Resolution of Generic Safety Issues: Issue 199: Implications of Updated Probabilistic Seismic Hazard Estimates in Central and Eastern U.S. for Existing Plants (Rev. 1) ( NUREG-0933, Main Report with Supplements 1–34 )

DESCRIPTION

Historical Background

On May 26, 2005, the Office of Nuclear Reactor Regulation (NRR), Division of Engineering, recommended that issues related to a closed generic seismic issue (Generic Issue (GI)-194, “Implications of Updated Probabilistic Seismic Hazard Estimates,” dated September 23, 2003) and the impact of higher seismic hazard on current nuclear power plants (NPPs) in the Central and Eastern United States (CEUS) region be examined under the GI identification and resolution process.¹⁹³⁰ On June 9, 2005, GI-199, “Implications of Updated Probabilistic Seismic Hazard Estimates in Central and Eastern United States,” joined the list of GIs.¹⁹³¹

Safety Significance

Recent data and models indicate that estimates of the potential for earthquake hazards for some NPPs in the CEUS may be larger than previous estimates. While it has been determined that currently operating plants remain safe, the recent seismic data and models warrant further study and analysis. This further analysis will allow the U.S. Nuclear Regulatory Commission (NRC) to better understand the current margins at plants for earthquakes.

Regulatory Guide 1.165, “Identification and Characterization of Seismic Sources and Determination of Safe Shutdown Earthquake Ground Motion,”¹⁹³² developed in the early 1990s, specifies a reference probability for exceedance of a safe-shutdown earthquake (SSE) ground motion (i.e., seismic hazard) at a median annual value of 1E-5. This reference probability value is based on the annual probability of exceeding the SSEs for 29 CEUS nuclear power sites and is used to establish the SSEs for future nuclear facilities. Based on preliminary results from work performed by the United States Geological Survey (USGS) in 2004, it appears that the reference probability for the 29 CEUS sites has increased to about 6 to 7E-5. The increase in the reference probability value is primarily due to recent developments in the modeling of earthquake ground motion in the CEUS. When the staff first identified this issue, no new plants had applied for a construction permit or early site permit (ESP) since Title 10 of the Code of Federal Regulations (10 CFR) Part 100, “Reactor Site Criteria,” was revised and Regulatory Guide 1.165¹⁹³² was issued in 1997. When the staff began review of the ESP applications, the staff realized the impact of the revised regulation and the regulatory guide as they relate to future plants and operating reactors.

From the staff’s review of the ESP applications with support from the 2004 USGS draft report, it appeared that the perception of seismic hazard for operating plants in the CEUS region had increased. Based on the evaluations of the Individual Plant Examination of External Events (IPEEE) Program, the staff had determined that seismic designs of operating plants in the CEUS provided an adequate level of protection. However, in light of the preliminary results from the USGS work of 2004 and the ESP applications, the staff also recognized that the probability of exceeding the SSE at some of the currently operating sites in the CEUS is higher than previously understood. Therefore, the staff initiated this GI to assess the impact of increased estimates of seismic hazards on selected current NPPs in the CEUS region that might be impacted by the updated seismic research, information, and models.

SCREENING ANALYSIS

In December 2007, the staff completed the screening analysis using guidance contained in Management Directive 6.4, “Generic Issues Program,”¹⁸⁵⁸ and SECY-07-0022, “Status Report on Proposed Improvements to the Generic Issues Program,” dated January 30, 2007.¹⁸⁸⁸ The screening panel reviewed the analysis in January 2008. On February 1, 2008, the Director of the Office of Nuclear Regulatory Research (RES) approved the screening panel recommendation¹⁹³³ to begin the safety/risk assessment stage of the generic issue process.

The screening panel’s recommendation was based on the screening analysis, which showed that GI-199 passed the seven GI screening criteria. The discussion under each criterion below provides the screening analysis for GI-199.

1. The issue affects public health and safety, the common defense and security, or the environment.
   
   The estimated risk to public health and safety and the environment associated with the occurrence of seismic events at some NPP sites might have increased from previous estimates. The issue stems from ongoing research being conducted by a number of scientists into the seismic history of the CEUS and the details of wave propagation and attenuation in this region. In particular, information submitted to the NRC by ESP applicants contained updated seismic information that included new models to estimate earthquake ground motion and updated models for earthquake sources in seismic regions such as eastern Tennessee and around both Charleston, South Carolina, and New Madrid, Missouri. In addition, information summarized by the USGS as part of the National Seismic Hazard Mapping Program indicates that the estimated likelihood of seismic activity (i.e., seismic hazard) in some CEUS locations has increased from previous estimates. Some of these locations are near existing NPP sites. An increase in the seismic hazard at these sites has the potential to adversely impact public health and safety if the estimated increased seismic hazard were to significantly exceed plant design capabilities; substantially reduce perceived safety margins for plant structures, systems, and components (SSCs) important to safety; or appreciably increase the risk associated with the plant’s response to a seismic event. From a qualitative perspective, if the increased hazard is significant at sites that have relatively small safety margins for seismic events, then the estimated risk for these sites could increase.

