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Don't feel bad if you think America's worst nuclear disaster was Three Mile Island. Everybody does. But a nuclear accident up to a thousand times worse than Three Mile Island occurred 20 years earlier, just over the sandstone formations of the Simi Hills northwest of Los Angeles. On top of a hill was the most secretive Area IV of the Santa Susana Field Laboratory, a lab that meant to prove nuclear power was not just for bombs and missiles, but that it could be used to provide electric power to America's cities-as in "atoms for peace."

On Monday July 13, 1959, an experimental nuclear reactor in Area IV was showing skyrocketing temperatures that would indicate a runaway reaction. Workers plunged control rods into the core to stop the reaction. It didn't work, so they shut it down manually. They released radioactive gases into the air to relieve pressure and prevent an explosion. Inexplicably, they turned the reactor back on. They had no idea a partial meltdown was occurring.

The radioactive gases they released could have been as much as 13,000 curies of iodine-131 and 2600 curies of cesium-137. By contrast, Three Mile Island released 17 curies of iodine-131 and no cesium.

The reactor was off and on for the next two weeks, then repaired in 1960. It continued to operate until it was shut down permanently in 1964.

Why Haven't We Heard?

Why Santa Susana received scant attention, as well as why it is not an icon of nuclear power like the mushroom cloud and the cooling towers of Three Mile Island, remains a burning question. One side answers with "It's no big deal." Santa Susana owners, currently Boeing and NASA, and federal agencies first stated there was no radiation released. They later said there was but downplayed the severity of the incident. On the opposite side, you hear "coverup" from scientists who conducted a five-year study; the state of California, which is trying clean up the mess; a local investigative news team; many living near or working in the facility; and anti-nuke activists, all alleging injury, death and disease from the radioactivity and toxic chemicals used on the site.

Could Be Worse - We Could Have a Fire

The Woolsey Fire in 2018 started on Santa Susana Field Laboratory and raged across almost a 100,000 acres in 2018, making it one of the most destructive fires in Southern California. It was not the first fire at the site. Another fire had swept through the Lab in 2005. Kim Kardashian tweeted that she was "shocked and furious" that a fire from a nuclear site would threaten her home in nearby Hidden Hills. By doing so, she may have thrust a simmering local feud between the site's owner, Boeing, and neighboring communities into the national spotlight.

A 2012 EPA report found that one out of every seven samples taken at Santa Susana Field Lab contained "concentrations of radioactive materials exceeding background levels." A 1989 Department of Energy report found contaminants in both the soil and plants, which are likely to get into the air by smoke, ash and embers of a fire.

But there was no radiation to worry about in the Woolsey Fire, according to California's Department of Toxic Substances, which said radiation levels were no higher than background levels in the fire damaged areas. Their findings were echoed by the LA County of Public Health. "We beg to differ," said a group of concerned scientists from Fairewinds Energy Education, who had made their own measurements and found more than 10 percent of samples to have elevated levels of radioactive materials, including radium-227, uranium and thorium.

Atoms for Peace

America in the late 1940s was a superpower that leveled the Japanese cities of Hiroshima and Nagasaki with nuclear bombs, the results of top-secret Manhattan Project. Instead of being mothballed, nuclear power gained new life from the Cold War that came next as both U.S, and the Soviet Union built formidable nuclear arsenals. While the militaries were considering "mutual assured destruction" a deterrent, the public was becoming increasingly anxious about living under the threat of a nuclear Armageddon. The U.S. government needed to turn the public's attention to peaceful application of nuclear power so they wouldn't close down nuclear power altogether. In 1953, President Eisenhower delivered an " Atoms of Peace " promise in a speech to the United Nations.

The task to fulfill the "Atoms for Peace" promise fell to The Atomic Energy Commission (AEC), which began a search for a place it could build and test nuclear reactors in California. The AEC enlisted North American Aviation (NAA), which later became part of Rockwell, to evaluate the sites. One of the sites NAA selected was one it already owned, the Santa Susana Field Labs, which had been testing liquid rocket fuels since 1947. The Apollo 11 fuel and rockets that landed men on the moon were a result of the Santa Susana Field Laboratory. Although worries about the air currents, water drainage and proximity to a growing population were expressed, the site was selected anyway because it was easy driving distance for scientists from UCLA and other universities. So, 209 acres at the top of a hill, right up the edge of the northern boundary of the Santa Susana Field Laboratory on the site NAA already owned and was using for noisy, dangerous work, would be designated Area IV for nuclear reactor experiments.

