Analysis of the Tokai-mura criticality accident

The accident at Tokai-mura occurred in the experimental uranium conversion plant of the Japan Nuclear Fuel Conversion Company, JCO Co. Ltd., one of th e many independent units operating at Tokai-mura. JCO has a capacity for pr ocessing annually up to 715 t of uranium with the usual low enrichment leve l of 5 percent for light water reactors, and up to 3 t of uranium with a me an enrichment level of approx. 18.8 percent normally used for fast reactors . The plant section in which the accident occurred performs the chemical co nversion of U3O8 into uranium dioxide (UO2) The plant operator seems to hav e neglected the procedures licensed by the regulatory authorities, as the u pper limit of enrichment of fissile material assumed to have been licensed by the authorities was greatly exceeded by the operator. The masses handled safely in the plant, which seem to hale been defined on the basis of the e nrichment level for internal purposes, were not observed by the staff, poss ibly because of a mix-up. The 16 kg filled make this a plausible assumption , as this was precisely the amount of low-enriched uranium covered by the p rocessing permit.