Dose estimation of JCO staffs based on behavior survey and calculations

The radiation doses of a few JCO staffs are evaluated directly by each personnel dosemeter, because of extremely high dose at the criticality accident on 30 September, 1999. The doses of most of JCO staffs are estimated by dose distributions of the scene based on monitoring data, calculation results, and each behavior survey during the accident. Variations of fission rate and dose equivalent rate of gamma ray, which consists of a prompt and a secondary gamma ray during the accident, are estimated by gamma ray-area monitors (11 ionization chambers) installed in each site of the JCO, monitoring stations (NaI(Tl) detector) of the NIRS, and monitoring posts (He-3 detector) of the JAERI. The fraction of the secondary gamma ray to total gamma ray dose equivalent is relatively small, and estimated within 4.6-6.6%. The measured values of the gamma ray-area monitors, which couldn't follow a sudden change of dose rate during one burst period (about 1 min) of the accident, are corrected. Exposed doses are estimated by the relationships of source-distance including absorption and scattering of medium. Neutron dose attenuation through shielding materials are calculated by using ANISN code. Aerial dose distributions in the conversion test building of the JCO are analyzed by Monte Carlo code MCNP-4B. Estimated doses for all of the staffs based on each behavior survey are compared with the measured values of film badge and whole body counter. (M. Suetake)