Outage Key Safety Functions Configuration risk assessment for a three loops Westinghouse PWR

Abstract

The methodology developed provides guidance on the use of Probabilistic Safety Assessment (PSA) for the risk-informed evaluation of Guides which ensure the compliment of Outage Key Safety Functions (OKSFs) in Nuclear Power Plants. The methodology has been applied to the 3rd and 13th Plant Operational States (POSs) as a Pilot experience. These POSs are within the operating mode 4 (Hot Shutdown) of a 3 loops Westinghouse Pressurized Water Reactor. The addressed Guide requires the operability of just one charge pump as boric acid supply source. PSA gives a Core Damage Frequency increase (¿CDF) of 1.19•10-6 year-1 for the unavailability of the charge pump in standby, consequently, the maximum exposure time (time for the increase of Core Damage Probability of the configuration to reach 1.0E-06) for this situation is T= 53.6 hours. Given an average time for the POSs of 40 hours, it is concluded that the charge pumps requirement is correct. However, it could be improved with the inclusion of an additional inventory replacement function. This would limit the effect on Risk of the charge pump unavailability. Furthermore, the need for the external electrical sources to be available during mode 4 is ratified. The procedure requires the operability of both supply sources during the POSs. The unavailability of one of supply sources them involves a ¿CDF equal to 1.64•10-5 year-1 and a maximum exposure time of T= 3.89 hours. This requirement is considered appropriate from the risk-informed regulation point of view.