ³ per day by April 2016. The schedule for fuel removal from the pond at unit 1 was postponed from late FY17 to FY20, while that for unit 2 was delayed from early FY20 to later the same fiscal year, and that at unit 3 from early FY15 to FY17. Fuel debris removal was to begin in 2021, as before. Tepco affirmed the revised plan.

In September 2017 the government updated the June 2015 decommissioning roadmap, with no changes to the framework, and confirming first removal of fuel debris from unit 1 in 2021. Treatment of all contaminated water accumulated in the reactor buildings is to be completed by 2020.

Tepco has a website giving updates on decommissioning work and environmental monitoring.

Storage ponds: First, debris has been removed from the upper parts of the reactor buildings using large cranes and heavy machinery. Covers will be built as required, and overhead cranes and fuel handling machines necessary to remove the spent fuel assemblies will be reinstalled, as already at unit 4. The fuel handling machine for unit 3 was built by Toshiba in January 2016 and delivered to the port at the plant. It is being fitted in 2017, with a cover, for the 566 fuel assemblies to be removed by mid-2018. Casks to transfer the removed fuel to the central spent fuel facility have been designed and manufactured using existing cask technology.

In July 2012 two unused fuel assemblies were removed from unit 4 pond, and were found to be in good shape, with no deformation or corrosion. Tepco started removal of both fresh and used fuel from the pond in November 2013, 22 assemblies at a time in each cask, with 1331 used and 202 new ones to be moved. This was uneventful, and the task continued through 2014. By 22 December 2014, all 1331 used as well as all 202 new fuel assemblies had been moved in 71 cask shuttles without incident, with weekly updates having been published. All of the radioactive used fuel was removed by early November, eliminating a significant radiological hazard on the site. The used fuel went to the central storage pond, from which older assemblies were transferred to dry cask storage - 1004 moved since the accident by mid 2014 to make way for new inputs from unit 4. The fresh fuel assemblies are stored in the pool of the undamaged unit 6.

From 2018 Tepco expects to move 514 used fuel assemblies from unit 3 to the central pool, as well as 52 new ones and then 292 used fuel assemblies and 100 new ones from unit 1. Finally unit 2 will have its 587 used assemblies and 28 fresh ones moved. The NRA has expressed concern about the unit 1 used fuel.

Reactors 1-3: First it is necessary to identify the locations of leaks from the primary containment vessels (PCVs) and reactor buildings using manual and remotely controlled dosimeters, cameras, etc., and indirectly analyse conditions inside the PCVs from the outside via measurements of gamma rays, echo soundings, etc. Any leakage points will be repaired and both reactor vessels (RPVs) and PCVs filled with water sufficient to achieve shielding. Then the vessel heads will be removed. The location of melted fuel and corium will then be established. In particular, the distribution of damaged fuel believed to have flowed out from the reactor pressure vessels (RPVs) into PCVs will be ascertained, and it will be sampled and analysed. After examination of the inside of the reactors, states of the damaged fuel rods and reactor core internals, sampling will be done and the damaged core material will be removed from the RPVs as well as from the PCVs. The whole process will be complex and slow, since safety remains paramount. In December Tepco estimates that the fuel will be removed from the reactors within 25 years - in line with US experience at Three Mile Island, though other estimates suggest ten years. Updated plans are on the IRID website.

The four reactors will be completely demolished in 30-40 years - much the same timeframe as for any nuclear plant. As noted above units 5-6 were decommissioned in 2014 and will be used for training.

Earlier, consortia led by both Hitachi-GE and Toshiba submitted proposals to Tepco for decommissioning units 1-4. This would generally involve removing the fuel and then sealing them for a further decade or two while the activation products in the steel of the reactor pressure vessels decay. They can then be demolished. As noted above, removal of the very degraded fuel will be a long process in units 1-3, but will draw on experience at Three Mile Island in USA. In January 2012 it was reported that an industry consortium (Hitachi GE Nuclear Energy, Mitsubishi Heavy Industries and Toshiba) would determine how to locate fuel debris inside units 1-3 and how to fill the pressure vessels with water.

Tepco has allocated ¥207 billion ($2.53 billion) in its accounts for decommissioning units 1-4. The government has allocated ¥1150 billion ($15 billion) for decontamination in the region, with the promise of more if needed.

The new International Research Institute for Nuclear Decommissioning (IRID) has a focus on Fukushima 1-4. See section above. Its schedule for units 1-4 is:
FY 20XX ends the following March
The Agency for Natural Resources and Energy (ANRE) has calculated that as of January 2017 Tepco needs an estimated ¥22,000 billion ($191 billion), double the original estimate, to implement fuel debris removal, cleanup and for compensation to firms and individuals in Fukushima prefecture. Of this amount, Tepco will pay ¥16,000 billion. Other Japanese nuclear operators will pay ¥4,000 billion through the Nuclear Damage Compensation and Decommissioning Facilitation Corp (NDF), and the Japanese government will pay ¥2,000 billion for cleanup in Fukushima prefecture.

A 12-member international expert team assembled by the IAEA at the request of the Japanese government has reported on remediation strategies for contaminated land. The mission focused on the remediation of the affected areas outside of the 20 km restricted area. The team said that it agreed with the prioritization and the general strategy being implemented, but advised the government to focus on actual dose reduction. They should "avoid over-conservatism" which "could not effectively contribute to the reduction of exposure doses" to people. It warned the government against being preoccupied with "contamination concentrations rather than dose levels," since this "does not automatically lead to reduction of doses for the public." The team's report calls on the Japanese authorities to "maintain their focus on remediation activities that bring best results in reducing the doses to the public."

Fukushima Daini plant

Units 1-4 were shut down automatically due to the earthquake. The tsunami - here only 9 m high - affected the generators and there was major interruption to cooling due to damaged heat exchangers, so the reactors were almost completely isolated from their ultimate heat sink. Damage to the diesel generators was limited and also the earthquake left one of the external power lines intact, avoiding a station blackout as at Daiichi 1-4. Staff laid and energised 8.8km of heavy-duty electric cables in 30 hours to supplement power.

In units 1, 2 & 4 there were cooling problems still evident on Tuesday 15th. Unit 3 was undamaged and continued to 'cold shutdown' status on 12th, but the other units suffered flooding to pump rooms where the equipment transfers heat from the reactor heat removal circuit to the sea. Pump motors were replaced in less than 30 hours. All units achieved 'cold shutdown' by16 March, meaning core temperature less than 100°C at atmospheric pressure (101 kPa), but still requiring some water circulation. The almost complete loss of ultimate heat sink for a day proved a significant challenge, but the cores were kept fully covered.

Radiation monitoring figures remained at low levels, little above background.

There is no technical reason for the Fukushima Daini plant not to restart. restart. However, Tepco in October 2012 said it planned to transfer the fuel from the four reactors to used fuel ponds, and this was done. In February 2015 the prime minister said that restarting the four units was essentially a matter for Tepco to decide. Local government is in favour of decommissioning. Tepco has not announced any decision about whether to retire or move towards restarting the four units.

International Nuclear Event Scale assessment

Japan's Nuclear & Industrial Safety Agency originally declared the Fukushima Daiichi 1-3 accident as Level 5 on the International Nuclear Events Scale (INES) - an accident with wider consequences, the same level as Three Mile Island in 1979. The sequence of events relating to the fuel pond at unit 4 was rated INES Level 3 - a serious incident.

However, a month after the tsunami the NSC raised the rating to 7 for units 1-3 together, 'a major accident', saying that a re-evaluation of early radioactive releases suggested that some 630 PBq of I-131 equivalent had been discharged, mostly in the first week. This then matched the criterion for level 7. In early June NISA increased its estimate of releases to 770 PBq, from about half that, though in August the NSC lowered this estimate to 570 PBq

For Fukushima Daini, NISA declared INES Level 3 for units 1, 2, 4 - each a serious incident.

Accident liability and compensation

Beyond whatever insurance Tepco might carry for its reactors is the question of third party liability for the accident. Japan was not party to any international liability convention but its law generally conforms to them, notably strict and exclusive liability for the operator. Early in 2015 Japan ratified the Convention on Supplementary Compensation for Nuclear Damage (CSC). Two laws governing liability are revised about every ten years: the Law on Compensation for Nuclear Damage and Law on Contract for Liability Insurance for Nuclear Damage. Plant operator liability is exclusive and absolute (regardless of fault), and power plant operators must provide a financial security amount of JPY 120 billion (US$ 1.46 billion) - it was half that to 2010. The government may relieve the operator of liability if it determines that damage results from "a grave natural disaster of an exceptional character" (which it did not do here), and in any case total liability is unlimited.

In mid-April 2011, the first meeting was held of a panel to address compensation for nuclear-related damage. The panel established guidelines for determining the scope of compensation for damage caused by the accident, and to act as an intermediary. It was established within the Ministry of Education, Culture, Sports, Science and Technology (MEXT), and was led by Law Professor Yoshihisa Nomi of Gakushuin, University in Tokyo.

On 11 May 2011, Tepco accepted terms established by the Japanese government for state support to compensate those affected by the accident at the Fukushima Daiichi plant. The scheme includes a new state-backed institution to expedite payments to those affected by the Fukushima Daiichi accident. The body receives financial contributions from electric power companies with nuclear power plants in Japan, and from the government through special bonds that can be cashed whenever necessary. The government bonds total JPY 5 trillion ($62 billion). Tepco accepted the conditions imposed on the company as part of the package. That included not setting an upper limit on compensation payments to those affected, making maximum efforts to reduce costs, and an agreement to cooperate with an independent panel set up to investigate its management.

This Nuclear Damage Compensation Facilitation Corporation, established by government and nuclear plant operators, includes representatives from other nuclear generators and will also operate as an insurer for the industry, being responsible to have plans in place for any future nuclear accidents. The provision for contributions from other nuclear operators is similar to that in the USA. The government estimates that Tepco will be able to complete its repayments in 10 to 13 years, after which it will revert to a fully private company with no government involvement. Meanwhile it will pay an annual fee for the government support, maintain adequate power supplies and ensure plant safety. Tepco estimated its extra costs for fossil fuels in 2011-12 (April-March) would be about JPY 830 billion ($10.7 billion).

On 14 June 2011, Japan's cabinet passed the Nuclear Disaster Compensation Bill, and a related budget to fund post-tsunami reconstruction was also passed subsequently.

In September 2011 the Nuclear Damage Compensation Facilitation Corporation started by working with Tepco to compile a business plan for the next decade. This was approved by the Ministry of Economy, Trade and Industry (METI) so that some JPY 900 billion ($11.5 billion) could be released to the company through bonds issued to the Nuclear Damage Facilitation Fund to cover compensation payments to March 2012. The plan also involved Tepco reducing its own costs by JPY 2545 billion ($32.6 billion) over the next ten years, including shedding 7400 jobs. This special business plan was superseded by a more comprehensive business plan in March 2012, involving compensation payments of JPY 910 billion ($11.6 billion) annually. Tepco wanted to include an electricity rate increase of 17% in the plan, to cover the additional annual fuel costs for thermal power generation to make up for lost capacity at idled nuclear power plants.

In February 2012 METI approved a further JPY 690 billion ($8.9 billion) in compensation support from the Nuclear Damage Liability Facilitation Fund, subject to Tepco's business plan giving the government voting rights. In June 2012 shareholders voted to sell the Japanese government 50.11% of Tepco's voting shares and an additional 25.73% with no voting rights, for JPY 1 trillion (about $12.5 billion), paid through the Nuclear Damage Liability Facilitation Fund. This was effected at the end of July, so that Tepco then became government-controlled, at least temporarily. Tepco said it appreciated the chance to 'transform to New Tepco'.

The government and 12 utilities are contributing funds into the new institution to pay compensation to individuals and businesses claiming damages caused by the accident. It received JPY 7 billion ($91 million) in public funds as well as a total of JPY 7 billion from 12 nuclear plant operators, the Tepco share of JPY 2379 million ($30 million) being largest. The percentage of utility contributions was fixed in proportion to the power output of their plants, so Kansai Electric Power Co. provided JPY 1229 million, followed by JPY 660 million by Kyushu Electric Power Co. and JPY 622 million by Chubu Electric Power Co. Japan Nuclear Fuel Ltd., which owns a used nuclear fuel reprocessing plant in Aomori Prefecture, provided JPY 117 million to the entity. The utility companies also pay annual contributions to the body. Tepco is required to make extra contributions, with the specific amount to be decided later.

