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Seismic margins issues
Nuclear Engineering and Design 107 (1988) 77-81 North-Holland, Amsterdam
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SEISMIC MARGINS ISSUES D a n J. G U Z Y a n d J a m e s E. R I C H A R D S O N Engineering Branch, Office of Nuclear Regulatory Research, U.S. Nuclear Regulatory Commission, Washington, DC 20555, USA Received 1 June 1987
The effect of very large earthquakes on the safe operation of nuclear power plants is discussed. The fundamental safety and regulatory issues of: (1) uncertainties in the seismic hazard, (2) earthquakes larger than the design basis, and (3) seismic vulnerabilities are described. Finally, the NRC-sponsored Seismic Design Margins Program is described in terms of approach and how this compares with probabilistic risk assessment.
1. Introduction Earthquakes are among the most severe of the natural hazards faced by nuclear power plants. Very large earthquakes can jeopardize the design concepts of redundancy and defense-in-depth through common mode failures. Because of its pervasive nature, an earthquake will "seek out" vulnerabilities such as design deficiencies or construction errors. Potential severe consequences have caused the designers and regulators of nuclear power plants to build in conservatisms in the seismic design to assure safe operation. Conservatisms are found at each step of seismic analysis chain [1]. Among the areas where significant conservatisms are introduced: (1) the specification of the design basis earthquake for SSE (although the SSE may not always be conservative as discussed below); (2) modeling and enveloping techniques used to account for soil-structure-interaction effects; (3) structural damping; (4) smoothed and broadened floor response spectra; (4) piping response analyses techniques; (5) pipe damping; (6) methods for combining dynamic loads; and (7) allowable stress criteria. Most seismic design experts agree that, in general, nuclear power plants are capable of withstanding earthquakes much larger than their original design basis without compromising their ability to safely shutdown and remain in a safe shutdown condition. That is to say, nuclear power plants have a margin built into their design. The question is: "How big is the seismic design margin?". The Advisory Committee on Reactor Safeguards (ACRS) has raised this question in several recent reviews of reactor operating licensing applica-
tions. The ACRS has also recommended that the N R C staff undertake research to determine how much inherent margin exists in the seismic design of operating nuclear power plants [2]. In response to this ACRS recommendation, the Executive Director for Operations established a working group within the N R C staff to address the seismic margins issues and to recommend a course of action that would lead to rational and sound regulatory decisions regarding the adequacy of the seismic design of operating nuclear power plants [3]. The purpose of this paper is to briefly describe the fundamental safety issues addressed by the NRC Working Group on Seismic Design Margins and to describe the research program and its resulting procedures and guidelines developed to address those issues.
2. Safety and regulatory issues 2.1. Uncertainties in the seismic hazard The fundamental seismic hazard issues are how to quantify and to reduce the uncertainties in seismic hazard assessments and how to develop techniques to deal with the uncertainties in a regulatory environment. Factors that contribute to the uncertainty in the seismic hazard assessment are (1) the uncertainty in establishing seismic source zones, (2) the uncertainty in the propagation of seismic energy, and (3) the uncertainty in the site-specific ground-motion response, including soil response. In addition to the fundamental seismic hazard issues, the issue of the recurrence of an 1886 Charleston-size
0 0 2 9 - 5 4 9 3 / 8 8 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
78
D.J. Guzy, .I.E. Richardson / Seismic margins issues
earthquake anywhere on the eastern seaboard has commanded significant NRC attention. In November 1982, the USGS clarified its position with respect to the 1886 Charleston earthquake [4]. The clarifying statement represents not so much a new understanding but rather a more explicit recognition of existing uncertainties with respect to the causative structure and mechanism of the 1886 Charleston earthquake. Many hypotheses have been proposed as to seismogenic mechanisms and potential location on the eastern seaboard of future Charlestonsize earthquakes. Some of these hypotheses would limit such an earthquake in both size and location while other would allow this earthquake to occur over very large areas of the Eastern United States and Canada. Presently, none of these hypotheses is definitive and all contain strong elements of speculation. Also, a policy issue memorandum for the Commission on the January 9, 1981 New Brunswick, Canada earthquake [5] stated in part: "Although all information relating to the size and location of the event is preliminary, it eventually may be concluded that this earthquake could have occurred anywhere within the New England Piedmont Tectonic Province and in accordance with Appendix A to 10 CFR Part 100, would represent the largest historical earthquake in that province.., which includes much of the New England and southern New York." If the NRC were to act conservatively on the Charleston and New Brunswick earthquake issues, using the procedures of Appendix A to 10 C F R Part 100 to establish revised values for the Safe Shutdown Earthquake (SSE), some Eastern United States nuclear power plants could have their review ground acceleration significantly increased over their original design value. The Commission would then be faced with decisions regarding possible plant shutdown or continued operation pending reanalysis of existing margins or requiring extensive structural and equipment modifications to meet the desired safety level.