Ten nuclear reactors found a home in Area IV. One of them was the Sodium Reactor Experiment (SRE), an experiment in cooling the core with molten sodium. It was to provide electricity for the nearby city of Moorpark, population 3,000, proving that atoms could really be used for peace.

Why Sodium?

Nuclear reactors normally are cooled by water. Modern nuclear reactors are enclosed in thick concrete domes to contain radioactivity in the event of a runaway reaction. The SRE had no concrete containment and was using molten sodium, not water, to cool the reactor cores.

Don't confuse sodium with table salt, sodium chloride. Elemental sodium is nasty. It burns in contact with air and explodes in contact with water. Why use it to cool a nuclear reactor? Sodium allows fission to continue without depleting the uranium, unlike water cooled reactors. The design of the SRE still used water to cool the pumps. Afraid of water leaking into the sodium, engineers used tetralin, an inorganic coolant to cool the pumps. That was a big mistake. Leaks in the pump seals introduced tetralin into the sodium. While it didn't explode in contact with the sodium like water would have, being bombarded by radioactive uranium turned the tetralin into a tarry substance that clogged the cooling channels. The SRE began overheating.

Over the next two weeks, 14 out of 43 fuel rods in the SRE were damaged. They were replaced by frantic -and exposed-workers. The SRE was turned back on. It stayed online, operating in fits and starts for several years until it was finally decommissioned in 1964.

Burn Pits

Sodium, ever so reactive, was disposed of in burn pits. A sodium contaminated part would be put in a pool of water where it would burn. Radioactive parts being burned in this way would have carried radiation into the air.

A million gallons of trichloroethylene (TCE), a toxic solvent, were used to wash off the rocket test stands. The TCE soaked up in the ground and found its way into the creeks, according to several sources.

Also within the Santa Susana Field Laboratory was the nation's biggest "hot lab," where radioactive substances were cut up and distributed to other U.S. nuclear sites. The hot lab was the scene of many spills, accidents and fires over the years, according to workers and internal documents.

The lab was also where plutonium was present. There is no record of a single plutonium accident, something Daniel Hirsch, retired director of the Program on Environmental and Nuclear Policy at the University of California, Santa Cruz finds very suspicious.

Hirsh deserves much of the credit for uncovering the Santa Susana Field Laboratory disaster, and he has been tireless in explaining it since. He founded and is president of The Committee to Bridge the Gap, a nuclear policy NGO, whose 29-page report on the Santa Susana Field Laboratory serves as a major source of this article.

Move Along. Nothing to See Here.

On Aug. 29, 1959, six weeks after the SRE incident, the Atomic Energy Commission, the U.S. government agency in charge at Santa Susana Field Laboratory, was quick to clear the air and assure the public of safety, issuing a press release saying it had inspected the SRE, evaluated the situation and "no release of radioactive materials to the plant or its environs occurred and operating personnel were not exposed to harmful conditions." The public did not find out until much later (20 years later!) that the AEC had known otherwise. Internal memos were circulating at Santa Susana Field Laboratory with full knowledge that something went horribly wrong. An internal memo issued by R.K. Owen, Health Physics Department at the lab to R.E. Durand of Atomics International tells of an air sample that measured radiation of 300 times the maximum permissible level.

20 Years Later

In 1979, UCLA students of Professor Hirsch at UCLA became very curious as to why the nuclear power initiative at Santa Susana that had promised so much had closed so quietly. Their search for answers led them to documents and photos of Santa Susana's damaged fuel rods buried in a dusty library annex. There was material donated by UCLA's own Dean of Engineering, Chauncey Starr. A staunch supporter of atomic energy, Starr had worked with Dr. J. Robert Oppenheimer on the Manhattan Project and then joined Rockwell as vice president after the war. He was president of Atomics International for 20 years.

That is when it was revealed that a third of the 47 fuel rods in the SRE were damaged and a meltdown had started. Copious amounts of radioactivity had been released, and not just by the SRE but also at other reactors on the site. More reactors suffered damage to their fuel rods for 10 years after the SRE meltdown.

The findings of these intrepid students were published in a pamphlet by Another Mother of Peace, an anti-war group. It failed to get any attention.

Hirsh founded the Committee to Bridge the Gap and renewed his efforts to bring the secret nuclear meltdown to light. His next report was picked up by the local NBC affiliate, which aired a week of exposés about the disaster that became part of their comprehensive and in-depth America's Nuclear Secret report.

