Abstract: For verifying the reasonability of combination of SSE and LOCA which affect the nuclear power plant primary piping during accident conditions, the simultaneous SSE/LOCA effect on primary piping should be demonstrated that can not occur. Concluding from probability statistical analysis, the probability of the simultaneous SSE/LOCA event is lower than that considered in the nuclear power plant design if the events are considered to be independent. According to deterministic methodology calculation, SSE is not the initiator of a LOCA even there is a detectable leakage flaw in the primary piping. Consequently, the assumption of that the SSE and LOCA can not act simultaneous is reasonable in the primary piping design, the load combination of SSE and LOCA which is combined as the square root of sum of square method is conservative in the primary piping stress analysis, and that can assure the protection of the health and safety of the public.