Gabriel R. Toro

Probabilistic Seismic Hazard Analyses for Ground Motions and Fault Displacement at Yucca Mountain, Nevada

Probabilistic seismic hazard analyses were conducted to estimate both ground motion and fault displacement hazards at the potential geologic repository for spent nuclear fuel and high-level radioactive waste at Yucca Mountain, Nevada. The study is believed to be the largest and most comprehensive analyses ever conducted for ground-shaking hazard and is a first-of-a-kind assessment of probabilistic fault displacement hazard. The major emphasis of the study was on the quantification of epistemic uncertainty. Six teams of three experts performed seismic source and fault displacement evaluations, and seven individual experts provided ground motion evaluations. State-of-the-practice expert elicitation processes involving structured workshops, consensus identification of parameters and issues to be evaluated, common sharing of data and information, and open exchanges about the basis for preliminary interpretations were implemented. Ground-shaking hazard was computed for a hypothetical rock outcrop at -300 m, the depth of the potential waste emplacement drifts, at the designated design annual exceedance probabilities of 10-3 and 10-4. The fault displacement hazard was calculated at the design annual exceedance probabilities of 10-4 and 10-5.

The Case for Using Mean Seismic Hazard

Complete probabilistic seismic hazard analyses incorporate epistemic uncertainties in assumptions, models, and parameters, and lead to a distribution of annual frequency of exceedance versus ground motion amplitude (the “seismic hazard”). For decision making, if a single representation of the seismic hazard is required, it is always preferable to use the mean of this distribution, rather than some other representation, such as a particular fractile. Use of the mean is consistent with modern interpretations of probability and with precedents of safety goals and cost-benefit analysis.

Central and Eastern United States (CEUS) Seismic Source Characterization (SSC) for Nuclear Facilities Project

This report describes a new seismic source characterization (SSC) model for the Central and Eastern United States (CEUS). It will replace the Seismic Hazard Methodology for the Central and Eastern United States, EPRI Report NP-4726 (July 1986) and the Seismic Hazard Characterization of 69 Nuclear Plant Sites East of the Rocky Mountains, Lawrence Livermore National Laboratory Model, (Bernreuter et al., 1989). The objective of the CEUS SSC Project is to develop a new seismic source model for the CEUS using a Senior Seismic Hazard Analysis Committee (SSHAC) Level 3 assessment process. The goal of the SSHAC process is to represent the center, body, and range of technically defensible interpretations of the available data, models, and methods. Input to a probabilistic seismic hazard analysis (PSHA) consists of both seismic source characterization and ground motion characterization. These two components are used to calculate probabilistic hazard results (or seismic hazard curves) at a particular site. This report provides a new seismic source model. Results and Findings The product of this report is a regional CEUS SSC model. This model includes consideration of an updated database, full assessment and incorporation of uncertainties, and the range of diverse technical interpretations from the larger technical community. The SSC model will be widely applicable to the entire CEUS, so this project uses a ground motion model that includes generic variations to allow for a range of representative site conditions (deep soil, shallow soil, hard rock). Hazard and sensitivity calculations were conducted at seven test sites representative of different CEUS hazard environments. Challenges and Objectives The regional CEUS SSC model will be of value to readers who are involved in PSHA work, and who wish to use an updated SSC model. This model is based on a comprehensive and traceable process, in accordance with SSHAC guidelines in NUREG/CR-6372, Recommendations for Probabilistic Seismic Hazard Analysis: Guidance on Uncertainty and Use of Experts. The model will be used to assess the present-day composite distribution for seismic sources along with their characterization in the CEUS and uncertainty. In addition, this model is in a form suitable for use in PSHA evaluations for regulatory activities, such as Early Site Permit (ESPs) and Combined Operating License Applications (COLAs). Applications, Values, and Use Development of a regional CEUS seismic source model will provide value to those who (1) have submitted an ESP or COLA for Nuclear Regulatory Commission (NRC) review before 2011; (2) will submit an ESP or COLA for NRC review after 2011; (3) must respond to safety issues resulting from NRC Generic Issue 199 (GI-199) for existing plants and (4) will prepare PSHAs to meet design and periodic review requirements for current and future nuclear facilities. This work replaces a previous study performed approximately 25 years ago. Since that study was completed, substantial work has been done to improve the understanding of seismic sources and their characterization in the CEUS. Thus, a new regional SSC model provides a consistent, stable basis for computing PSHA for a future time span. Use of a new SSC model reduces the risk of delays in new plant licensing due to more conservative interpretations in the existing and future literature. Perspective The purpose of this study, jointly sponsored by EPRI, the U.S. Department of Energy (DOE), and the NRC was to develop a new CEUS SSC model. The team assembled to accomplish this purpose was composed of distinguished subject matter experts from industry, government, and academia. The resulting model is unique, and because this project has solicited input from the present-day larger technical community, it is not likely that there will be a need for significant revision for a number of years. See also Sponsors Perspective for more details. The goal of this project was to implement the CEUS SSC work plan for developing a regional CEUS SSC model. The work plan, formulated by the project manager and a technical integration team, consists of a series of tasks designed to meet the project objectives. This report was reviewed by a participatory peer review panel (PPRP), sponsor reviewers, the NRC, the U.S. Geological Survey, and other stakeholders. Comments from the PPRP and other reviewers were considered when preparing the report. The SSC model was completed at the end of 2011.

