A report of the investigating committee for JCO criticality accident

Aiming to make through investigation of the causal factors for the criticality accident in JCO uranium processing factory, an investigating committee was set up in Nuclear Safety Commission under the decision of countermeasure head office in the government authorities on Oct.4. This report contains the outline of the criticality accident, its cause and actual situations, general measures devised to deal with the disaster, on-the-spot measures and consideration of the background of accident as well as countermeasure in future. The committee concluded that the accident occurred because of human error and serious violation against the safety principle and standard. Moreover, it was demonstrated that the radiation continued for more than 20 hours after the beginning of criticality in the conversion building, but merely trace amount of rare gases and gaseous iodine were released from the building. The radioactivity outside the building rapidly returned to the basal level after the termination of the criticality. It was thus concluded as an irradiation accident but not pollution one. Since the accident was caused by operations under the conditions extremely deviated from the defined working standard, the disaster was not caused by unavoidable force due to an accumulation of unpredictable accident. Therefore, not only direct factors but also indirect ones leading to the violation against the working standard were discussed to prevent recurrence. (M.N.)