2. The issue applies to two or more facilities and/or licensees/certificate holders or holders of other regulatory approvals.
   
   The updated information described above results in increased estimates of the seismic hazard that could occur at multiple, although not all, NPP sites in the CEUS. Specifically, updated models for earthquake sources in seismic regions such as eastern Tennessee and around both Charleston, South Carolina, and New Madrid, Missouri, indicate that the rate of earthquake occurrence in these regions is greater than previously recognized. Because this change applies to several large regions, it has the potential to affect more than one NPP site. Further, new models used to estimate earthquake ground motion have been revised relative to those used in the 1980s. This change also has the potential to affect more than one NPP site. Updated estimates of seismic hazard values at some of the sites could potentially exceed the design basis as well as the review-level earthquake spectrum used as part of the IPEEE Program.

3. The issue cannot be readily addressed through other regulatory programs and processes; existing regulations, policies, or guidance; or voluntary industry initiatives.
   
   In a memorandum to RES dated May 26, 2005, NRR identified this issue and recommended that it be examined under the Generic Issues Program.¹⁹³⁴ In this memorandum, the staff concluded that the seismic designs of operating plants in the CEUS still provide adequate safety margins while the staff continues to evaluate new seismic hazard data and models and their potential impact on plant risk estimates. At the same time, the staff also recognized that these new seismic data and models could reduce available safety margins due to increased estimates of the probability associated with seismic hazards at some of the currently operating sites in the CEUS. Therefore, to help assess the potential reduction in available safety margins using a probabilistic approach, the NRR staff recommended that the new data and models on CEUS seismic hazards be examined under the Generic Issues Program.¹⁹³⁴ Accordingly, at that time, the NRR staff determined that this issue was not sufficiently characterized to be addressed under existing licensing processes for licensees of plants that might be impacted.
   
   Based on the limited evaluation of available information, this issue does not appear to be adequately characterized for complete treatment under existing regulatory programs and processes. Examples of regulatory programs and processes that might apply after obtaining additional information and performing further evaluations are listed below. Additional analysis will help determine whether this issue is amenable to these or other regulatory programs or industry initiatives.

• LIC-100, “Control of Licensing Bases for Operating Reactors”
• LIC-105, “Managing Regulatory Commitments Made by Licensees to the NRC”
• LIC-202, “Procedures for Managing Plant-Specific Backfits and 50.54(f) Information Requests”
• LIC-300, “Rulemaking Procedures”
• LIC-400, “Procedures for Controlling the Development of New and Revised Generic Requirements for Power Reactor Licensees”
• LIC-401, “NRR Reactor Operating Experience Program”
• LIC-501, “Program Coordination for Risk-Informed Activities”
• LIC-503, “Generic Communications Affecting Nuclear Reactor Licensees”
• LIC-504, “Integrated Risk-Informed Decision-Making Process for Emergent Issues”

4. The issue can be resolved by new or revised regulation, policy, or guidance.
   
   Further analysis of the risk or safety impact would provide sufficient additional information to properly characterize the issue and its potential impact on CEUS plants and support consideration under other existing regulatory programs or industry initiatives. The regulatory office has authority to take appropriate regulatory action(s) as necessary to protect the public health and safety and the environment. Depending on the outcome of the additional analysis, as well as industry initiatives to address any safety issues, the regulatory office could address this issue through one or more actions involving regulation, policy, or guidance.

5. The issue’s risk or safety significance can be adequately determined (i.e., it does not involve phenomena or other uncertainties that would require long-term studies and/or experimental research to establish the risk or safety significance).
   
