Tokaimura nuclear accident

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There have been two Tokaimura nuclear accidents at the nuclear facility at Tōkai, Ibaraki, Japan: on 11 March 1997, an explosion occurred in a Dōnen plant, and on 30 September 1999, a serious criticality accident happened in a JCO plant.

The Tokaimura nuclear plant[edit]

Tokaimura Nuclear Plant

The Tōkai Nuclear Power Plant located in Tokai, Ibaraki Prefecture, Japan was founded in 1966. It served as a fuel conversion company and producer of nuclear reactor fuel rods until 1999. The second accident[clarification needed] occurred on 30 September 1999 in a JCO plant and was classified a serious criticality accident.[1] Both incidents were due to a lack of safety culture, improper training of technicians and procedural handling errors. This facility shut down after the last incident in 1999. There are fourteen other nuclear facilities in Tokaimura employing approximately one-third of the population.[2]

Nuclear power in Tokaimura[edit]

Nuclear power has become an important energy alternative for natural-resource-poor Japan to limit dependence on imported energy. Tokaimura's location and available land space made it an ideal investment for nuclear power production. This facility was established to provide Tokaimura's electricity and nuclear fuel conversion needs. Nuclear power provided approximately 30% of Japan’s electricity[3] up until the Fukushima nuclear disaster of 2011, after which nuclear electricity production fell into sharp decline.[4]

Foundation of Tokaimura nuclear plant[edit]

The Tōkai Nuclear Power Plant was commissioned in 1966, decommissioned later and reestablished in the 1970s. It is located in the village Tokaimura approximately seventy miles from Tokyo. It served as Japan's first commercial nuclear power plant.[5] Nearly one-third of Tokaimura’s population rely upon nuclear industry related employment.[6] Originally, the Tokaimura plant was established as a small-scale fuel preparation plant and not associated with electricity production or manufacturing operation routines.

The production process for nuclear power at Tokaimura[edit]

The Power Reactor and Nuclear Fuel Development Corporation (Dōnen) plant served as a bitumen solidification facility, disposing of low-level radioactive waste through combining it with asphalt in drums.[7] The second facility owned by JCO converted enriched uranium hexafluoride into uranium dioxide fuel. This served as the first step in producing nuclear reactor fuel rods for neighboring power plants.[2] Handling nuclear products imposes extreme risks to technicians; it requires precision and seasoned employees to safely conduct procedures. The process of combining nuclear products involves a fission process with potential to produce radiation and explosive energy.[8] In order to produce the necessary uranium fuel, a purified chemical procedure with three crucial steps is required. The steps included feeding small batches of uranium oxide powder into a designated dissolving tank in order to produce uranyl nitrate using nitric acid.[9] Next, the mixture is carefully transported to a specially-crafted buffer tank. The buffer tank containing the combined ingredients is specially designed to prevent fission activity from reaching criticality. In a precipitation tank, ammonia is added forming a solid product. This tank is meant to capture any remaining nuclear waste contaminants. In the final process, uranium oxide is placed in the dissolving tanks until purified, without enriching the isotopes, in a wet-process technology specialized by Japan.[9]

The nuclear accidents[edit]

There have been two Tokaimura nuclear accidents at the nuclear facility at Tōkai. The first accident occurred on 11 March 1997 producing an explosion after improperly heated nuclear waste products caught on fire at the Dōnen plant. Over twenty people were exposed to radiation. The second incident on 30 September 1999 happened at the JCO plant. It was classified a serious criticality accident. The incident resulted in radiation exposure for 667 people and the death of two workers.[10]

1997 accident[edit]

On 11 March 1997, the first nuclear-related incident reported in Tokaimura occurred in a Dōnen (Power Reactor and Nuclear Fuel Development Corporation) nuclear fuel reprocessing plant. It is sometimes referred to as the Dōnen accident (動燃事故, Dōnen jiko). This facility rendered nuclear fuel by-products inactive and safe to store through combination with asphalt. To render the waste ready for storage, it is heated no higher than 195 degrees. On 11 March, the waste was being processed for storage when the heat control measures failed and set the mixture on fire.[11] Most likely due to improper extinguishing of the fire, a small explosion occurred. The explosion broke windows allowing smoke and radiation to escape into the surrounding area. Plant surveillance confirmed the windows and doors affected by the explosion had not been repaired for a few hours subjecting the surrounding area to radioactivity.[12]

