A combined deterministic and probabilistic procedure for safety assessment of beyond design basis accidents in nuclear power plant: Application to ECCS performance assessment for design basis LOCA redefinition

•We propose the procedure for safety assessment of the beyond design basis accidents.•Deterministic (BEPU) and probabilistic (PSA) approaches are combined in the procedure.•We premised that LOCAs for any breaks larger than TBS would be regarded as BDBA.•Performance of ECCS is assessed against BDB LOCA for original design/design changes.•It is shown that the proposed methodology is applicable to safety assessment of BDBA.

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) is proposed for safety assessment of the beyond design basis accidents (BDBAs). In the CDPP, the conditional exceedance probability 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 and conditional core damage probability. To verify applicability of the methodology, performance of the APR-1400 emergency core cooling system is 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. In addition, discussions are made for analysis results for allowable NPP changes of emergency diesel generator start time extension and power uprating. It is concluded that the proposed CDPP is applicable to safety assessment of BDBAs in NPPs without significant erosion of the safety margin.