Valorization of probabilistic seismic hazard results in Finland

Probabilistic seismic hazard analysis (PSHA) is currently the standard method for assessing seismic hazards for nuclear power plants (NPPs) in Finland. To obtain values of ground-motion parameters for design purposes, two decisions are made regarding which annual frequency of exceedance (AFE) should be adopted and from which hazard curve the ground-motion value should be read for the design-basis earthquake (DBE) and the design-extension condition earthquake (DEC EQ, DEC C for short). The current regulatory status in Finland, given in the guide YVL B.7 (STUK 2019) by the Radiation and Nuclear Safety Authority of Finland (STUK), is that the median- confidence seismic hazard at AFE 10−5 is used for DBEs at NPPs with a minimum horizontal peak-ground acceleration (PGA) of 0.1 g. The high variability of ground-motion shaking patterns and various examples of exceedance of the DBE ground motion from that of natural earthquakes throughout the world have resulted in upgrades to meet new definitions of the requirements for ground motion beyond that of DBEs. Exceptional earthquake effects, with an estimated frequency of occurrence less than 10−5/year are postulated in DEC C for NPPs in Finland. Here, we outline the PSHA and its consequent use in risk assessment and risk-informed decision-making. We review arguments about the mean and median hazard curves in the PSHA. We draw particularly on the outcomes of the SENSEI (SENsitivity study of SEIsmic hazard prediction in Finland) project conducted under the auspices of STUK in 2019−2020, present new figures based on the SENSEI set of hazard calculations, and analyze them. We focus on the ratio of the mean and median hazard of PGA, spectral acceleration at 1 Hz, 5 Hz, and 25 Hz at AFE levels 10−4, 10−5, 10−6, 10−7 and 10−8. We analyze possible options for DBE and DEC C for consideration of the various stakeholders. Since the use of median hazards has a long tradition in Finland, an update is no trivial undertaking. The suite of options is not necessarily exhaustive.