Probabilistic Safety Assessment (PSA) of Natural External Hazards Including Earthquakes. Workshop Proceedings, Prague, Czech Republic, 17-20 June 2013

The Fukushima Dai-ichi accident triggered discussions about the significance of external hazards and their treatment in safety analyses. In addition, stress tests results have shown vulnerabilities and potential of cliff-edge effects in plant responses to external hazards and have identified possibilities and priorities for improvements and safety measures' implementation at specific sites and designs. In order to address these issues and provide relevant conclusions and recommendations to CSNI and CNRA, the CSNI Working Group on Risk Assessment (WGRISK) directed, in cooperation with the CSNI Working Group on Integrity and Ageing of Components and Structures (WGIAGE), a workshop hosted by UJV Rez. The key objectives of the workshop were to collect information from the OECD member states on methods and approaches being used, and experience gained in probabilistic safety assessment of natural external hazards, as well as to support the fulfillment of the CSNI task on 'PSA of natural external hazards including earthquakes'. These objectives are described more in detail in the introduction in Chapter 1 of this report. The WGRISK activities preceding the workshop and leading to the decision to organize it are described in Chapter 2 of this report. The focus of the workshop was on external events PSA for nuclear power plants, including all modes of operation. The workshop scope was generally limited to external, natural hazards, including those hazards where the distinction between natural and man-made hazards is not sharp. The detailed information about the presentations, discussions, and results of the workshop is presented in Chapter 3 of this report. Some general conclusions were agreed on during the workshop, which are presented in the following paragraphs. - The lessons learned from the Fukushima Dai-ichi reactor accidents and related actions at the national, regional, and global level have emphasized the importance to assess risks associated (authors) with external hazards, including combinations of those hazards, and their impacts. It is important that such an analysis covers not only individual plant units, but also the site as a whole, including all dependent effects and impacts. - While systematic approaches for addressing external hazards in PSA currently exist and there is a well-developed state-of-practice (e.g., with respect to external flooding, seismic, high winds), additional work is needed; for example, the use of conservative approaches to address uncertainties might be practical, but more realistic evaluations provide better view on the real problems. - Some methods and guides are available for seismic hazard determination, identification of external hazards, screening of external events for detailed consequence analysis, including several lists of screening criteria. However, additional development is also needed in area of consensus standards and guides; for example IAEA continues developing the methodological support for external hazards analysis. - The major areas of concern in external hazards studies are: 1. Scope of the PSA for external events in terms of plant operation regimes (e.g., full power, low power and shutdown operational states), 2. Combinations of external hazards impacts, 3. Multi-unit impacts, and 4. Screening procedure for site specific hazards. - The following are significance technical/methodological challenges for external hazard PSA: 1) Fragility analysis of non-seismic external hazards, 2) Correlation effects and consequent damage scenarios, 3) Human Reliability Analysis (HRA) for external events PSA, 4) PSA mission times for long-term external event scenarios, and 5) Significance and magnitude of the effects of climate changes on the hazard frequencies and magnitudes. - The following aspects are considered to be important good practice attributes for external hazard modeling in PRA: 1) Critically challenging assumptions, 2) Calibrating models, 3) Accounting for underlying physical processes, 4) Fully treating dependencies, 5) Involving multidisciplinary teams, and 6) Prompt ly and broadly disseminating information. Recognizing the impetus for action provided by actual operational events (including the Fort Calhoun flooding as well as the Fukushima Dai-ichi reactor accidents), it has been noted that WGRISK can provide stronger (and better-focused) cases for action by increasing its use of operating experience feedback. Among other things, this could imply strengthening ties with associated international working groups, particularly the NEA/Committee of Nuclear Regulatory Authorities (CNRA) Working Group on Operating Experience (WGOE). An additional action for WGRISK suggested by our review concerns the tracking of past recommendations. It appears that increased efforts by the WGRISK leadership to systematically track and dispose report recommendations would help ensure that each task performed by the group more strongly supports the group's overall objectives, and would help WGRISK improve its strategic planning processes. Detailed information about the conclusions made during the workshop is presented in Chapter 4 of this report