Three Dimensional Analysis of RIA in PWR and BWR with High Burnup Fuel

摘要

The results of recent reactivity insertion accident (RIA) experiments with high burnup fuel performed in France (CABRI) and in Japan (NSRR) indicated that some fuel failed at the lower deposited energy in the fuel than was previously assumed. Taking it seriously, Japanese Nuclear Safety Commission (NSC) had reviewed the current licensing criteria for RIA, and the failure threshold for high burnup fuel was revised based on the new experimental data. The new failure threshold related to PCMI is defined as function of fuel burnup and it decreases stepwise with increasing fuel burnup. This change would directly affect the safety assessment of RIA. Historically, the point kinetics model or one dimensional kinetics model using core wide coefficients having a significant conservatism has been employed for the safety assessment of RIA. This approach is insufficient to evaluate the distribution of fuel enthalpy in the core considering the variation of fuel burnup. In order to investigate the influences of fuel burnup and decreased threshold on the safety assessment of RIA, the realistic analysis by using detailed three dimensional method is necessary. The three dimensional analysis of RIA in typical PWR and BWR with high burnup fuel was performed using the EUREKA-JINS/S code to understand the realistic fuel behavior during the transient and to evaluate the safety margin accurately. The fraction of fuel failure was evaluated applying the newly revised failure threshold. For the analysis of RIA in BWR, the void reactivity feedback was considered to understand how it affects on the fuel behavior under the RIA. The results of three dimensional analysis of control rod ejection accident for a typical 4-loop PWR showed that none of fuel failure occurs under this event. The results of three dimensional analysis of control rod drop accident for a typical BWR considering the void reactivity feedback indicated that the void reactivity feedback significantly influenced to decrease the maximum fuel enthalpy and the fraction of fuel failure. This paper presents the outline of three dimensional neutronic transient code "EUREKA-JINS/S" and calculated results of the RIA in typical PWR and BWR. This study was performed under the sponsorship of the Ministry of Economy, Trade and Industry (METI).