In June 2013 Tepco requested a further JPY 666 billion ($6.7 billion) in government support through the Nuclear Damage Liability Facilitation Fund, bringing the total amount requested by Tepco to JPY 3.79 trillion ($38 billion). The company said that more than half of the latest request - some JPY 370 billion ($3.7 billion) - resulted from the re-evaluation of the evacuation zone around the damaged plant and a re-examination of the estimated amount "regarding compensation for mental damages, loss or depreciation of valuables such as housing lands and buildings." About JPY 43 billion ($431 million) was due to a higher estimate of compensation coming from damages to the agriculture, forestry and fisheries industries, as well as the food processing and distribution industries. This, it said, also resulted from "harmful rumours" about the possible health effects of consuming food products from the region near the damaged power plant. As restrictions on the transport of foodstuffs from the Fukushima area seemed set to continue, an additional JPY 240 billion ($2.4 billion) was included to cover for the further compensation claims resulting from this.

By mid-May 2014, Tepco had paid JPY 3808 billion ($38 billion) in compensation, fairly evenly split between businesses and individuals, based on decisions of the Nuclear Damage Compensation Facilitation Corporation, and covered by loans from the NDLF Fund. Some $16 billion of this was distributed evenly among 85,000 evacuees - $188,200 each person including children, as directed early in 2011.

In July 2015 the government approved Tepco's recovery plan, including compensation payments of JPY 7075 billion ($57.2 billion), enabling it to receive JPY 950 billion more than the JPY 6125 billion estimated in April, according to METI.

In December 2013 the government raised the upper limit of its financial assistance to Tepco from JPY 5 trillion to JPY 9 trillion ($86 billion). Early in 2014 the government estimated it would take JPY11 trillion and 40 years to clean up the Fukushima site. The 2013 Japan trade deficit was JPY 11.5 trillion.

Inquiries and reports: the accident itself and decommissioning

In October 2014 the NRA published its Analysis of the TEPCO Fukushima Daiichi NPS Accident, Interim Report. A provisional translation in English was published in February 2015. This focuses on a number of questions which remained unexplained in the 2012 National Diet Investigation Commission report.

In May 2011 a team of 18 experts from 12 countries spent a week at the plant on behalf of the International Atomic Energy Agency (IAEA), and that mission's final report was presented to the IAEA Ministerial Conference in Vienna in June. At the IAEA General Conference in 2012 the Director General promised a comprehensive report which would be "an authoritative, factual and balanced assessment, addressing the causes and consequences of the accident as well as the lessons learned." The IAEA published this report in September 2015, accompanied by five technical volumes.

In February 2015 the IAEA completed its third review mission (as follow-up to that of late 2013, and involving some 180 experts from 42 IAEA member states and other organizations over two years) and reported on decommissioning to METI. In May 2015 its final report was delivered to member states, and was published in September. It is broadly positive regarding progress since 2013, but said that some challenging issues remain. It contains advisory points on topics such as long-term radioactive waste management, measures concerning contaminated water, and issues related to the removal of used fuel and fuel debris.

The Foreword to the Director General's report in 2015 states: "A major factor that contributed to the accident was the widespread assumption in Japan that its nuclear power plants were so safe that an accident of this magnitude was simply unthinkable. This assumption was accepted by nuclear power plant operators and was not challenged by regulators or by the government. As a result, Japan was not sufficiently prepared for a severe nuclear accident in March 2011." The accident exposed "certain weaknesses" in Japan's regulatory framework, with responsibilities divided among a number of bodies. It also said there were certain weaknesses "in plant design, in emergency preparedness and response arrangement and in planning for the management of a severe accident".

The Director General said: "I am confident that the legacy of the Fukushima Daiichi accident will be a sharper focus on nuclear safety everywhere. I have seen improvements in safety measures and procedures in every nuclear power plant that I have visited. There is widespread recognition that everything humanly possible must be done to ensure that no such accident ever happens again." He added: "This is all the more essential as global use of nuclear power is likely to continue to grow in coming decades." Furthermore, "there can be no grounds for complacency about nuclear safety in any country. Some of the factors that contributed to the Fukushima Daiichi accident were not unique to Japan. Continuous questioning and openness to learning from experience are key to safety culture and are essential for everyone involved in nuclear power."

The Executive Summary includes recommendations, but the following paragraphs indicate some salient points from the actual investigation.

Before the accident, there was a basic assumption in Japan that the design of nuclear power plants and the safety measures that had been put in place were sufficiently robust to withstand external events of low probability and high consequences. Because of the basic assumption that nuclear power plants in Japan were safe, there was a tendency for organizations and their staff not to challenge the level of safety. The reinforced basic assumption among the stakeholders about the robustness of the technical design of nuclear power plants resulted in a situation where safety improvements were not introduced promptly.

Before the accident, the operator had conducted some reassessments of extreme tsunami flood levels, using a consensus based methodology developed in Japan in 2002, which had resulted in values higher than the original design basis estimates. Based on the results, some compensatory measures were taken, but they proved to be insufficient at the time of the accident.

There were no indications that the main safety features of the plant were affected by the vibratory ground motions generated by the earthquake on 11 March 2011. This was due to the conservative approach to earthquake design and construction of nuclear power plants in Japan, resulting in a plant that was provided with sufficient safety margins. However, the original design considerations did not provide comparable safety margins for extreme external flooding events, such as tsunamis.

Despite the efforts of the operators at the Fukushima Daiichi nuclear power plant to maintain control, the reactor cores in units 1-3 overheated, the nuclear fuel melted and the three containment vessels were breached. Hydrogen was released from the reactor pressure vessels, leading to explosions inside the reactor buildings in units 1, 3 and 4 that damaged structures and equipment and injured personnel. Radionuclides were released from the plant to the atmosphere and were deposited on land and on the ocean. There were also direct releases into the sea.

Venting of the containment was necessary to relieve pressure and prevent its failure. The operators were able to vent units 1 and 3 to reduce the pressure in the primary containment vessels. However, this resulted in radioactive releases to the environment. Even though the containment vents for units 1 and 3 were opened, the primary containment vessels for units 1 and 3 eventually failed. Containment venting for unit 2 was not successful, and the containment failed, resulting in radioactive releases.

People within a radius of 20 km of the site and in other designated areas were evacuated, and those within a radius of 20-30 km were instructed to shelter before later being advised to voluntarily evacuate. Restrictions were placed on the distribution and consumption of food and the consumption of drinking water. At the time of writing, many people were still living outside the areas from which they were evacuated.

No early radiation induced health effects were observed among workers or members of the public that could be attributed to the accident.

Earlier reports 2011-2012

Early in June 2011 the independent Investigation Committee on the Accident at the Fukushima Nuclear Power Stations ( ICANPS), a panel of ten experts, mostly academics and appointed by the Japanese cabinet, began meeting. It has two technological advisers. An initial report was published in December 2011 and a final report in July 2012. The panel set up four teams to undertake investigations on the causes of the accident and ensuing damage and on measures to prevent the further spread of damage caused by the accident, but not to pursue the question of responsibility for the accident.

The national Diet later set up a legally-constituted Nuclear Accident Independent Investigation Commission ( NAIIC, or National Diet Investigation Commission) of ten members which started its work in December 2011. One of the purposes of NAIIC is to provide suggestions including the "re-examination of an optimal administrative organization" for nuclear safety regulation based on its investigation of the accident. NAIIC reported in July 2012, harshly criticizing the government, the plant operator and the country's national culture. After conducting 900 hours of public hearings and interviews with more than 1,100 people and visiting several nuclear power plants, the commission's report concluded that the accident was a "manmade disaster," the result of "collusion between the government, the regulators and Tokyo Electric Power Co." It said the "root causes were the organizational and regulatory systems that supported faulty rationales for decisions and actions." The NAIIC criticized the regulator for insufficiently maintaining independence from the industry in developing and enforcing safety regulations, the government for inadequate emergency preparedness and management, and Tepco for its poor governance and lack of safety culture. The report called for fundamental changes across the industry, including the government and regulators, to increase openness, trustworthiness and focus on protecting public health and safety.

The NAIIC Chairman wrote: "What must be admitted - very painfully - is that this was a disaster 'Made in Japan.' 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 mindset of government and industry led the country to avoid learning the lessons of the previous major nuclear accidents at Three Mile Island and Chernobyl. "The consequences of negligence at Fukushima stand out as catastrophic, but the mindset that supported it can be found across Japan. In recognizing that fact, each of us (every Japanese citizen) should reflect on our responsibility as individuals in a democratic society."

NAIIC reported that Tepco had been aware since 2006 that Fukushuima Daiichi could face a station blackout if flooded, as well as the potential loss of ultimate heat sink in the event of a major tsunami. However, the regulator, NISA, gave no instruction to the company to prepare for severe flooding, and even told all nuclear operators that it was not necessary to plan for station blackout. During the initial response to the tsunami, this lack of readiness for station blackout was compounded by a lack of planning and training for severe accident mitigation. Plans and procedures for venting and manual operation of emergency cooling were incomplete and their implementation in emergency circumstances proved very difficult as a result. NISA was also criticised for its "negligence and failure over the years" to prepare for a nuclear accident in terms of public information and evacuation, with previous governments equally culpable. Then Tepco's difficulty in mitigation was compounded by government interference which undermined NISA.

On 7 June 2011 the government submitted a 750-page report to IAEA compiled by the nuclear emergency taskforce, acknowledging reactor design inadequacies and systemic shortcomings. It said that "In light of the lessons learned from the accident, Japan has recognized that a fundamental revision of its nuclear safety preparedness and response is inevitable."

On 11 September 2011 a second report was issued by the government and submitted to the IAEA, summarising both on-site work and progress and off-site responses. It contained further analysis of the earthquake and tsunami, the initial responses to manage and cool the reactors, the state of spent fuel ponds and the state of reactor pressure vessels. It also summarised radioactive releases and their effects.

Meanwhile a July 2011 report from MIT's Centre for Advanced Nuclear Energy Systems provided a useful series of observations, questions raised, and suggestions. Its Appendix has some constructive comment on radiation exposure and balancing the costs of dose avoidance in circumstances of environmental contamination.

In November 2011 the US Institute of Nuclear Power Operators (INPO) released its Special Report on the Nuclear Accident at the Fukushima Daiichi Nuclear Power Station, with timeline. This 97-page report gives a valuable and detailed account of events.

Also in November 2011 the Japan Nuclear Technology Institute published a 280-page report on the accident, with proposals to be addressed in the future.

On 2 December 2011 Tepco released its interim investigation report on the accident (in Japanese).

The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) is undertaking a 12-month study on the magnitude of radioactive releases to the atmosphere and ocean, and the range of radiation doses received by the public and workers.

An analysis by the Carnegie Endowment in March 2012 said that if best practices from other countries had been adopted by Tepco and NISA at Fukushima, the serious accident would not have happened, underlining the need for greater international regulatory collaboration.

In April 2012 the US Electric Power Research Institute (EPRI) published Fukushima Daiichi Accident - Technical Causal Factor Analysis, which identified the root cause beyond the flooding and its effects as a failure to consider the possibility of the rupture of combinations of geological fault segments in the vicinity of the plant.

Inquiries and reports: radiation effects

A preliminary report from the World Health Organisation (WHO) in May 2012 estimated the radiation doses that residents of Japan outside the evacuated areas received in the year following the accident. The report's headline conclusion is that most people in Fukushima prefecture would have received a radiation dose of between 1 and 10 mSv during the first year after the accident. This compares with levels of about 2.4 mSv they would have received from unavoidable natural sources. In two places the doses were higher - between 10 and 50 mSv, still below any harmful level. Almost all were "below the internationally-agreed reference level for the public exposure due to radon in dwellings" (about 10 mSv/yr).

The UN Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) in May 2012 reported that despite skin contamination of several workers, no clinically-observable effects have been reported and there is no evidence of acute radiation injury in any of the 20,115 workers who participated in Tepco's efforts to mitigate the accident at the plant. Eighteen UNSCEAR member states provided 72 experts for the assessment. UNSCEAR also surveyed Fukushima prefecture tol compare its data with Japanese measurements of exposures of some 2 million people living there at the time of the accident.

The results of UNSCEAR's 12-month study on the magnitude of radioactive releases to the atmosphere and ocean, and the range of radiation doses received by the public and workers were announced in May 2013 are reported above in the subsection on Radiation Exposure.

