- Development of Implementation Guidance for SSHAC Level 3 & 4 Assessments used in Probabilistic Seismic Hazard Analyses for Nuclear Facilities
- A. Kammerer ; J.J. Bommer ; K.J. Coppersmith ; J.P. Ake
- Book Title / Journal: Transactions Structural Mechanics in Reactor Technology (SMiRT) Conference, New Delhi, India
- Year: 2011
- Risk & Reliability
- Seismic risk analysis for nuclear facilities requires probabilistic characterization of both the earthquake loading and the fragility of structures, systems and components, including consideration of the important contributors to uncertainty. The seismic hazard is determined through a probabilistic seismic hazard analysis (PSHA), which requires demonstration that the analyses have identified, quantified and incorporated both aleatory and epistemic uncertainties. The explicit characterization of uncertainty contributes to regulatory assurance by reducing the likelihood of unforeseen circumstances that have not been considered in the safety evaluation. Aleatory (random) variability in both the degree and timing of future seismicity and the ground shaking generated by specific earthquakes is accounted for through an integration process within PSHA. However, the associated epistemic (modeling or interpretation) uncertainty requires expert judgment and the use of logic trees. For critical facilities such as nuclear power plants (NPPs), the judgments of multiple experts are required to capture the complete distribution of technically defensible interpretations of the available Earth science data. The guidelines developed by the Senior Seismic Hazard Analysis Committee (SSHAC) as described in NUREG/CR-6372 provide a structured framework for conducting multiple expert assessments. Following 15 years of experience in applying the SSHAC guidelines for hazard studies for critical facilities, the US Nuclear Regulatory Commission (NRC) conducted a study of the lessons learned from practice. These lessons have now been distilled into a new US NRC NUREG-series report that provides additional practical guidance on implementing the SSHAC assessment process. The NUREG focuses primarily on the higher levels of SSHAC process (Levels 3 and 4), which are the most complex but provide a higher degree of regulatory assurance. The new NUREG gives clear guidance on the requirements for such studies, particularly SSHAC Level 3, which received relatively little attention in the original SSHAC guidelines (where the emphasis was on Level 4 studies). This NUREG also corrects the misperception that the most significant increase in complexity and likelihood of regulatory assurance occurs between Levels 3 and 4. The actual increase occurs between Levels 2 and 3. Indeed, for new nuclear sites the NRC makes no distinction between Level 3 and 4 studies, both of which are viewed as appropriate processes for conducting new PSHA studies.
- Implementation Guidance for SSHAC Level 3 & 4 Processes
- K.J. Coppersmith ; J.J. Bommer ; A. Kammerer ; J.P. Ake
- Book Title / Journal: 10th International Probabilistic Safety Assessment & Management Conference
- Year: 2010
- Risk & Reliability
- Keywords: PSHA ; Epistemic uncertainty ; SSHAC
- Risk analysis for critical facilities requires a probabilistic assessment of the hazards that could affect the installation. The complexity of the processes that generate geological hazards such as seismic ground shaking and volcanic events is such that there is inevitably large uncertainty associated in the hazard assessment. This uncertainty is reflected in the range of legitimate technical interpretations made by informed technical experts based on the available data. Procedures to develop multiple expert assessments for seismic hazards in a structured process have been established in the SSHAC (Senior Seismic Hazard Analysis Committee) guidelines. The objective of the present paper is to capture and clarify the insights gained from performing a number of detailed assessments using the SSHAC approach over the past 10-15 years. Unlike classical expert elicitation, which attempts to extract information from independent experts, the SSHAC process encourages interaction amongst experts and fosters learning by the experts throughout the process, with the ultimate objective of capturing the full community distribution of technical interpretations. The SSHAC guidelines, written largely in abstract, have now been implemented in practice several times. In these studies valuable lessons have been learned, which are now being distilled into a new U.S. Nuclear Regulatory Commission NUREG-series report to provide practical guidance on implementing SSHAC processes for hazard assessments. A key lesson from these studies is that higher level SSHAC processes (Levels 3 and 4) which specify the use of a Participatory Peer Review Panel (PPRP), provide a higher degree of regulatory assurance and stability for the initial development of hazard models for safety-critical installations. Also, significant technically-informed participation by project sponsors and regulators throughout the process enhances the likelihood of regulatory acceptance.