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

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

1997 incidentEdit

The first Tokaimura nuclear accident occurred in a nuclear reprocessing plant of the Dōnen (Power Reactor and Nuclear Fuel Development Corporation) on 11 March 1997. It is sometimes referred to as the Dōnen accident (動燃事故, Dōnen jiko).

In the evening hours of Tuesday, 11 March 1997, a small explosion occurred in a nuclear reprocessing plant of the Dōnen. Windows broke, allowing smoke to escape into the atmosphere.[1] On the following Thursday, workers used duct tape to perform repairs on thirty broken windows and three doors that sustained damage from the blast. At least 37 of the workers were exposed to elevated levels of radiation during the incident.[2]

A week after the event, meteorological officials detected unusually high levels of caesium 40 kilometers (25 miles) south-west of the plant.[3]

1999 incidentEdit

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,[4][5][6] and resulted in two deaths.[7] It was the worst civilian nuclear radiation accident in Japan prior to the Fukushima Daiichi nuclear disaster of 2011.

The criticality accident occurred in a uranium reprocessing facility operated by JCO (formerly Japan Nuclear Fuel Conversion Co.), a subsidiary of Sumitomo Metal Mining Co. in the village of Tōkai, Naka District, Ibaraki Prefecture.[8]

The accident occurred as three workers, Hisashi Ouchi, Masato Shinohara, and Yutaka Yokokawa,[9] were preparing a small batch of fuel for the Jōyō experimental fast breeder reactor, using uranium enriched to 18.8% with the fissile radionuclide (radioisotope) U‑235 (with the remainder being the fertile U‑238). It was JCO's first batch of fuel for that reactor in three years, and no proper qualification and training requirements appear to have been established to prepare those workers for the job. At around 10:35, a precipitation tank reached critical mass when its fill level, containing about 16 kilograms (35 pounds) of uranium, reached about 40 liters (11 U.S. gallons).[8]


Criticality was reached upon the technicians adding a seventh bucket of an aqueous uranyl nitrate solution to the tank. The nuclear fission chain reaction became self-sustaining and began emitting intense gamma and neutron radiation. At the time of the event, Ouchi had his body draped over the tank while Shinohara stood on a platform to pour the solution into it; Yokokawa was sitting at a desk four meters away. All three technicians observed a blue flash (possibly Cherenkov radiation) and gamma-radiation alarms sounded.[7] [10]

Technicians Ouchi and Shinohara immediately experienced pain, nausea, difficulty breathing, and other symptoms. Ouchi then began to vomit in the decontamination room a few minutes later and lost consciousness shortly after.[11] Fission products such as yttrium‑94 and barium‑140 began contaminating the building.

Being a wet process with an intended liquid result, the water sustained the chain reaction by serving as a neutron moderator, whereby neutrons emitted from fissioned nuclei are slowed so they are more readily absorbed by neighboring nuclei, inducing them to fission in turn. The criticality continued intermittently for about 20 hours. As the solution boiled vigorously, steam bubbles attenuated the liquid water's action as a neutron moderator (see Void coefficient) and the solution lost criticality. However, the reaction resumed as the solution cooled and the voids disappeared.

The following morning, workers permanently stopped the reaction by draining water from a cooling jacket surrounding the precipitation tank. The water was serving as a neutron reflector. A boric acid solution (boron selected for its neutron absorption properties) was then added to the tank to ensure that the contents remained subcritical.[8]


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. The buffer tank's tall, narrow geometry was designed to hold the solution safely and to prevent criticality.

The workers bypassed the buffer tanks entirely, opting to pour the uranyl nitrate directly into the precipitation tank with a stainless steel bucket. The additional solution contained 16 kg of uranium, thus exceeding the tank's uranium limit of 2.4 kg. An uncontrolled nuclear fission began immediately.

