Probabilistic safety assessment of support systems on CANDU stations

Atomic Energy of Canada Limited (AECL) has performed safety and reliability assessments (otherwise known as probabilistic safety assessments (PSAs)) of support systems since 1975. The purpose of this paper is to briefly describe AECL's experience in the use of PSAs on support systems and the application of PSA for a new plant design called the Canada deuterium uranium (CANDU) 3. Between 1978 and 1983, a exhaustive PSA program was undertaken for the following CANDU stations: Gentilly-2, Point Lepreau-1, Wolsung-1, Pickering B; and Bruce B (Ontario Hydro). All the above PSA work reviewed a series of initiating events, which included loss of service water and loss of instrument air as initiating events during full-power operation and during plant shutdown conditions. The key results from the above work were the following: (1) for both service water and instrument air, the failure frequency was dominated by passive component failures; (2) the plant response dictated the need for design changes to meet the probabilistic acceptance criteria used for licensing at that time. For the CANDU-3, to avoid design changes and backfits during the construction and commissioning phases of the project, reliability targets and interface requirements are set early and are required to be met.