Evaluation of dose equivalent rate distribution in JCO critical accident by radiation transport calculation

In the prevention of nuclear disaster, there needs the information on the dose equivalent rate distribution inside and outside the site, and energy spectra. The three dimensional radiation transport calculation code is a useful tool for the site specific detailed analysis with the consideration of facility structures. It is important in the prediction of individual doses in the future countermeasure that the reliability of the evaluation methods of dose equivalent rate distribution and energy spectra by using of Monte Carlo radiation transport calculation code, and the factors which influence the dose equivalent rate distribution outside the site are confirmed. The reliability of radiation transport calculation code and the influence factors of dose equivalent rate distribution were examined through the analyses of critical accident at JCO's uranium processing plant occurred on September 30, 1999. The radiation transport calculations including the burn-up calculations were done by using of the structural information about the conversion test facility and other facilities inside JCO site, and the established 'trend pattern of neutron counting rates'. Radiation transport calculational parameters were searched to reproduce the dose equivalent rate measured at the second floor of the office building looked down conversion test facility directly. The addition of water content of 0.15 g centre dot cm sup - sup 3 to the autoclaved light weight concrete used in early analysis brought reasonable result. As the consideration of the building structure at measuring point and the instrumental structure of the precipitation tank, it was possible of lower water content in autoclaved light weight concrete. The calculated results were obtained within factor 3 of dose equivalent rates in the other monitoring points. There needs the more detail modeling of the internal facility structure in the conversion test facility and the other facility structure in the JCO site to achieve th