Sensitivity Analysis on the Effect of Shutdown Period Extension in Low Power & Shutdown Core Damage Frequency

One of the most important differences between LPSD PSA and PSA for full power operations is that plant configurations and conditions significantly change during LPSD operations. To model the unique plant configuration and operational conditions during LPSD operation, an outage is divided into numerous Plant Operating States (POSs) based on Reactor Coolant System (RCS) level, temperature, pressure, decay heat load, and some other parameters. The most challenging issue in definition of POSs is estimating the duration of each POS. This study aims at investigating the impact of POS duration on LPSD Core Damage Frequency (CDF). A comprehensive LPSD PSA model has been developed for the under design IR-360 NPP. Total LPSD CDF is estimated to be 2.27E-06/year, assuming refueling outage as shutdown representative. CDF is re-evaluated for sensitivity cases with different POS durations (shorter shutdown period) the results represent 13% decrease in total CDF of LPSD conditions. Changes in CDF of POSs are also compared. Change in CDF of POSs is proportional to change in POS duration. However, for POS 9 and POS 7, with the higher primary contribution in total CDF of LPSD conditions, the percent change is more conspicuous. The study also shows the proportionality of the change in percent contributions of POS_CDF in total CDF of LPSD conditions with that of POS duration in total duration of LPSD conditions. Therefore, it is concluded that shortening of POSs with higher contributions in total duration of LPSD conditions, leads to more obvious decrease in total CDF of LPSD conditions.