   The screening analysis was performed based on the staff’s review of updated seismic data and models submitted by ESP applicants and also updated seismic hazard data and models available from the USGS as part of the National Seismic Hazard Mapping Program. The seismic hazard at CEUS plant sites of interest can be evaluated using an approach like the detailed assessment performed by the Electric Power Research Institute (EPRI)¹⁹³⁵ for 28 of the 29 sites included in Regulatory Guide 1.165.¹⁹³² This study used updated attenuation models and incorporated updates to the EPRI seismic source model developed during the preparation of the ESPs. The risk significance of the updated seismic hazard information can be evaluated for CEUS plant sites of interest by performing a comparison of uniform hazard spectra or other hazard results to the beyond-design-basis review-level earthquake or hazard curve used as part of the IPEEE evaluation.¹⁷⁹⁸ The available IPEEE Program results would allow a general assessment of the potential safety impact of increases in seismic hazard at specific sites. This analysis was performed later as part of the safety/risk assessment under the Generic Issues Program.

6. The issue is well defined, discrete, and technical.
   
   The seismic hazard will be adequately defined upon detailed assessment of available updated seismic data and models submitted by ESP applicants and also updated seismic hazard data and models available for other CEUS plant sites of interest using an approach like that performed by EPRI¹⁹³⁵ for 28 of the 29 sites included in Regulatory Guide 1.165.¹⁹³² This will allow the seismic hazard estimates for CEUS plant sites of interest to reflect the state of current knowledge. As new information and research becomes available, future updates might be warranted. The plants’ response to seismic hazards involves technical analyses using established techniques.

7. Resolution of the issue may potentially involve review, analysis, or action by the affected licensees, certificate holders, or holders of other regulatory approvals.
   
   After further characterization of site-specific seismic hazards and an analysis of the plant’s response to the increased seismic hazard, some plants may be identified as having a vulnerability that must be addressed to maintain adequate safety margins. Determining a plant’s margin and potential need for action to maintain an adequate margin could involve regulatory actions (e.g., requests for information from plant licensees, reviews, additional analysis, mitigation actions, physical enhancements, administrative controls) for some plant licensees or could involve actions by industry stakeholders.
   
   The screening analysis showed that the estimated increase in spectral acceleration for some existing CEUS plant sites might exceed the design basis and values used for the NRC’s review of IPEEE submittals. This translates into an equivalent increase in seismic demand on plant SSCs. As a result, this issue has the potential to result in increased seismic core damage frequency (SCDF) estimates for some plants. However, the screening analysis provided a limited evaluation that did not assess the safety response of the plants.
   
   The limited scope screening analysis concluded that the seismic designs of operating plants in the CEUS provided adequate safety margins while the staff continued to evaluate new seismic hazard data and models and their potential impact on plant risk estimates. Specific reasons for this conclusion included the following:

• The estimated annual probability of exceedance of seismic hazard is small in an absolute sense.

• Earthquakes cause ground motion over a range of frequencies. Lower frequency motions are more damaging to buildings and equipment than higher frequency motions. Based on the NRC staff’s reviews associated with ESPs, the staff was confident that the recent seismic data and models would show that increased estimates of the seismic hazards would occur primarily in the higher ground motion frequencies. Accordingly, the staff anticipated that these increased estimates of seismic hazards would primarily have little impact on previous estimates of the potential damage to buildings and equipment.

• The plants are designed to withstand anticipated earthquakes with substantial design margins. Plants may have seismic margins beyond those reflected in their IPEEE submittals, and these could compensate for the increase in estimated seismic load. Such additional seismic margins at plants may be inherent in the design and construction, realized from improved data and analysis methods, or result from plant modifications or enhancements completed since the IPEEE submittals.

Based on the knowledge of this issue at the time of the screening analysis and its potential effect on CEUS plants, this issued passed the seven GI screening criteria and, therefore, warranted further analysis under the Generic Issues Program.

Safety/Risk Assessment

RES staff developed and implemented a methodology to determine the implications of updated probabilistic seismic hazard estimates in the CEUS on existing plants. The methodology, analyses, results, and limitations of the safety risk assessment are summarized below. A detailed discussion of the safety/risk assessment is documented in the NRC’s “Safety/Risk Assessment Results for Generic Issue 199, Implications of Updated Probabilistic Seismic Hazard Estimates in Central and Eastern United States on Existing Plant: Safety/Risk Assessments,” dated August 2010. ¹⁹⁷⁴

Risk Methodology

SCDF was chosen as the appropriate risk metric because it is expected to be more sensitive than other metrics (either large early release fraction or public dose) to changes in the seismic hazard. In addition, SCDF can be estimated using IPEEE information. Conversely, the IPEEE Program did not produce sufficient quantitative information to estimate alternate risk metrics.

