External doses to 350 m zone residents around the Tokai-mura criticality accident site

A criticality accident at the uranium conversion facilities in Tokai-mura Japan on September 30, 1999 caused neutron exposure of residents. The distance between the nearest residence and the uranium source was only about 100 m. Evacuation of people in the 350 m zone was started after 5 h. There was a remarkable directional distribution of the neutron beam which depended on the complex internal and external structures of the buildings surrounding the critical uranium. The neutron flux changed five-fold over various directions. External effective doses to residents in the 350 m-zone near the JCO industrial accident site are estimated, taking into account the directional dependence of the radiation. The maximum doses for outdoors and indoor in the 350 m zone are estimated at 30 and 12 mSv, respectively. Average outdoor and indoors doses in the 350 m zone are estimated to have been 5.6 and 2.2 mSv, respectively. If all the residents were indoors during the accident, 47% of those in the 350 m zone might have received external doses of less than 1 mSv.