UNSCEAR's final report of radiation effects was released in April 2014. This concluded that the rates of cancer or hereditary diseases were unlikely to show any discernible rise in affected areas because the radiation doses people received were too low. People were promptly evacuated from the vicinity of the nuclear power plant, and later from a neighbouring area where radionuclides had accumulated. This action reduced their radiation exposure by a factor of ten, to levels that were "low or very low." Overall, people in Fukushima are expected on average to receive less than 10 mSv due to the accident over their whole lifetime, compared with the 170 mSv lifetime dose from natural background radiation that people in Japan typically receive. "The most important health effect is on mental and social well-being, related to the enormous impact of the earthquake, tsunami and nuclear accident, and the fear and stigma related to the perceived risk of exposure to radiation." UNSCEAR's follow-up white paper in October 2015 said that none of the new information appraised after the 2013 report "materially affected the main findings in, or challenged the major assumptions of, the 2013 Fukushima report."

In October 2013 a 16-member IAEA mission visited at government request and reported on remediation and decontamination in particular. Its preliminary report said that decontamination efforts were commendable but driven by unrealistic targets.

'Stress tests' on Japanese reactors and new regulatory authority

The government ordered nuclear risk and safety reassessments - so-called 'stress tests' - based on those in the EU for all Japan's nuclear reactors except Fukushima's before they restart following any shutdown, including for routine checks. These were in two stages, and are described in the Japan Nuclear Power paper.

The government then created a separate Nuclear Regulatory Agency (NRA) under the authority of the Environment Ministry and combining the roles of NISA and NSC, commissioned in September 2012. A new Nuclear Regulatory Commission (NRC) replaced the NSC and will review the effectiveness of the NRA and be responsible for the investigation of nuclear accidents.

Notes & references

General sources

Tepco
NISA
IAEA
METI
JAIF
NSC
Acton J.M. & Hibbs M, Why Fukushima was preventable, March 2012 Carnegie Paper.
INPO 11-005 Addendum, Aug 2012, Lessons learned from the Nuclear Accident at Fukushima Daiichi Nuclear Power Station.
Government's Decision on Addressing the Contaminated Water Issue at TEPCO's Fukushima Daiichi NPS, 3 Sept 2013, on Ministry of Foreign Affairs website
K. Tateiwa, Jan 2014, Decommissioning Fukushima Daiichi NPS
UNSCEAR webpage on The Fukushima-Daiichi nuclear power plant accident
IAEA Report by the Director General on The Fukushima Daiichi Accident, STI/PUB/1710 (ISBN:978-92-0-107015-9), September 2015
A. Komori, Current status and the future of Fukushima Daiichi NP station, World Nuclear Association 2015 Symposium presentation

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Following a major earthquake, a 15-metre tsunami disabled the power supply and cooling of three Fukushima Daiichi reactors, causing a nuclear accident on 11 March 2011. All three cores largely melted in the first three days. The accident was rated 7 on the INES scale, due to high radioactive releases over days 4 to 6, eventually a total of some 940 PBq (I-131 eq).

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The World Nuclear Association's report on the Chernobyl accident

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The World Nuclear Association's report on the Chernobyl accident

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In February 2022, Russia launched a military offensive against Ukraine. For further information see page on Russia-Ukraine War and Nuclear Energy.

The Chernobyl accident in 1986 was the result of a flawed reactor design that was operated with inadequately trained personnel.

The resulting steam explosion and fires released at least 5% of the radioactive reactor core into the environment, with the deposition of radioactive materials in many parts of Europe.

Two Chernobyl plant workers died due to the explosion on the night of the accident, and a further 28 people died within a few weeks as a result of acute radiation syndrome.

The United Nations Scientific Committee on the Effects of Atomic Radiation has concluded that, apart from some 5000 thyroid cancers (resulting in 15 fatalities), "there is no evidence of a major public health impact attributable to radiation exposure 20 years after the accident."

Some 350,000 people were evacuated as a result of the accident, but resettlement of areas from which people were relocated is ongoing.

On 24 February Ukraine informed the International Atomic Energy Agency that Russian forces had taken control of all facilities at Chernobyl (see below).

On 9 March the Chernobyl nuclear plant was disconnected from the electricity grid. The IAEA stated that it did not see a critical impact on safety as a result.

The April 1986 disaster at the Chernobyl nuclear power plant in Ukraine was the product of a flawed Soviet reactor design coupled with serious mistakes made by the plant operators. It was a direct consequence of Cold War isolation and the resulting lack of any safety culture.

The accident destroyed the Chernobyl 4 reactor, killing 30 operators and firemen within three months and several further deaths later. One person was killed immediately and a second died in hospital soon after as a result of injuries received. Another person is reported to have died at the time from a coronary thrombosis. Acute radiation syndrome (ARS) was originally diagnosed in 237 people onsite and involved with the clean-up and it was later confirmed in 134 cases. Of these, 28 people died as a result of ARS within a few weeks of the accident. Nineteen more workers subsequently died between 1987 and 2004, but their deaths cannot necessarily be attributed to radiation exposure. Nobody offsite suffered from acute radiation effects although a significant, but uncertain, fraction of the thyroid cancers diagnosed since the accident in patients who were children at the time are likely to be due to intake of radioactive iodine fallout. Furthermore, large areas of Belarus, Ukraine, Russia, and beyond were contaminated in varying degrees. See also sections below and Chernobyl Accident Appendix 2: Health Impacts.

The Chernobyl disaster was a unique event and the only accident in the history of commercial nuclear power where radiation-related fatalities occurred. The design of the reactor is unique and in that respect the accident is thus of little relevance to the rest of the nuclear industry outside the then Eastern Bloc. However, it led to major changes in safety culture and in industry cooperation, particularly between East and West before the end of the Soviet Union. Former President Gorbachev said that the Chernobyl accident was a more important factor in the fall of the Soviet Union than Perestroika - his program of liberal reform.

The Chernobyl site and plant

The Chernobyl Power Complex, lying about 130 km north of Kiev, Ukraine, and about 20 km south of the border with Belarus, consisted of four nuclear reactors of the RBMK-1000 design (see information page on RBMK Reactors). Units 1 and 2 were constructed between 1970 and 1977, while units 3 and 4 of the same design were completed in 1983. Two more RBMK reactors were under construction at the site at the time of the accident. To the southeast of the plant, an artificial lake of some 22 square kilometres, situated beside the river Pripyat, a tributary of the Dniepr, was constructed to provide cooling water for the reactors.

This area of Ukraine is described as Belarussian-type woodland with a low population density. About 3 km away from the reactor, in the new city, Pripyat, there were 49,000 inhabitants. The old town of Chornobyl, which had a population of 12,500, is about 15 km to the southeast of the complex. Within a 30 km radius of the power plant, the total population was between 115,000 and 135,000 at the time of the accident.

The RBMK-1000 is a Soviet-designed and built graphite moderated pressure tube type reactor, using slightly enriched (2% U-235) uranium dioxide fuel. It is a boiling light water reactor, with two loops feeding steam directly to the turbines, without an intervening heat exchanger. Water pumped to the bottom of the fuel channels boils as it progresses up the pressure tubes, producing steam which feeds two 500 MWe turbines. The water acts as a coolant and also provides the steam used to drive the turbines. The vertical pressure tubes contain the zirconium alloy clad uranium dioxide fuel around which the cooling water flows. The extensions of the fuel channels penetrate the lower plate and the cover plate of the core and are welded to each. A specially designed refuelling machine allows fuel bundles to be changed without shutting down the reactor.

The moderator, the function of which is to slow down neutrons to make them more efficient in producing fission in the fuel, is graphite, surrounding the pressure tubes. A mixture of nitrogen and helium is circulated between the graphite blocks to prevent oxidation of the graphite and to improve the transmission of the heat produced by neutron interactions in the graphite to the fuel channel. The core itself is about 7 m high and about 12 m in diameter. In each of the two loops, there are four main coolant circulating pumps, one of which is always on standby. The reactivity or power of the reactor is controlled by raising or lowering 211 control rods, which, when lowered into the moderator, absorb neutrons and reduce the fission rate. The power output of this reactor is 3200 MW thermal, or 1000 MWe. Various safety systems, such as an emergency core cooling system, were incorporated into the reactor design.

One of the most important characteristics of the RBMK reactor is that it can possess a 'positive void coefficient', where an increase in steam bubbles ('voids') is accompanied by an increase in core reactivity (see information page on RBMK Reactors). As steam production in the fuel channels increases, the neutrons that would have been absorbed by the denser water now produce increased fission in the fuel. There are other components that contribute to the overall power coefficient of reactivity, but the void coefficient is the dominant one in RBMK reactors. The void coefficient depends on the composition of the core - a new RBMK core will have a negative void coefficient. However, at the time of the accident at Chernobyl 4, the reactor's fuel burn-up, control rod configuration, and power level led to a positive void coefficient large enough to overwhelm all other influences on the power coefficient.

The 1986 Chernobyl accident

On 25 April, prior to a routine shutdown, the reactor crew at Chernobyl 4 began preparing for a test to determine how long turbines would spin and supply power to the main circulating pumps following a loss of main electrical power supply. This test had been carried out at Chernobyl the previous year, but the power from the turbine ran down too rapidly, so new voltage regulator designs were to be tested.

A series of operator actions, including the disabling of automatic shutdown mechanisms, preceded the attempted test early on 26 April. By the time that the operator moved to shut down the reactor, the reactor was in an extremely unstable condition. A peculiarity of the design of the control rods caused a dramatic power surge as they were inserted into the reactor (see Chernobyl Accident Appendix 1: Sequence of Events).

The interaction of very hot fuel with the cooling water led to fuel fragmentation along with rapid steam production and an increase in pressure. The design characteristics of the reactor were such that substantial damage to even three or four fuel assemblies would - and did - result in the destruction of the reactor. The overpressure caused the 1000 t cover plate of the reactor to become partially detached, rupturing the fuel channels and jamming all the control rods, which by that time were only halfway down. Intense steam generation then spread throughout the whole core (fed by water dumped into the core due to the rupture of the emergency cooling circuit) causing a steam explosion and releasing fission products to the atmosphere. About two to three seconds later, a second explosion threw out fragments from the fuel channels and hot graphite. There is some dispute among experts about the character of this second explosion, but it is likely to have been caused by the production of hydrogen from zirconium-steam reactions.

Two workers died as a result of these explosions. The graphite (about a quarter of the 1200 tonnes of it was estimated to have been ejected) and fuel became incandescent and started a number of fires, causing the main release of radioactivity into the environment. A total of about 14 EBq (14 x 10 ¹⁸ Bq) of radioactivity was released, over half of it being from biologically-inert noble gases.*
* The figure of 5.2 EBq is also quoted, this being "iodine-131 equivalent" - 1.8 EBq iodine and 85 PBq Cs-137 multiplied by 40 due its longevity, and ignoring the 6.5 EBq xenon-33 and some minor or short-lived nuclides.
About 200-300 tonnes of water per hour was injected into the intact half of the reactor using the auxiliary feedwater pumps but this was stopped after half a day owing to the danger of it flowing into and flooding units 1 and 2. From the second to tenth day after the accident, some 5000 tonnes of boron, dolomite, sand, clay, and lead were dropped on to the burning core by helicopter in an effort to extinguish the blaze and limit the release of radioactive particles.
The damaged Chernobyl unit 4 reactor building
The 1991 report by the State Committee on the Supervision of Safety in Industry and Nuclear Power on the root cause of the accident looked past the operator actions. It said that while it was certainly true the operators placed their reactor in a dangerously unstable condition (in fact in a condition which virtually guaranteed an accident) it was also true that in doing so they had not in fact violated a number of vital operating policies and principles, since no such policies and principles had been articulated. Additionally, the operating organization had not been made aware either of the specific vital safety significance of maintaining a minimum operating reactivity margin, or the general reactivity characteristics of the RBMK which made low power operation extremely hazardous.

Immediate impact of the Chernobyl accident

The accident caused the largest uncontrolled radioactive release into the environment ever recorded for any civilian operation, and large quantities of radioactive substances were released into the air for about 10 days. This caused serious social and economic disruption for large populations in Belarus, Russia, and Ukraine. Two radionuclides, the short-lived iodine-131 and the long-lived caesium-137, were particularly significant for the radiation dose they delivered to members of the public.