On 12 March in the early morning, Dōnen (PNC) officials confirmed at least 21 workers had been exposed to radioactivity during the incident. Dutch radio news broadcast higher numbers of workers that inhaled the contaminants.[13] In response, officials prohibited entry to the facility and barricaded a 30,000 square foot area around the plant despite reporting no abnormal radioactivity. PNC leadership claimed normal levels of radioactivity in the area the morning of 13 March. A week after the event, meteorological officials detected unusually high levels of caesium 40 kilometers (25 miles) south-west of the plant.[13] Aerial views over the nuclear processing plant building showed a damaged roof from the fire and explosion allowing continued external radiation exposure.[14]

PNC management mandated workers to falsely report the chronological events leading to the fire in order to cover up lack of proper supervision.[15] Dōnen leadership failed to immediately report the fire to the Science and Technology Agency (STA). This delay was due to their own internal investigation of the fire causing hampered immediate emergency response teams and prolonged radioactivity exposure. Dōnen facility officials initially reported a 20 percent increase of radiation levels in the area surrounding the reprocessing plant but later revealed the true percent was ten times higher than initially published.[7] Tokaimura residents demanded criminal prosecution of PNC officials, reorganization of company leadership and closure of the plant itself.[15] Following the public's outcry, the Dōnen nuclear fuel reprocessing plant closed until reopening in November 2000 when it was reinstated as a nuclear fuel reprocessing plant.[16]

Later Prime Minister Ryutaro Hashimoto criticized the delay that allowed radiation to continue to impact local areas.[17] JCO failed to correct the improper handling procedures and substandard training of workers that created the fire and explosion. This failure led to a devastating incident two years later at a JCO plant.

1999 accident[edit]

Tokaimura nuclear accident
Tōkai-mura is located in Japan
Location of Tōkai-mura in Japan
Date30 September 1999
LocationTōkai, Ibaraki, Japan
Coordinates36°28′47.00″N 140°33′13.24″E / 36.4797222°N 140.5536778°E / 36.4797222; 140.5536778Coordinates: 36°28′47.00″N 140°33′13.24″E / 36.4797222°N 140.5536778°E / 36.4797222; 140.5536778
TypeNuclear criticality accident
CauseUncontrolled nuclear fission after uranyl nitrate overload
OutcomeINES Level 4 (accident with local consequences)
Non-fatal injuries667 contaminated
InquiriesWorld Nuclear Association

The second, more serious Tokaimura nuclear accident (Japanese: 東海村JCO臨界事故 Tōkai-mura JCO-rinkai-jiko) occurred on 30 September 1999 in a JCO uranium processing plant. The event was classified a criticality accident. The incident took place in a conversion building operated by JCO (formerly Japan Nuclear Fuel Conversion Co.) a subsidiary of Sumitomo Metal Mining Company in the village of Tōkai.[10] It was the worst civilian nuclear radiation accident in Japan prior to the Fukushima Daiichi nuclear disaster of 2011.[7] The incident exposed the surrounding population to hazardous nuclear radiation after the uranium mixture reached criticality. Two of the three technicians mixing fuel lost their lives. The incident was caused by lack of regulatory supervision, inadequate safety culture and improper technician training and education.[18]

The JCO facility started to dissolve and mix high-purity enriched uranium oxide with nitric acid to produce uranyl nitrate for shipping on 28 September 1999. The highly-enriched uranium in production was improperly prepared for fuel conversion. Pressure placed upon staff to prepare uranyl nitrate for shipping led to several errors including pouring the solution (uranium oxide in nitric acid). The technicians opted to pour the product by hand in stainless-steel buckets directly into a precipitation tank.[7] This process inadvertently contributed to a critical mass level incident triggering uncontrolled nuclear chain reactions over the next several hours.

