The Mission Time Analysis in Level 2 Probabilistic Safety Assessment of the Third-Generation Nuclear Power Plant

Abstract Mission times have to be defined for the probabilistic system or equipment failure analysis. Severe accident mitigating systems may need to be in operation for a long time period, thus mission time considered in Level-2 probabilistic safety assessment (L2PSA) is usually longer than in Level-1 probabilistic safety assessment (LIPSA). By simplication, the mission times in the LIPSA are often defined globally as a maximized delay before obtaining a stable state without core damage. A common mission time is often defined as 24 hours for all systems and all accidents in LIPSA. L2PSA has another perspective. In practice, most LIPSAs with the issue of containment challenge due to core concrete interaction which may pressurize the containment or melt through the containment bottom within typical time scales of one to several days. Due to the complexity of the severe accidents and the diversification of the mitigation measures, there is no common time in L2PSA. Different times are used, like 24 hours or 72 hours. Selection principles of the mission time in L2PSA were researched and the modeling methods were developed. As an example of generation III NPP, the mission time was chosen and modeled, and the sensitivity was analyzed.