against seismically induced failure for those similar systems and equipment used. The performance of conventional power plants in past earthquakes confirms the existence of substantial seismic capacity in nuclear power plants. A sound, practical seismic margins program using margins-to-failure analysis and seismic probabilistic risk assessment techniques will serve to minimize the need for changing requirements and licensing actions as estimates of the seismic hazard and system response change. In addition, seismic margin studies can provide a sound basis for establishing confidence in the seismic capacity of nuclear power plants and serve to indicate, if necessary, places where risk should be reduced.
2.2. Earthquakes larger than the design basis
In 1984, the NRC Seismic Design Margins Program was initiated to provide the technical bases for better addressing seismic margins issues. A panel of consultants was selected that provided expertise in the areas of seismic design, earthquake experience and testing, and seismic probabilistic risk assessments (seismic PRAs). With support from the Lawrence Livermore National Laboratory (LLNL) and its subcontractors, this Expert Panel on the Qualification of Seismic Margins developed a recommended program plan [8] that was endorsed by the NRC Working Group on Seismic Design Margins and now serves as the basic outline for our N R C research program.
New seismological information coupled with uncertainties in defining the seismic hazard as discussed above, sometimes results in the need to consider increasing the level of earthquake magnitude or intensity used in the original design of nuclear power plants. The seismic margin issue relates to whether these changes can be accommodated within the inherent capacity of the original design on whether plant modifications are warranted. Recent studies such as the RES sponsored Seismic Safety Margins Research Program (SSMRP) [6] indicate that nuclear plants generally have high margins
2.3. Seismic vulnerabilities To achieve the main goal of the Severe Accident Policy Statement [7], namely, to bring stability to licensing and regulation with regard to all severe accident issues, it is important that all initiators of severe accidents receive appropriate consideration. The Policy Statement includes events initiated within the plant and events caused by external initiators such as earthquake and flood. With regard to external events, the identification of seismic vulnerabilities is of utmost importance since current risk assessments indicate that risk from earthquakes could be a major contributor. A plant review procedure, reflecting insight from seismic probabilistic risk assessments and earthquake experience and test data will identify key elements (components and systems) that make the plant vulnerable to seismic events. Using a conservatively estabfished acceptance criteria to find seismic vulnerability, may later require a more detailed review to establish the plant's seismic capacity.
3. Seismic Design Margins Research Program
D.J. Guzy, J.E. Richardson / Seismic margins issues The Expert Panel has also served as the chief innovator of a new approach for assessing the adequacy of nuclear power plant seismic margins [9]. This approach has made use of the results and insights that have come from both the Seismic Safety Margins Research Program and from industry-sponsored seismic risk studies of the Zion, Indian Point 2, Indian Point 3, Millstone 3, Limerick, Seabrook, and Oconee nuclear plants. The seismic fragility (i.e., failure) information that was used in these studies [10] has been reassessed in light of new dynamic test data for structures and equipment and recent systematic evaluations of earthquake experience in other heavy industrial facilities. The ongoing studies by the Seismic Qualification Utilities Group (SQUG) have been particularly useful in establishing lower bounds of fragility estimates for equipment. Current individual issues involving structure, equipment, and systems design were also factored into the recommended approach. The LLNL staff and its subcontractors worked with the Expert Panel to develop guidelines for seismic margins reviews [11] based on the panel's recommended approach. These guidelines have been used in a trial plant seismic margins review of the Maine Yankee Plant [13] and later, with some expanded systems guidance added, will be used in the trial plant review of a boiling water reactor plant. 3.1. The seismic margins approach An important and difficult first step in developing the approach was to determine what "seismic margin" should mean. The following working definition was adopted by the Expert Panel: "A general definition of seismic margin is expressed in terms of the earthquake motion level that compromises plant safety, specifically leading to melting of the reactor core. In this context, margin needs to be defined for the whole plant. The margin concept also can be extended to any particular structure, function, system, equipment, item, or component, for which "'compromising safety" means sufficient loss of safety function to contribute to core melting if combined with other failures". It was decided that peak ground acceleration (pga), defined as the average of the two horizontal peak components of free-field groundsurface acceleration, could most conveniently be used as a primary description of "earthquake motion level". It should be noted that while seismic margins definition above has used reactor core melt as the measure of importance (rather than, say, "exceedence of code allowable" or "risk"), the new review procedure does
