Under the auspices of the Standards Developing Organizations (SDOs), the nuclear industry is currently in the process of developing standards for Probabilistic Risk Assessments (PRAs) to support risk-informed applications of nuclear power plants, including PRA standards for Low Power and Shutdown (LP/SD) operations. A particularly challenging part of building a LP/SD PRA model is the definition of quasi-steady Plant Operating States (POSs). This document provides guidance for developing the POSs needed for the creation and use of a LP/SD risk model based on a standard set of 15 generally applicable POSs. NUREG/CR-6144, prepared by the Brookhaven National Laboratory for the Nuclear Regulatory Commission (NRC) (for the Surry plant), was used as a foundation to build the set of standard set of 15 POSs presented here. Within these standard POSs, there is room for modification for plant- and application-specific purposes such as the specific LP/SD risk application of transition risk analysis. Specifically, modifications are accomplished by subdividing POSs. Another particularly challenging part of building a LP/SD PRA model is defining and modeling periods of transition which for the suggested guidance, are defined as risk-significant activities between quasi-steady POSs which involve significant configuration changes. This guidance provides support for developing the models needed for the evaluation of the risk associated with plant transitions occurring in the process of the plant shutdown and startup. Specifically, guidance is provided to lead a PRA modeler through the identification and characterization of the transitions and the selection and generation of appropriate transition PRA modeling elements and data. The transition modeling guidelines are applicable for procedure-based transitions (as opposed to physical-phenomenon-type transitions such as the drop in decay heat level) occurring during LP/SD operations. The guidance presented is intended to be applicable for all Light-Water Reactors (LWRs). The detailed guidance processes described are currently focused on Pressurized-Water Reactors (PWRs).

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