- Tokaimura nuclear accident
Japan's worst nuclear radiation accident took place at a uranium reprocessing facility in Tokaimura, Ibaraki prefecture, northeast of Tokyo, Japan, on 30 September, 1999. 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. [http://www.uic.com.au/nip52.htm Tokaimura Criticality Accident: Nuclear Issues Briefing Paper # 52, June 2000] ] The direct cause of the criticality accidentwas workers putting uranyl nitratesolution containing about 16.6 kg of uranium, which exceeded the critical mass, into a precipitation tank. The tank was not designed to dissolve this type of solution and was not configured to prevent eventual criticality. Three workers were exposed to (neutron) radiation doses in excess of allowable limits, and two of these workers died; a further 119 received lesser doses of 1 mSvor greater.
Dozens of emergency workers and nearby residents were hospitalised and hundreds of thousands of others were forced to remain indoors for 24 hours. [ [http://www.physicstoday.org/dec99/toka1.htm In The Wake of Tokaimura, Japan Rethinks its Nuclear Picture] ]
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.
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.
Five hours after the start of the criticality, evacuation commenced of some 161 people from 39 households within a 350 metre 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.
A total of 119 people received a radiation dose over 1 mSv from the accident, but only the three operators' doses were above permissible limits, 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 IAEA.
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Nuclear power in Japan
* [http://www.physicstoday.org/dec99/toka2.htm What Happened at Tokaimura?]
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