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- / UNITED STATES g j' ,g NUCLEAR REGULATORY COMMISSION WASHINGTON, D. C. 20555 t f y
DEC 2 41984
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REC ; yp o f MEMORANDUM FOR: Chairman Palladino TC,
((;]3 g{R C. [M Commissioner Roberts Comissioner Asselstine [ ~ ' "n
" r g Commissioner Bernthal AH Comissioner Zech $;Ag;;g FROM: William J. Dircks j Executive Director for Operations
SUBJECT:
PROGRESS REPORT AND SCHEDU;E CHANGE IN NRC PROGRAM PLAN RELATING TO CLARIFICATION OF U. S. GE0 LOGICAL SURVEY POSITION REGARDING SEISMIC DESIGN EARTHQUAKES IN THE EASTERN SEAB0ARD OF THE UNITED STATES For the purpose of licensing nuclear facilities in the Southeastern U. S.,
the NRC staff has taken a position, based primarily on the advice of the U. S. Geological Survey (USGS), that any reoccurrence of the 1886 Charleston, S.C. earthquake would be confined to the Charleston area; that is, the Charleston earthquake is assumed to be associated with a geologic structure in the Charleston area. The effect of this position is that nuclear power plants in the region east of the Appalachian Mountains are usually controlled in their seismic design, according to Appendix A to 10 CFR Part 100, by the maximum historical earthquake not associated with a geologic structure. This controlling earthquake is typically a Modified Mercalli Intensity (MMI) VII or VIII whereas the Charleston earthquake was a MMI X. Since 1974, the NRC has funded an extensive research project to gain further information on the causative mechanism of the Charleston earthquake.
In a letter dated November 18, 1982 from James F. Devine, USGS to Robert E. Jackson, NRC, the USGS clarified its position indicating that:
"Because the geologic and tectonic features of the Charleston region are similar to those in other regions of the eastern seaboard, we conclude that although there is no recent or historical evidence that other regions have experienced strong earthquakes, the historical record is not, of itself, sufficient grounds for ruling out the occurrence in these other regions of strong seismic ground motions similar to those experienced near Charleston in 1886. Although the probability of strong ground motion due to an earthquake in any given year at a particular location in the eastern seaboard may be very low, deterministic and probabilistic evaluations of the seismic hazard should be made for individual sites in the eastern seaboard to establish the seismic engineering parameters-for~ critical
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-2 In a Comission Paper dated February 5,1982 (SECY-82-53), we informed the Comission of the possibility of modification in the USGS position and in a memorandum dated November 19, 1982, the USGS clarification was forwarded to the Comission along with an assessment of significance and a pre-liminary plan to address the clarified USGS position. This plan was discussed with the Commission in its November 19, 1983 meeting and a joint ,
NRR/RES program was initiated. The joint program consisted of a short term probabilistic program that has as its core a Lawrence Livermore Laboratory (LLNL) estimation of seismic hazard at all nuclear power plant sites east of the Rocky Mountains, and a long term determinis'.ic program through RES to determine the causes of large earthquakes, such as the~
Charleston earthquake, in the eastern seaboard.
With regard to the short term probabilistic program, final calculations by LLNL are almost complete for 10 test sites. As a means of comparison we recommended in our original program plan that a utility sponsored study also be carried out. The utility study, being conducted through the Electric. Power Research Institute (EPRI), is currently scheduled for completion in April 1985. We have decided to defer LLNL's estimation of the seismic hazard at the 65 remaining eastern sites for approximately 1 year so as to await completion of the EPRI program and allow a thorough comparison of EPRI and other seismic hazard estimates to be made. The original plan called for this comparison to take place concurrent with LLNL's calculations, however, we believe this change to be useful in light of the rapidly evolving technology of seismic hazard estimation. As a result the probabilistic portion of the plan to address the clarification of the U. S. Geological Survey's Position on the 1886 Charleston Earth-quake will be available at the end of 1986, instead of in 1985 us originally scheduled.
The log tenn detenninistic program is also progressing. Most signif-icantly, several teams of investigators, mainly funded by NRC, have found evidence of paleoliquefaction in the Charleston area. This is the first indication of large to moderate earthquakes in the eastern seaboard in prehistoric times. It is possible that a seismic recurrer.ce interval for large earthquakes in the Charleston area may be developed and that a determination can be made whether or not this area is seismically unique.
These conclusions await further investigations and assessment.