It is estimated that all of the xenon gas, about half of the iodine and caesium, and at least 5% of the remaining radioactive material in the Chernobyl 4 reactor core (which had 192 tonnes of fuel) was released in the accident. Most of the released material was deposited close by as dust and debris, but the lighter material was carried by wind over Ukraine, Belarus, Russia, and to some extent over Scandinavia and Europe.

The casualties included firefighters who attended the initial fires on the roof of the turbine building. All these were put out in a few hours, but radiation doses on the first day caused 28 deaths - six of which were firemen - by the end of July 1986. The doses received by the firefighters and power plant workers were high enough to result in acute radiation syndrome (ARS), which occurs if a person is exposed to more than 700 milligrays (mGy) within a short time frame (usually minutes). Common ARS symptoms include gastrointestinal problems ( e.g. nausea, vomiting), headaches, burns and fever. Whole body doses between 4000 mGy and 5000 mGv within a short time frame would kill 50% of those exposed, with 8000-10,000 mGy universally fatal. The doses received by the firefighters who died were estimated to range up to 20,000 mGy.

The next task was cleaning up the radioactivity at the site so that the remaining three reactors could be restarted, and the damaged reactor shielded more permanently. About 200,000 people ('liquidators') from all over the Soviet Union were involved in the recovery and clean-up during 1986 and 1987. They received high doses of radiation, averaging around 100 millisieverts (mSv). Some 20,000 liquidators received about 250 mSv, with a few receiving approximately 500 mSv. Later, the number of liquidators swelled to over 600,000, but most of these received only low radiation doses. The highest doses were received by about 1000 emergency workers and onsite personnel during the first day of the accident.

According to the most up-to-date estimate provided by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), the average radiation dose due to the accident received by inhabitants of 'strict radiation control' areas (population 216,000) in the years 1986 to 2005 was 31 mSv (over the 20-year period), and in the 'contaminated' areas (population 6.4 million) it averaged 9 mSv, a minor increase over the dose due to background radiation over the same period (about 50 mSv).

Initial radiation exposure in contaminated areas was due to short-lived iodine-131; later caesium-137 was the main hazard. (Both are fission products dispersed from the reactor core, with half lives of 8 days and 30 years, respectively. 1.8 EBq of I-131 and 0.085 EBq of Cs-137 were released.) About five million people lived in areas of Belarus, Russia and Ukraine contaminated (above 37 kBq/m ² Cs-137 in soil) and about 400,000 lived in more contaminated areas of strict control by authorities (above 555 kBq/m ² Cs-137). A total of 29,400 km ² was contaminated above 180 kBq/m ².

The plant operators' town of Pripyat was evacuated on 27 April (45,000 residents). By 14 May, some 116,000 people that had been living within a 30-kilometre radius had been evacuated and later relocated. About 1000 of these returned unofficially to live within the contaminated zone. Most of those evacuated received radiation doses of less than 50 mSv, although a few received 100 mSv or more.

In the years following the accident, a further 220,000 people were resettled into less contaminated areas, and the initial 30 km radius exclusion zone (2800 km ²) was modified and extended to cover 4300 square kilometres. This resettlement was due to application of a criterion of 350 mSv projected lifetime radiation dose, though in fact radiation in most of the affected area (apart from half a square kilometre close to the reactor) fell rapidly so that average doses were less than 50% above normal background of 2.5 mSv/yr. See also following section on Resettlement of contaminated areas.

Long-term health effects of the Chernobyl accident

Video: Experts talk about the health effects of Chernobyl (Recorded 2011)

Several organizations have reported on the impacts of the Chernobyl accident, but all have had problems assessing the significance of their observations because of the lack of reliable public health information before 1986.

In 1989, the World Health Organization (WHO) first raised concerns that local medical scientists had incorrectly attributed various biological and health effects to radiation exposure. Following this, the Government of the USSR requested the International Atomic Energy Agency (IAEA) to coordinate an international experts' assessment of accident's radiological, environmental and health consequences in selected towns of the most heavily contaminated areas in Belarus, Russia, and Ukraine. Between March 1990 and June 1991, a total of 50 field missions were conducted by 200 experts from 25 countries (including the USSR), seven organizations, and 11 laboratories. In the absence of pre-1986 data, it compared a control population with those exposed to radiation. Significant health disorders were evident in both control and exposed groups, but, at that stage, none was radiation related.

In February 2003, the IAEA established the Chernobyl Forum, in cooperation with seven other UN organisations as well as the competent authorities of Belarus, the Russian Federation, and Ukraine. In April 2005, the reports prepared by two expert groups - "Environment", coordinated by the IAEA, and "Health", coordinated by WHO - were intensively discussed by the Forum and eventually approved by consensus. The conclusions of this 2005 Chernobyl Forum study (revised version published 2006) are in line with earlier expert studies, notably the UNSCEAR 2000 report which said that "apart from this [thyroid cancer] increase, there is no evidence of a major public health impact attributable to radiation exposure 14 years after the accident. There is no scientific evidence of increases in overall cancer incidence or mortality or in non-malignant disorders that could be related to radiation exposure." There is little evidence of any increase in leukaemia, even among clean-up workers where it might be most expected. Radiation-induced leukemia has a latency period of 5-7 years, so any potential leukemia cases due to the accident would already have developed. A low number of the clean-up workers, who received the highest doses, may have a slightly increased risk of developing solid cancers in the long term. To date, however, there is no evidence of any such cancers having developed. Apart from these, the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) said: "The great majority of the population is not likely to experience serious health consequences as a result of radiation from the Chernobyl accident. Many other health problems have been noted in the populations that are not related to radiation exposure."

The Chernobyl Forum report says that people in the area have suffered a paralysing fatalism due to myths and misperceptions about the threat of radiation, which has contributed to a culture of chronic dependency. Some "took on the role of invalids." Mental health coupled with smoking and alcohol abuse is a very much greater problem than radiation, but worst of all at the time was the underlying level of health and nutrition. Apart from the initial 116,000, relocations of people were very traumatic and did little to reduce radiation exposure, which was low anyway. Psycho-social effects among those affected by the accident are similar to those arising from other major disasters such as earthquakes, floods, and fires.

A particularly sad effect of the misconceptions surrounding the accident was that some physicians in Europe advised pregnant women to undergo abortions on account of radiation exposure, even though the levels concerned were vastly below those likely to have teratogenic effects. Robert Gale, a hematologist who treated radiation victims after the accident, estimated that more than 1 million abortions were undertaken in the Soviet Union and Europe as a result of incorrect advice from their doctors about radiation exposure and birth defects following the accident.

Some exaggerated figures have been published regarding the death toll attributable to the Chernobyl disaster, including a publication by the UN Office for the Coordination of Humanitarian Affairs (OCHA). However, the Chairman of UNSCEAR made it clear that "this report is full of unsubstantiated statements that have no support in scientific assessments", and the Chernobyl Forum report also repudiates these claims.

The number of deaths resulting from the accident are covered most fully in the account of health effects provided by an annex to the UNSCEAR 2008 report, released in 2011. The report concluded: "In summary, the effects of the Chernobyl accident are many and varied. Early deterministic effects can be attributed to radiation with a high degree of certainty, while for other medical conditions, radiation almost certainly was not the cause. In between, there was a wide spectrum of conditions. It is necessary to evaluate carefully each specific condition and the surrounding circumstances before attributing a cause."

According to an UNSCEAR report in 2018, about 20,000 cases of thyroid cancer were diagnosed 1991-2015 in patients who were 18 and under at the time of the accident. The report states that a quarter of the cases (5000 cases) were "probably" due to high doses of radiation, and that this fraction was likely to have been higher in earlier years, and lower in later years. However, it also states that the uncertainty around the attributed fraction is very significant - at least 0.07 to 0.5 - and that the influence of annual screenings and active follow-up make comparisons with the general population problematic. Thyroid cancer is usually not fatal if diagnosed and treated early; the report states that of the diagnoses made between 1991 and 2005, 15 proved to be fatal.

Progressive closure of the Chernobyl plant

In the early 1990s, some $400 million was spent on improvements to the remaining reactors at Chernobyl, considerably enhancing their safety. Energy shortages necessitated the continued operation of one of them (unit 3) until December 2000. (Unit 2 was shut down after a turbine hall fire in 1991, and unit 1 at the end of 1997.) Almost 6000 people worked at the plant every day, and their radiation dose has been within internationally accepted limits. A small team of scientists works within the wrecked reactor building itself, inside the shelter.

Workers and their families now live in a new town, Slavutich, 30 km from the plant. This was built following the evacuation of Pripyat, which was just 3 km away.

Ukraine depends upon, and is deeply in debt to, Russia for energy supplies, particularly oil and gas, but also nuclear fuel. Although this dependence is gradually being reduced, continued operation of nuclear power stations, which supply half of total electricity, is now even more important than in 1986.

When it was announced in 1995 that the two operating reactors at Chernobyl would be closed by 2000, a memorandum of understanding was signed by Ukraine and G7 nations to progress this, but its implementation was conspicuously delayed. Alternative generating capacity was needed, either gas-fired, which has ongoing fuel cost and supply implications, or nuclear, by completing Khmelnitski unit 2 and Rovno unit 4 ('K2R4') in Ukraine. Construction of these was halted in 1989 but then resumed, and both reactors came online late in 2004, financed by Ukraine rather than international grants as expected on the basis of Chernobyl's closure.

Chernobyl today

Russian military operation 2022

On 24 February 2022, Russian forces took control of all facilities of the Chernobyl nuclear plant. Control levels of gamma radiation dose rates in the Chernobyl exclusion zone were exceeded. The SNRIU said that the rise in radiation levels was likely due to "disturbance of the top layer of soil from movement of a large number of heavy military machinery through the exclusion zone and increase of air pollution." It added: "The condition of Chernobyl nuclear facilities and other facilities is unchanged." Radiation readings from the site were assessed by the IAEA to be low and in line with near background levels.

On 9 March at 11.22 the Chernobyl plant lost connection to the grid. The SNRIU said that backup diesel generators were running and had 48 hours of fuel. The IAEA stated that, based on the heat load of spent fuel in the ISF-1 storage pool, and the volume of cooling water it contained, there would be sufficient heat removal without electrical supply. It said that it saw no critical impact on safety as a result of the loss of power, but said that the loss of power would likely create additional stress for the about 210 staff who have not been able to rotate for the past two weeks.

Professor Geraldine Thomas, director of the Chernobyl Tissue Bank, said: "They [the used fuel bundles] will not be producing significant amounts of heat, making a release of radiation very unlikely. In the unlikely event of a release of any radiation, this would be only to the immediate local area, and therefore not pose any threat to western Europe - there would be no radioactive cloud."

On 13 March Energoatom reported that transmission system operator Ukrenergo had succeeded in repairing a power line needed to restore external electricity supplies to Chernobyl. The site was due to be reconnected to the grid a day later, but Ukrenergo reported in the morning of 14 March that the line had sustained further damage "by the occupying forces". Later on 14 March Ukrenergo said that external power had been restored at 13.10 local time, and at 16.45 the plant was reconnected to Ukraine's electricity grid.

On 31 March control of the site was returned to Ukrainian personnel.

For more detailed information, see page on Russia-Ukraine War and Nuclear Energy.

Unit 4 containment

Chernobyl unit 4 was enclosed in a large concrete shelter which was erected quickly (by October 1986) to allow continuing operation of the other reactors at the plant. However, the structure is neither strong nor durable. The international Shelter Implementation Plan in the 1990s involved raising money for remedial work including removal of the fuel-containing materials. Some major work on the shelter was carried out in 1998 and 1999. About 200 tonnes of highly radioactive material remains deep within it, and this poses an environmental hazard until it is better contained.

The New Safe Confinement (NSC) structure was completed in 2017, having been built adjacent and then moved into place on rails. It is an arch 110 metres high, 165 metres long and spanning 260 metres, covering both unit 4 and the hastily-built 1986 structure. The arch frame is a lattice construction of tubular steel members, equipped with internal cranes. The design and construction contract for this was signed in 2007 with the Novarka consortium and preparatory work onsite was completed in 2010. Construction started in April 2012. The first half, weighing 12,800 tonnes, was moved 112 metres to a holding area in front of unit 4 in April 2014. The second half was completed by the end of 2014 and was joined to the first in July 2015. Cladding, cranes, and remote handling equipment were fitted in 2015. The entire 36,000 tonne structure was pushed 327 metres into position over the reactor building in November 2016, over two weeks, and the end walls completed. The NSC is the largest moveable land-based structure ever built.

