Probabilistic Safety Assessment (PSA) for the plant shutdown state of nuclear power plants is performed based on Plant Operational States (POSs) which depend on periodic inspections and maintenance. For a typical BWR, the shutdown POSs are defined in consideration of various plant conditions: boundaries of Primary Containment Vessel (PCV)/ Reactor Pressure Vessel (RPV), reactor water level, availability of mitigation systems and temperature/pressure of primary coolant. The shutdown POSs for a BWR plant may involve conservative simplifications. One of the simplifications is treatment of PCV/RPV heads conditions during the POSs involving their opening/closing operations. Usually the PCV/RPV heads conditions are assumed to be the most onerous in terms of accident sequence modelling on both Level 1 and 2 PSA.

In this study, a sensitivity analysis is performed to understand the significance of the conservatism by refining the POSs which involve opening/closing operations of PCV/RPV heads.

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