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William J. Dircks Executive Director for Operations cc: SECY OPE OGC c
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5 October 1981 ,
Comissioner Victor Gilinsky Nuclear Regulatory Comission Room H1113
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Dear Comissioner Gilinsky:
In response to a telephone request by Ed Abott, I will try to lay out in.a few pages how I believe the earthquake resistant design criteria should be developed for major projects siich~ as nuclear plants, if this could be
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cone in the absence of regulatory or historical' constraints. The outline will necessarily be brief and the fine points will be glossed over. In preparing this letter I am relying en my general background plus recent experiences with the seismic design of offshore drilling platfo ns for the Santa Barbara Channel and with the Seismic Review Panel the Californit Public Utilities Comission has formed for the proposed LNG facility at Point Conception. The major differences between what the NRC does and
(. what I recomend is that some oversimplified definitions and practices' of the NRC are not used, and the judgments are made differently.
- tss the first step, seismological and geological studies are under-taken to determine the possible sources of ground motion and surface faulting (the latter is assumed to be avoided by site selection). One of the results of these studies would be a list of faults, with estimates of the geologic time of their last movement and their activity rates.
This gradation of importance sidesteps the " capable fault" definition and does not require equal treatment of a fault with abundant evidence of movement and one that may have moved a few centimeters in the last
, half-million years. The estimate of current activity rates, which typically would vary by orders of magnitude, would be useful for deter-mining both the upper level (SSE) and lower level (DBE) earthquakes. If
- the larger historic earthquakes near the site could not be tied to an identified fault, this fact would afftet tne conservatism used in applying the seismological and geological assessments.
The major result I would expect from the geoscientists would be, l
for each source, their estimate of the biggest earthquake expected, on the average, every 200 years, for example, and also that expected, on <
the average, every few thousands of years (say 2000 years). The 200 l
and the 2000 define, in effect, the OBE and the SSE and the NRC should decide the numbers depending on its desired conservatism. An earthquake with a return period of thousands of years approaches the " maximum I~ credible," but represents a more meaningful way to address the problem.
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. Page Two 5 October 1981 .
Also, the geologist and seismologist should pr. ovide an estimate of the biggest earthquake that could occur anywhere near the site, but not clearly identified with the faults they have found.
- From these design earthquakes the earthquake engineer would develop the expected level of response spectra for shaking at the site. In most
, cases, one or two design earthquakes wo'uld govern -- each in different parts of the frequency domain. The design spectra could then be set conservatively with respect to the average of the expected response spectra, with some tailoring for limitations of the methods used to
, estimate the response spectra. (Seesketch.)
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One could justify using the avotage curve, since the design earthquakes are chosen conservatively, but more common practice is to take arother conservative step and to set the design spectra above the expected average of the response spectra, e.g., near the estimate of the mean plus one standard deviation. This process would be done for both the SSE and OBE. -
In this step, attention is concentrated on that part of the frequency ,
domain which is important for the project. The peak acceleration of the expected ground motion is usually of minor importance in this pro-cedure since it is associated with a frequency higher than the important -
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5 October 1981 .
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i After the design spectra for both levels of earthquakes are constructed for the required values of' damping, the next step is to specify the allowable response of the structure in the two cases. For the lower level, only elastic response is usually permitted. For the upper level earthquake, the allowable response is larger and the amount-permitted should bear some relation to the probability of experiencing the motion as well as the potential hazard of the structure. Generally, the more improbable the event, the more the structure should be allowed to respond beyond the 1
elastic limit, as long as structural integrity against collapse or cata-
' strophic failure is retained. In the case of conventional steel-framed offshore drilling platforms, levels of response around twice the yield 1
level are allowed, provided a collapse mechanism (plastic hinges in all legs) does not develop. Other types of structures can take more or less:
- the amount of reliable ductility is an engineering question that depends on the structure's forn, the materials of construction, the details of elements and joints, etc. This last' stage, at which the allowable response is set, is also the point where the overall conservatism of the design is evaluated and any needed adjustments made. Note that this is
! in the hands of the earthquake engineers and designers, where it belongs.
(- With the design spectra determined and the danping and allowable response fixed, the design criteria are complete. The next step is to see that they are implemented correctly. Most major projects employ three means for doing this. First, they have a consulting board (s) that helps resolve questions that arise. Second, the design is ssually reviewed by an independent firm; this is often a condition for insurance. Third, there is, or should be, an active inspection process during construction.
- A final coment: Once the design criteria are reasonably conserva-tive, the most generally effective way to increase the seismic capacity of l structures is not to simply raise the criteria, but to concentrate on
! the detailing of members and joints to insure a tough, ductile structure.