Nuclear criticality event chronology[edit]

JCO facility technicians Hisashi Ouchi, Masato Shinohara, and Yutaka Yokokawa were speeding up the last few steps of the fuel/conversion process to meet shipping requirements. It was JCO's first batch of fuel for that reactor in three years; no proper qualification and training requirements were established to prepare for the process.[10] To save processing time, and for convenience, the team mixed the chemicals in stainless-steel buckets. The workers followed JCO operating manual guidance in this process but were unaware it was not approved by the STA.[9] Under correct operating procedure, uranyl nitrate would be stored inside a buffer tank and gradually pumped into the precipitation tank in 2.4 kg increments.[18]

At around 10:35, the precipitation tank reached critical mass when its fill level, containing about 16 kg of uranium, reached criticality in the tall and narrow buffer tank.[2] The hazardous level was reached after the technicians added a seventh bucket containing aqueous uranyl nitrate, enriched to 18.8% 235U, to the tank.[19] The solution added to the tank was almost seven times the legal mass limit specified by the STA.[19]

The nuclear fuel conversion standards specified in the 1996 JCO Operating Manual dictated the proper procedures regarding dissolution of uranium oxide powder in a designated dissolution tank.[20] The buffer tank's tall, narrow geometry was designed to hold the solution safely and to prevent criticality. In contrast, the precipitation tank had not been designed to hold unlimited quantities of this type of solution. The designed wide cylindrical shape made it favorable to criticality. The workers bypassed the buffer tanks entirely, opting to pour the uranyl nitrate directly into the precipitation tank. An uncontrolled nuclear fission began immediately. The resulting nuclear fission chain became self-sustaining emitting intense gamma and neutron radiation in the nuclear facility.[10] At the time of the event, Ouchi had his body draped over the tank while Shinohara stood on a platform to assist in pouring the solution. Yokokawa was sitting at a desk four meters away.[9] All three technicians observed a blue flash (possibly Cherenkov radiation) and gamma-radiation alarms sounded.[21] Over the next several hours the fission reaction produced continuous chain reactions.

Technicians Ouchi and Shinohara immediately experienced pain, nausea, and difficulty breathing. Ouchi received the largest radiation exposure, resulting in problems with mobility, coherence, and loss of consciousness.[9] Upon the point of critical mass, large amounts of high-level gamma radiation set off alarms in the building, prompting the three technicians to evacuate.[22] All three of the workers were unaware of the impact of the accident or reporting criteria. A worker in the next building became aware of the injured employees and contacted emergency medical assistance; an ambulance escorted them to the nearest hospital. The fission products contaminated the fuel reprocessing building and immediately outside the nuclear facility.[1] Emergency service workers arrived and escorted other plant workers outside of the facility's muster zones.[10]

The next morning, workers ended the nuclear chain reaction by draining water from the surrounding cooling jacket installed on the precipitation tank. The water served as a neutron reflector. A boric acid solution was added to the precipitation tank to reduce all contents to sub-critical levels (boron selected for its neutron absorption properties).[1]

Timeline of 1999 Accident
Day Time Event/action Affected parties
30 September 1999 10:35 Criticality event occurred, setting off radiation monitors and alarms; evacuation begins and employees exposed to radiation[23] 3 Workers; Hisashi Ouchi, Masato Shinohara and Yutaka Yokokawa
30 September Until 23:30 (5 hours later) STA confirms continuing chain reactions; Tokaimura sets up headquarters for the incidents, (12 hours later) broadcasts all surrounding residents to evacuate, informs Japan's leadership and ceased all crop and water usage Tokaimura City and National Leadership
1 October 1999 All day Road blocks implemented; shelter in place lifted but schools closed all day; water drainage initiated to stop chain reaction. All residents
2 October 1999 All day Health checks conducted on all residents, measuring radiation; schools reopened and government press conferences held All residents

Tokaimura evacuation[edit]

By mid-afternoon the plant workers and surrounding residents were asked to evacuate. Five hours after the start of the criticality, evacuation commenced of some 161 people from 39 households within a 350-meter radius from the conversion building. Twelve hours after the incident, 300,000 surrounding residents of the nuclear facility were told to stay indoors and cease all agricultural production.[24] This restriction was lifted the following afternoon. Almost 15 days later, the facility instituted protection methods with sandbags and other shielding to protect from residual gamma radiation.


