Tokaimura nuclear accident
There have been two Tokaimura nuclear accidents at the nuclear facility at Tōkai, Ibaraki.
- On 11 March 1997, a small explosion in a Dōnen plant.
- On 30 September 1999, a serious criticality accident in a JCO plant.
The first Tokaimura nuclear accident was the accident which occurred on 11 March 1997, in a nuclear reprocessing plant of the Dōnen (Power Reactor and Nuclear Fuel Development Corporation). Another name is the Dōnen accident (動燃事故 (Dōnen jiko?)).
On the night of Tuesday 11 March 1997, a small explosion occurred in a nuclear reprocessing plant of the Dōnen. Windows were smashed and smoke escaped to the atmosphere. On Thursday, workers repaired thirty broken windows and three doors with duct tape. They had been damaged during the blast. At least 37 workers were exposed to elevated levels of radiation during the incident.
A week after the event, meteorological officials detected unusually high levels of Cesium 25 miles south-west of the plant.
The second and more serious Tokaimura nuclear accident (Japanese: 東海村JCO臨界事故Tōkai-mura JCO-rinkai-jiko) indicates the nuclear disaster which occurred on 30 September 1999, resulting in two deaths. It was the worst civilian nuclear radiation accident in Japan prior to the Fukushima Daiichi nuclear disaster of 2011.
The criticality accident occurred in an 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.
The accident occurred as three workers, Hisashi Ouchi, Masato Shinohara, and Yutaka Yokokawa, 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) known as U‑235 (with the remainder being the fissionable-only 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.m., a precipitation tank reached critical mass when its fill level, containing about 16 kilograms of uranium, reached about 40 liters (about 35 pounds and 11 US gallons respectively).
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 to emit intense gamma and neutron radiation. At the time of the criticality 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. The technicians closest to the tank (Ouchi and Shinohara) immediately experienced pain, nausea, difficulty breathing, and other symptoms. The technician closest to the tank (Ouchi) lost consciousness in the decontamination room a few minutes later and began to vomit. There was no explosion, but fission products (fission fragments of U‑235 with atomic masses typically around 95 and 137, such as yttrium‑94 and barium‑140) were progressively released inside the building.
Being a wet process with an intended liquid result, the water promoted 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 since that water was serving as a neutron reflector. A boric acid solution (boron being a good neutron absorber) was then added to the tank to ensure that the contents remained subcritical. These operations exposed 27 workers to radioactivity.
The direct cause of the criticality accident was workers putting uranyl nitrate solution containing about 16 kg of uranium, which exceeded the critical mass for the precipitation tank. Unlike the buffer tank, which had a tall and narrow geometry to prevent criticality, the precipitation tank had not been designed to hold this type of solution and was not configured to prevent criticality.
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, and this restriction was lifted the following afternoon.
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. At least 667 workers, emergency responders, and nearby residents were exposed to excess radiation as a result of the accident.
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. By comparison, a dose of 50 millisieverts (mSv — thousandths of a sievert) is the maximum allowable annual dose for Japanese nuclear workers. A dose of 8 sieverts (800 rem) is normally fatal and more than 10 sieverts almost invariably so. Normal background radiation amounts to an annual exposure of about 3 mSv. 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).
The two technicians who received the higher doses, Ouchi and Shinohara, died several months later. Ouchi suffered serious burns to most of his body, experienced severe damage to his internal organs, and had a near-zero white blood cell count.
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- Memorial University of Newfoundland: “The Tokaimura Accident (28 September 1999)”
- Tokaimura Criticality Accident
- Michael E. Ryan. "The Tokaimura Accident: Nuclear Energy and Reactor Safety". Department of Chemical Engineering, University at Buffalo, SUNY.
- 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).
- 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).
- In The Wake of Tokaimura, Japan Rethinks its Nuclear Picture
- What Happened at Tokaimura?
- Tokaimura Criticality Accident – What happened in Japan
- International Atomic Energy Agency: “Report on the preliminary fact finding mission following the accident at the nuclear fuel processing facility in Tokaimura, Japan”, 1999 (9.5 MB PDF, here )
- Criticality accident at Tokai nuclear fuel plant (Japan) Wise Uranium project