- This is often not a question of much cost, but involves instead choices of layout for the facility, the location of structural members, the
. choice of materials, the placement of reinforcing steel in critical l areas, etc. Because of the many possibilities, these factors cannot bc
- covered by codes or general criteria; this is where the quality of the i engineering is important. '
1 I sense in drafting this letter that I have reached, if not exceeded, the length of document desired, and I will now stop. If I :
i can help further, or if you want to discuss these questions further, please do not hesitate to call.
SINCERELY YOURS,
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PAUL C. JENNINGS ;
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O O got. cot April 27, 1979 SECY-79-300 4
UNITED ST ATES MD 1 NUCLEAH REGULATORY COMMISSION m c.n ce C ITTH UNNM mox s3nca:tes na INFORMATION REPORT 3b For: The Commissioners iJ d From: Robert B. Minogue, Director I, , u 14 Office of Standards Development 7 Thru: Lee V. Gossick, Executive Director for Operationsr' . . . .
Subject:
IDENTIFICATION OF ISSUES PERTAINING TO SEISMIC AND GEOLOGIC SITING REGULATION, POLICY, AND PRACTICE FOR NUCLEAR POWER PLANTS
Purpose:
To inform the Commission of the status of the staff's reassess-ment of Appendix A " Seismic and Geologic Siting Criteria For Nuclear Power Plants," to 10 CFR Part 100, " Reactor Site Criteria."
Backaround: This paper is a sequel to SECY 77-288A, which described current licensing practice and regulatory requirescats in the seismic and geologic siting area. Appendix A to Part 100 sets forth a framework that guides the staff in its evaluation of the ade-Dj
- D Q;,: . Lad quacy of applicants' investigations of geologic and earthquake c/n @ (i~iO phenomena and proposed plant design parameters. The bases for E,3h4I O I6 Appendix A were established in the late 60's and it became effective in December 1973. Since then, with advances in the c:e. 8 "F- 1 d k',j sciences of seismology and geology along with the occurrence of 7 3;gy some issues in licensing cases not foreseen in the development
@f.0.* r- [p , . of Appendix A, a number of significant difficulties have arisen
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! in the application of this regulation. As a result of these
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. difficulties, the staff began a reassessment of Appendix A. This
- [, o j 'd stage of the staff reassesswent involvc.d identifyir.g problem areas
- . -) ,. 6 ' A . needing resolution.
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t.O? ,""O M -- Issues identified in the enclosures have been synthesized from comments by the staff, meetings with the Seismic Subcommittee cf
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$;!iil the Advisory Committee on Reactor Safeguards and its consultants,
-9, ., . , b % {J ~.; interested persons who responded to a staff FEDERAL REGISTER
" " ">7"1 MWt notice on January 19, 1978 (eighteen comments were received),
and other sources. Enclosure E describes in more detail the back-ground for this paper and sources of information used.
Discussion: Issues that have been identified have been divided into three categories and presented in Enclosures A, B, and C. Enclosure A contains issues that stem directly f rom geost:ience requirements out forth in Appendix A. Enclosure B,contains issues arising from engineering requirements in Appendix A, procedures for pro-viding an interface of these requirements wi,th geologic and
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NUCLEAR REGULATORY COMMISSION gyggg. i W.**iTTTL ON o'.m sm.ms o, u, 1
INFORMATION REPORT 1N For: The Commissioners L f
4 From: Robert 8. Minogue, Director Office of Standards Development
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-Thru: Lee V. Gossick, Executive Director.for Operations ....
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Subject:
. IDENTIFICATION OF ISSUES PERTAINING TO SEISMIC AND GE0 LOGIC SITING REGULATION, POLICY, AND PRACTICE FOR NUCLEAR POWER
! PLANTS i
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Purpose:
To inform the Commission of the status of the staff's reassess-
- ment of Appendix A " Seismic and Geologic Siting Criteria For Nuclear Power Plants," to 10 CFR Part 100, " Reactor Site Criteria.
i 8ackground: This paper is a sequel to SECY 77-288A, which described current licensing practice and regulatory requirements in the seismic l and geologic siting area. App adix A to Part 100 sets forth a l framework that guides the staff in its evaluation of the ade-quacy of applicants' investigations of geologic and earthquake l(
- , phenomena and proposed plant design parameters. The bases for l
Appendix A were established in the late 60's and it became effective in December 1973. Since then, with advances in the l
sciences of seismology and geology along with the occurrence of
- some issues in licensing cases not foreseen in the development of Appendix A, a number of significant difficulties have arisen i in the application of this regulation. As a result of these l
difficulties, the staff began a reassessment of Appendix A. This j
stage of the staff reassessment involved identifying problem areas ;
needing resolution.
