In the aftermath of the Fukushima accident, the nuclear community has been re-evaluating seismic hazard at nuclear facilities worldwide. One of the major challenges in this effort is quantifying and reducing epistemic uncertainties;  these uncertainties can result in unrealistic hazard estimates having significant implications. To this end, licensees and regulators need a reliable framework to evaluate the characteristics of earthquake ground motions at rock sites, especially at high frequencies. Ground motions at high frequencies are driven by the combination of rock amplification and attenuation.

Energy dissipation effects are measured by kappa, the slope of amplitude decay at high frequencies in the spectral domain. Kappa exerts a major influence on the results of Probabilistic Seismic Hazard Analysis – very consequential at low probabilities, for nuclear plant components that are sensitive to high frequencies. It is believed to be correlated with site conditions – harder rock has lower kappa: thus less attenuation of high frequency energy. This issue is of critical importance in defining input ground motions for earthquake safety assessments of nuclear facilities worldwide.

For this purpose, the OECD Nuclear Energy Agency (NEA) organised a virtual workshop in collaboration with the Canadian Nuclear Safety Commission (CNSC) to discuss the current state of the practice to evaluate the characteristics of ground motions at rock sites.

Opening remarks were delivered by the NEA Head of the Division of Nuclear Safety, Technology and Regulation, Véronique Rouyer and the CNSC Vice-President and Chief Scientific Officer, Peter Elder.

The workshop brought together the most important experts in this area from academia, regulatory bodies, international organisations, and industry representatives involved in the seismic hazard analysis and seismic design and assessment of nuclear facilities. During the four-day workshop, participants exchanged the most recent research results and provided consensus conclusions and recommendations on how to reduce epistemic uncertainties in high-frequency ground motions based on the analysis of seismic records.

DAY 1 - 25 May 2021

16:00 – 16:15 (CET)

• Opening remarks
  • Véronique Rouyer, OECD/NEA Head of Division Nuclear Safety, Technology and Regulation
  • Peter Elder, CNSC Vice-President and Chief Scientific Officer
• Context of the Workshop and Goals
  • Neb Orbovic, CNSC, Chair and the Moderator of the Workshop

All presenters have 30 min for the presentation and 15 min for the discussion

16:15 – 17:45 (CET)

• Fabrice Hollender (CEA, Cadarache, France), Paola Traversa (EDF, Aix-en-Provence, France), Emeline Maufroy (Universite Grenoble Alpes, France), Zafeiria Roumelioti (University of Patras, Greece): What are the local features that influence the high frequency content of ground motion records? Lessons learnt from the French accelerometric network and other sites
• Fabrice Hollender (CEA, Cadarache, France), Paola Traversa (EDF, Aix-en-Provence, France), Hussein Shible (CEA, Cadarache, France), Pierre-Yves Bard (Universite Grenoble Alpes, France): Tackling the ‘reference motion’ issue: alternative approaches to Vs-Kappa corrections

17:45 – 18:00 (CET)    BREAK

18:00 – 19:30 (CET)

• Norman Abrahamson (UC Berkeley, USA): New method for Kappa scaling factors
• Olga-Joan Ktenidou (National Observatory of Athens, Athens, Greece), Erion-Vasilis Pikoulis (University of Patras, Greece): Kappa estimation in challenging conditions

DAY 2 - 26 May 2021

16:00 – 17:30 (CET)

• Samantha Palmer and Gail Atkinson (University of Western Ontario, London, Ontario, Canada): Kappa at rock sites in eastern Canada (CNSC/Western Research Project)
• Olga-Joan Ktenidou (National Observatory of Athens, Athens, Greece), Robert Darragh (Pacific Engineering & Analysis, El Cerrito, CA, USA), Paola Traversa (EDF, Aix-en-Provence, France), Yousef Bozorgnia (UCLA, Los Angeles, California, USA), Walter Silva (Pacific Engineering & Analysis, El Cerrito, CA, USA): Kappa estimation at hard-rock sites

17:30 – 17:45 (CET)   BREAK

17:45 – 19:15 (CET)

• Chiara Felicetta et al. (INGV, Milano, Italy): High-frequency attenuation parameter k in Italy: estimation methods and regional features
• Paola Traversa (EDF, Aix-en-Provence, France), Fabrice Hollender (CEA, Cadarache, France), Celine Gélis (IRSN, Fontenay aux Roses, France), Aurore Laurendeau (IRSN, Fontenay aux Roses, France) and Jessy Mayor (EDF, Saclay, France): Ground motion at reference sites in weak to moderate seismicity areas: case study of France

DAY 3 - 27 May 2021

16:00 – 17:30 (CET)

• Fabrice Cotton, Annabel Haendel, Marco Pilz and Chuanbin Zhu (GFZ German Research Centre for Geosciences, Potsdam, Germany): Variability of high-frequency site responses at surface and depth
• Annabel Haendel (GFZ German Research Centre for Geosciences, Potsdam, Germany), John Anderson (University of Reno, Reno, Nevada, USA), Marco Pilz, Fabrice Cotton (GFZ German Research Centre for Geosciences, Potsdam, Germany): A frequency-dependent model for the shape of the Fourier amplitude spectrum at high frequencies

17:30 – 17:45 (CET) BREAK

17:45 – 19:15 (CET)

• Lauri Rinne (AFRY, Espoo, Finland): Seismic wave attenuation and spectral decay parameter, kappa, in crystalline bedrock at Olkiluoto, SW Finland
• Yoshi Fukushima (IAEA, Vienna, Austria): Necessary consideration of theoretical backgrounds for high frequency decay on the site effect

Day 4 - 28 May 2021

16:00 – 17:30 (CET)

• Discussion - Proposal for Conclusions and Recommendations