The staff performed a two-stage assessment to determine the implications of updated probabilistic seismic hazards in the CEUS on existing NPPs. The first stage involved evaluating the change in seismic hazard with respect to previous estimates at individual plants. The second stage estimated the change in SCDF as a result of the change in the seismic hazard for each operating plant in the CEUS. The seismic hazard at each NPP site depends on the unique seismology and geology surrounding the site, which necessitated separately determining the implications of updated probabilistic seismic hazard for each of the 96 operating NPPs in the CEUS.

Evaluation of Changes in Seismic Hazard Estimates

In the first stage of the assessment, the NRC staff evaluated the potential significance of changes in seismic hazards in a stepwise fashion by assessing the degree to which the seismic hazard estimates developed using the most recent seismic hazard information and NRC staff guidance deviate from previously developed assessments. The comparison of results indicated an increase in the seismic hazard estimates relative to previous assessments for a number of plants.

Evaluation of Changes in Seismic Core Damage Frequency

In the second stage, the NRC staff developed SCDF estimates using three sets of mean seismic hazard curves (the 1989 EPRI study, the 1994 Lawrence Livermore National Laboratory study, and a 2008 USGS study) and plant-level fragility curves developed from information provided in the IPEEE submittals. This method had previously been used by the staff in the resolution of GI-194 and during reviews of various risk-informed license amendments.¹⁹⁷⁵ The changes in the NRC’s SCDF for a number of plants lie in the range of 10^-4 per year to 10^-5 per year, which meets the numerical risk criteria for an issue to proceed to the regulatory assessment phase of the Generic Issues Program.

Overall seismic risk estimates remain small in an absolute sense. All operating plants in the CEUS have a SCDF less than or equal to 10^-4/year, confirming that there is no immediate concern for adequate protection.

The approach used to estimate SCDF in the safety/risk assessment does not provide insight into which SSCs are important to seismic risk. Such knowledge provides the basis for postulating plant backfits and conducting a value/impact analysis of potential backfits during a regulatory analysis. For a number of plants, especially those that performed reduced-scope seismic margin analysis, detailed information is presently not available to the NRC regarding plant seismic capacity (the ability of a plant’s SSCs to successfully withstand an earthquake) beyond the required design-basis level.

Safety/Risk Assessment Panel Conclusions and Observations

In accordance with Management Directive 6.4, a safety/risk assessment panel was established to determine, on a generic basis, if the risk associated with GI-199 warranted further investigation for potential imposition as a cost-justified backfit and to provide a recommendation for the next step.

The panel completed its independent review of the safety/risk assessment for GI-199 in September 2010.¹⁹⁷⁶ The panel reached the following conclusions and observations:

• Overall seismic core damage risk estimates are consistent with the Commission’s safety goal policy statement because they are within the subsidiary objective of 10^-4/year for core damage frequency. The GI-199 safety/risk assessment, based in part on information from the NRC’s IPEEE Program, indicates that no concern exists about adequate protection and that the seismic design of operating reactors provides a safety margin to withstand potential earthquakes exceeding the original design basis.

• The changes in SCDF estimated in the safety/risk assessment stage of GI-199 for numerous plants lie in the range of 10^-4/year to 10^-5/year, which meet the numerical risk criteria for an issue to proceed to the regulatory assessment stage of the generic issues program.

• New consensus seismic-hazard estimates will become available in 2011 (these are a product of a joint NRC, U.S. Department of Energy, USGS, and EPRI project). These consensus seismic hazard estimates will supersede the existing EPRI, Lawrence Livermore National Laboratory, and USGS hazard estimates used in the GI-199 safety/risk assessment.

• Certain factors that affect the development of realistic SCDF estimates will remain unresolved even after the new consensus seismic hazard estimates are developed. The issue is primarily that many IPEEEs did not produce SCDF estimates and so lack some of the information needed to produce such estimates.

  • For a number of the plants that performed reduced-scope seismic margin analyses as part of the IPEEE Program, limited detailed information exists about plant seismic capacity (the ability of a plant’s SSCs to successfully withstand an earthquake) beyond the required design-basis level.
    
    
  • The approach used in the safety/risk assessment to estimate SCDF considered the plant-level seismic capacity and, therefore, did not provide insight into which SSCs were important to seismic risk. Such knowledge would be required in order to postulate potential cost-beneficial backfits.
    
    

• IPEEE submittals generally provided limited, qualitative information about the seismic capability of containments. Any regulatory analysis of GI-199 should consider potential plant modifications for reducing the probability of seismically induced containment failure as discussed in Section 3.3.1 of NUREG/BR-0058, “Regulatory Analysis Guidelines of the U.S. Nuclear Regulatory Commission,” Revision 4, issued September 2004. ¹⁹⁷⁷

CONCLUSION

The panel recommended transferring the lead responsibility for subsequent GI-199 actions to NRR for regulatory office implementation and taking further actions to address GI-199 outside the Generic Issues Program (i.e., obtain information and develop methods, as needed, to complete plant-specific value/impact analyses of potential backfits to reduce seismic risk).