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Issues identified in the enclosures have been synthesized frera '
l comments by the staff, meetings with the Seismic Subcommittee of the Advisory Cosmiittee on Reactor Safeguards and its consultants, i fr.terested persons who responded to a staff FEDERAL REGISTER i
notice on January 19, 1978 (eighteen comments were received), ,
and other sources. Enclosure E describes in more detail the back '
ground for this paper and sources of information used.
[ Discussion: Issues that have besn identified have been divided into three categories and presented in Enclosures A, 8, and C. Enclosure A.
contcins issues that stas directly from geoscience requirements
! put forth in Appendix A. Enclosure 8 contains issues arising from engineering requirements in Appendix A, procedures for pro-l(
l widing an interface of these requirements wi,th geologic and
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( The Commissioners 2 seismic input and with matters involving scientific and engineer-ing conservatism. Enclosure C contains broad policy and technica issues bearing on the implemeiitation of Appendix A and its revision. Enclosure F memo from Minogue to Commissioner Mason, dated October 8,19767,providesfurtherinformationonseismic I issues (Operating Basis Earthquake Concept).
It In making geoscience assessments, there is a need for consider-able latitude and judgement. This latitude and judgement is required because of limitations in data, the state of the art of geologic and seismic analyses, and the rapid evolution taking place in the geosciences in terms of accumulating knowledge and in modifying concepts. This appears to have been recognized when Appendix A was developed. However, having geoscience assess ments detailed and cast in Appendix A, a regulation, has created difficulty for applicants and the staff in terms of inhibiting the use of.needed judgement and latitude. Also, it has inhibited flexibility in applying basic principles to new situations and the use of evolving methods of analyses in the licensing process.
Additionally, various sections of Appendix A lack clarity and are subject to different interpretations and dispute. Also,
(- some sections in the Appendix do not provide sufficient informa-tion for implementation. As a result of being both overly detailed in some areas and not detailed enough in others, the Appendix has been the source of licensing delays and debate, has inhibited the use of some types of analyses and has inhibited the development of regulatory guidance.
In other siting areas, such as hydrology, regulatory guidance has been handled effectively through the use of regulatory guides and a program for their continuous updating. Many problems encountered in implementing Appendix A could best be alleviated through the use of regulatory guides and a program for contin-uous updating. The best course of action appears to be that Appendix A be revised to express the general intent of geologic and seismic assessments and that details presently in Appendix A be incorporated into a set of "1st generation" regulatory guides which would provide at least the equivalent of what is now in Appendix A. The "1st generation" guides could then be updated and supplemented with further guides to keep pace with advances in the state of the art and staff experience gained in the review of license applications.
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The Commissioners 3 r
The subsequent stage of the staff reassessment is discussed in Enclosure D. In brief, the next stage will consist of a value-impact analysis of issues, the development of a revised regu-lation and supplemental regulatory guides, as was stated in the prior paragraph, and the development of a policy paper making g, specific recommendations for rule making.
Coordination: The enclosures to this paper were prepared jointly by the Offices of NRR and OSD. The Office of NRR concurs in this paper; the Offices of I&E, RES and OELD, and the Seismic Subcommittee of the ACRS were consulted. DELD has no legal objections to this paper.
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Robert B. Minogue, Dir Office of Standards Development f
Enclosures:
\ A. Geoscience Issues Originating from Appendix A to 10 CFR Part 100 B. Engineering Design Issues Related to Vibratory Ground Motion C. Broad Policy and Technical Issues Bearing on the Implementation and Revision of Appendix A D. Summary of Subsequent Stage in the Assessment of Current Seismic and Geologic Siting Criteria, Policy,~and Practice E. Background and Sources of Information Used in this Paper F. Memo from R.B. Minogue to Commissioner Mason, 10/8/76, "The Relationship between Safe Shutdown Earthquakes and Operating Basis Earthquakes" DISTRIBUTION:
Commissioners Commission Staff Offices Exec. Dir for Opers.
