As a part of the efforts to develop the risk-informed regulation, alternative rulemaking of 10CFR50.46 is underway. In the rule, USNRC divided the current spectrum of LOCA break sizes into two regions, by determining a transition break size (TBS), and the LOCAs for any breaks larger than TBS would be regarded as beyond design basis accident (BDBA). A combined deterministic and probabilistic procedure (CDPP) was proposed for safety assessment of BDBAs. The performance of the APR-1400 emergency core cooling system (ECCS) performance was assessed against large break LOCA applying CDPP. It was confirmed that current APR-1400 ECCS design has capability to mitigate BDB LOCA by analyzing ECCS cooling performance for BDB LOCA. The proposed CDPP was also applied to design changes of the emergency diesel generator (EDG) start time extension and power uprates with simplified assumption that the probabilistic safety assessment (PSA) data are still valid. By assumptions and considerations, the CDPP to assess ECCS performance for plant design modification was reduced to calculating conditional exceedance probability (CEP) of one sequence and comparing allowable value. The allowable CEP was used to determine whether the design change is acceptable or not, and discussions were made for acceptable nuclear power plant changes.

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