Valorisation of probabilistic seismic hazard results in Finland (VALERI)

Probabilistic seismic hazard analysis (PSHA) is the standard method to assess seismic hazard for nuclear power plants (NPPs). In Finland, the median confidence seismic hazard at annual frequency of exceedance (AFE) 10-5 is used for designbasis earthquake (DBE), with a minimum threshold of the horizontal peak-ground acceleration of 0.1g. Exceptional earthquakes for design extension conditions (DECC) are proposed with median confidence at AFE 10-7/year (STUK, 2019). In this work, we explore the possibilities of DBE being anchored to other confidence level hazards, since the use of median is the minority position in Europe. We outline PSHA as a tool for hazard calculations, and how hazards are used in risk assessment and risk-informed decision-making. We particularly focus on the treatment of uncertainties and arguments about the mean and fixed-confidence hazards. The goal is to probe if regulatory transition away from median confidence hazard is (i) desirable, (ii) possible and (iii) identify the foreseeable difficulties. We discuss possible options for DBE and DEC C, for the consideration of the different stakeholders. Since the use of median hazard has a long tradition in Finland, an update would be no trivial undertaking.