Regional Offices ACRS
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TOPICS OF ENCLOSURES ENCLOSURE A: GEOSCIENCE ISSUES ORIGINATING FROM APPENDIX A TO 10 CFR PART 100
- 1. INTRODUCTION y 1.1 Impacts of Issues
- Impaired efficiency of licensing process (delays)
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- Expenditure of manpower
- Restricted ability to use advances in science and engineering
- Impacts on backfitting
- Difficulties for applicants created by the regulation
- Impacts on safety
- 2. ISSUES 2.1 Tectonic Provinces and Associated Concepts 2.2 Correlation of Seismicity and Tectonic Structure 2.3 Capable Fault 2.4 Specification of Safe Shutdown Earthquake (SSE) and Operating Basis Earthquake (OBE) Vibratory Ground Motion
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ENCLOSURE 8: ENGINEERING DESIGN ISSUES RELATED TO VIBRATORY GROUND MOTION
- 1. INTRODUCTION
- 2. ENGINEERING REQUIREMENTS IN THE REGULATION 2.1 Specification of Vibratory Ground Motion 2.1.1 Site Specific vs Generalized Response Spectra 2.1.2 Variation of Ground Motion with Depth 2.1.3 Specification of Time History 2.1.4 Duration of Shaking
- 2. 2 OBE Use in Engineering 2.3 Consideration of Aftershocks 2.4 Consideration of Potential Damage from Earthquakes Less than
! the SSE j 2. 5 Use of Probability for Considering Combinations of Loads 1 2.6 Need for Seismic Scram 4
- 3. ISSUES REGARDING CONSERVATISM 3.1 Deterministic vs. Probabilistic Approach 3.2 Empirical Relations 8etween Earthquake Size and Ground Motion Parameters 3.3 General Lack of Definition of Overall Seismic Design Conservatism
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ENCLOSURE C: BROAD POLICY AND TECHNICAL ISSUES 8 EARING ON THE ,
IMPLEMENTATION AND REVISION OF APPENDIX A
- 1. INTRODUCTION
- 2. IMPEDIMENTS TO IMPLEMENTING APPENDIX A IN THE LEGAL CONTEXT
- 3. INTERFACE OF ISSUES WITH OTHER NRC POLICY h 3.1 Lack of Policy Statements Concerning Early Site Reviews, Limited Work Authorizations, and Alternative Site Reviews 3.2 Seismic Design of Fuel Cycle Facilities 3.3 Consideration of Seismic Design of Nonradiological Safety Structures, Systems and Components
- 4. ISSUES PERTAINING TO NATIONAL POLICIES AND PRACTICES 4.1 Consistency of NRC Seismic and Geologic Siting Policy and Practice with Other National Policies and Practices 4.1.1 Earthquake Hazards Reduction Act of 1977 (EHRA) 4.1.2 Presidential Directives 4.1.2.1 Executive Order - Improving Government Regulations 4.1.2.2 National Energy Policy
. 4.1.3 Draft Congressional Legislation 4.1.4 Comparison of NRC and Other Federal Agency Critical
( Facility Seismic and Geologic Siting Policy and Practice
- 5. EXTENT AND NATURE OF REVISIONS TO APPENDIX A ENCLOSURE 0:
SUMMARY
OF SUBSEQUENT STAGE IN THE ASSESSMENT OF CURRENT SEISMIC AND GEOLOGIC SITING CRITERIA, POLICY, AND PRACTICE A. Preliminary Value Impact Statement (PVIS)
- 8. Revision of Appendix A to 10 CFR Part 100 and Development of Supplemental Gui, des ENCLOSURE E: BACXGROUNO AND SOURCES OF INFORMATION USED IN THIS PAPER I. Background II. Sources of Information ENCLOSURE F: MEMO FROM R.8. MINOGUE TO COMMISSIONER MASON, 10/8/76, "THE RELA- l TIONSHIP BETWEEN SAFE SHUT 00WN EARTHQUAKES AND OPERATING BASIC EARTHQUAKES" l
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ENCLOSURE A
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. ENCLOSURE A GEOSCIENCE ISSUES ORIGINATING FROM APPENDIX A TO 10 CFR PART 100
- 1. INTRODUCTION General Design Criterion 2 of Appendix A to 10 CFR Part 50 requires that nuclear power plant structures, systems, and components important to safety be designed to withstand the effects of natural phenomena such as earthquakes, and tornadoes, without loss of capability to perform their functions. Appendix A to 10 CFR Part 100, Seismic and Geologic Siting Criteria for Nuclear Power Plants, sets forth criteria pertaining to site investigations to assess the effects of earthquakes and other geologic phenomena to meet the requirements of General Design Criterion 2. Appendix A
( sets forth considerations which guide the Commission in its evaluation of:
(1) the suitability of a proposed site; (2) the suitability of plant design bases established in consideration of site characteristics; and (3) reasonable assurance that a nuclear power plant can be constructed and operated at a proposed site without undue risk to the health and safety of the public. When Appendix A was developed, it was recognized that limita-tions in data and the state of the art would necessitate future modifica-tions as data increased and the state of the art advanced (see SCOPE of Appendix A and Statement of Considerations).
