The Nuclear criticality accident in Japan, Revisited

Okuno, Hiroshi  ; Suyama, Kenya

The criticality accident that resulted in the evacuation of residents occurred on September 30, 1999, at JCO's nuclear fuel fabrication plant in Ibaraki prefecture, Japan. This paper presents the outline, technical issues and background of the accident and the situations that followed. The review of this accident was one of the hot issues in the last ICNC2003 organized in Tokai-mura. At the this turn of ICNC in Japan, we would like to revisit the accident to reaffirm and share the idea that the response and preparedness should be strengthened to protect people and the environment from nuclear disasters. The accident occurred in a factory of JCO during the informal and unusual process of preparing a uranium nitrate solution of medium-enriched (18.8 weight% U) uranium from UO using the precipitation tank of 45 cm in diameter, which exceeded the critical diameter (ca. 23 cm) of an infinite cylinder with a full thick water reflector. A 2.2-cm-thick water "jacket" surrounded and enclosed the tank, and the jacket was connected to the cooling tower beside the factory. The jacket not only functioned as the neutron reflector but also prevented the evaporation of the solution, and then the criticality continued for about 20 hours. Because JCO's plant had not anticipated the criticality accident, the response to the accident was confusing. During the accident, the JAERI and JNC, both the predecessors of the Japan Atomic Energy Agency (JAEA), acted to terminate the criticality and reduce the residents' exposure to radiation. After the accident, the JAERI and the National Institute of Radiological Sciences provided the telephone consultation at the village office of Tokai-mura. The JNC did the same things at the prefectural building of Ibaraki to advise the residents. The presentation may include issues of applying the Slide rule, identifying a nuclear criticality accident to occur, and responding to the emergency.