How Safe Were the Neighbors?

The heart-rending personal accounts of those who were victimized by Santa Susana Field Laboratory are included in the NBC investigation.

Arline Matthew's son, Bobby, a high school cross country runner who ran in the hills around Santa Susana, died of a glioblastoma, a rare form of brain cancer. Matthew's grandson has been diagnosed with leukemia.

Three sisters who played in and drank from a creek that ran from the lab have all battled cancer, despite no family history of cancer. One is on her third case of cancer, a lymphoma.

Bonnie Klea, who worked in the Lab and whose job had her visiting all parts of the lab, was diagnosed with bladder cancer. She says 14 of the 15 houses on her block have someone with cancer.

Ralph Powell, who worked at the lab and was exposed to the burn pits, thinks he may have carried enough radioactive material home on his clothes to kill his son with leukemia at the age of 11.

Krista Slack grew up in Simi Valley and had no family history of cancer, is not Jewish or African-American, but is afflicted with "triple negative" breast cancer, which usually affects Jewish and African-American women.

In addition to the above, there is Lisa Cass, who reached out to engineering.com to tell her story. A one-time real estate agent, Cass is perplexed that her family received no formal disclosure, as is required by California law, of a toxic/radioactive site so close to the Simi Valley home they bought when she was 4. She remembers watching the light and hearing the sound of the "Santa Soo" burn pits, as entertaining as a fireworks display, with her three sisters. Her family has been riddled with cancers, including double mastectomies, despite having no family history of cancer and no genetic markers indicating a predisposition to cancer.

In 1990, the State of California Department of Health Services, or CDHS, published the Cancer Incidence Rates in Five Los Angeles County Census Tracts within 5 miles of the Santa Susana Field Lab, revealing higher rates of bladder cancer, leukemia and Hodgkin's lymphoma, all diseases associated with radiation. The bladder cancer rates were highest in the tracts closest to the lab. It was an incriminating report as any could be. But its conclusions were soon attacked by a series of other reports, one of them, strangely enough, by the CDHS.

Two years after its first report, the CDHS issued an 11-year study of the cancers in the same area, calling its previous report "preliminary" and claimed that because the rates of bladder cancer were higher in men than in women, the cause must have been something besides radiation.

California Environmental Protection Agency (Cal/EPA) studied the incriminating CDHS reports and concluded that the "extremely modest cancer incidence increases associated with known radiosensitive tumors could be easily explained by "uncontrolled confounding or impression in the data" and "the results do not support the presence of any major environmental hazard."

In 1997, UCLA published an exhaustive study of death certificates of the 4,607 people who worked at Santa Susana between 1950 and 1993. Of the 4,607 people enrolled in Rockedyne's Health Physics Radiation Monitoring Program, they found that those who received higher doses of radiation had an increased risk of dying from leukemia, lymphoma and lung cancer. The threat of all cancers rose as did the exposure to radiation. Some workers were monitored for internal radiation. Those with higher doses died from the same cancers plus cancers of the mouth, throat, esophagus and stomach.

Boeing, current owner of the site, refuted the UCLA study above, finding a thousand workers who were not in Rockedyne's radiation monitoring program and occasionally monitored, and reported no detectable increase in cancer deaths.

The Tri-Counties Regional Cancer Registry reported on the cancer incidence in the Simi Valley between 1988-1995 for 19 census tracts and found residents to have cancer risks similar to overall rates in the region except for lung cancers.

Then came Hirsch, the UCLA and UC Santa Cruz lecturer who may have known more about California's reactor program than anyone who was not working on it, declared Santa Susana Field Laboratory to be one of the more dangerous radioactive sites in the U.S., and founded the activist group Committee to Bridge the Gap from which he draws no salary, which published the 29-page report laying out the history of the lab and its many transgressions, radioactive and chemical.

Dr. Hal Morgenstern, former UCLA researcher and author of the 2007 federally funded study, " Cancer Incidence in the Community Surrounding the Rocketdyne Facility in Southern California," was unable to find an association for cancers from radioactivity among adults but found that nine specific cancers caused by chemical exposure for persons living within 2 miles of SSFL were as much as 60 percent higher for cancers of blood and lymph tissue, bladder, thyroid and upper aero-digestive tract.