Without an emergency plan or public communication from JCO, confusion and panic followed the event.[24] Authorities warned not to harvest crops or drink well water.[24] In order to ease public concerns, officials began radiation testing of residents living approximately 6 miles from the facility. Over the next 10 days, approximately 10,000 medical check-ups were conducted.[24] Dozens of emergency workers and nearby residents were hospitalized and hundreds of thousands of others were forced to remain indoors for 24 hours. Testing confirmed 39 of the workers were exposed to the radiation.[1] At least 667 workers, first-responders, and nearby residents were exposed to excess radiation as a result of the accident.[18]

Ultimately the incident was classified as an “irradiation” not “contamination” accident under Level 4 on the Nuclear Event Scale.[10] This determination labeled the situation low risk outside of the facility.[10] The technicians and workers in the facility were measured for radiation contamination. The three technicians measured significantly higher levels of radiation than the measurement designated the maximum allowable dose (0.05 sieverts) for Japanese nuclear workers.[1] Many employees of the Company and local population suffered accidental radiation exposure exceeding safe levels. Over fifty plant workers tested up to 0.23 Sv and local residents up to 0.15 Sv.[10] Fatal doses of radiation ended the lives of two technicians, Ouchi and Shinohara.

Impact on technicians[edit]

According to the radiation testing by the STA, Ouchi was exposed to 17 Sv of radiation, Shinohara 10 Sv, and Yokokawa received 3 Sv.[24] The two technicians who received the higher doses, Ouchi and Shinohara, died several months later.

Hisashi Ouchi, 35, was transported and treated at the University of Tokyo Hospital.[25] Ouchi suffered serious radiation burns to most of his body, experienced severe damage to his internal organs, and had a near-zero white blood cell count. Without a functioning immune system, Ouchi was vulnerable to hospital-borne pathogens and was placed in a special radiation ward to limit the risk of contracting an infection.[26] Doctors attempted to restore some functionality to Ouchi's immune system by administering peripheral blood stem cell transplantation, which at the time was a new form of treatment.[18] After receiving the transplant from his sister, Ouchi initially experienced increased white blood cell counts temporarily but succumbed to his other injuries shortly thereafter.[25] The leukocytes being produced by the transplanted tissue were found to have been mutated by the residual radiation present in his body, triggering autoimmune responses that exacerbated his rapidly deteriorating condition, and white blood cell counts began to decrease.[18] Numerous other interventions were conducted in an attempt to arrest further decline of Ouchi's severely damaged body, including repeated use of cultured skin grafts and pharmacological intervention with painkillers, broad-spectrum antibiotics and granulocyte colony-stimulating factor, though each of these attempts at intervention failed.[27]

At the wishes of his family, doctors repeatedly revived Ouchi when his heart stopped, even as it became clear the damage his body had sustained through radiation was untreatable. Despite their efforts, his condition deteriorated into multiple organ failure resulting from extensive radiation damage, exacerbated by the repeated incidents where Ouchi's heart stopped. He died on 21 December 1999 following an unrecoverable cardiac arrest.[28]

Masato Shinohara, 40, was transported to the same facility where he died on 27 April 2000 of multiple organ failure. He endured radical cancer treatment, numerous successful skin grafts, and a transfusion from congealed umbilical cord blood (to boost stem cell count). Despite his seven month battle, he was unable to fight radiation induced infections and internal bleeding, resulting in fatal lung and kidney failure.

Their supervisor, Yutaka Yokokawa, 54, received treatment from the National Institute of Radio-logical Sciences (NIRS) in Chiba. He was released three months later with minor radiation sickness. He faced negligence charges in October 2000.

Contributors to both accidents[edit]

According to the International Atomic Energy Agency, the cause of the accidents were "human error and serious breaches of safety principles".[1] Several human errors caused the incident in 1997 including careless material handling procedures, inexperienced technicians, inadequate supervision and obsolete safety procedures on the operating floor.[18] The company had not had any incidents for over 15 years making company employees complacent in their daily responsibilities.

The 1999 incident resulted from poor management of operation manuals, failure to qualify technicians and engineers, and improper procedures associated with handling nuclear chemicals. The lack of communication between the engineers and workers contributed to lack of reporting when the incident arose.[9] Had the company corrected the errors after the 1997 incident, the 1999 incident would have been considerably less devastating or may not have happened.

Comments within the 2012 Report by the National Diet of Japan Fukushima Nuclear Accident Independent Investigation Commission notice regulatory and nuclear industry overconfidence, and governance failures may equally apply to the Tokaimura nuclear accident.

