Neutron dose and power released by the JCO criticality accident in Tokai-mura

There was no monitor and no instrumentation to measure neutrons in the JCO facilities when the criticality accident began. Two neutron monitoring posts located at 1.7 and 2.0 km from the JCO site, however, recorded the process of the criticality accident from its beginning. Through the analysis of the monitoring data, neutron doses of 0.096 and 0.014 μSv were evaluated at 1.7 and 2.0 km, respectively. Neutron transport in air-over-ground geometry was calculated to over 2 km from the building where the criticality accident took place, assuming simple models for shielding. The neutron dose calculated at 1 m above ground was fitted to the measured data. The distances corresponding to neutron doses of 1, 0.1, 0.01 and 0.001 mSv were about 450, 700, 1000 and 1300 m, respectively. The neutron dose at 100 m was estimated to have been 81 mSv. The total power released by the accident was estimated to have been about 17 kWh corresponding to 1.9×1018 fissions of 235U.