The Curious Case of the Jewish Camp

In 1941, Dr. Shlomo Bardin founded the Brandeis Camp Institute and, in 1947, bought 2,200 acres of land in the foothills of the Santa Susanna mountains. The hills reminded him of the Judean Hills outside Jerusalem. It became the Brandeis-Bardin Institute (BBI) and is thought to be the largest Jewish-owned land holding outside of Israel. Today, BBI is part of the American Jewish University.

1947 was the same year 2,800 acres to the south of BBI were chosen for Santa Susana Field Laboratory.

Santa Susana Field Laboratory calculated a danger zone around nuclear reactors to by this formula:

Where K is the power in kilowatts and R is radius in miles.

It was presumed that the reactors would be no bigger than a 1,000 kilowatts, making the danger zone 0.32 miles, so they planned for a third of a mile on all sides of reactors to be fenced and guarded and away from the nearest population concentration a mile away. But the SRE was 20,000 kilowatts, making a danger zone of a 1.4 mile radius.

Using Google Maps, we calculated the least distance between areas of activity between the sites as 1.07 miles (see picture).

The proximity to Santa Susana Field Laboratory was too close for BBI. In 1991, an internal Rockwell International memo stamped "Confidential" showed radiation and chemical contamination in BBI wells. An EPA-reviewed study found radioactivity in soil samples in 1993. Although the EPA said at the time the radioactivity levels were safe, the National Academies of Sciences later stated that there are no safe levels of radiation. The EPA tested and found radiation and chemical contamination in the water in the wells, fruit on its trees and milk in the cows. By then, 30,000 children had sleepovers at BBI's Camp Alonin.

BBI sued Rocketdyne and received $3.2 million in a hush-hush, out-of-court settlement in 1997, along with $200,000 for a strip of land closest to SSFL. They promised never to bother Boeing, the next owner of SSFL, with any further damage claims.

BBI and the American Jewish University (AJU), which BBI is now part of, now insists its grounds are perfectly safe for workers and visitors.

Let's Compare the China Syndrome and Three Mile Island

The nuclear disasters that have received the most attention have been, in reverse chronological order, Fukushima Daiichi nuclear plant in Japan (2011), Chernobyl in Ukraine (1986) and Three Mile Island in Pennsylvania (1979).

Three Mile Island, a partial meltdown of one of the reactors, was a result of a valve stuck open, human errors and confusing instruments, leading to a shutoff of an automatic cooling system that would have avoided the disaster The plant came within one hour and 1,000 degrees of a complete meltdown-the worst-case scenario, the dreaded China syndrome.

The China Syndrome movie is fiction too close to fact. In the movie, a physicist describes a full meltdown as something that would flow through the Earth's core "all the way to China," giving the movie its name. When he is asked how bad a nuclear meltdown could be, he says it would "render a state the size of Pennsylvania uninhabitable." Creepy he would say that because A)the movie is based on a nuclear incident in Illinois and set in California, and B) the movie was released two weeks before the Three Mile Island incident in Pennsylvania.

A nuclear fireball would never do that, of course. Even with the full meltdown at Chernobyl, the worst nuclear disaster ever, the fireball cooled while still in the Earth's crust.

While the damage to the TMI core was significant, the partial meltdown was eventually contained. Some radioactive gas was released into the air but not enough to matter, according to the History Channel.

A Little Clean Up Wouldn't Kill You

In 2007, California, under Governor Arnold Schwarzenegger, signed SB990, a bill that would have all parties of Santa Susana Field Laboratory clean up the site to a level considered safe for residential buildings or agricultural use, whichever was more stringent.

Boeing, by then the owner of most of Santa Susana Field Laboratory, fought the bill, arguing that the State of California was picking on it while ignoring every other hazardous waste site in the state. NASA and the Department of Energy agreed to clean up parts they felt responsible for. Boeing acquiesced to restore the site to a level considered safe as "open space," full of wildlife and respectful of the Native Americans, where hikers and mountain bikers would visit.

Hirsch cared little for the Boeing plan, saying that the aerospace giant was basically shirking a big cleanup bill. Bringing the radioactivity down to a level safe for residences was 10 times more expensive than bringing it to a level for open space, he said. It also leaves 90 percent of the radioactive material in the ground.

California's most recent hope was U.S. Energy Secretary Rick Perry, who promised a less secretive Department of Energy and visited the Santa Susana Field Laboratory in September 2019. Unfortunately, Perry resigned a month later.

Hike, Anyone?