79
not determine a median core melt probability. This is because of two features of the seismic margins approach that make it distinct from the seismic PRA approach, and which purposely make it easier and more acceptable for use by those with chiefly engineering design backgrounds. First, the seismic margins approach does not involve a probabilistic seismic hazard evaluation. A single reference earthquake is selected for the seismic margins review. In contrast, the estimation of the seismic hazard curves (annual exceedence probability versus ground motion) is an integral part of seismic PRA calculations. The results of the PRAs studied by the Expert Panel have indicated that the earthquake range of roughly 2 through 4 times SSE contributes most to the total probability of seismically-induced core melt. The uncertainty of seismic hazard curves in this range influences the PRA results more than the uncertainty in seismic fragility estimates. By making the margin review process independent of the site seismic hazard curve, the evaluation gives a robust measure of the inherent seismic capacity of nuclear plants. Secondly, unlike seismic PRA studies which use median (or mean) failure estimates as the key parameters, the seismic margin approach has chosen as its figure of merit a high-confidence of low probability of failure (HCLPF). The HCLPF is a conservative representation of capacity and in simple terms corresponds to the earthquake level at which it is extremely unlikely that failure will occur. HCLPF values for specific types of components are derived from a combination of engineering data (from either testing or real earthquake experience) and engineering analysis. As a point of reference, the HCLPF values can be thought to correspond approximately to a 95 percent confidence (probability) that the probability of failure will be less than five percent. Generally, the median capacity is at least a factor of 2 greater than the HCLPF value and thus, there is no sudden failure which is expected to occur immediately above the HCLPF value. It should be strongly noted, however, that fragility estimates do not have to be developed to derive HCLPF values. In fact, the alternative Conservative Deterministic Failure Margins (CDFM) approach used in margins reviews avoids making median failure estimates. Whereas there is much earthquake experience showing nonfailures (and which helps validate lower-bound estimates), little data exists in the estimated median fragility range. Seismic designers and reviewers are more comfortable with a lower bound approach since this is more in line with current licensing practices for the seismic design of structures and components.
80
D.J. Guzy, J.E. Richardson / Seismic margins issues
The HCLPF concept is used at the component, systems (or function) and plant level in the seismic margins approach. When HCLPF values are used to represent the plant as a whole, they give a lower bound estimation of what peak ground acceleration would likely cause core melt. Since the shape of the related plant fragility curve probably would be unknown, one could not directly calculate the peak ground acceleration corresponding to median core melt frequency. In order to make the seismic margins review process efficient, as well as thorough and systematic, the screening concept was adopted. Here, the screening uses a series of "filters" that enable the reviewer to sort the relevant elements (components and functions) into several classes. The purposes of these filters is to eliminate from any detailed evaluation those components that either are known to inherently have very high seismic capacity (i.e., concrete containments) or are in systems judged not to be important contributors to seismic-induced core melt. The "seismic capacity filter" sorts components into two HCLPF classes, either higher than or lower than the earthquake review level. To aid in this, the Expert Panel developed guidance on the generic attributes of broad classes of components. They used recent SQUG experience data plus test data and seismic PRA fragility information. The "systems filter" sorts the components into different classes depending on the nature of the plant functions they serve. The Expert Panel's review of eight seismic PRA studies enabled them to reach conclusions regarding the relative importance of plant systems and safety function for pressurized water reactors (PWRs). (A similar study of new BWR seismic PRA's is currently underway). The Expert Panel's conclusion about PWRs was that only two plant functions must be considered for estimating seismic margin. These two functions are shutting down the nuclear chain reaction and providing cooing to the reactor core in the time period immediately following the seismic event (that is, the injection phase or the pre-residual heat removal time period). The systems screening filter is therefore based on evaluating only components in systems that serve these functions. The Expert Panel focused its efforts on earthquakes that could occur east of the Rocky Mountains. Because of limited data on large magnitude events, the approach's assessment of component capacities is now limited to earthquakes of less than a magnitude of about 6.5, which are characterized by three to five strong motion cycles with a total duration of 10 to 15 seconds. The frequency content of the earthquake
