The Effect on APR1400 SI Optimization according to the Changes of ECCS Acceptance Criteria of USNRC

In early 2006, U.S. NRC has finalized the Option 3 approach for Large Break LOCA(LBLOCA) requirement. This is recognize that the initiating frequency of LBLOCA, specially double-ended guillotine(DEG) break, is small and small break and transients are high contributor to the risk. The rule determines the transition break size and for the size above the transition break, it is to be considered as beyond design basis accident. In this case, the best estimate analysis will be performed without single failure constraint. In this study, we perform sensitivity analyses for transition break size to optimize safety injection system and to evaluate the effect of the new 10CFR50.46(ECCS acceptance criteria) for APR1400 using RELAP5/Mod 3.3 code. To evaluate ECCS performance, LBLOCA has been set up best estimate methods such as CSAU, KREM and so on. However, there is no uncertainty quantification method for small break LOCA. Therefore, in this study, we apply Limit Value Approach for the transition break LOCA and best estimate approach for the LBLOCA