Urban expansion continues to encroach to the edges of the Santa Susana Field Laboratory site. New developments are springing up in Los Angeles' outlying areas all around the site as a result of California's housing shortage. KB Homes has built over 400 homes in their Arroyo Vista at the Woodlands development in Simi Valley, just 1.5 miles northwest of Area IV, the site of the failed nuclear reactor. Two story homes are going for $800,000. KB Homes introduces home buyers to Simi Valley as a place for "outdoor recreation, including hiking, biking and equestrian trails" and that the development is "bordered by the Santa Susana Mountains" but fails to mention the Santa Susana Field Laboratory on its website. Buyers are given a disclosure form that tells them to "conduct their own review," according to NBC News in their Living in the Shadow of a Toxic Mess report in 2016.

We found no awareness of the site's danger with new and longtime residents. A real estate agent contacted for this article felt no remorse about not disclosing the radioactivity in the deserted Santa Susana Field Laboratory, saying that there were "conflicting reports."

The present owner of Santa Susana Field Laboratory, Boeing, continues to downplay the significance of the disaster, the contamination of the site and urgency of the cleanup. Boeing is currently claiming the site is safe for use for public use as well as a sanctuary for wildlife, like mountain lions. The company recently organized an Earth Day nature walk to show the public how safe it is-perhaps thinking mountain lions are safer than toxic chemicals and radioactivity.

References

Report of the Santa Susana Field Laboratory Advisory Panel, Daniel Hirsch, et. Al, October 2006

LA's Nuclear Secret, Joel Grover and Matthew Glasser, NBC4 Investigative Team

The History Channel, which devoted one segment of Engineering Disasters to Santa Susana Field Laboratory disaster.

What Happened At The Santa Susana Nuclear Site During The Woolsey Fire?, Elina Shatkin, LAist.com, November 13, 2018

Mad Science: The Nuclear Power Experiment, Joseph Mangano, 2012

1. From REPORT OF THE SANTA SUSANA FIELD LABORATORY ADVISORY PANEL, October 2006, where David Lochbaum, a nuclear safety engineer, studied the pathways of possible release of radioactivity at the damaged reactor and concluded that between 15 percent to 30 percent, or a maximum of 13,000 curies of iodine-131 and 2600 curies of cesium-137, were released. By contrast, Three Mile Island released 17 curies of iodine-131 and no cesium. The higher bound of the SSFL release is 918 times that of Three Mile Island.

2. With No Cleanup Plan in Place, Santa Susana Field Lab Still Stokes Contamination Fears, Robert Kerbeck, Los Angeles Magazine, December 4, 2019

3. North American Aviation was a division of Rocketdyne. Rocketdyne merged with North American Rockwell, which became Rockwell International. In 1996, Boeing acquired the aerospace division of Rockwell International, which included all of Santa Susana Field Laboratory except for a small area owned by NASA.

4. Nuclear Disaster at Three Mile Island, The History Channel

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America's Worst Nuclear Disaster Was in California. Who Knew?

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Don't feel bad if you think America's worst nuclear disaster was Three Mile Island. Everybody does. But a nuclear accident up to a thousand times worse than Three Mile Island occurred 20 years earlier, just over the sandstone formations of the Simi Hills northwest of Los Angeles.

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San Onofre Nuclear Power Plant, Old Pacific Highway, Pendleton, CA, USA

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San Onofre Nuclear Power Plant, Old Pacific Highway, Pendleton, CA, USA

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San Onofre Nuclear Power Plant, Old Pacific Highway, Pendleton, CA, USA

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Windscale Works, Sellafield, Seascale, UK

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Windscale Works, Sellafield, Seascale, UK

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Windscale Works, Sellafield, Seascale, UK

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This article was most recently revised and updated by

Windscale fire, accident in 1957 at the Windscale nuclear reactor facility and plutonium-production plant in the county of Cumberland (now part of Cumbria), in northwestern England, that was the United Kingdom's most serious nuclear power accident. The Windscale plant consisted of two gas-cooled nuclear reactors. The accident occurred on October 8, 1957, when a routine heating of the No. 1 reactor's graphite control blocks got out of control, causing adjacent uranium cartridges to rupture. The uranium thus released began to oxidize, releasing radioactivity and causing a fire that burned for 16 hours before it was put out. The fire left about 10 tons of radioactive fuel melted in the reactor core. The Windscale fire also caused the release of sizable amounts of radioactive iodine into the atmosphere. As a consequence, the government banned for several weeks the sale of milk produced in a 500-square-km (200-square-mile) area around the reactor site. At the time, the British government released only sketchy details of the accident and in general tried to minimize its seriousness. The contaminated Windscale reactor was subsequently sealed until the late 1980s, when a cleanup of it was begun (cleanup expected to be completed in 2015).