ground motion is assumed to be represented by median broadband response spectra. Ref. [11] provides the guidance on what should consist of a seismic margins review following the approach discussed above. Assuming the seismic margins review is to be performed for a 0.3g pga earthquake at an eastern U.S. PWR plant on a rock site, the time to perform such a review is estimated to be about 8 months. The total effort should be slightly less than that for the seismic part of a typical full-scope PRA, however, the component capacity evaluation will in fact be more rigorous then for a PRA. The savings are made in having no seismic hazard analysis, no consequence analysis, and a lesser effort on the systems analysis. 3.2. Comparison of the seismic margins and PRA approaches
In theory, a seismic PRA can provide complete answers regarding the seismic safety of the plant. However, a seismic PRA done for risk estimation purposes is a cumbersome way to assess the inherent capability of nuclear plants to withstand given earthquake levels. Large uncertainties in the "bottom hne" risk numbers arise from the seismic hazard analysis and from the fact that the fragility evaluation relies heavily (at least for past PRAs) on analysis and judgment in the absence of actual fragility test data. The extensive use of subjective input in fragility prediction has been necessary since a "best-estimate" rather than "lower-bound" approach is used in PRA's. (Although the total uncertainty in the seismic risk estimates is considered by some to be larger than the uncertainties of risks from internal accident initiators, one possible explanation is that seismic PRAs explicitly include modeling uncertainties.) By contrast, the seismic margins review process is independent of probabiIstic seismic hazard estimation, and thus the evaluation gives a more robust measure of the inherent seismic capacity of nuclear plants. Because the scope of a seismic margins review is narrower and more focused than a seismic PRA, this allows a more thorough evaluation of the components and systems for the earthquake level in question. In fact, the recommended walk-down procedures involve two plant visits (versus a single visit for a PRA) and much more attention is given to systems interaction, anchorages, etc., for the systems and components being studied. Thus, a seismic margins review gives more assurance of finding plant vulnerabilities important to potential core melt scenarios for earthquake levels in the moderately high range.
D.J. Guzy, J.E. Richardson / Seismic margins issues
Current seismic margins procedures are limited to earthquakes up to 0.5g, because this is the upper range of the earthquake experience used in the component capacity screening guidance. Thus, only seismic PRA's could be used now to evaluate very high level earthquakes such as would be considered for some California plants. It should also be noted that you can not use seismic margins results to make risk estimates. Since no seismic hazard curve is developed and since the shape of the related plant fragility curve probably would be unknown, one could not directly calculate median core melt frequency. Research activities in the seismic margins area are currently directed towards the joint EPRI and NRC review of a BWR plant.
References [1] P.D. Smith et al., LLNL/DOR Seismic Conservatism Programs: Investigations of the Conservatisms in the Seismic Design of Nuclear Power Plants, UCID-20572 (October 1985). [2] Letter from J.J. Ray (ACRS Chairman) to NRC Chairmen Nunzio, Palladino, Qualification of Seismic Design Margins (January 11, 1983). [3] Letter from William Dircks (NRC EDO) to NRC Chairman Palladino, Quantification of Seismic Design Margins (April 12, 1984).
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[4] Letter from J.F. Devine, USGS to R.E. Jackson, NRC (November 18, 1982). [5] Memorandum from W. Dircks (NRC EDO), Possible Relocation of Design Controlling Earthquakes in the Eastern U.S. SECY-82-53 (February 5, 1982). [6] G.E. Cummings, Summary Report on the Seismic Safety Margins Research Program, NUREG/CR-4431 (January 1986). [7] Memorandum from Victor Stello (NRC EDO) to the Commissioners, Treatment of External Events in the Implementation of the Severe Accident Policy Statement, SECY-86-162 (May 22, 1986). [8] G.E. Cummings, J.J. Johnson and R.J. Budnitz, NRC Seismic Design Margins Program Plan, LLNL Report UCID-20247 (October 1984). [9] R.J. Budnitz, P.J. Amico, C.A. Cornell, W.J. Hall, R.P. Kennedy, J.W. Reed and M. Shinozuka, An Approach to the Quantification of Seismic Margins in Nuclear Power Plants, NUREG/CR-4334 (August 1985). [10] R.D. Campbell, M.K. Ravindra, and A. Bhatia, R.C. Murray, Compilation of Fragility Information From Available Probabilistic Risk Assessments, LLNL Report UCID-20571 (September 1985). [11] P.G. Prassinos, M.K. Ravindra and J.B. Savy, Recommendations to the Nuclear Regulatory Commission on Trial Guidelines for Seismic Margin Reviews of Nuclear Power Plants - Draft for Comment, NUREG/4482 (March 1986). [12] LLNL, EQE, EI, Seismic Margin Review of the Maine Yankee Atomic Power Station, NUREG/CR-4826 (March 1987).
