Dose Evaluation to Workers at JCO Criticality Accident Based on Whole Body Measurement of Sodium-24 Activity and Area Monitoring

Abstract

A criticality accident occurred at nuclear fuel processing facility of JCO Company Ltd. at 10:35 local time on September 30, 1999. In this accident, the doses to few workers were evaluated directly with individual dosimeters. Japan Nuclear Cycle Development Institute (JNC) estimated the dose to the JCO workers in cooperation with the Criticality Accident Investigation Committee. JNC carried out the dose evaluation using the following methods.
(1) Dose evaluation based on ²⁴Na activities produced in the human body due to neutron capture reaction, which were measured with whole body counter.
(2) Dose evaluation by the calculation from the change of the radiation dose rate which is estimated by the area monitors, and the behavior of the employee at the criticality accident.
To validate the results of the dose evaluation, dose calculation was carried out to several workers whose dose had been measured with individual dosimeters or a whole body counter. As the result, the dose calculation technique was almost applicable for the dose estimation of JCO workers.