The role of external events PSA in the Finnish regulatory approach

PSA has been used in Finland to support regulatory decision making since the late 1980's. Seismic events, harsh weather and other off-site external events were included in the PSA models of the operating units in 1990's and several updates and extensions have been carried out since then. For new units a preliminary full scope PSA is required at the design phase and it shall be refined during construction. The Finnish environmental conditions are, in general, moderate with no recorded destructive earthquakes and no experiences from extensive destruction by other natural events. However, even moderate external events may result in significant risks, if they are not properly considered in the design. For example, the northern climate poses some challenges to the design of nuclear power plants. According to the current view, the original design basis of the operating units for seismic and other external events was not in all respects adequate. External events PSA has had an important role in evaluating the safety of the units and in making decisions on safety improvements. For new units external events have been considered extensively starting from the early design phase. External events PSAs were also used in the national and EU 'stress tests' after the Fukushima accident. According to the Finnish experience, external events PSA is a useful tool in the safety management and regulatory control on nuclear power plants. It has been used to identify and prioritize possible safety improvements and to evaluate the adequacy of the design basis. It has also been used in support of licensing of new units and periodic safety reviews of operating units. (authors)