Introduction

[Illustration from Figure 5: Expert interaction for SSHAC Level 4.]

In April 1997, after four years of deliberations, the Senior Seismic Hazard Analysis Committee released its report “Recommendations for Probabilistic Seismic Hazard Analysis: Guidance on Uncertainty and Use of Experts” through the U.S. Nuclear Regulatory Commission as NUREG/CR–6372, hereafter SSHAC (1997). Known informally ever since as the “SSHAC Guidelines,” SSHAC (1997) addresses why and how multiple expert opinions—and the intrinsic uncertainties that attend them—should be used in Probabilistic Seismic Hazard Analyses (PSHA) for critical facilities such as commercial nuclear power plants.

Ten years later, in September 2007, the U.S. Geological Survey (USGS) entered into a 13-month agreement with the U.S. Nuclear Regulatory Commission (NRC) titled “Practical Procedures for Implementation of the SSHAC Guidelines and for Updating PSHAs.” The NRC was interested in understanding and documenting lessons learned from recent PSHAs conducted at the higher SSHAC Levels (3 and 4) and in gaining input from the seismic community for updating PSHAs as new information became available. This study increased in importance in anticipation of new applications for nuclear power facilities at both existing and new sites. The intent of this project was not to replace the SSHAC Guidelines but to supplement them with the experience gained from putting the SSHAC Guidelines to work in practical applications. During the course of this project, we also learned that updating PSHAs for existing nuclear power facilities involves very different issues from the implementation of the SSHAC Guidelines for new facilities. As such, we report our findings and recommendations from this study in two separate documents, this being the first.

The SSHAC Guidelines were written without regard to whether the PSHAs to which they would be applied were site-specific or regional in scope. Most of the experience gained to date from high-level SSHAC studies has been for site-specific cases, although three ongoing (as of this writing) studies are regional in scope. Updating existing PSHAs will depend more critically on the differences between site-specific and regional studies, and we will also address these differences in more detail in the companion report.

Most of what we report here and in the second report on updating PSHAs emanates from three workshops held by the USGS at their Menlo Park facility: “Lessons Learned from SSHAC Level 3 and 4 PSHAs” on January 30-31, 2008; “Updates to Existing PSHAs” on May 6-7, 2008; and “Draft Recommendations, SSHAC Implementation Guidance” on June 4-5, 2009. These workshops were attended by approximately 40 scientists and engineers familiar with hazard studies for nuclear facilities. This company included four of the authors of SSHAC (1997) and four other experts whose contributions to this document are mentioned in the Acknowledgments section; numerous scientists and engineers who in one role or another have participated in one or more high-level SSHAC PSHAs summarized later in this report; and representatives of the nuclear industry, the consulting world, the regulatory community, and academia with a keen interest and expertise in hazard analysis. This report is a community-based set of recommendations to NRC for improved practical procedures for implementation of the SSHAC Guidelines.

In an early publication specifically addressing the SSHAC Guidelines, Hanks (1997) noted that the SSHAC Guidelines were likely to evolve for some time to come, and this remains true today. While the broad philosophical and theoretical dimensions of the SSHAC Guidelines will not change, much has been learned during the past decade from various applications of the SSHAC Guidelines to real PSHAs in terms of how they are implemented. We anticipate that, in their practical applications, the SSHAC Guidelines will continue to evolve as more experience is gained from future SSHAC applications. Indeed, to the extent that every PSHA has its own particular wrinkles to iron out, some flexibility must be maintained in interpreting SSHAC (1997) and what we recommend here. For the same reason, it will surely be helpful for present and future projects to take stock of their own experience in implementing the SSHAC Guidelines.

The next section, The SSHAC Guidelines, recounts the reason for SSHAC (1997) and a brief summary of it, emphasizing what we consider to be the essential element of the SSHAC Guidelines, the SSHAC process of expert interaction. We then briefly describe the types of participants in the PSHAs conducted according to SSHAC (1997), how they function on the four different levels of SSHAC, and a brief description of high-level SSHAC PSHAs completed to date. The bulk of this report resides in the subsequent section, which presents our recommendations based on what we have learned from these completed studies and issues that arose while these studies were being conducted. We conclude with a glossary of terminology and appendices containing information about the workshops conducted as part of this project.

The SSHAC Guidelines are a complex tapestry woven from many different threads, occasionally imperfectly. Dealing with both the individual threads and the fabric as a whole in the sequential form of a written report presents significant challenges and, ultimately, arbitrary decisions as to which thread comes first, the most common commentary we received in the 17 reviews of the draft of this report. As is the case for the SSHAC Guidelines readers of this report must exercise some patience in working through it.

Version 1.0

First posted May 4, 2009