The hermetically sealed building will allow engineers to remotely dismantle the 1986 structure that has shielded the remains of the reactor from the weather since the weeks after the accident. It will enable the eventual removal of the fuel-containing materials (FCM) in the bottom of the reactor building and accommodate their characterization, compaction, and packing for disposal. This task represents the most important step in eliminating nuclear hazard at the site - and the real start of dismantling. The NSC will facilitate remote handling of these dangerous materials, using as few personnel as possible. During peak construction of the NSC some 1200 workers were onsite.

The Chernobyl Shelter Fund, set up in 1997, had received €864 million from international donors by early 2011 towards this project and previous work. It and the Nuclear Safety Account (NSA), set up in 1993, are managed by the European Bank for Reconstruction and Development (EBRD). The total cost of the new shelter was in 2011 estimated to be €1.5 billion. In November 2014 the EBRD said the overall €2.15 billion Shelter Implementation Plan including the NSC had received contributions from 43 governments but still had a funding shortfall of €615 million. The following month the EBRD made an additional contribution of €350 million in anticipation of a €165 million contribution by the G7/European Commission, which was confirmed in April 2015. This left a balance of €100 million to come from non-G7 donors, and €15 million of this was confirmed in April 2015.
Chernobyl New Safe Confinement under construction and before being moved into place (Image: EBRD)
Funding other Chernobyl work

The Nuclear Safety Account (NSA), had received €321 million by early 2011 for Chernobyl decommissioning and also for projects in other ex-Soviet countries. At Chernobyl it funds the construction of used fuel and waste storage (notably ISF-2, see below) and decommissioning units 1-3. In April 2016 the European Commission pledged €20 million to the NSA, the largest part of €45 million expected from the G7 and the European Commission. A further €40 million was expected from the EBRD in May 2016.

In total, the European Commission has committed around €730 million so far to Chernobyl projects in four ways. First, €550 million for assistance projects, out of which €470 million were channelled through international funds, and €80 million implemented directly by the European Commission. Secondly, power generation support of €65 million. Thirdly, €15 million for social projects. And finally, €100 million for research projects.

Chernobyl used fuel: ISF-1 & ISF-2

Used fuel from units 1-3 was stored in each unit's cooling pond, and in an interim spent fuel storage facility pond (ISF-1). A few damaged assemblies remained in units 1&2 in 2013, with the last of these removed in June 2016. ISF-1 now holds most of the spent fuel from units 1-3, allowing those reactors to be decommissioned under less restrictive licence conditions. Most of the fuel assemblies were straightforward to handle, but about 50 are damaged and required special handling.

In 1999, a contract was signed with Framatome (now Areva) for construction of the ISF-2 radioactive waste management facility to store 25,000 used fuel assemblies from units 1-3 and other operational waste long-term, as well as material from decommissioning units 1-3 (which are the first RBMK units decommissioned anywhere). However, after a significant part of the dry storage facility had been built, technical deficiencies in the concept emerged in 2003, and the contract was terminated amicably in 2007.

Holtec International became the contractor in September 2007 for the new interim spent nuclear fuel storage facility (ISF-2 or SNF SF-2) for the state-owned Chernobyl NPP. Design approval and funding from the NSA was confirmed in October 2010, and the final €87.5 million of €400 million cost was pledged in April 2016. Construction was completed in January 2020. Hot and cold tests took place during 2020, and the facility received an operating licence in April 2021.

ISF-2 is the world's largest dry used fuel storage facility, accommodating 21,217 RBMK fuel assemblies in dry storage for at least a 100-year service life.

The project includes a processing facility, able to cut the RBMK fuel assemblies* and to put the material in double-walled canisters, which are then filled with inert gas and welded shut. They will then be transported to concrete dry storage vaults in which the fuel containers will be enclosed for up to 100 years. This facility, treating 2500 fuel assemblies per year, is the first of its kind for RBMK fuel.
* According to Holtec: "Unique features of the Chernobyl dry storage facility include the world's largest 'hot cell' for dismembering the conjugated RBMK fuel assembly and a (Holtec patented) forced gas dehydrator designed to run on nitrogen."
Other Chernobyl radwaste

Industrial Complex for Radwaste Management (ICSRM): In April 2009, Nukem handed over this turnkey waste treatment centre for solid radioactive waste. In May 2010, the State Nuclear Regulatory Committee licensed the commissioning of this facility, where solid low- and intermediate-level wastes accumulated from the power plant operations and the decommissioning of reactor blocks 1-3 is conditioned. The wastes are processed in three steps. First, the solid radioactive wastes temporarily stored in bunkers is removed for treatment. In the next step, these wastes, as well as those from decommissioning reactor blocks 1-3, are processed into a form suitable for permanent safe disposal. Low- and intermediate-level wastes are separated into combustible, compactable, and non-compactable categories. These are then subject to incineration, high-force compaction, and cementation respectively. In addition, highly radioactive and long-lived solid waste is sorted out for temporary separate storage. In the third step, the conditioned solid waste materials are transferred to containers suitable for permanent safe storage.

As part of this project, at the end of 2007, Nukem handed over an Engineered Near Surface Disposal Facility for storage of short-lived radioactive waste after prior conditioning. It is 17 km away from the power plant, at the Vektor complex within the 30-km zone. The storage area is designed to hold 55,000 m ³ of treated waste which will be subject to radiological monitoring for 300 years, by when the radioactivity will have decayed to such an extent that monitoring is no longer required.

Another contract has been let for a Liquid Radioactive Waste Treatment Plant (LRTP), to handle some 35,000 cubic metres of low- and intermediate-level liquid wastes at the site. This will be solidified and eventually buried along with solid wastes on site. Construction of the plant has been completed and the start of operations was due late in 2015. LRTP is also funded through EBRD's Nuclear Safety Account (NSA).

Non-Chernobyl used fuel

The Central Spent Fuel Storage Facility (CSFSF) Project for Ukraine's VVER reactors is being built by Holtec International within the Chernobyl exclusion area, between the resettled villages Staraya Krasnitsa, Buryakovka, Chistogalovka, and Stechanka, southeast of Chernobyl and not far from ISF-2. This will not take any Chernobyl fuel, though it will become a part of the common spent nuclear fuel management complex of the state-owned company Chernobyl NPP.

Decommissioning units 1-3

After the last Chernobyl reactor shut down in December 2000, in mid-2001 a new enterprise, SSE ChNPP was set up to take over management of the site and decommissioning from Energoatom. (Its remit includes eventual decommissioning of all Ukraine nuclear plants.)

In January 2008, the Ukraine government announced a four-stage decommissioning plan which incorporated the above waste activities and progresses towards a cleared site.

In February 2014 a new stage of this was approved for units 1-3, involving dismantling some equipment and putting them into safstor condition by 2028. Then, to 2046, further equipment will be removed, and by 2064 they will be demolished.

See also official website.

Resettlement of contaminated areas

In the last two decades there has been some resettlement of the areas evacuated in 1986 and subsequently. Recently the main resettlement project has been in Belarus.

In July 2010, the Belarus government announced that it had decided to settle back thousands of people in the 'contaminated areas' covered by the Chernobyl fallout, from which 24 years ago they and their forbears were hastily relocated. Compared with the list of contaminated areas in 2005, some 211 villages and hamlets had been reclassified with fewer restrictions on resettlement. The decision by the Belarus Council of Ministers resulted in a new national program over 2011-15 and up to 2020 to alleviate the Chernobyl impact and return the areas to normal use with minimal restrictions. The focus of the project is on the development of economic and industrial potential of the Gomel and Mogilev regions from which 137,000 people were relocated.

The main priority is agriculture and forestry, together with attracting qualified people and housing them. Initial infrastructure requirements will mean the refurbishment of gas, potable water and power supplies, while the use of local wood will be banned. Schools and housing will be provided for specialist workers and their families ahead of wider socio-economic development. Overall, some 21,484 dwellings are slated for connection to gas networks in the period 2011-2015, while about 5600 contaminated or broken down buildings are demolished. Over 1300 kilometres of road will be laid, and ten new sewerage works and 15 pumping stations are planned. The cost of the work was put at BYR 6.6 trillion ($2.2 billion), split fairly evenly across the years 2011 to 2015 inclusive.

The feasibility of agriculture will be examined in areas where the presence of caesium-137 and strontium-90 is low, "to acquire new knowledge in the fields of radiobiology and radioecology in order to clarify the principles of safe life in the contaminated territories." Land found to have too high a concentration of radionuclides will be reforested and managed. A suite of protective measures was set up to allow a new forestry industry whose products would meet national and international safety standards. In April 2009, specialists in Belarus stressed that it is safe to eat all foods cultivated in the contaminated territories, though intake of some wild food was restricted.

Protective measures will be put in place for 498 settlements in the contaminated areas where average radiation dose may exceed 1 mSv per year. There were also 1904 villages with annual average effective doses from the pollution between 0.1 mSv and 1 mSv. The goal for these areas is to allow their re-use with minimal restrictions, although already radiation doses there from the caesium are lower than background levels anywhere in the world.

The Belarus government decision was an important political landmark in an ongoing process. Studies reviewed by UNSCEAR show that the Chernobyl disaster caused little risk for the general population. A UN Development Program report in 2002 said that much of the aid and effort applied to mitigate the effects of the Chernobyl accident did more harm than good, and it seems that this, along with the Chernobyl Forum report, finally persuaded the Belarus authorities. In 2004 President Lukashenko announced a priority to repopulate much of the Chernobyl-affected regions of Belarus, and then in 2009 he said that he "wants to repopulate Chernobyl's zone quickly".

In 2011 Chernobyl was officially declared a tourist attraction, with many visitors.

In 2015 the published results of a major scientific study showed that the mammal population of the exclusion zone (including the 2162 sq km Polessian State Radiation-Ecological Reserve - PSRER in Belarus) was thriving, despite land contamination. The "long-term empirical data showed no evidence of a negative influence of radiation on mammal abundance." The data "represent unique evidence of wildlife's resilience in the face of chronic radiation stress." ( Current Biology, Elsevier) . Other studies have concluded that the net environmental effect of the accident has been much greater biodiversity and abundance of species, with the exclusion zone having become a unique sanctuary for wildlife due to the absence of humans.

What has been learned from the Chernobyl disaster?

Leaving aside the verdict of history on its role in melting the Soviet 'Iron Curtain', some very tangible practical benefits have resulted from the Chernobyl accident. The main ones concern reactor safety, notably in eastern Europe. (The US Three Mile Island accident in 1979 had a significant effect on Western reactor design and operating procedures. While that reactor was destroyed, all radioactivity was contained - as designed - and there were no deaths or injuries.)

While no-one in the West was under any illusion about the safety of early Soviet reactor designs, some lessons learned have also been applicable to Western plants. Certainly the safety of all Soviet-designed reactors has improved vastly. This is due largely to the development of a culture of safety encouraged by increased collaboration between East and West, and substantial investment in improving the reactors.

Modifications have been made to overcome deficiencies in all the RBMK reactors still operating. In these, originally the nuclear chain reaction and power output could increase if cooling water were lost or turned to steam, in contrast to most Western designs. It was this effect which led to the uncontrolled power surge that led to the destruction of Chernobyl 4 (see Positive void coefficient section in the information page on RBMK Reactors). All of the RBMK reactors have now been modified by changes in the control rods, adding neutron absorbers and consequently increasing the fuel enrichment from 1.8 to 2.4% U-235, making them very much more stable at low power (see Post accident changes to the RBMK section in the information page on RBMK Reactors). Automatic shut-down mechanisms now operate faster, and other safety mechanisms have been improved. Automated inspection equipment has also been installed. A repetition of the 1986 Chernobyl accident is now virtually impossible, according to a German nuclear safety agency report.

Since 1989, over 1000 nuclear engineers from the former Soviet Union have visited Western nuclear power plants and there have been many reciprocal visits. Over 50 twinning arrangements between East and West nuclear plants have been put in place. Most of this has been under the auspices of the World Association of Nuclear Operators (WANO), a body formed in 1989 which links 130 operators of nuclear power plants in more than 30 countries (see also information page on Cooperation in the Nuclear Power Industry).

Many other international programmes were initiated following Chernobyl. The International Atomic Energy Agency (IAEA) safety review projects for each particular type of Soviet reactor are noteworthy, bringing together operators and Western engineers to focus on safety improvements. These initiatives are backed by funding arrangements. The Nuclear Safety Assistance Coordination Centre database lists Western aid totalling almost US$1 billion for more than 700 safety-related projects in former Eastern Bloc countries. The Convention on Nuclear Safety adopted in Vienna in June 1994 is another outcome.

