Fault tree conditioning methods to trace system configuration changes for the application to low-power/shutdown PSA

There have been few Probabilistic Safety Assessments (PSAs) for a whole low power and shutdown (LPSD) period since the difficulties and the cost for these assessments are considerable when compared with those for a normal operation period. The frequent changes in system configurations and the reactor coolant system (RCS) states make it difficult to estimate a risk for the LPSD period. We propose a new fault tree (FT) construction method for the LPSD PSA by using reference FTs, usually for a full-power mode, and by applying conditioning methods to the reference FTs. The conditioning method includes a condition gate function, an alternative FT structure, and the substitutions of an event to a reference FT. We used the FT of a full power PSA as the starting point to maintain a consistency between the full-power and the LPSD PSA model. Maintaining the original structure of the FT for a full-power PSA and providing flexibility to the developed FT for the LPSD are the key factors of the present study. The developed method was applied to the LPSD PSA model of the UCN 3&4 units in Korea, which showed that the FTs for the LPSD PSA model of a nuclear power plant (NPP) can be generated easily from those for a full-power mode as well as maintaining a consistency between the two models.