Study on the classification and optimization of non isolated small LOCA accidents of EPR nuclear power units under shutdown conditions

This paper focuses on the classification of non-isolated small break loss-of-coolant accidents (LOCA) for the EPR nuclear power unit at Taishan Nuclear Power Plant under shutdown conditions. Through the calculation of primary circuit pipe break frequencies and the use of event tree analysis to determine the frequency of stuck-open safety valve events in the pressurizer, the study determines the frequency of non-isolated small breaks under shutdown conditions. Probabilistic safety assessment methods are employed to determine the risk importance of non-isolated small breaks under shutdown conditions. Based on the event frequency and risk importance analysis results, non-isolated small LOCA accidents under operational states C, D, and E are categorized as design extension accidents. This analysis provides technical support for the modification of the Final Safety Analysis Report (FSAR) and operating technical specifications for the EPR nuclear power unit, thereby achieving the goal of simplifying requirements in the operating technical specifications, shortening major maintenance durations, and enhancing the capacity factor of the unit.