Proceedings of the 12th International INQUA meeting on paleoseismology, active tectonic and archaeoseismology

384 PATA Days 2024 Regulatory requirements and objectives of the review The main objective of SSM’s review of SKB’s spent fuel repository application is to provide the government with documentation, including a recommendation, to either grant a license or not. Hence, the suitability of the site to host a spent fuel repository, both at present and for the long term after repository closure, is assessed. Therefore, the assessment of the seismic activity is not only based on the present conditions, but also on how they will change during a glacial cycle. SSM’s review is not only the basis for the statement to the government, but serve also as information to the public such as the municipality that will host the spent nuclear fuel repository. The regulatory requirements are formulated on a general level, including requirements on barrier system robustness and a risk criterion. Hence, there are no descriptive criteria that directly relate to different parts of the analysis as, for instance, seismic hazard. Review strategies When siting a spent fuel repository, it’s imperative to quantify the spatial and temporal variations in seismicity, in order to assess the suitability of a site. In stable continental regions (SCR) such as the Baltic Shield, this pose a challenge due to the absence of larger events and the short period of observations. To extent the knowledge of the spatial and temporal variability of earthquakes rates and magnitudes beyond the historical and current observations, the level of seismicity beyond the historical recordings needs to be included. However, there is significant uncertainty and discussion on the level of paleoseismicity in Sweden as a whole, and in central and southern Sweden in particular. This pose a challenge when reviewing an application for a sensitive facility such as a spent fuel repository. SSM addressed this challenge with field excursion to key sites claimed to show credible evidence of pre- historic earthquakes of considerable magnitudes, expert panel elicitation and expert consultation. These three topics are addressed in following section. Field excursion In May 2009, SSM arranged a 2-day field excursion led by Nils-Axel Mörner, who for many years have strongly argued for an abundance of paleoearthquakes since the Weischelian ice sheet disappeared, 11,000 – 9,500 years before present (Mörner, 2009). The excursion was part of the excursion led by Mörner during the 33rd International Geological Congress that was held in Oslo the year before (Mörner et al., 2008). The objective of the excursion arranged by SSM was to evaluate the validity of the claims by Mörner. SSM also invited four experts for additional examination during the excursion. Expert Panel Elicitation In 2005, the authority selected several experts addressing the frequency of earthquakes greater the magnitude 6 within a 10 km radius from the potential site (SSI 2005:20). The expert’s used different methods that can be grouped into two different conceptual approaches, earthquake estimations based on observational data respective model calculations. The first approach is based on observations from the present and the past including non-verified claims of paleoseismic activity. Also, present seismicity in SCR comparable to the Baltic shield were used. The second approach is based on ice sheet characteristics, such as ice sheet extent and thickness, and how the basement could have responded to the glacially imposed stresses. Expert consultation SSM (and prior to 2008 the Swedish Nuclear Power Inspectorate and the Swedish Radiation Protection Authority) early on decided that the site investigations should be followed by an independent expert group formed for this purpose called Independent Site Investigation Tracking and Evaluation (INSITE and OVERSITE groups, SSM2010). Several of the experts contracted by SSM also have undertaken work for other authorities, which has given insights into other nuclear waste programs and has increased their competence and also given value to SSM. Numerous reports were delivered spanning many disciplines, including