This paper provides an overview of the experience in developing and using the initiating event fault trees (IEFTs) in the base and EOOS Probabilistic Safety Assessment (PSA) models at the Entergy Nuclear South (ENS) sites. Simple fault tree models can be developed and linked to the master fault tree in order to provide a better estimate of plant-specific initiating event frequencies. If developed considering certain aspects, there can be significant advantages to using IEFT methodology for some initiators, especially in the plant EOOS models. The most important advantage is to have EOOS re-calculate and use such initiating event frequencies for different alignments taking the components that are out of service into consideration while keeping the quantification time and complexity of the cutsets at an acceptable level. The important steps for developing the IEFTs, some insights gained through the experience at the ENS sites, and the advantages of using IEFTs will be discussed in this paper.

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