André Munro.

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Windscale fire | Nuclear Disaster, Cumbrian Village

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Windscale fire, accident in 1957 at the Windscale nuclear reactor facility and plutonium-production plant in the county of Cumberland (now part of Cumbria), in northwestern England, that was the United Kingdom's most serious nuclear power accident. The Windscale plant consisted of two gas-cooled

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Fukushima accident, accident in 2011 at the Fukushima Daiichi ("Number One") plant in northern Japan, the second worst nuclear accident (after the Chernobyl disaster of 1986) in the history of nuclear power generation. The site is on Japan's Pacific coast, in northeastern Fukushima prefecture about 100 km (60 miles) south of Sendai. The facility, operated by the Tokyo Electric and Power Company (TEPCO), was made up of six boiling-water reactors constructed between 1971 and 1979. At the time of the accident, only reactors 1-3 were operational, and reactor 4 served as temporary storage for spent fuel rods.

TEPCO officials reported that tsunami waves generated by the main shock of the Japan earthquake on March 11, 2011, damaged the backup generators at the Fukushima Daiichi plant. Although all three of the reactors that were operating were successfully shut down, the loss of power caused cooling systems to fail in each of them within the first few days of the disaster. Rising residual heat within each reactor's core caused the fuel rods in reactors 1, 2, and 3 to overheat and partially melt down, leading at times to the release of radiation. Melted material fell to the bottom of the containment vessels in reactors 1 and 2 and bored sizable holes in the floor of each vessel-a fact that emerged in late May. Those holes partially exposed the nuclear material in the cores. Explosions resulting from the buildup of pressurized hydrogen gas occurred in the outer containment buildings enclosing reactors 1 and 3 on March 12 and March 14, respectively. Workers sought to cool and stabilize the three cores by pumping seawater and boric acid into them. Because of concerns over possible radiation exposure, government officials established a 30-km (18-mile) no-fly zone around the facility, and a land area of 20-km (12.5-mile) radius around the plant-which covered nearly 600 square km (approximately 232 square miles)-was evacuated.

A third explosion occurred on March 15 in the building surrounding reactor 2. At that time the explosion was thought to have damaged the containment vessel housing the fuel rods. (In actuality, the explosion punched a second hole in the containment vessel; the first hole had been created earlier by melted nuclear material that passed through the bottom of the vessel.) In response, government officials designated a wider zone, extending to a radius of 30 km around the plant, within which residents were asked to remain indoors. The explosion, along with a fire touched off by rising temperatures in spent fuel rods stored in reactor 4, led to the release of higher levels of radiation from the plant.

In the days that followed, some 47,000 residents left their homes, many people in areas adjacent to the 20-km evacuation warning zone also prepared to leave, and workers at the plant made several attempts to cool the reactors using truck-mounted water cannons and water dropped from helicopters. Those efforts met with some success, which temporarily slowed the release of radiation; however, they were suspended several times after rising steam or smoke signaled an increased risk of radiation exposure.

As workers continued their attempts to cool the reactors, the appearance of increased levels of radiation in some local food and water supplies prompted Japanese and international officials to issue warnings about their consumption. At the end of March, the evacuation zone was expanded to 30 km around the plant, and ocean water near the plant was discovered to have been contaminated with high levels of iodine-131, which resulted from leakage of radioactive water through cracks in trenches and tunnels between the plant and the ocean. On April 6 plant officials announced that those cracks had been sealed, and later that month workers began to pump the irradiated water to an on-site storage building until it could be properly treated.

On April 12 nuclear regulators elevated the severity level of the nuclear emergency from 5 to 7-the highest level on the scale created by the International Atomic Energy Agency-placing it in the same category as the Chernobyl accident, which had occurred in the Soviet Union in 1986. It was not until the middle of December 2011 that Japanese Prime Minister Noda Yoshihiko declared the facility stable, after the cold shutdown of the reactors was completed.