Appendix A criteria, procedures and methods are directed toward the following major objectives:
- a. The estimation of the severity of ground shaking at a site due to potential earthquakes for use in nuclear power plant design;
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i 1 Enclosure "A" 4
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( b. The assessment of the potential for ground rupture that could affect plant structures due to fault movement;
- c. The evaluation of the effect on the site of phenomena associated with earthquakes such as seismically generated sea waves and ground failure; and
- d. The assessment of the potential for other geologic hazards at a site such as landslides and subsidence.
The principal issues discussed in this enclosure relate to tectonic provinces, tectonic structures, capable faults, and specification of the Safe Shutdown Earthquake and Operating Basis Earthquake. These concepts have been put forth in Appendix A to achieve the above objectives.
Difficulties have arisen with regard to the application of these concepts.
( An additional issue that has been identified but is not discussed further in this paper concerns volcanic hazards. Appendix A states that volcanic hazards are to be addressed on a case-by-case basis; however, it has been suggested that generic regulatory guidance be provided.
1.1 Impacts of Issues Issues identified in this enclosure have far-reaching impacts on siting policy and practice, plant design and construction, and ultimately on safety margins presently applied in these areas. Major impacts are summarized below.
2 Enclosure "A"
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Impaired efficiency of licensina process (delays)
Seismic difficulties in cases and debate over requirements in Appendix A have led to considerable delays. The extent of impact in this area is difficult to quantify. However, NUREG-0292, " Nuclear Power Plant Licensing:
Opportunities for Improvement," indicates considerable schedule slippage due to delays in geology / seismology reviews. In the reviews of Indian
! Point 2 and 3 WPPSS 1 and 4, Skagit, Pebble Springs, and Pilgrim 2, debate over satisfying Appendix A requirements caused delays in excess of a year. In addition, faulting on the site, and perceived difficulties by the applicant in meeting the requirements of Appendix A was the reason given for the withdrawal of the Sears Island proposed site.
( Expenditure of manpower 8ecause of difficulties encountered, considerable manpower is required for case review, preparation and response to interrogatories, preparation of testimony, and appearance at hearings. Often problems requiring con-siderable manpower stem from difficulties in making geologic or seismic assessments because of limitations in data and the state of the art.
However, difficulties have been encountered which arise more from attempts
! to meet requirements in Appendix A than from a given seismic or geologic assessment. For example, the Indian Point 2 and 3 case before the ASLAB (Atomic Safety and Licensing Appeal Board) dealt with many fundamental issues stemming from Appendix A requirements and involved approximately 12 members of the staff (technical review and legal) over a 2 year period.
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i Restricted ability to use advances in science and engineerina i
1 The staff is inhibited in certain parts of its review from using l l state-of-the-art analysis because Appendix A, as a regulation, cannot be
- easily modified to accommodate developments in science or engineering i'
methodology. The future development of the review process is also inhib-9-
ited because results of NRC reseuch or state-of-the-art procedures and l methods, which might be incorporated into regulatory guides, are often not i
compatible with the requirements of Appendix A. The NRC has spent millions j of dollars for research and contract support work in the earth science area. Much of this work has been necessitated by difficulties in applying the requirements of Appendix A to cases. As an important example, the NRC is sponsoring geologic and seismic research over a period of 5 years in the J
( approximate amount of $3.5 million to find a definitive means of implement-ing the tectonic province concept (a basic concept introduced in Appendix A to determine seismic potential, but not developed). Even if such a means is found, Appendix A is worded such that the results of this research could be i incorporated into the licensing process only by further rulemaking.
l The validity of various procedures employed in the geologic and seismic review process has been increasingly questioned by experts in the earth l
l science community familiar with these current procedures. In particular, the concepts of tectonic province and capable faulting, stated as regula-1 tions, have been criticized by experts because they imply that certain procedures involving interpretation and professional judgment are defini-l l tive with respect to their solutions to a particular problem, l l l i
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4 Enclosure "A" l
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Impacts on backfittina Evolution of staff practice in an attempt to meet requirements in t
Appendix A and to incorporate state-of-the-art procedures have raised questions concerning the need for backfitting of previously licensed g facilities. For example, a number of TVA plants were recently reassessed as to their adequacy in light of evolving staff practice in assessing earthquake ground motion (response spectra, intensity-acceleration relation-ships). Because Appendix A lacks guidance on a quantitative measure of conservatism to be met, it inhibits staff reassessments of existing facil-ities in terms of assessing whether design or construction modifications are needed, and creates uncertainty for applicants, as to whether plants
( having construction permits will receive operating licenses in light of new data.