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SEISMIC MARGINS ISSUES D a n J. G U Z Y a n d J a m e s E. R I C H A R D S O N Engineering Branch, Office of Nuclear Regulatory Research, U.S. Nuclear Regulatory Commission, Washington, DC 20555, USA Received 1 June 1987
The effect of very large earthquakes on the safe operation of nuclear power plants is discussed. The fundamental safety and regulatory issues of: (1) uncertainties in the seismic hazard, (2) earthquakes larger than the design basis, and (3) seismic vulnerabilities are described. Finally, the NRC-sponsored Seismic Design Margins Program is described in terms of approach and how this compares with probabilistic risk assessment.
1. Introduction Earthquakes are among the most severe of the natural hazards faced by nuclear power plants. Very large earthquakes can jeopardize the design concepts of redundancy and defense-in-depth through common mode failures. Because of its pervasive nature, an earthquake will "seek out" vulnerabilities such as design deficiencies or construction errors. Potential severe consequences have caused the designers and regulators of nuclear power plants to build in conservatisms in the seismic design to assure safe operation. Conservatisms are found at each step of seismic analysis chain [1]. Among the areas where significant conservatisms are introduced: (1) the specification of the design basis earthquake for SSE (although the SSE may not always be conservative as discussed below); (2) modeling and enveloping techniques used to account for soil-structure-interaction effects; (3) structural damping; (4) smoothed and broadened floor response spectra; (4) piping response analyses techniques; (5) pipe damping; (6) methods for combining dynamic loads; and (7) allowable stress criteria. Most seismic design experts agree that, in general, nuclear power plants are capable of withstanding earthquakes much larger than their original design basis without compromising their ability to safely shutdown and remain in a safe shutdown condition. That is to say, nuclear power plants have a margin built into their design. The question is: "How big is the seismic design margin?". The Advisory Committee on Reactor Safeguards (ACRS) has raised this question in several recent reviews of reactor operating licensing applica-
tions. The ACRS has also recommended that the N R C staff undertake research to determine how much inherent margin exists in the seismic design of operating nuclear power plants [2]. In response to this ACRS recommendation, the Executive Director for Operations established a working group within the N R C staff to address the seismic margins issues and to recommend a course of action that would lead to rational and sound regulatory decisions regarding the adequacy of the seismic design of operating nuclear power plants [3]. The purpose of this paper is to briefly describe the fundamental safety issues addressed by the NRC Working Group on Seismic Design Margins and to describe the research program and its resulting procedures and guidelines developed to address those issues.
2. Safety and regulatory issues 2.1. Uncertainties in the seismic hazard The fundamental seismic hazard issues are how to quantify and to reduce the uncertainties in seismic hazard assessments and how to develop techniques to deal with the uncertainties in a regulatory environment. Factors that contribute to the uncertainty in the seismic hazard assessment are (1) the uncertainty in establishing seismic source zones, (2) the uncertainty in the propagation of seismic energy, and (3) the uncertainty in the site-specific ground-motion response, including soil response. In addition to the fundamental seismic hazard issues, the issue of the recurrence of an 1886 Charleston-size
0 0 2 9 - 5 4 9 3 / 8 8 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
78
D.J. Guzy, .I.E. Richardson / Seismic margins issues
earthquake anywhere on the eastern seaboard has commanded significant NRC attention. In November 1982, the USGS clarified its position with respect to the 1886 Charleston earthquake [4]. The clarifying statement represents not so much a new understanding but rather a more explicit recognition of existing uncertainties with respect to the causative structure and mechanism of the 1886 Charleston earthquake. Many hypotheses have been proposed as to seismogenic mechanisms and potential location on the eastern seaboard of future Charlestonsize earthquakes. Some of these hypotheses would limit such an earthquake in both size and location while other would allow this earthquake to occur over very large areas of the Eastern United States and Canada. Presently, none of these hypotheses is definitive and all contain strong elements of speculation. Also, a policy issue memorandum for the Commission on the January 9, 1981 New Brunswick, Canada earthquake [5] stated in part: "Although all information relating to the size and location of the event is preliminary, it eventually may be concluded that this earthquake could have occurred anywhere within the New England Piedmont Tectonic Province and in accordance with Appendix A to 10 CFR Part 100, would represent the largest historical earthquake in that province.., which includes much of the New England and southern New York." If the NRC were to act conservatively on the Charleston and New Brunswick earthquake issues, using the procedures of Appendix A to 10 C F R Part 100 to establish revised values for the Safe Shutdown Earthquake (SSE), some Eastern United States nuclear power plants could have their review ground acceleration significantly increased over their original design value. The Commission would then be faced with decisions regarding possible plant shutdown or continued operation pending reanalysis of existing margins or requiring extensive structural and equipment modifications to meet the desired safety level.
