Deterministic and probabilistic approaches in the supervision of Swiss nuclear power plants

The report presents an overview of the planned approach to be taken by the Central Department for the Safety of Nuclear Facilities (HSK) in systematically integrating probabilistic safety analyses (PSA) into the supervision of Swiss nuclear power plants which, so far, has been largely deterministic in character. On the basis of a description of the present rank of PSA, the principles of risk-informed super-vision as sought by HSK are outlined. In addition, practical applications of PSA are shown by a number of examples implemented alongside the development of the concept. HSK recognized the importance of PSA early on and required all Swiss nuclear power plants to conduct level-1 and level-2 PSA studies for full-power operation as early as in 1987. In early 1990, level-1 PSA was extended to cover also analyses of startup, shutdown, and outage modes of operation. PSA has become more important also as a consequence of the periodic safety reviews (PSR) carried out since 1995 within the framework of ongoing supervisory activities. It has become a permanent part in a holistic system evaluating the safety status of nuclear power plants. The next step planned by HSK is the introduction of risk-informed supervision in which probabilistic considerations will be employed side by side with deterministic approaches.