Difficulties for applicants created by the regulation Appendix A was developed prior to the present-day concepts of early site review, limited work authorization and alternative site review.
Because the present regulation emphasizes a case-by-case approach, it is not compatible with the application of these new concepts which use a generic approach. Uncertainty for applicants concerning the licensing process is caused by a lack of clear guidance in the regulation regarding what constitutes acceptability for various aspects and stages of the current review process. Additionally, there are instances where the regulation has fostered nonuniformity and redundancy in SAR submittals,
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_ _ . _ _ _ _ _ _ _ _ _ _ _ _ , _ _ _ _ __ 5 _ _ ._ _ _ _ Enclosure "A" _ _ _ _ _
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particularly with regard to defining tectonic provinces. A number of differing tectonic province schemes have been submitted by applicants that meet the requirements in Appendix A but are of questionable value for sites located in the same geographic area. On the other hand, for sites'in the same area, the same tectonic province scheme is resubmitted with extensive documentation that is already available in other SARs.
Impacts on safety The issues identified in this enclosure have impact on the margin of safety presently being applied in seismic design. The degree of this impact is hard to ascertain in a rigorous sense. Some issues bear on increasing while others bear on decreasing margins of safety presently applied. Most issues relate to the application of professional judgment and experience, and differing opinions exist as to the adequacy of deter-minations and the level of conservatism achieved. Our understanding of the impact of issues on safety is limited because safety margins and over-all conservatism are not quantitatively defined.
- 2. ISSUES 2.1 Tectonic Provinces And Associated Concepts Four principal conceptual elements contained in Appendix A govern the determination of the maximum intensity of ground shaking due to earth-quakes to be considered appropriate at a site. These four elements are the concepts of tectonic provinca, tectonic structure, capable fault k
6 Enclosure "A"
(
. (discussed in Section 2.3), and " reasonable" correlation of seismicity !
with geologic structure. A tectonic province is defined in Appendix A as:
a region of the North American continent characterized by a relative consistency of the geologic structural features contained therein."
f-The concept of tectonic province was developed to provide an appropriate design basis for earthquakes whose cause is presently indeterminate. The staff interprets this concept as employed in Appendix A to imply regions of uniform earthquake hazard.
A tectonic structure is defined as:
"a large-scale dislocation or distortion within the earth's crust. Its extent is measured in miles."
(
The concept of tectonic structure is employed in Appendix A to ensure consideration of structure which might localize seismicity in the vicinity of a site, and therefore, might require special attention in assessing the seismic design bases.
What constitutes a reasonable correlation between seismicity and structure is not defined in Appendix A. However, Appendix A requires:
" correlation of epicenters or locations of highest intensity of historically reported earthquakes, where possible, with tectenic structures any part of which is located within 200 miles of the site. Epicenters or locations of highest intensity which cannot
- be reasonably correlated with tectonic structures shall be
! identified with tectonic provinces any part of which is located within 200 miles of the site."
k I
7 Enclosure "A"
- - , - . , - - ,--,m---- . - - , - - e.,-, ,, - - - - - - , - - - -----,,,n_ -
, , _ _ -------....,,,.---------n,-n.---. -- - - - , ,--
(
Epicenters that cannot be reasonably correlated represent events which then must be assumed to have the potential for occurring randomly within a tectonic j province.
The definition of tectonic province contained in Appendix A mentions l 1
only geologic structural features but implies that areas of uniform seiscic potential will be delineated. Use of information restricted solely to geologic structure and without regard for its geochronological or seismological significance has led to a variety of tectonic province schemes, particularly in the East. Some of these schemes conform to the classical Paleozoic (250-600 million years before the present) geological provinces depicted on most maps. However, these maps, such as those by King (1969, 1974),
Eardley (1962) and Rodgers (1970), were not developed with any attention
( to the possible distribution of seismically active structures. In fact, a study sponsored by the regulatory staff of the Atomic Energy Commission and carried out by the U.S. Geological Survey (Hadley and Devine, 1974),
and motivated by concern about this issue, shows that there is a very limited correlation between the classical Paleozoic structural provinces and earthquake activity. Other schemes proposed by applicants and based on Paleozoic geology may meet the definition in Appendix A but suffer from the limited correlation between earthquakes and Paleozoic structure. The type of assessment called for in Appendix A does not provide adequately for vitally important factors bearing on the determination of a tectonic province and the earthquake ground motioa for a given site. These factors are:
(
8 Enclosure "A"
(
- a. Seismicity The pattern, frequency, and intensity of historic and instru-mentally recorded seismicity is probably the best and most direct indicator of present-day tectonic activity in the East.