against seismically induced failure for those similar systems and equipment used. The performance of conventional power plants in past earthquakes confirms the existence of substantial seismic capacity in nuclear power plants. A sound, practical seismic margins program using margins-to-failure analysis and seismic probabilistic risk assessment techniques will serve to minimize the need for changing requirements and licensing actions as estimates of the seismic hazard and system response change. In addition, seismic margin studies can provide a sound basis for establishing confidence in the seismic capacity of nuclear power plants and serve to indicate, if necessary, places where risk should be reduced.
2.2. Earthquakes larger than the design basis
In 1984, the NRC Seismic Design Margins Program was initiated to provide the technical bases for better addressing seismic margins issues. A panel of consultants was selected that provided expertise in the areas of seismic design, earthquake experience and testing, and seismic probabilistic risk assessments (seismic PRAs). With support from the Lawrence Livermore National Laboratory (LLNL) and its subcontractors, this Expert Panel on the Qualification of Seismic Margins developed a recommended program plan [8] that was endorsed by the NRC Working Group on Seismic Design Margins and now serves as the basic outline for our N R C research program.
New seismological information coupled with uncertainties in defining the seismic hazard as discussed above, sometimes results in the need to consider increasing the level of earthquake magnitude or intensity used in the original design of nuclear power plants. The seismic margin issue relates to whether these changes can be accommodated within the inherent capacity of the original design on whether plant modifications are warranted. Recent studies such as the RES sponsored Seismic Safety Margins Research Program (SSMRP) [6] indicate that nuclear plants generally have high margins
2.3. Seismic vulnerabilities To achieve the main goal of the Severe Accident Policy Statement [7], namely, to bring stability to licensing and regulation with regard to all severe accident issues, it is important that all initiators of severe accidents receive appropriate consideration. The Policy Statement includes events initiated within the plant and events caused by external initiators such as earthquake and flood. With regard to external events, the identification of seismic vulnerabilities is of utmost importance since current risk assessments indicate that risk from earthquakes could be a major contributor. A plant review procedure, reflecting insight from seismic probabilistic risk assessments and earthquake experience and test data will identify key elements (components and systems) that make the plant vulnerable to seismic events. Using a conservatively estabfished acceptance criteria to find seismic vulnerability, may later require a more detailed review to establish the plant's seismic capacity.
3. Seismic Design Margins Research Program
D.J. Guzy, J.E. Richardson / Seismic margins issues The Expert Panel has also served as the chief innovator of a new approach for assessing the adequacy of nuclear power plant seismic margins [9]. This approach has made use of the results and insights that have come from both the Seismic Safety Margins Research Program and from industry-sponsored seismic risk studies of the Zion, Indian Point 2, Indian Point 3, Millstone 3, Limerick, Seabrook, and Oconee nuclear plants. The seismic fragility (i.e., failure) information that was used in these studies [10] has been reassessed in light of new dynamic test data for structures and equipment and recent systematic evaluations of earthquake experience in other heavy industrial facilities. The ongoing studies by the Seismic Qualification Utilities Group (SQUG) have been particularly useful in establishing lower bounds of fragility estimates for equipment. Current individual issues involving structure, equipment, and systems design were also factored into the recommended approach. The LLNL staff and its subcontractors worked with the Expert Panel to develop guidelines for seismic margins reviews [11] based on the panel's recommended approach. These guidelines have been used in a trial plant seismic margins review of the Maine Yankee Plant [13] and later, with some expanded systems guidance added, will be used in the trial plant review of a boiling water reactor plant. 3.1. The seismic margins approach An important and difficult first step in developing the approach was to determine what "seismic margin" should mean. The following working definition was adopted by the Expert Panel: "A general definition of seismic margin is expressed in terms of the earthquake motion level that compromises plant safety, specifically leading to melting of the reactor core. In this context, margin needs to be defined for the whole plant. The margin concept also can be extended to any particular structure, function, system, equipment, item, or component, for which "'compromising safety" means sufficient loss of safety function to contribute to core melting if combined with other failures". It was decided that peak ground acceleration (pga), defined as the average of the two horizontal peak components of free-field groundsurface acceleration, could most conveniently be used as a primary description of "earthquake motion level". It should be noted that while seismic margins definition above has used reactor core melt as the measure of importance (rather than, say, "exceedence of code allowable" or "risk"), the new review procedure does