The NRC issued information notices in September 2010 to inform stakeholders of the issuance of the GI-199 safety and risk assessment report. Information Notice 2010-18, “Generic Issue 199, ‘Implications of Updated Probabilistic Seismic Hazard Estimates in Central and Eastern United States on Existing Plants,’” dated September 2, 2010, ¹⁹⁷⁸ was issued to NPPs and independent spent fuel storage installations (ISFSI). It stated that the NRC will follow the appropriate regulatory process to request operating plants and ISFSIs to provide specific information relating to their facilities to enable the NRC staff to complete the regulatory assessment where candidate backfits are identified and evaluated. Information Notice 2010-19, “Updated Probabilistic Seismic Hazard Estimates in Central and Eastern United States,” dated September 16, 2010, 1979 was issued to fuel cycle facilities. GI-199 is in the regulatory office implementation stage, in accordance with Management Directive 6.4.¹⁸⁵⁸

REFERENCES

1798. NUREG-1742, “Perspectives Gained from the Individual Plant Examination of External Events (IPEEE) Program,” U.S. Nuclear Regulatory Commission, (Volumes 1 and 2) April 2002.

1858. Management Directive 6.4, “Generic Issues Program,” U.S. Nuclear Regulatory Commission, November 17, 2009.

1888. SECY-07-0022, “Status Report on Proposed Improvements to the Generic Issues Program,” U.S. Nuclear Regulatory Commission, January 30, 2007. [ML063460239]

1930. Memorandum for F. Eltawila from M. Mayfield, “Identification of a Generic Seismic Issue,” May 26, 2005. [ML051450456]

1931. Memorandum for M. Mayfield from F. Eltawila, “Generic Issue 199, ‘Implications of Updated Probabilistic Seismic Hazard Estimates in Central and Eastern United States,’” June 9, 2005. [ML051600272]

1932. Regulatory Guide 1.165 (Draft was DG-1032), “Identification and Characterization of Seismic Sources and Determination of Safe Shutdown Earthquake Ground Motion,” March 1997 [ML003740084]

1933. Memorandum for B. Sheron from P. Hiland, “Results of Initial Screening of Generic Issue 199, ‘Implications of Updated Probabilistic Seismic Hazard Estimates in Central and Eastern United States on Existing Plants,’” February 1, 2008. [ML073400477]

1934. Memorandum for F. Eltawila from M. Mayfield, “Identification of a Generic Seismic Issue,” May 26, 2005. [ML051450456]

1935. EPRI Report Nos.1012044 and 1012045, “Program on Technology Innovation: Assessment of a Performance-Based Approach for Determining Seismic Ground Motions for New Plant Sites,” Vol. 1 and 2, Electric Power Research Institute, 2005.

1974. Generic Issue 199, “Implications of Updated Probabilistic Seismic Hazard Estimates in Central and Eastern United States on Existing Plants: Safety/Risk Assessments,” U.S. Nuclear Regulatory Commission, August 2010. [ML100270639]

1975. Kennedy, R.P., “Overview of Methods for Seismic PRA and Margins Including Recent Innovations,” Proceedings of the Organization for the Economic Cooperation and Development/Nuclear Energy Agency Workshop on Seismic Risk, August 10–12, 1997, Tokyo, Japan.

1976. Memorandum for B. Sheron from P. Hiland, “Safety/Risk Assessment Results for Generic Issue 199, ‘Implications of Updated Probabilistic Seismic Hazard Estimates in Central and Eastern United States on Existing Plants,’” September 2, 2010. [ML100270598]

1977. NUREG/BR-0058, “Regulatory Analysis Guidelines of the U.S. Nuclear Regulatory Commission,” U.S. Nuclear Regulatory Commission, Revision 4, September 2004. [ML042820192]

1978. Information Notice 2010-18, “Generic Issue 199, ‘Implications of Updated Probabilistic Seismic Hazard Estimates in Central and Eastern United States on Existing Plants,’” U.S. Nuclear Regulatory Commission, September 2, 2010. [ML101970221]

1979. Information Notice 2010-19, “Updated Probabilistic Seismic Hazard Estimates in Central and Eastern United States,” U.S. Nuclear Regulatory Commission, September 16, 2010 [ML102160735]