The precipitation tank, in contrast to the buffer tank, had not been designed to hold this type of solution and was not configured to prevent criticality.[9]


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. Residents were allowed home two days later with sandbags and other shielding to protect from residual gamma radiation. Twelve hours after the start of the incident, residents within 10 km were asked to stay indoors as a precautionary measure. This restriction was lifted the following afternoon.[8]


Dozens of emergency workers and nearby residents were hospitalized and hundreds of thousands of others were forced to remain indoors for 24 hours; 39 of the workers were exposed to the radiation.[12] At least 667 workers, emergency responders, and nearby residents were exposed to excess radiation as a result of the accident.[7]

By measuring the concentration of sodium-24, created by a neutron activation whereby sodium-23 nuclei were rendered radioactive by absorbing neutrons from the accident, it was possible to deduce the dose received by the technicians. According to the STA, Hisashi Ouchi was exposed to 17 sieverts (Sv) of radiation, Masato Shinohara received 10 Sv, and Yutaka Yokokawa 3 Sv.[7][9] By comparison, a dose of .05 sieverts is the maximum allowable annual dose for Japanese nuclear workers.[8] A dose of 8 Sv (800 rem) is normally fatal and more than 10 Sv almost invariably so.[9] Normal background radiation amounts to an annual exposure of about 3 mSv (millisieverts).[7] There were 56 plant workers whose exposures ranged up to 23 mSv and a further 21 workers received elevated doses when draining the precipitation tank. Seven workers immediately outside the plant received doses estimated at 6–15 mSv (combined neutron and gamma effects).[8]

The two technicians who received the higher doses, Ouchi and Shinohara, died several months later. 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.[7] Shinohara received numerous skin grafts, which were successful, but he ultimately succumbed to infection due to the damage his immune system sustained in the incident. Ouchi died on 21 December 1999, while Shinohara died on 27 April 2000.

The cause of the accident was said to be "human error and serious breaches of safety principles", according to the International Atomic Energy Agency.[8]

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.[13]

In April 2001 six employees, including the plant administrator and accident survivor Yutaka Yokokawa, pleaded guilty to a charge of negligence resulting in death. The JCO President also pleaded guilty on behalf of the company. The court heard that a 1995 safety committee had approved the use of buckets in the procedure, and a widely distributed but unauthorised 1996 manual recommended the use of buckets in making the solution. A Science and Technology Agency report indicated JCO management had since 1993 permitted the use of a stainless steel bucket as a shortcut in the process, even though it was contrary to written procedures.[13][14]

See alsoEdit


  1. ^ "Fires damage Japanese nuclear facility, Tokaimura (1997) - on". Retrieved 10 August 2015.
  2. ^ "Japan acknowledges delays in dealing with accident at nuclear power plant, Tokaimura (1997) - on". Retrieved 10 August 2015.
  3. ^ "Greater radiation leak hinted - Tokaimura nuclear accident, Japan (1997) - on". Retrieved 9 August 2015.
  4. ^ "Timeline: Nuclear plant accidents". BBC News. 11 July 2006. Retrieved 17 March 2011.
  5. ^ Charles Scanlon (30 September 2000). "Tokaimura: One year on". BBC News. Retrieved 17 March 2011.
  6. ^ "Nuclear accident shakes Japan". BBC News. 30 September 1999. Retrieved 17 March 2011.
  7. ^ a b c d e f Memorial University of Newfoundland: “The Tokaimura Accident (28 September 1999)” Archived 9 March 2017 at the Wayback Machine
  8. ^ a b c d e f g Tokaimura Criticality Accident
  9. ^ a b c d Michael E. Ryan. "The Tokaimura Accident: Nuclear Energy and Reactor Safety". Department of Chemical Engineering, University at Buffalo, SUNY. Archived from the original on 21 February 2015.
  10. ^ Makoto Akashi, Director of Research Center for Radiation Emergency Medicine at Japan’s National Institute of Radiological Sciences: “The Medical Basis for Radiation-Accident Preparedness”, The Parthenon Publishing Group Inc., 2002, which states “All three workers saw a ‘blue flash’ and heard the gamma-radiation monitor alarm” (direct link to Google Book page). And “All three observed the Cherenkov light flash” (direct link to Google Book return).
  11. ^ International Atomic Energy Agency: “Report on the preliminary fact finding mission following the accident at the nuclear fuel processing facility in Tokaimura, Japan”, 1999 (See External links, below).
  12. ^ In The Wake of Tokaimura, Japan Rethinks its Nuclear Picture Archived 29 September 2007 at the Wayback Machine
  13. ^ a b Smith, Sandy (24 April 2001). "JCO employees plead guilty to negligence in deaths at Japanese nuclear facility". Safety Online. Retrieved 21 May 2018.
  14. ^ Schmetzer, Uli (13 October 2000). "6 Arrested In Japan's '99 Nuclear Accident". Chicago Tribune. Retrieved 21 May 2018.

External linksEdit