In November, I visited JCO Co.'s nuclear fuel-processing plant -- a subsidiary of Sumitomo Metal Mining Co. -- where Sept. 30 a level-5 nuclear incident took place. The plant is located 110 km from Tokyo in the small town of Tokaimura, Ibaraki Prefecture. The plant is in an area that is a blend of residences and light industry. The nearest industry is less than 80 meters from the site of the accident and the nearest residential housing only 20 meters further away. There is a school in the vicinity, and when I stood outside the small, concrete building where the accident took place, I could hear children's shrill voices from the nearby playground.
The rather simple, official version of what happened holds that the accident was caused by employee negligence. Three employees were engaged in a solvent extraction process. Failing to follow procedures in the operating manual, the employees used a bucket to transfer uranium oxide into a precipitating tank. The uncontrolled transfer led to a situation where the critical mass of uranium in the tank became sufficient for a self-sustaining nuclear reaction to start.
At this point, a blue flash took place and vast amounts of radiation were released. One worker, who recently died, received 17,000 times the recommended maximum annual dose. Because no emergency procedures had been established to deal with such an event, criticality continued for more than 20 hours until action could be taken to drain the water from the cooling jacket surrounding the precipitation tank. This was necessary because the water had been reflecting neutrons coming from the nuclear reaction in the precipitating tank, helping to sustain the criticality.