An Analysis of the Effect of Correlation of Response in the System Reliability Analysis in Seismic PSA of Nuclear Power Plants

Abstract

In seismic probabilistic safety assessments of nuclear power plants. it has been recognized important to consider the effect of correlation among responses and/or capacities of components. The authors developed a new method to calculate a failure probability of a system considering the effect of correlation of component failures by a direct Fault Tree quantification using the Monte Carlo method. By this method, the failure probability of a system can be calculated with the effect of arbitrary correlation not only on the occurrence probabilities of intersections of component failures but also on the occurrence probability of a union of component failures, which has been ignored by the calculation method developed in the phase-1 of the Seismic Safety Margins Research Program in the U.S.A. The usefulness of this method was demonstrated by the calculation of the failure probability of a Residual Heat Removal (RHR) system in a Boiling Water Reactor. This result showed that the correlation of response significantly lowered the occurrence probability of the union of component failures and influenced the calculated failure probability of the RHR system, indicating that the neglect of effect of correlation on the union of component failures might cause an excessive overestimation of failure probability of a system.