As the fallout pattern became better understood, an additional corridor of land covering roughly 207 square km (80 square miles) and stretching away from the initial 20-km zone was also designated for evacuation in the months following the disaster. Months later, radiation levels remained high in the evacuation zone, and government officials remarked that the area might be uninhabitable for decades. However, they also announced that radiation levels had declined enough in some towns located just beyond the original 20-km evacuation warning zone that residents could return to their homes there. Although many areas located within the 20-km evacuation warning zone and the expanded zone (an area called the "difficult-to-return" zone) continued to remain off-limits due to high radiation levels, officials began to allow limited activities (business activities and visitation but no lodging) in other previously evacuated areas with moderately high radiation levels. Beginning in July 2013, evacuation orders were lifted in some areas characterized by lower levels of radiation both within and beyond the 20-km evacuation warning zone. By March 2017 all evacuation orders in the areas outside the difficult-to-return zone (which continued to sequester some 371 square km [about 143 square miles]) had been lifted. A 2016 study on the effects of the accident on fish and marine products showed that the contamination level had decreased dramatically over time, though the radioactivity of some species, especially sedentary rockfish, remained elevated within the exclusion zone.

A second, but smaller, nuclear accident took place in August 2013 when approximately 300 tonnes (330 tons) of irradiated water used in ongoing cooling operations in reactors 1, 2, and 3 was discharged into the landscape surrounding the Fukushima Daiichi facility. TEPCO officials reported that the leak was the result of an open valve in the short barrier wall that surrounded several of the tanks used in radioactive water storage. The leak was severe enough to prompt Japan's Nuclear Regulation Authority to classify it as a level-3 nuclear incident.

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Fukushima accident | Summary, Date, Effects, & Facts

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Fukushima accident, disaster that occurred in 2011 at the Fukushima Daiichi ('Number One') nuclear power plant on the Pacific coast of northern Japan, which was caused by a severe earthquake and powerful series of tsunami waves and was the second worst nuclear power accident in history.

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Exelon Three Mile Island Nuclear Generating Station, Pennsylvania 441, Middletown, PA, USA

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Exelon Three Mile Island Nuclear Generating Station, Pennsylvania 441, Middletown, PA, USA

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Exelon Three Mile Island Nuclear Generating Station, Pennsylvania 441, Middletown, PA, USA

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Table of Contents

This article was most recently revised and updated by

Three Mile Island accident, accident in 1979 at the Three Mile Island nuclear power station that was the most serious in the history of the American nuclear power industry. The Three Mile Island power station was named after the island on which it was situated in the Susquehanna River near Harrisburg, Pa. At 4:00 am on March 28, an automatically operated valve in the Unit 2 reactor mistakenly closed, shutting off the water supply to the main feedwater system (the system that transfers heat from the water actually circulating in the reactor core). This caused the reactor core to shut down automatically, but a series of equipment and instrument malfunctions, human errors in operating procedures, and mistaken decisions in the ensuing hours led to a serious loss of water coolant from the reactor core. As a result, the core was partially exposed, and the zirconium cladding of its fuel reacted with the surrounding superheated steam to form a large accumulation of hydrogen gas, some of which escaped from the core into the containment vessel of the reactor building. Very little of this and other radioactive gases actually escaped into the atmosphere, and they did not constitute a threat to the health of the surrounding population. In the following days adequate coolant water circulation in the core was restored.

The accident at Three Mile Island, though minuscule in its health consequences, had widespread and profound effects on the American nuclear power industry. It resulted in the immediate (though temporary) closing of seven operating reactors like those at Three Mile Island. A moratorium on the licensing of all new reactors was also temporarily imposed, and the whole process of approval for new plants by the Nuclear Regulatory Commission was significantly slowed for years after the accident. No new reactors were ordered by utility companies in the United States from 1979 through the mid-1980s. The accident increased public fears about the safety of nuclear reactors and strengthened public opposition to the construction of new plants. The unharmed Unit 1 reactor at Three Mile Island did not resume operation until 1985. The cleanup of Unit 2 continued until 1990; damage to the unit was so severe, however (52 percent of the core melted down), that it remained unusable.

Erik Gregersen.

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Three Mile Island accident | 1979, Pennsylvania, US

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Three Mile Island accident, accident in 1979 at the Three Mile Island nuclear power station that was the most serious in the history of the American nuclear power industry. The Three Mile Island power station was named after the island on which it was situated in the Susquehanna River near

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Table of Contents

When did the Chernobyl disaster occur?

What happened in the Chernobyl disaster?

How many people died as a result of the Chernobyl disaster?

How big was the exclusion zone created after the Chernobyl disaster?