Comparison of Historical and Deductive Methods for the Calculation of Low-Probability Seastates in the Gulf of Mexico

Historical methods are typically used to determine the 100-year seastates used in traditional design practice. In these methods, one fits a probability distribution to the historical record of Hs and wind speed. With the development of more rational design standards, interest has shifted to rare events-with annual exceedence probabilities of 10−4 or lower. If one uses historical methods to determine the seastates associated with these probabilities, one typically encounters high uncertainties as a result of limited sample size and modeling uncertainty associated with the choice of distribution shape. This paper presents the results from Phase-I of an industry-sponsored project aimed at the development of more reliable methods for the calculation of seastates (particularly Hs) associated with rare probabilities. These methods construct deductive (or physically based) probabilistic models of the storm characteristics and their effects, and then use these models (together with the laws of probability theory) to calculate the probabilities of rare seastates. The objective of this first (exploratory) phase was to develop and apply a deductive model for hurricanes affecting platform sites in the western Gulf of Mexico, compare the predictions of this model to those of the historical approach, investigate and compare the uncertainties in both approaches, and explore other benefits of using the deductive method. For the sake of efficiency and internal consistency, a simple parametric-hindcast method was used for both the deductive and historical approaches. Results from this project indicate that the deductive and historical methods predict identical values of Hs for annual exceedence probabilities of 10−2 per year. For lower probabilities (particularly for 10−4 ), the predictions by the deductive method are within the range of predictions by the historical method using different assumptions about distribution shape. The statistical uncertainty of the deductive method is comparable to that of the historical method (if the latter uses pooled data coming from the entire static region). If no pooling is performed with the historical method, the uncertainty in the deductive method is lower. In addition, the deductive method shows less sensitivity to distribution-shape assumptions. This paper also summarizes ongoing efforts to adapt the deductive approach to extra-tropical storms in the North Sea — which have more complicated wind fields than hurricanes — and to the entire Gulf of Mexico — with the associated geographical variation in storm rate and characteristics.Copyright

Approaches that use seismic hazard results to address topics of nuclear power plant seismic safety, with application to the Charleston earthquake issue

Abstract Plant seismic safety indicators include seismic hazard at the SSE (safe shut-down earthquake) acceleration, seismic margin, reliability against core damage, and reliability against offsite consequences. This work examines the key role of hazard analysis in evaluating these indicators and in making rational decisions regarding plant safety. The paper outlines approaches that use seismic hazard results as a basis for plant seismic safety evaluation and applies one of these approaches to the Charleston earthquake issue. This approach compares seismic hazard results that account for the Charleston tectonic interpretation, using the Electric Power Research Institute (EPRI)–Seismicity Owners Group (SOG) methodology, with hazard results that are consistent with historical tectonic interpretations accepted in regulation. Based on hazard results for a set of 21 eastern U.S. nuclear power plant sites, the comparison shows that no systematic “plant-to-plant” increase in hazard accompanies the Charleston hypothesis; differences in mean hazards for the two interpretations are generally insignificant relative to current uncertainties in seismic hazard.

Issues in Strong Ground‐Motion Estimation in Eastern North America

‘The estimation of quantitative characteristics of strong earthquake ground motion in eastern North America (ENA) is dificult. No instrumental records arc available from destructive shaking in the region, and there are perceived differences between shaking in ENA and in California, thereby invalidating the use of empirical equations from California (where data are abundant) in ENA. Quantitative estimation of ground motion is a critical step in the mitigation of risk for existing facilities and in the selection of design criteria for new facilities. Therefore in the absence of empirical observations of strong shaking we must evaluate and use relevant theories, low-amplitude seismographic observations, and data from other regions to deduce what might be the range of strong ground-motion characteristics in ENA. Correct decisions (in the sense of optimal decisions under uncertainty) about seismic-risk mitigation will be made only if available alternatives are judged on their scientific merits and if current Uncertainties on groundmotion characteristics are reported honestly. This paper summarizes the current scientific issues causing uncertainty in ground-rnotion estimation for ENA. and evaluates methods of ground-motion prediction on their ability to incorporate and represent current alternative viewpoints.