79
not determine a median core melt probability. This is because of two features of the seismic margins approach that make it distinct from the seismic PRA approach, and which purposely make it easier and more acceptable for use by those with chiefly engineering design backgrounds. First, the seismic margins approach does not involve a probabilistic seismic hazard evaluation. A single reference earthquake is selected for the seismic margins review. In contrast, the estimation of the seismic hazard curves (annual exceedence probability versus ground motion) is an integral part of seismic PRA calculations. The results of the PRAs studied by the Expert Panel have indicated that the earthquake range of roughly 2 through 4 times SSE contributes most to the total probability of seismically-induced core melt. The uncertainty of seismic hazard curves in this range influences the PRA results more than the uncertainty in seismic fragility estimates. By making the margin review process independent of the site seismic hazard curve, the evaluation gives a robust measure of the inherent seismic capacity of nuclear plants. Secondly, unlike seismic PRA studies which use median (or mean) failure estimates as the key parameters, the seismic margin approach has chosen as its figure of merit a high-confidence of low probability of failure (HCLPF). The HCLPF is a conservative representation of capacity and in simple terms corresponds to the earthquake level at which it is extremely unlikely that failure will occur. HCLPF values for specific types of components are derived from a combination of engineering data (from either testing or real earthquake experience) and engineering analysis. As a point of reference, the HCLPF values can be thought to correspond approximately to a 95 percent confidence (probability) that the probability of failure will be less than five percent. Generally, the median capacity is at least a factor of 2 greater than the HCLPF value and thus, there is no sudden failure which is expected to occur immediately above the HCLPF value. It should be strongly noted, however, that fragility estimates do not have to be developed to derive HCLPF values. In fact, the alternative Conservative Deterministic Failure Margins (CDFM) approach used in margins reviews avoids making median failure estimates. Whereas there is much earthquake experience showing nonfailures (and which helps validate lower-bound estimates), little data exists in the estimated median fragility range. Seismic designers and reviewers are more comfortable with a lower bound approach since this is more in line with current licensing practices for the seismic design of structures and components.
80
D.J. Guzy, J.E. Richardson / Seismic margins issues
The HCLPF concept is used at the component, systems (or function) and plant level in the seismic margins approach. When HCLPF values are used to represent the plant as a whole, they give a lower bound estimation of what peak ground acceleration would likely cause core melt. Since the shape of the related plant fragility curve probably would be unknown, one could not directly calculate the peak ground acceleration corresponding to median core melt frequency. In order to make the seismic margins review process efficient, as well as thorough and systematic, the screening concept was adopted. Here, the screening uses a series of "filters" that enable the reviewer to sort the relevant elements (components and functions) into several classes. The purposes of these filters is to eliminate from any detailed evaluation those components that either are known to inherently have very high seismic capacity (i.e., concrete containments) or are in systems judged not to be important contributors to seismic-induced core melt. The "seismic capacity filter" sorts components into two HCLPF classes, either higher than or lower than the earthquake review level. To aid in this, the Expert Panel developed guidance on the generic attributes of broad classes of components. They used recent SQUG experience data plus test data and seismic PRA fragility information. The "systems filter" sorts the components into different classes depending on the nature of the plant functions they serve. The Expert Panel's review of eight seismic PRA studies enabled them to reach conclusions regarding the relative importance of plant systems and safety function for pressurized water reactors (PWRs). (A similar study of new BWR seismic PRA's is currently underway). The Expert Panel's conclusion about PWRs was that only two plant functions must be considered for estimating seismic margin. These two functions are shutting down the nuclear chain reaction and providing cooing to the reactor core in the time period immediately following the seismic event (that is, the injection phase or the pre-residual heat removal time period). The systems screening filter is therefore based on evaluating only components in systems that serve these functions. The Expert Panel focused its efforts on earthquakes that could occur east of the Rocky Mountains. Because of limited data on large magnitude events, the approach's assessment of component capacities is now limited to earthquakes of less than a magnitude of about 6.5, which are characterized by three to five strong motion cycles with a total duration of 10 to 15 seconds. The frequency content of the earthquake