Victim compensation and plant closure[edit]

Over 600 plant workers, firemen, emergency personnel and local residents were exposed to radioactivity following the incident.[24] In October 1999, JCO set up advisory booths to process compensation claims and inquiries of those affected.[24] By July 2000, over 7,000 compensation claims were filed and settled. In September 2000 JCO agreed to pay $121 million in compensation to settle 6,875 claims from people exposed to radiation and affected agricultural and service businesses.[29] All residents within 350 meters of the incident and those forced to evacuate received compensation if they agreed to not sue the company in the future.[24]

In late March 2000, the STA cancelled JCO’s credentials for operation serving as the first Japanese plant operator to be punished by law for mishandling nuclear radiation.[30] This suit was followed by the company president’s resignation. In October, six officials from JCO were charged with professional negligence derived from failure to properly train technicians and knowingly subverting safety procedures.[28]

Resulting legal suits[edit]

In April 2001 six employees, including the chief of production department at the time, pleaded guilty to a charge of negligence resulting in death.[29] Among those arrested was Yokokawa for his failure to supervise proper procedures.[31] The JCO President also pleaded guilty on behalf of the company.[29] During the trial, the jury learned that a 1995 JCO safety committee had approved the use of steel buckets in the procedure. Furthermore, a widely distributed but unauthorized 1996 manual recommended the use of buckets in making the solution. A STA report indicated JCO management had permitted these hazardous practices beginning in 1993 to shortcut the conversion process, even though it was contrary to approved nuclear chemical handling procedures.[18]

As a response to the incidents, special laws were put in place stipulating operational safety procedures and quarterly inspection requirements.[18] These inspections focused on the proper conduct of workers and leadership. This change mandated both safety education and quality assurance of all facilities and activities associated with nuclear power generation. Starting in 2000, Japan's atomic and nuclear commissions began regular investigations of facilities, expansive education regarding proper procedures and safety culture regarding handling nuclear chemicals and waste.[18]

Efforts to comply with emergency preparedness procedures and international guideline requirements continued. New systems were put in place for handling a similar incident with governing legislature and institutions in an effort to prevent further situations from occurring.[18]

Japan relies heavily on imports for 80% of all energy requirements, due to this shortage, mounting pressures to produce self-sustaining energy sources remain. In 2014, the Japanese government decided to establish the "Strategic Energy Plan" naming nuclear power as an important power source that can safely stabilize and produce the energy supply and demand of the country.[3] This event contributed to antinuclear activist movements against production of nuclear energy in Japan.[10] To this day, the tensions between the need for produced power outside of nonexistent natural resources and the safety of the country’s population remain. Advocacy for acute nuclear disease victims and eradication of nuclear related incidents has led to several movements across the globe promoting human welfare and environmental conservation.[32]

See also[edit]