The Chernobyl Forum report said that some seven million people are now receiving or eligible for benefits as 'Chernobyl victims', which means that resources are not targeting those most in need. Remedying this presents daunting political problems however.

Notes & references

Notes

a. Chernobyl is the well-known Russian name for the site; Chornobyl is preferred by Ukraine. [ Back]

b. Much has been made of the role of the operators in the Chernobyl accident. The 1986 Summary Report on the Post-Accident Review Meeting on the Chernobyl Accident (INSAG-1) of the International Atomic Energy Agency's (IAEA's) International Nuclear Safety Advisory Group accepted the view of the Soviet experts that "the accident was caused by a remarkable range of human errors and violations of operating rules in combination with specific reactor features which compounded and amplified the effects of the errors and led to the reactivity excursion." In particular, according to the INSAG-1 report: "The operators deliberately and in violation of rules withdrew most control and safety rods from the core and switched off some important safety systems."

However, the IAEA's 1992 INSAG-7 report, The Chernobyl Accident: Updating of INSAG-1, was less critical of the operators, with the emphasis shifted towards "the contributions of particular design features, including the design of the control rods and safety systems, and arrangements for presenting important safety information to the operators. The accident is now seen to have been the result of the 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." But the report goes on to say that the International Nuclear Safety Advisory Group "remains of the opinion that critical actions of the operators were most ill judged. As pointed out in INSAG-1, the human factor has still to be considered as a major element in causing the accident."

It is certainly true that the operators placed the reactor in a dangerous condition, in particular by removing too many of the control rods, resulting in the lowering of the reactor's operating reactivity margin (ORM, see information page on RBMK Reactors). However, the operating procedures did not emphasize the vital safety significance of the ORM but rather treated the ORM as a way of controlling reactor power. It could therefore be argued that the actions of the operators were more a symptom of the prevailing safety culture of the Soviet era rather than the result of recklessness or a lack of competence on the part of the operators.

In what is referred to as his Testament - which was published soon after his suicide two years after the accident - Valery Legasov, who had led the Soviet delegation to the IAEA Post-Accident Review Meeting, wrote: "After I had visited Chernobyl NPP I came to the conclusion that the accident was the inevitable apotheosis of the economic system which had been developed in the USSR over many decades. Neglect by the scientific management and the designers was everywhere with no attention being paid to the condition of instruments or of equipment... When one considers the chain of events leading up to the Chernobyl accident, why one person behaved in such a way and why another person behaved in another etc, it is impossible to find a single culprit, a single initiator of events, because it was like a closed circle." [ Back]

c. The initial death toll was officially given as two initial deaths plus 28 from acute radiation syndrome. One further victim, due to coronary thrombosis, is widely reported, but does not appear on official lists of the initial deaths. The 2006 report of the UN Chernobyl Forum Expert Group "Health", Health Effects of the Chernobyl Accident and Special Health Care Programmes, states: "The Chernobyl accident caused the deaths of 30 power plant employees and firemen within a few days or weeks (including 28 deaths that were due to radiation exposure)." [ Back]

d. Apart from the initial 31 deaths (two from the explosions, one reportedly from coronary thrombosis - see Note c above - and 28 firemen and plant personnel from acute radiation syndrome), the number of deaths resulting from the accident is unclear and a subject of considerable controversy. According to the 2006 report of the UN Chernobyl Forum's 'Health' Expert Group: "The actual number of deaths caused by this accident is unlikely ever to be precisely known."

On the number of deaths due to acute radiation syndrome (ARS), the Expert Group report states: "Among the 134 emergency workers involved in the immediate mitigation of the Chernobyl accident, severely exposed workers and fireman during the first days, 28 persons died in 1986 due to ARS, and 19 more persons died in 1987-2004 from different causes. Among the general population affected by the Chernobyl radioactive fallout, the much lower exposures meant that ARS cases did not occur."

According to the report: "With the exception of thyroid cancer, direct radiation-epidemiological studies performed in Belarus, Russia and Ukraine since 1986 have not revealed any statistically significant increase in either cancer morbidity or mortality induced by radiation." The report does however attribute a large proportion of child thyroid cancer fatalities to radiation, with nine deaths being recorded during 1986-2002 as a result of progression of thyroid cancer. [ Back]

e. There have been fatalities in military and research reactor contexts, e.g. Tokai-mura. [ Back]

f. Although most reports on the Chernobyl accident refer to a number of graphite fires, it is highly unlikely that the graphite itself burned. Information on the General Atomics website (but now deleted) stated: "It is often incorrectly assumed that the combustion behavior of graphite is similar to that of charcoal and coal. Numerous tests and calculations have shown that it is virtually impossible to burn high-purity, nuclear-grade graphites." On Chernobyl, the same source stated: "Graphite played little or no role in the progression or consequences of the accident. The red glow observed during the Chernobyl accident was the expected color of luminescence for graphite at 700°C and not a large-scale graphite fire, as some have incorrectly assumed."

A 2006 Electric Power Research Institute Technical Report states that the International Atomic Energy Agency's INSAG-1 report is
...potentially misleading through the use of imprecise words in relation to graphite behaviour. The report discusses the fire-fighting activities and repeatedly refers to "burning graphite blocks" and "the graphite fire". Most of the actual fires involving graphite which were approached by fire-fighters involved ejected material on bitumen-covered roofs, and the fires also involved the bitumen. It is stated: "The fire teams experienced no unusual problems in using their fire-fighting techniques, except that it took a considerable time to extinguish the graphite fire." These descriptions are not consistent with the later considered opinions of senior Russian specialists... There is however no question that extremely hot graphite was ejected from the core and at a temperature sufficient to ignite adjacent combustible materials.

There are also several referrals to a graphite fire occurring during the October 1957 accident at Windscale Pile No. 1 in the UK. However, images obtained from inside the Pile several decades after the accident showed that the graphite was relatively undamaged. [ Back]

g. The International Chernobyl Project, 1990-91 - Assessment of Radiological Consequences and Evaluation of Protective Measures, Summary Brochure, published by the International Atomic Energy Agency, reports that, in June 1989, the World Health Organization (WHO) sent a team of experts to help address the health impacts of radioactive contamination resulting from the accident. One of the conclusions from this mission was that "scientists who are not well versed in radiation effects have attributed various biological and health effects to radiation exposure. These changes cannot be attributed to radiation exposure, especially when the normal incidence is unknown, and are much more likely to be due to psychological factors and stress.Attributing these effects to radiation not only increases the psychological pressure in the population and provokes additional stress-related health problems, it also undermines confidence in the competence of the radiation specialists." [ Back]

h. Image taken from page 31 of The International Chernobyl Project Technical Report, Assessment of Radiological Consequences and Evaluation of Protective Measures, Report by an International Advisory Committee, IAEA, 1991 (ISBN: 9201291914) [ Back]

i. A 55-page summary version the revised report, Chernobyl's Legacy: Health, Environmental and Socio-Economic Impacts and Recommendations to the Governments of Belarus, the Russian Federation and Ukraine, The Chernobyl Forum: 2003-2005, Second revised version, as well as the Report of the UN Chernobyl Forum Expert Group "Environment" and the Report of the UN Chernobyl Forum Expert Group "Health" are available through the IAEA's webpage on the Chernobyl accident ( https://www.iaea.org/topics/chornobyl) [ Back]

j. The United Nations Scientific Commission on the Effects of Atomic Radiation (UNSCEAR) is the UN body with a mandate from the General Assembly to assess and report levels and health effects of exposure to ionizing radiation. Exposures and effects of the Chernobyl accident , Annex J to Volume II of the 2000 United Nations Scientific Committee on the Effects of Atomic Radiation Report to the General Assembly, is available at the UNSCEAR 2000 Report Vol. II webpage (www.unscear.org/unscear/en/publications/2000_2.html). It is also available (along with other reports) on the webpage for UNSCEAR's assessments of the radiation effects of The Chernobyl accident (www.unscear.org/unscear/en/chernobyl.html). The conclusions from Annex J of the UNSCEAR 2000 report are in Chernobyl Accident Appendix: Health Impacts

k. The quoted comment comes from a 6 June 2000 letter from Lars-Erik Holm, Chairman of UNSCEAR and Director-General of the Swedish Radiation Protection Institute, to Kofi Annan, Secretary-General of the United Nations. [

l. A reinforced concrete casing was built around the ruined reactor building over the seven months following the accident. This shelter - often referred to as the - was intended to contain the remaining fuel and act as a radiation shield. As it was designed for a lifetime of around 20 to 30 years, as well as being hastily constructed, a second shelter - known as the New Safe Confinement - with a 100-year design lifetime is planned to be placed over the existing structure. See also ASE keeps the lid on Chernobyl , World Nuclear News (19 August 2008). [

m. The UNSCEAR committee in 2018 estimated that the fraction of the incidence of thyroid cancer attributable to radiation exposure among non-evacuated residents of Belarus, Ukraine and the four most contaminated oblasts of the Russian Federation, who were under 18 at the time of the accident, is in the order of 0.25 . The committee states that the uncertainty range of the fraction is large, at least from 0.07 to 0.5. [ Back]

References

1. 7. The Accident and the Safety of RBMK-Reactors, Gesellschaft für Anlagen und Reaktorsicherheit (GRS) mbH, GRS-121 (February 1996) [9. Health Effects of the Chernobyl Accident and Special Health Care Programmes, Report of the UN Chernobyl Forum, Expert Group "Health", World Health Organization, 2006 (ISBN: 9789241594172)
2. Appendix D, Graphite Decommissioning: Options for Graphite Treatment, Recycling, or Disposal, including a discussion of Safety-Related Issues , EPRI, Palo Alto, CA, 1013091 (March 2006)
The International Chernobyl Project, 1990-91 - Assessment of Radiological Consequences and Evaluation of Protective Measures, Summary Brochure , International Atomic Energy Agency, IAEA/PI/A32E, 1991; The International Chernobyl Project, An Overview , Assessment of Radiological Consequences and Evaluation of Protective Measures, Report by an International Advisory Committee , IAEA, 1991 (ISBN: 9201290918); The International Chernobyl Project Technical Report , Assessment of Radiological Consequences and Evaluation of Protective Measures, Report by an International Advisory Committee , IAEA, 1991 (ISBN: 9201291914) [
4. Mikhail Balonov, Malcolm Crick and Didier Louvat, Update of Impacts of the Chernobyl Accident: Assessments of the Chernobyl Forum (2003-2005) and UNSCEAR (2005-2008) , Proceedings of the Third European IRPA (International Radiation Protection Association) Congress held in Helsinki, Finland (14-18 June 2010) [
5. UNSCEAR, 2011, UNSCEAR 2008 Report, vol II, annex D (lead author: M. Balanov) [
6. Chernobyl - A Continuing Catastrophe, United Nations Office for the Coordination of Humanitarian Affairs (OCHA), 2000 [

8. Deryabina, T.G. Long-term census data reveal abundant wildlife populations at Chernobyl Health Effects due to Radiation from the Chernobyl Accident, , Current Biology, Volume 25, Issue 19, pR824-R826, Elsevier (5 October 2015) [
Back] Evaluation of data on thyroid cancer in regions affected by the Chernobyl accident, UNSCEAR (2018) [ Back]

General sources

INSAG-7, The Chernobyl Accident: Updating of INSAG-1, A report by the International Nuclear Safety Advisory Group, International Atomic Energy Agency, Safety Series No. 75-INSAG-7, 1992, (ISBN: 9201046928)

Chernobyl's Legacy: Health, Environmental and Socio-Economic Impacts and Recommendations to the Governments of Belarus, the Russian Federation and Ukraine, The Chernobyl Forum: 2003-2005, Second revised version, International Atomic Energy Agency, IAEA/PI/A.87 Rev.2/06-09181 (April 2006)

Environmental Consequences of the Chernobyl Accident and their Remediation: Twenty Years of Experience, Report of the Chernobyl Forum Expert Group 'Environment', International Atomic Energy Agency, 2006 (ISBN 9201147058)

Health Effects of the Chernobyl Accident and Special Health Care Programmes, Report of the UN Chernobyl Forum Expert Group "Health", World Health Organization, 2006 (ISBN: 9789241594172)

The Chernobyl accident, UNSCEAR's assessments of the radiation effects on the UNSCEAR (United Nations Scientific Committee on the Effects of Atomic Radiation) radiation website

Exposures and effects of the Chernobyl accident, Annex J of Sources and Effects of Ionizing Radiation, UNSCEAR 2000 Report to the General Assembly Vol. II

Ten Years after Chernobyl: what do we really know? (based on the proceedings of the IAEA/WHO/EC International Conference, Vienna, April 1996), International Atomic Energy Agency

Chernobyl: Assessment of Radiological and Health Impacts - 2002 Update of Chernobyl: Ten Years On, OECD Nuclear Energy Agency (2002).

