Analysis for Impact of Seismic Capacity Uncertainty on System Reliability

It is important for nuclear power plant safety whether the safety system can work well or not in seismic condition. Seismic capacity data are important parameters to describe the component reliability, based on which the High Confidence, Low Probability of Failure (HCLPF) for component can be obtained by seismic margin evaluation. However, other than median ground acceleration capacity, the aleatory and epistemic uncertainties are also key factors for component reliability, so the seismic hazard curve is one of the important affecting factors and should be taken into account in the system reliability analysis. In this paper, passive residual heat removal system in AP1000 is calculated as an example, Fault Tree(FT) method is used to analyze the system reliability at different ground acceleration levels, the contribution to the system failure of components having different uncertainty parameters are given. And Monte Carlo(MC) simulation is used to evaluate the system reliability in seismic situation based on different seismic hazard curves, the effect of seismic hazard curve and seismic capacity uncertainty are put forward. Then the following conclusion is gotten: since the impact of seismic capacity uncertainty on the system reliability is affected by the relationship between the median ground acceleration capacity of the component and the ground acceleration level, that is, when the median ground acceleration capacity of the component is much higher than the ground acceleration level, the higher is the uncertainty, the higher is the component failure probability, when the median ground acceleration capacity of the component is close to or even higher than the ground acceleration level, the component seismic capacity uncertainty may decrease the component failure probability, so the system reliability and the main contributors are decided by the seismic capacity data and seismic hazard curve synthetically.