A study on the safety assessment methodology of nuclear power plant for beyond design basis accidents

Abstract

The concept and assessment approach of nuclear safety in nuclear power plants (NPPs) have been evolved with the technological progress and the lessons learned from the major events. Recently, studies on the integrated approach of deterministic and probabilistic method have been done. In this study, a combined deterministic and probabilistic procedure (CDPP) was proposed for safety assessment of the beyond design basis accidents (BDBAs). In the CDPP, the conditional exceedance probability (CEP) obtained by the best estimate plus uncertainty method acts as go-between deterministic and probabilistic safety assessments, resulting in more reliable values of core damage frequency (CDF) and conditional core damage probability (CCDP). To verify applicability of the methodology, performance of the APR-1400 emergency core cooling system (ECCS) was assessed against large break loss of coolant accident (LOCA), under the premise that LOCAs for any breaks larger than transition break size would be regarded as BDBA. It was confirmed that current APR-1400 ECCS design has capability to mitigate BDB LOCA. In addition, discussions were made for analysis results for allowable NPP changes of emergency diesel generator (EDG) start time extension and power uprating. The safety assessment of OPR-1000 for station blackout (SBO) accident, which is a typical BDBA and important contributor to overall plant risk, was performed by applying the CDPP. It was confirmed that the SBO risk should be reevaluated by eliminating excessive conservatism in existing probabilistic safety assessment (PSA) to meet the targeted CDF and CCDP. By estimating the offsite power restoration time appropriately applying CDPP, the SBO risk was reevaluated, and it was finally confirmed that current OPR-1000 system has capability to mitigate the SBO. Therefore, it is concluded that the proposed CDPP is applicable to safety assessment of BDBAs in NPPs without significant erosion of the safety margin.