Approximate formulas for treating asymmetrical common cause failure events

A parameter estimation of common cause failure (CCF) events for parametric models such as the alpha factor and Multiple Greek letter methods is based on a symmetry assumption that the probabilities of CCF events involving similar components are the same. However, many components in nuclear power plants (NPPs) have asymmetrical characteristics for their failures. In this paper, we derived the approximate formulas for modeling asymmetrical CCF events and estimating their parameters by using a decomposition approach. The total failure events of a component including the CCF events were divided into their symmetry part and their asymmetrical part. Under the assumption that the CCF events of each divided part were the symmetrical CCF events within them, the alpha factor method and the basic parameter method were employed for a parameter estimation of the asymmetrical CCF events. The derived approximate formulas were applied to the CCF analysis of three emergency diesel generators (EDGs) to demonstrate their adequacy.