An approach toward evaluation of fp behavior in npps under severe accidents

Procedures for evaluation of fission product (FP) behavior under severe accident (SA) conditions should be established to determine radioactive contamination in the major areas in Fukushima Daiichi Nuclear Power Plant for suitable preparation for decommissioning works and, at the same time, to prepare for validation of SA evaluation codes for the BSAF Project Phase II. An approach toward evaluation of FP behavior consisting of forward evaluation and backward evaluation procedures is proposed. The forward evaluation procedures are carried out by applying mechanistic SA evaluation codes, in which FP behavior is divided into 5 categories. But there are several parameters not determined by the mechanistic models, e.g., sizes and locations of defects of the primary containment vessels (PCVs) to determine accidental FP leakage rate from PCVs to the major areas. Those parameters can be determined by applying backward evaluation procedures, which consist of mass balance analysis to determine the amounts of suspending and depositing FPs by applying guess values for accidental FP leakage into the area, dose rate evaluation by applying the calculated FP amounts as radiation sources and then comparison of calculated and measured dose rates to determine the most realistic leakage rate. They can determine the leakage rates as solutions of the inverse problems. There are some of the backward evaluation procedures which have been successfully applied to estimate FP effluent to the environment and FP sources for the contaminated water. In the paper, main frames of the FP behavior evaluation procedures are introduced with the backward evaluation procedures to determine PCV leakage parameters.