The Tokaimura nuclear accident (Japanese: Tōkai-mura JCO-rinkai-jiko), which occurred on 30 September 1999,[1][2][3] resulted in two deaths.[4] At that time, it was Japan's worst civilian nuclear radiation accident. 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.[5]
The accident occurred as three workers 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 non-fissile 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 gallons respectively).[5]
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Criticality was reached upon the technicians adding a seventh bucket of an aqueous uranium solution known as uranyl nitrate to the tank. The nuclear fission chain reaction became self-sustaining and began to emit intense gamma and neutron radiation. The technicians, one of whom had his body draped over the tank, observed a blue flash of Cherenkov radiation and gamma-radiation alarms sounded.[4][6] The two technicians closest to the tank immediately experienced pain, nausea, difficulty breathing, and other symptoms. The technician closest to the tank lost consciousness in the decontamination room a few minutes later and began to vomit.[7] 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.[5]
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. The tank was not 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 after sandbags and other shielding ensured no hazard 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.[5]
Dozens of emergency workers and nearby residents were hospitalized and hundreds of thousands of others were forced to remain indoors for 24 hours.[8] At least 667 workers, emergency responders, and nearby residents were exposed to excess radiation as a result of the accident.[4]
A dose of 50 millisieverts (mSv) is the maximum allowable annual dose for Japanese nuclear workers.[5] For context, 8000 mSv (800 rem) is normally a fatal dose and more than 10,000 mSv is almost invariably fatal. Normal background radiation amounts to an annual exposure of about 3 mSv.[4] 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).[9] The three operators' doses were far above permissible limits at 3,000, 10,000, and 17,000 mSv; the two receiving the higher doses died several months later.[4] The most severely exposed worker had his body draped over the tank when it went critical. He suffered serious burns to most of his body, experienced severe damage to his internal organs, and had a near-zero white blood cell count.[4]
The cause of the accident was said to be "human error and serious breaches of safety principles", according to the International Atomic Energy Agency.[5]