Time-independent and time-dependent contributions to the unavailability of standby safety system components

The unavailability of standby safety system components due to failures in nuclear power plants is considered to involve a time independent and a time dependent part. The former relates to the component`s unavailability from demand stresses due to usage, and the latter represents the component`s unavailability due to standby time stresses related to the environment. In this paper, data from the nuclear plant reliability data system (NPRDS) were used to partition the component`s unavailability into the contributions from standby time stress (i.e., due to environmental factors) and demand stress (i.e., due to usage). Analyses are presented of motor operated valves (MOVs), motor driven pumps (MDPs), and turbine driven pumps (FDPs). MOVs fail predominantly (approx. 78%) from environmental factors (standby time stress failures). MDPs fail slightly more frequently from demand stresses (approx. 63%) than standby time stresses, while TDPs fail predominantly from standby time stresses (approx. 78%). Such partitions of component unavailability have many uses in risk informed and performance based regulation relating to modifications to Technical Specification, in-service testing, precise determination of dominant accident sequences, and implementation of maintenance rules.