Development of a practical approach to estimate the failure rate of a system using a fault tree method in PSA

An accident sequence in the PSA (Probabilistic Safety Assessment) is expressed in production of an initiating event and a mitigating system. To calculate the occurrence rate of the accident sequence in case that the initiating event is modeled using a fault tree, we have to combine two fault tree models for the initiating event and the mitigating system. Note that the initiating event is modeled in terms of the failure rate and the mitigating system is modeled in terms of the failure probability. Currently, no practical approach is available to combine the two different kinds of fault trees where one is for the failure rate and the other is for the failure probability. This report presents a practical approach how to model fault trees for the initiating event and mitigating system for the purpose of estimating the occurrence rate of the accident sequence. The basic idea of the approach is to estimate the failure rate from the failure probability during the mission time. Furthermore, in case that a system is included in both the initiating event and the mitigating system, the fault tree model already developed for the mitigating system in the PSA can be used directly for the initiating event with the proposed approach