What effects did the Chernobyl disaster have?

This article was most recently revised and updated by

Chernobyl disaster, accident in 1986 at the Chernobyl nuclear power station in the Soviet Union, the worst disaster in the history of nuclear power generation. The Chernobyl power station was situated at the settlement of Pryp'yat, 10 miles (16 km) northwest of the city of Chernobyl (Ukrainian: Chornobyl) and 65 miles (104 km) north of Kyiv, Ukraine. The station consisted of four reactors, each capable of producing 1,000 megawatts of electric power; it had come online in 1977-83.

The explosion at Unit 4 and initial containment efforts

The disaster occurred on April 25-26, 1986, when technicians at reactor Unit 4 attempted a poorly designed experiment. Workers shut down the reactor's power-regulating system and its emergency safety systems, and they withdrew most of the control rods from its core while allowing the reactor to continue running at 7 percent power. These mistakes were compounded by others, and at 1:23 am on April 26 the chain reaction in the core went out of control. Several explosions triggered a large fireball and blew off the heavy steel and concrete lid of the reactor. This and the ensuing fire in the graphite reactor core released large amounts of radioactive material into the atmosphere, where it was carried great distances by air currents. A partial meltdown of the core also occurred.

Disasters of Historic Proportion

On April 27 the 30,000 inhabitants of Pryp'yat began to be evacuated. A cover-up was attempted, but on April 28 Swedish monitoring stations reported abnormally high levels of wind-transported radioactivity and pressed for an explanation. The Soviet government admitted there had been an accident at Chernobyl, thus setting off an international outcry over the dangers posed by the radioactive emissions. By May 4 both the heat and the radioactivity leaking from the reactor core were being contained, albeit at great risk to workers. Radioactive debris was buried at some 800 temporary sites, and later in the year the highly radioactive reactor core was enclosed in a concrete-and-steel sarcophagus (which was later deemed structurally unsound).

Deaths, radioactivity, and the creation of the Chernobyl Exclusion Zone

Some sources state that two people were killed in the initial explosions, whereas others report that the figure was closer to 50. Dozens more people contracted serious radiation sickness; some of them later died. Between 50 and 185 million curies of radionuclides (radioactive forms of chemical elements) escaped into the atmosphere-several times more radioactivity than that created by the atomic bombs dropped on Hiroshima and Nagasaki, Japan. This radioactivity was spread by the wind over Belarus, Russia, and Ukraine and soon reached as far west as France and Italy. Millions of acres of forest and farmland were contaminated, and, although many thousands of people were evacuated, hundreds of thousands more remained in contaminated areas. In addition, in subsequent years many livestock were born deformed, and among humans several thousand radiation-induced illnesses and cancer deaths were expected in the long term. The Chernobyl disaster sparked criticism of unsafe procedures and design flaws in Soviet reactors, and it heightened resistance to the building of more such plants. Chernobyl Unit 2 was shut down after a 1991 fire, and Unit 1 remained on-line until 1996. Chernobyl Unit 3 continued to operate until 2000, when the nuclear power station was officially decommissioned.

Following the disaster, the Soviet Union created a circle-shaped exclusion zone with a radius of about 18.6 miles (30 km) centred on the nuclear power plant. The exclusion zone covered an area of about 1,017 square miles (2,634 square km) around the plant. However, it was later expanded to 1,600 square miles (4,143 square km) to include heavily radiated areas outside the initial zone. Although no people actually live in the exclusion zone, scientists, scavengers, and others may file for permits that allow them to enter for limited amounts of time. With the dissolution of the Soviet Union in 1991, control of the site passed to Ukraine. In 2011 the Ukrainian government opened parts of the exclusion zone to organized tour groups, and Chernobyl and the abandoned city of Pryp'yat became popular destinations for so-called "dark tourists." During the Russian invasion of Ukraine in 2022, Russian forces attacking from Belarus captured Chernobyl after a brief but pitched battle. Combat at the site of the world's worst nuclear disaster led to concerns about damage to the containment structure and the possibility of widespread radioactive contamination.

The Editors of Encyclopaedia Britannica Michael Ray.

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Chernobyl disaster | Causes, Effects, Deaths, Videos, Location, & Facts

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Chernobyl disaster, accident at the Chernobyl nuclear power station in the Soviet Union in 1986, the worst disaster in nuclear power generation history. Between 2 and 50 people were killed in the initial explosions, and dozens more contracted serious radiation sickness, some of whom later died.