  1. ^ a b c d e f IAEA. (1999). Report on the preliminary fact finding mission following the accident at the nuclear fuel processing facility in Tokaimura, Japan (1st ed., Vol. 1, pp. 32–34). Vienna.
  2. ^ a b c "The Tokaimura Incident". Retrieved 27 April 2020.
  3. ^ a b "Why is Nuclear Energy Necessary in Japan? - The Federation of Electric Power Companies of Japan(FEPC)". Retrieved 27 April 2020.
  4. ^
  5. ^ "Tōkai Nuclear Power Plant", Wikipedia, 26 April 2020, retrieved 27 April 2020
  6. ^ "Nuclear Workers Appeared Unaware of Dangers". Los Angeles Times. 7 October 1999. Retrieved 29 April 2020.
  7. ^ a b c d Iovino, Peter Aldhous and Zena. "Japan's record of nuclear cover-ups and accidents". New Scientist. Retrieved 27 April 2020.
  8. ^ Ryan, Michael (1 September 2001). "The Tokaimura Nuclear Accident: A Tragedy of Human Errors" (PDF). Journal of College Science Teaching. 31: 42 – via Amazon.[dead link]
  9. ^ a b c d e f Ryan, Michael (1 September 2001). "The Tokaimura Nuclear Accident: A Tragedy of Human Errors" (PDF). Journal of College Science Teaching. 31: 43 – via Amazon.[dead link]
  10. ^ a b c d e f g h i "Tokaimura Criticality Accident - World Nuclear Association". Retrieved 27 April 2020.
  11. ^ Author, No (21 November 2000). "Tokai nuclear fuel plant reopens after 1997 fire". The Japan Times. Retrieved 27 April 2020.
  12. ^ "#468 - March 14, 1997". Retrieved 29 April 2020.
  13. ^ a b "#468 - March 14, 1997". Retrieved 29 April 2020.
  14. ^ Ryan, Michael (June 2001). "The Tokaimura Accident: Nuclear Energy and Reactor Safety" (PDF). National Center for Case Study Teaching in Science: 5 – via Science Cases.
  15. ^ a b Heenan, Patrick (2014). The Japan Handbook: Regional Handbooks of Economic Development. Routledge. p. 56. ISBN 9781135925338.
  16. ^ Author, No (21 November 2000). "Tokai nuclear fuel plant reopens after 1997 fire". The Japan Times. Retrieved 27 April 2020.
  17. ^ "Explosion at PNC Tokai reprocessing plant | Wise International". Retrieved 27 April 2020.
  18. ^ a b c d e f g h i j Akashi, M., Aoki, H., Endo, A., Fujimoto, K., Homma, T., Kukita, Y., … Zombori, P. (2000). Lessons Learned From the Jco Nuclear Criticality Accident in Japan in 1999. International Atomic Energy Agency. Retrieved from
  19. ^ a b Ryan, Michael (25 June 2001). "The Tokaimura Accident: Nuclear Energy and Reactor Safety" (PDF). National Center for Case Study Teaching in Science: 3.
  20. ^ Akashi, M., Aoki, H., Endo, A., Fujimoto, K., Homma, T., Kukita, Y., … Zombori, P. (2000). Lessons Learned From the Jco Nuclear Criticality Accident in Japan in 1999. International Atomic Energy Agency. Retrieved from
  21. ^ Ryan, Michael (June 2001). "The Tokaimura Accident: Nuclear Energy and Reactor Safety" (PDF). National Center for Case Study Teaching in Science: 5 – via Science Cases.
  22. ^ Ryan, Michael (25 June 2001). "The Tokaimura Accident: Nuclear Energy and Reactor Safety" (PDF). National Center for Case Study Teaching in Science: 3.
  23. ^ Smith, Sandy (24 April 2001). "JCO employees plead guilty to negligence in deaths at Japanese nuclear facility". Safety Online. Retrieved 21 May 2018.
  24. ^ a b c d e f g h Ryan, Michael (1 November 2001). "The Tokaimura Nuclear Accident: A Tragedy of Human Errors" (PDF). Journal of College Science Teaching. 31: 45 – via Amazon.[dead link]
  25. ^ a b Ryan, Michael (25 June 2001). "The Tokaimura Nuclear Accident: A Tragedy of Human Errors" (PDF). Journal of College Science Teaching. 1: 44 – via Amazon.[dead link]
  26. ^ Street Journal, Peter LandersStaff Reporter of The Wall (1 October 1999). "Nuclear Accident in Tokai Is Among Japan's Worst". Wall Street Journal. ISSN 0099-9660. Retrieved 24 December 2020.
  27. ^ "Lessons learned from the JCO Nuclear Criticality Accident in Japan in 1999" (PDF). International Atomic Energy Agency. Retrieved 24 December 2020.
  28. ^ a b "Japanese Nuclear Accident Timeline of Events". Retrieved 28 April 2020.
  29. ^ a b c "JCO employees plead guilty to negligence in deaths at Japanese nuclear facility". Retrieved 28 April 2020.
  30. ^ Ryan, Michael (June 2001). "The Tokaimura Accident: Nuclear Energy and Reactor Safety" (PDF). National Center for Case Study Teaching in Science: 5 – via Science Cases.
  31. ^ Ryan, Michael (June 2001). "The Tokaimura Accident: Nuclear Energy and Reactor Safety" (PDF). National Center for Case Study Teaching in Science: 5 – via Science Cases.
  32. ^ Eiko Maruko Siniawer. Consuming Postwar Japan. Ithaca and London: Cornell University Press, 2018

External links[edit]