ground motion is assumed to be represented by median broadband response spectra. Ref. [11] provides the guidance on what should consist of a seismic margins review following the approach discussed above. Assuming the seismic margins review is to be performed for a 0.3g pga earthquake at an eastern U.S. PWR plant on a rock site, the time to perform such a review is estimated to be about 8 months. The total effort should be slightly less than that for the seismic part of a typical full-scope PRA, however, the component capacity evaluation will in fact be more rigorous then for a PRA. The savings are made in having no seismic hazard analysis, no consequence analysis, and a lesser effort on the systems analysis. 3.2. Comparison of the seismic margins and PRA approaches
In theory, a seismic PRA can provide complete answers regarding the seismic safety of the plant. However, a seismic PRA done for risk estimation purposes is a cumbersome way to assess the inherent capability of nuclear plants to withstand given earthquake levels. Large uncertainties in the "bottom hne" risk numbers arise from the seismic hazard analysis and from the fact that the fragility evaluation relies heavily (at least for past PRAs) on analysis and judgment in the absence of actual fragility test data. The extensive use of subjective input in fragility prediction has been necessary since a "best-estimate" rather than "lower-bound" approach is used in PRA's. (Although the total uncertainty in the seismic risk estimates is considered by some to be larger than the uncertainties of risks from internal accident initiators, one possible explanation is that seismic PRAs explicitly include modeling uncertainties.) By contrast, the seismic margins review process is independent of probabiIstic seismic hazard estimation, and thus the evaluation gives a more robust measure of the inherent seismic capacity of nuclear plants. Because the scope of a seismic margins review is narrower and more focused than a seismic PRA, this allows a more thorough evaluation of the components and systems for the earthquake level in question. In fact, the recommended walk-down procedures involve two plant visits (versus a single visit for a PRA) and much more attention is given to systems interaction, anchorages, etc., for the systems and components being studied. Thus, a seismic margins review gives more assurance of finding plant vulnerabilities important to potential core melt scenarios for earthquake levels in the moderately high range.
D.J. Guzy, J.E. Richardson / Seismic margins issues
Current seismic margins procedures are limited to earthquakes up to 0.5g, because this is the upper range of the earthquake experience used in the component capacity screening guidance. Thus, only seismic PRA's could be used now to evaluate very high level earthquakes such as would be considered for some California plants. It should also be noted that you can not use seismic margins results to make risk estimates. Since no seismic hazard curve is developed and since the shape of the related plant fragility curve probably would be unknown, one could not directly calculate median core melt frequency. Research activities in the seismic margins area are currently directed towards the joint EPRI and NRC review of a BWR plant.
References [1] P.D. Smith et al., LLNL/DOR Seismic Conservatism Programs: Investigations of the Conservatisms in the Seismic Design of Nuclear Power Plants, UCID-20572 (October 1985). [2] Letter from J.J. Ray (ACRS Chairman) to NRC Chairmen Nunzio, Palladino, Qualification of Seismic Design Margins (January 11, 1983). [3] Letter from William Dircks (NRC EDO) to NRC Chairman Palladino, Quantification of Seismic Design Margins (April 12, 1984).
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[4] Letter from J.F. Devine, USGS to R.E. Jackson, NRC (November 18, 1982). [5] Memorandum from W. Dircks (NRC EDO), Possible Relocation of Design Controlling Earthquakes in the Eastern U.S. SECY-82-53 (February 5, 1982). [6] G.E. Cummings, Summary Report on the Seismic Safety Margins Research Program, NUREG/CR-4431 (January 1986). [7] Memorandum from Victor Stello (NRC EDO) to the Commissioners, Treatment of External Events in the Implementation of the Severe Accident Policy Statement, SECY-86-162 (May 22, 1986). [8] G.E. Cummings, J.J. Johnson and R.J. Budnitz, NRC Seismic Design Margins Program Plan, LLNL Report UCID-20247 (October 1984). [9] R.J. Budnitz, P.J. Amico, C.A. Cornell, W.J. Hall, R.P. Kennedy, J.W. Reed and M. Shinozuka, An Approach to the Quantification of Seismic Margins in Nuclear Power Plants, NUREG/CR-4334 (August 1985). [10] R.D. Campbell, M.K. Ravindra, and A. Bhatia, R.C. Murray, Compilation of Fragility Information From Available Probabilistic Risk Assessments, LLNL Report UCID-20571 (September 1985). [11] P.G. Prassinos, M.K. Ravindra and J.B. Savy, Recommendations to the Nuclear Regulatory Commission on Trial Guidelines for Seismic Margin Reviews of Nuclear Power Plants - Draft for Comment, NUREG/4482 (March 1986). [12] LLNL, EQE, EI, Seismic Margin Review of the Maine Yankee Atomic Power Station, NUREG/CR-4826 (March 1987).