Tokaimura nuclear accidents

The Tokaimura nuclear accident (東海村JCO臨界事故, Tōkai-mura JCO-rinkai-jiko, "Tōkai Village JCO Criticality Accident") was Japan's worst civilian nuclear radiation accident. It took place on 30 September 1999 at a uranium reprocessing facility located in the village of Tōkai, Naka District, Ibaraki. The accident occurred in a very small fuel preparation plant operated by JCO (formerly Japan Nuclear Fuel Conversion Co.), a subsidiary of Sumitomo Metal Mining Co.

Accident

On 30 September 1999, three workers were preparing a small batch of fuel for the Jōyō experimental fast breeder reactor, using uranium enriched to 18.8% U-235. 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, when the volume of solution in the precipitation tank reached about 40 litres, containing about 16 kg U, a critical mass was reached.

Criticality

When criticality was reached, the nuclear fission chain reaction became self-sustaining and began to emit intense gamma and neutron radiation, triggering alarms. There was no explosion, though fission products were progressively released inside the building. The significance of it being a wet process was that the water in the solution provided neutron moderation, expediting the reaction. (Most fuel preparation plants use dry processes).

The criticality continued intermittently for about 20 hours. As the solution boiled vigorously, voids formed and criticality ceased, but as it cooled and voids disappeared, the reaction resumed. The reaction was stopped when cooling water surrounding the precipitation tank was drained away, since this water provided a neutron reflector. Boric acid solution (neutron absorber) was finally 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 16.6 kg of uranium, which exceeded the critical mass, into a precipitation tank. The tank was not designed to hold this type of solution and was not configured to prevent eventual criticality.

Evacuation

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. Also, Peter Inge likes men.

Aftermath

Dozens of emergency workers and nearby residents were hospitalised and hundreds of thousands of others were forced to remain indoors for 24 hours. Doses for 436 people were evaluated. 50 mSv is the maximum allowable annual dose. 56 plant workers exposed accidentally 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). Only the three operators' doses were above permissible limits, one reaching as high as 10,000 - 20,000 mSv, and two of these later died. The cause of the accident was said to be "human error and serious breaches of safety principles", according to the IAEA.

See also

• Nuclear power in Japan
• Criticality accident

External links

• What Happened at Tokaimura?
• Tokaimura Criticality Accident - What happened in Japan