Comparative study on the construction of support system initiating event fault trees for a fire probabilistic safety assessment

This study compared two different fire-induced support system initiating event fault tree (SSIE FT) models: one with only fire IE initiators, and the other with both fire IE initiators and enabling events. Enabling events place a system in a state such that it transitions from the operating state to the failed state when the initiating event occurs. To develop the fire event probabilistic safety assessment (PSA) models for these two different SSIE FT models, hypothetical internal event accident scenarios were used based on the assumption that a fire included in the fire event PSA causes a reactor shutdown. The results obtained indicate that SSIE FT models with only fire IE initiators, in terms of quantification results of fire event PSA models, are equivalent to those with both fire IE initiators and enabling events under the assumption that a fire included in the fire event PSA results in a reactor shutdown. For the case where a fire in a nuclear power plant (NPP) does not lead to a reactor shutdown, on the other hand, this study showed that SSIE FT models with only fire IE initiators might underestimate the real fire-induced risk, whereas SSIE FT models with both fire IE initiators and enabling events could lead to reasonable quantification results for risk due to fires.