Zbigniew Jaworowski, Lessons of Chernobyl with particular reference to thyroid cancer, Australasian Radiation Protection Society Newsletter No. 30 (April 2004). The same article appeared in Executive Intelligence Review (EIR), Volume 31, Number 18 (7 May 2004). An extended version of this paper was published as Radiation folly, Chapter 4 of Environment & Health: Myths & Realities, Edited by Kendra Okonski and Julian Morris, International Policy Press (a division of International Policy Network), June 2004 (ISBN 1905041004). See also Chernobyl Accident Appendix 2: Health Impacts

The chernobyl.info website www.chernobyl.info - out of date but some useful information

Chernobyl Forum information on IAEA website

Mikhail Balonov, The Chernobyl Forum: Major Findings and Recommendations, presented at the Public Information Materials Exchange meeting held in Vienna, Austria on 12-16 February 2006

GreenFacts webpage on Scientific Facts on the Chernobyl Nuclear Accident (www.greenfacts.org/en/chernobyl)

European Centre of Technological Safety's Chernobyl website (www.tesec-int.org/Chernobyl) and its webpage on Sarcophagus and Decommissioning of the Chernobyl NPP

Chernobyl Legacy website (www.chernobyllegacy.com)

David Mosey, Looking Beyond the Operator, Nuclear Engineering International, 26 Nov 2014

Chernobyl 25th anniversary, Frequently Asked Questions, World Health Organization (23 April 2011)

Environmental Consequences of the Chernobyl Accident and their Remediation: Twenty Years of Experience, Report of the UN Chernobyl Forum Expert Group "Environment", STI/PUB/1239, International Atomic Energy Agency (2006)

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(Image credit: Igor Kostin/Laski Diffusion/Getty Images)

In the early morning hours of April 26, 1986, the Chernobyl Nuclear Power Plant in Ukraine (formerly part of the Soviet Union) exploded, creating what many consider the worst nuclear disaster the world has ever seen.

Even after many years of scientific research and government investigation, there are still many unanswered questions about the Chernobyl accident - especially regarding the long-term health impacts that the massive radiation leak will have on those who were exposed.
Related: 5 weird things you didn't know about Chernobyl
Where is Chernobyl?

The city of Pripyat, founded in 1970, was the nearest town to the power plant at just under 2 miles (3 km) away and housed almost 50,000 people in 1986. A smaller and older town, Chernobyl, was about 9 miles (15 km) away and home to about 12,000 residents. The remainder of the region was primarily farms and woodland.

The power plant

(Image credit: Sovfoto/Universal Images Group via Getty Images)

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The Chernobyl plant used four Soviet-designed RBMK-1000 nuclear reactors - a design that's now universally recognized as inherently flawed. RBMK reactors were of a pressure tube design that used an enriched U-235 dioxide fuel to heat water, creating steam that drives the reactors' turbines and generates electricity, according to the World Nuclear Association.

What happened during the nuclear explosion?

Reactor 4 had been shut down the day before in order to perform the maintenance checks to safety systems during potential power outages, according to the Nuclear Energy Agency (opens in new tab) (NEA). While there is still some disagreement over the actual cause of the explosion, it is generally believed that the first was caused by an excess of steam and the second was influenced by . The excess steam was created by the reduction of the cooling water, which caused steam to build up in the cooling pipes - the positive-void coefficient - which caused an enormous power surge that the operators could not shut down.

(Image credit: Sergeev Kirill/Shutterstock)

The explosions occurred at 1:23 a.m. on April 26, destroying reactor 4 and initiating a booming fire, according to the NEA. Radioactive debris of fuel and reactor components rained over the area while fire spread from the building housing reactor 4 to adjacent buildings. Toxic fumes and dust were carried by the blowing wind, bringing fission products and the noble gas inventory of naturally-occurring odorless and colorless gases with it.

Radioactive fallout

The explosions killed two plant workers - the first of several workers to die within hours of the accident. For the next several days, as emergency crews tried desperately to contain the fires and radiation leaks, the death toll climbed as plant workers succumbed to acute radiation sickness.

The initial fire was stifled by about 5 a.m., but the resulting graphite-fueled fire took 10 days and 250 firefighters to extinguish, according to the NEA. However, toxic emissions continued to be pumped into the atmosphere for an additional 10 days.

Most of the radiation released from the failed nuclear reactor was from fission products -131, -134 and cesium-137. Iodine-131 has a relatively short half-life of eight days, according to UNSCEAR, but it is rapidly ingested through the air and tends to localize in the thyroid gland. Cesium isotopes have longer half-lives (cesium-137 has a half-life of 30 years) and are a concern for years after their release into the environment.

Evacuations of Pripyat commenced on April 27 - about 36 hours after the accident had occurred. By that time, many residents were already complaining about vomiting, headaches and other signs of radiation sickness. Officials closed off an 18-mile (30 km) area around the plant by May 14, evacuating another 116,000 residents. Within the next few years, 220,000 more residents were advised to move to less contaminated areas, according to the World Nuclear Association.

(Image credit: Anton Petrus/Getty Images)

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Health effects

(Image credit: Dreamstime)

Within three months of the Chernobyl accident, a total of 31 people died from radiation exposure or other direct effects of the disaster, according to the NRC. Between 1991 and 2015, as many as 20,000 cases of thyroid cancer cases were diagnosed in patients who were under the age of 18 in 1986, according to a 2018 (opens in new tab) report. While there may still be additional cases of cancer that emergency workers, evacuees and residents may experience throughout their lifetimes, the known overall rate of cancer deaths and other health effects directly related to Chernobyl's radiation leak is lower than was initially feared. "The majority of the five million residents living in contaminated areas ... received very small radiation doses comparable to natural background levels (0.1 rem per year)," according to an NRC report. "Today, the available evidence does not strongly connect the accident to radiation-induced increases of or solid cancer, other than thyroid cancer."

Environmental impacts

The plant, the ghost towns of Pripyat and Chernobyl, and the surrounding land make up a 1,000-square-mile (2600 square kilometers) "exclusion zone," which is restricted to nearly everyone except for scientists and government officials.

Despite the dangers, several people returned to their homes shortly after the disaster, with some sharing their stories with news sources such as the (opens in new tab), (opens in new tab) and The Guardian (opens in new tab). And in 2011, Ukraine opened up the area to tourists wanting to see the after-effects of the disaster firsthand.

Chernobyl today
Related: Infographic: Chernobyl nuclear disaster 25 years later
Today, the region, including within the exclusion zone, is filled with a variety of wildlife that have thrived without interference from humans, according to National Geographic (opens in new tab). Thriving populations of , , , , eagles, , , and other animals have been documented in the dense woodlands that now surround the silent power plant. Nonetheless, a handful of radiation effects, such as stunted trees growing in the zone of highest radiation and animals with high levels of cesium-137 in their bodies, are known to occur.

The area has recovered to some extent, but is far from returning to normal. But in the areas just outside the exclusion zone, people are beginning to . Tourists continue to visit the site, with visitation rates jumping 30% to 40% thanks to a 2019 HBO series based on the disaster. And the catastrophe that occurred at Chernobyl resulted in a few significant changes for the nuclear industry: concern about reactor safety increased in eastern Europe as well as around the world; the remaining RBMK reactors were modified to reduce the risk in another disaster; and many international programs including the International Atomic Energy Agency (opens in new tab) (IAEA) and the World Association of Nuclear Operators (opens in new tab) (WANO) were founded as a direct result of Chernobyl, according to the World Nuclear Association. And around the globe, experts have continued researching ways to prevent future nuclear disasters.

Russian invasion

(Image credit: Hennadii Minchenko/ Ukrinform/Future Publishing via Getty Images)

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On Feb. 24, 2022, during a full invasion of Ukraine ordered by Russian president Vladimir Putin, Russian troops captured the Chernobyl Nuclear Power Plant, taking its staff hostage.

Just a day later (Feb. 25), after heavy fighting between Ukrainian and Russian forces, increased radiation levels were detected at the power plant, according to Ukrainian officials and online data (opens in new tab) from Chernobyl exclusion zone's automated radiation-monitoring system. Gamma radiation, a high-energy type of electromagnetic radiation, increased 20 times above typical levels at multiple inspection points. This radiation spike was likely due to radioactive dust that was thrown into the air due to nearby disturbances from war equipment and fighting.

"If it's a resuspension of dust, this is generally stuff that was not that mobile, or it would have blown away," Edwin Lyman, director of nuclear power safety at the Union of Concerned Scientists, previously told Live Science. "So it's probably heavier particles of soil that don't disperse very far."

Even with this radiation spike however, "the dose rates they're finding are not that much greater than the usual dose rates in that area, which, admittedly, are probably about a hundred times the background dose of anywhere else in the world," Lyman said. "But even so, if [the troops] don't spend that much time in the area, it's not going to have a significant impact on their health compared to the threat of dying in war."

On March 9, Ukraine's state energy company announced (opens in new tab) that Chernobyl's nuclear power plant and all the facilities in the exclusion zone had been completely disconnected and were without electricity. This led Ukrainian officials to express their concern (opens in new tab) that the spent nuclear material kept in the plant's cooling pools could heat up and evaporate into its immediate surroundings. But nuclear energy experts cautioned that the plant's roughly 20,000 spent nuclear fuel units, which are 22 years old, were fairly cold and that an event of this kind would be highly unlikely.

Sometime during the Russian occupation, looters stole radioactive material and isotopes from a radiation monitoring laboratory near the defunct nuclear power plant, according to the Institute for Safety Problems of Nuclear Power Plants (ISPNPP). As it contains no plutonium or uranium, the stolen material cannot be used to make nuclear weapons, but it could possibly make a dirty bomb, although this risk is also low, Live Science previously reported.

Additional resources

Read more about how water cools and moderates nuclear reactors from the International Atomic Energy Agency (opens in new tab). Or find the latest news about the Chernobyl Nuclear Power Plant on the power plant's website (opens in new tab). To learn about longstanding health effects from the Chernobyl disaster, go to the Canadian Safety Nuclear Commission (opens in new tab).

Bibliography

BBC. "The people who refused to leave Chernobyl." April 26, 2016.

Live Science. "Is It Safe to Visit Chernobyl?" June 7, 2019.

Live Science. "Disaster Tourists Are Flocking to Chernobyl, Thanks to HBO Series." June 5, 2019.
This article was updated on June 20, 2019 by Live Science Contributor Rachel Ross.
Live Science. "Who Will Prevent the Next Chernobyl? (Op-Ed)" April 25, 2016.

Live Science. "Russian troops have taken over Chernobyl power plant, Ukrainian official says." Feb. 24, 2022.

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Chernobyl: Facts About the Nuclear Disaster

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(Image credit: Igor Kostin/Laski Diffusion/Getty Images) In the early morning hours of April 26, 1986, the Chernobyl Nuclear Power Plant in Ukraine (formerly part of the Soviet Union) exploded, creating what many consider the worst nuclear disaster the world has ever seen.

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Newspaper article on Chernobyl. Retrieved from the 1966 New York Times

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Newspaper article on Chernobyl. Retrieved from the 1966 New York Times

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Soviet Announces Nuclear Accident at Electric Plant

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Construction of the Chernobyl plant began in the early 1970's and the first reactor was commissioned in 1977. Work has been lagging behind plans. In April 1983, the Ukrainian Central Committee chastised the Chernobyl plant, along with the Rovno nuclear power station at Kuznetsovsk, for ''inferior quality of construction and installation work and low operating levels.'' ---- U.S.

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The Nuclear Energy Institute - Fact Sheet on Chernobyl 

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The Nuclear Energy Institute - Fact Sheet on Chernobyl 

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Key Facts

The 1986 accident at the Chernobyl nuclear power plant in Ukraine, then part of the former Soviet Union, is the only accident in the history of commercial nuclear power to cause fatalities from radiation. It was the product of a severely flawed Soviet-era reactor design combined with human error. Key differences in U.S. reactor design, regulation and emergency preparedness make it highly unlikely that a Chernobyl-type accident could occur in the United States.

Twenty-eight highly exposed reactor staff and emergency workers died from radiation and thermal burns within four months of the accident. Officials believe the accident also was responsible for nearly 7,000 cases of thyroid cancer among individuals who were under 18 years of age at the time of the accident. As of 2005, 15 children had died of thyroid cancer. Improved monitoring has been implemented to help ensure that thyroid cancer is detected early, when it is highly treatable.

Most emergency workers and people living in contaminated areas received relatively low whole-body radiation doses, according to a United Nations study published in 2011. The study found no evidence of increases in solid cancers, decreased fertility or congenital malformations. However, there is "some evidence of a detectable increase" in leukemia and cataract risk among workers who received higher radiation doses when engaged in recovery at the site. Long-term health monitoring of these workers is ongoing.

What Happened

The accident, which occurred in the early morning of April 26, 1986, resulted when operators took actions in violation of the plant's technical specifications. Operators ran the plant at very low power, without adequate safety precautions and without properly coordinating or communicating the procedure with safety personnel.

The four Chernobyl reactors were pressurized water reactors of the Soviet RBMK design, or Reactor BolshoMoshchnosty Kanalny, meaning "high-power channel reactor." Designed to produce both plutonium and electric power, they were very different from standard commercial designs, employing a unique combination of a graphite moderator and water coolant.

The reactors also were highly unstable at low power, primarily owing to control rod design and "positive void coefficient," factors that accelerated nuclear chain reaction and power output if the reactors lost cooling water.

These factors all contributed to an uncontrollable power surge that led to Chernobyl 4's destruction. The power surge caused a sudden increase in heat, which ruptured some of the pressure tubes containing fuel.

The hot fuel particles reacted with water and caused a steam explosion, which lifted the 1,000-metric-ton cover off the top of the reactor, rupturing the rest of the 1,660 pressure tubes, causing a second explosion and exposing the reactor core to the environment. The fire burned for 10 days, releasing a large amount of radiation into the atmosphere.

The Chernobyl plant did not have the massive containment structure common to most nuclear power plants elsewhere in the world. Without this protection, radioactive material escaped into the environment.

The crippled Chernobyl 4 reactor now is enclosed in a concrete structure that is growing weaker over time. Ukraine and the Group of Eight industrialized nations have agreed on a plan to stabilize the existing structure by constructing an enormous new sarcophagus around it, which is expected to last more than 100 years.

Officials shut down reactor 2 after a building fire in 1991 and closed Chernobyl 1 and 3 in 1996 and 2000, respectively.

Dealing With the Consequence

Soviet scientists reported that the Chernobyl 4 reactor contained about 190 metric tons of uranium dioxide fuel and fission products. An estimated 13 percent to 30 percent of this escaped into the atmosphere. Contamination from the accident scattered irregularly, depending on weather conditions. Reports from Soviet and western scientists indicate that Belarus received about 60 percent of the contamination that fell on the former Soviet Union. A large area in the Russian Federation south of Bryansk also was contaminated, as were parts of northwestern Ukraine.

Soviet authorities started evacuating people from the area around Chernobyl within 36 hours of the accident. In 1986, 115,000 local people were evacuated. The government subsequently resettled another 220,000 people.

However, the United Nations study found significant shortcomings in the Soviet Union's implementation of countermeasures. "In the first few weeks, management of animal fodder and milk production (including prohibiting the consumption of fresh milk) would have helped significantly to reduce doses to the thyroid due to radioiodine," according to the study. "There is no doubt that a substantial contributor to the excess incidence of thyroid cancer has been exposure to radioiodine released during the Chernobyl accident."

While the Soviets' initial countermeasures were deemed inadequate, over the next few years the government implemented extensive measures to protect the public. These measures included:

decontaminating settlements

removing substantial amounts of food from human consumption

treating pasture

providing clean fodder to farm animals.

"In part because of the countermeasures taken, the resulting radiation doses were relatively low ... and should not lead to substantial health effects in the general population that could be attributed to radiation exposure from the accident," the study concluded. The average radiation dose in "contaminated areas" was about equivalent to that from a computed tomography (CT) scan, according to the study.

A Safety Comparison With the United States

A 2004 report by the National Academy of Sciences identified two important differences between the conditions that led up to the Chernobyl disaster and the U.S. nuclear energy program.

The first key difference is in how the plants are designed and built. All U.S. power reactors have extensive safety features to prevent large-scale accidents and radioactive releases. The Chernobyl reactor had no such features and was unstable at low power levels.

Second, federal regulations require extensive emergency preparedness planning for all U.S. nuclear energy facilities. NAS cited three factors:

Stringent emergency preparedness plans. Even with the Chernobyl reactor's poor design, officials could have averted many radioactive exposures to the population with an effective emergency response. Key personnel at all U.S. power reactors work with surrounding populations on an ongoing basis to prepare for an orderly and speedy evacuation in the unlikely event of an accident.

Alert and notification. Chernobyl plant operators concealed the accident from authorities and the local population, and thus the government did not even begin limited evacuations until about 36 hours after the accident. In the United States, nuclear power plant operators are required to alert local authorities and make recommendations for protecting the public within 15 minutes of identifying conditions that might lead to a significant release-even if such a release has not occurred. The U.S. Nuclear Regulatory Commission posts resident inspectors at every nuclear power plant site to ensure the plants are following federal safety requirements.

Protecting the food chain. Since authorities did not promptly disclose details of the Chernobyl accident, many people unknowingly consumed contaminated milk and food. This would not be the case in the United States. As it did following the Three Mile Island nuclear accident in 1979, the federal government would carefully monitor and test food and water supplies that potentially could become contaminated. Under existing federal programs and regulations, the government would quarantine and remove from public consumption any unsafe food or water. The accident at Three Mile Island caused the release of a small amount of radioactive material into the atmosphere, but it was too small to cause discernible health effects to the population living near the plant.

For more information, see NEI's fact sheet on emergency preparedness at U.S. nuclear energy facilities.

Additional Resources

Sources and Effects of Ionizing Radiation, UNSCEAR 2008 Report to the General Assembly, Vol. II, Scientific Annex D, "Health effects due to radiation from the Chernobyl accident," United Nations Scientific Committee on the Effects of Atomic Radiation, 2011.

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Chernobyl Accident And Its Consequences - Nuclear Energy Institute

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The 1986 accident at the Chernobyl nuclear power plant in Ukraine, then part of the former Soviet Union, is the only accident in the history of commercial nuclear power to cause fatalities from radiation. It was the product of a severely flawed Soviet-era reactor design combined with human error.

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In the early hours of 26 April 1986, one of four nuclear reactors at the Chernobyl power station exploded.

Moscow was slow to admit what had happened, even after increased radiation was detected in other countries.

The lack of information led to exaggerated claims of the number killed by the blast in the immediate area.

Contamination is still a problem, however, and disputes continue about how many will eventually die as a result of the world's worst nuclear accident.

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In the early hours of 26 April 1986, one of four nuclear reactors at the Chernobyl power station exploded. Moscow was slow to admit what had happened, even after increased radiation was detected in other countries. The lack of information led to exaggerated claims of the number killed by the blast in the immediate area.

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The official report of 'The Fukushima Nuclear Accident Independent Investigation Commission

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The official report of 'The Fukushima Nuclear Accident Independent Investigation Commission

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The official report of Executive summary The Fukushima Nuclear Accident Independent Investigation Commission

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On March 28, 1979, TMI 2 near Harrisburg, Pa., was operating at about 100 percent power when it automatically shut down after a pump that provided cooling water stopped operating. Pressure and temperature increased in the reactor, causing a pressure relief valve to open. The valve opened as designed, and water and steam began flowing out of the reactor to a tank in the basement of the reactor building.

As pressure returned to normal, the valve should have closed. But, unknown to the operators, the valve stuck open. It remained open for more than two hours, allowing water that covered and cooled the fuel core to escape from the reactor system. This caused the fuel to begin to overheat.

However, instrumentation in the TMI control room indicated to the operators that the relief valve was closed and that too much water was being injected into the reactor vessel. Consequently, operators did not replace the water that was lost as a result of the open relief valve.

As pressure continued to drop, more and more coolant turned to steam, causing excessive vibration in the main coolant pumps. The vibration prompted operators, who did not realize the reactor was experiencing a loss of coolant, to shut down the pumps.

The loss of pressure and water caused a large steam bubble to form in the top of the reactor vessel, further preventing the flow of cooling water through the core. Without coolant, core temperatures rose above the melting point of the fuel cladding and the uranium fuel.

About half of the fuel melted before coolant flow was restored. The colder cooling water also shattered some of the hot fuel rods. All of the fuel was damaged.

As a result of the TMI 2 accident, 700,000 gallons of radioactive cooling water ended up in the basement of the reactor building and in tanks in the auxiliary building, contaminating them.

In addition, a small amount of radioactive material was released into the atmosphere from the ventilation stack of an auxiliary building to relieve pressure inside the reactor building.
Health Effects
The TMI 2 accident caused no injuries, and at least a dozen epidemiological studies conducted since 1981 have found no discernible direct health effects to the population in the vicinity of the plant. (For details, see the appendix that follows this fact sheet.)

A federal appeals court in December 2003 dismissed the consolidated cases of 2,000 plaintiffs seeking damages against the plant's former owners for health problems they alleged the accident caused. The court said the plaintiffs had failed to present evidence they had received a radiation dose large enough to cause possible health effects.

Decades of research and scientific studies have shown no negative health effects on the population surrounding the plant.

People who suffered financial losses as a result of the precautionary evacuation following the incident were promptly paid, demonstrating the effectiveness of the industry's liability insurance protection under the Price-Anderson Act. In addition, businesses were compensated for loss of revenue, and the state and local community were compensated for the expenses incurred during the response to the threat.
Current Situation
After cleaning up the damaged TMI 2 reactor, GPU Nuclear placed the plant in monitored storage in December 1993. In December 1999, GPU sold TMI 1 to AmerGen Energy Co., a joint venture of Exelon and British Energy Co. British Energy subsequently sold its interest in TMI 1 to Exelon. In 2008, AmerGen Energy Co. was integrated into Exelon Generation, and the AmerGen legal entity was dissolved.

Under the terms of the sale, GPU retained ownership of TMI 2. GPU subsequently merged with FirstEnergy, making First Energy financially responsible for the decommissioning of TMI 2. In-plant and off-site monitoring of TMI 2 will continue until it is fully decommissioned, with regular reports made to the U.S. Nuclear Regulatory Commission, the commonwealth of Pennsylvania and the public.

The two reactors will be decommissioned jointly when TMI 1 is taken out of service.
Actions to Improve Safety
Two weeks after the 1979 accident, President Jimmy Carter appointed a 12-member commission, headed by the late John Kemeny, then president of Dartmouth College, to investigate what had happened and the possible impact on the health and safety of the public and plant personnel.

The commission's report on its investigation, issued in October 1979, recommended that the industry develop its own standards of excellence. The commission also cited a need for agency-accredited training institutions for nuclear plant operators and immediate supervisors of operations.

The NRC also moved quickly, setting up a group to study the accident. The group, headed by attorney Mitchell Rogovin, reached many of the same conclusions as the Kemeny Commission.

Within nine months of the accident, the industry had formed INPO, whose mission is to promote the highest levels of safety and reliability in the operation of nuclear power plants.

To improve training, INPO in 1985 formed the National Academy for Nuclear Training. The academy reviews and accredits nuclear utilities' training programs for all key positions at each plant.

INPO has had a profound impact on the way nuclear plants are managed and operated. The proof is the steady improvement in plant performance in the nearly 30 years since the accident at TMI.

Today, the country's 104 operating reactors maintain high levels of safety and reliability, as evidenced by the NRC's reactor oversight program and performance indicators tracked by the World Association of Nuclear Operators.
Source:
Nuclear Energy Institute, http://www.nei.org/Master-Document-Folder/Backgrounders/Fact-Sheets/The-Tmi-2-Accident-Its-Impact-Its-Lessons.

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On March 28, 1979, TMI 2 near Harrisburg, Pa., was operating at about 100 percent power when it automatically shut down after a pump that provided cooling water stopped operating. Pressure and temperature increased in the reactor, causing a pressure relief valve to open.