As such, it is clearly the most relevant parameter for defining areas of uniform seismic hazard. However, as presently written, Appendix A doesn't speak to the application of this data base to the tectonic province concept.
- b. Post-Paleozoic tectonics The post-Paleozoic and particularly neotectonic (15 million years and younger) development of a region is important to the assessment of tectonic provinces. In areas of relatively high seismicity on a world-wide basis, there is a good correlation between earthquakes and structures formed during this period.
Additionally, the theory of plate tectonics indicates that the current pattern of tectonic driving forces that affects the stress pattern in the North American continent occurred during post-Paleozoic time. In the eastern U. S., post-Paleozoic deformational effects are not as pervasive or well exposed as are those of the Paleozoic. These subtle effects have not gen-erally been considered in the past mapping of tectonic elements to be of the same level of importance as older deformational features. In part, this is caused by a bias in mapping toward large-scale geologic structures.
9 Enclosure "A"
t t
4
(
- c. Advances in Scientific Understanding Since Appendix A was first drafted in 1966, took much of its current form in 1969, and was formally adopted in 1973, progress has been made in our understanding of intraplate seismicity, and the expansion of the data base is proceeding at P-a rapid pace. Important information has been collected and synthesized on stress distribution and relief, seismic sources, deep crustal structures, and microseismicity. Because of its incorporation of detail in the form of a regulation, Appendix A j does not permit such advances in science to be readily incor-porated into the licensing process.
Reliance solely on geologic structure to define areas of uniform k earthquake hazard, as Appendix A can be construed, is an over simplified approach and has led to a number of problems. The selection of the appro-i priate geologic structures as boundaries of tectonic provinces is contro-versial in almost every case. Lack of guidance in this area has permitted applicants and the staff to consider widely varying province configurations for sites located in thu same geographic area and has led to assessments of uncertain value with regard to conservatism and scientific validity.
To date, the NRC has not succeeded in developing a tectonic province siting map, and in fact efforts to do so have brought about further com-
).
plexities. Controversy about size and distribution of tectonic provinces
- has led to the recommendation (e.g., the ASLAS in their conclusions on i Indian Point) that NRC establish such a generic map for siting purposes.
( Although several Federal agencies have adopted maps to establish the a
i i
10 Enclosure "A"
t
(
. seismic design basis for various types of structures, it would not appear appropriate to apply these to nuclear reactors. For the Commission to do
- so, a number of fundamental policy issues must first be addressed. All the state-of-the-art seismic zoning maps being developed rely to some
- degree on probabilistic considerations. At present, there is no consistent 1
NRC policy in the geoscience area regarding the use of probabilistic methods for nuclear power pla,nts. Furthermore, adoption of any particular map based on probabilistic considerations will necessarily require that a specific level of conservatism or confidence be defined. To date, no policy has been established stating the specific level of conservatism i required in the geoscience area. Even when these issues have been addres-( sed, the adoption of a tectonic province map based on any factors other lr than " consistency of geologic structure" will run contrary to a literal interpretation of the present regulation.
Appendix A allows for more conservative assessments than might nor-mally result from using the tectonic province procedures set forth in the regulation in areas having " complex geology" and "high seismicity" or "where geologic and seismic data warrant." " Complex geology" and "high seismicity" are relative terms and are not defined in Appendix A; thus they become items subject to dispute. Additionally, situations where
" geological and seismological data warrant" consideration of larger earthquakes are undefined in Appendix A and, again, are open to dispute.
Appendix A also requires the most severe earthquakes associated with k -
11 Enclosure "A"
.\
structures and provinces be identified considering the historical earth-quakes that can be associated with the structures and provinces and "other relevant factors." No guidance is given as to what is meant by "other relevant factors."
I-2.2 Correlation of Seismicity and Tectonic Structure i
Fundamental problems arise in the application of Appendix A because of a lack of guidance regarding the concept of tectonic structure and correlation with seismicity. The definition of tectonic structure given in Appendix A is broad and little guidance is given as to how it is to be interpreted. Section IV (Required Investigations) does mention the need to evaluate tectonic structure "whether buried or expressed at the surface,"
(
implying that tectonic structure may include features that are interpre-tive and not necessarily susceptible to traditional methods of surface geologic mapping. This view that tectonic structure may be interpretative rather than demonstrated is generally held in the geologic community, and was originally intended in Appendix A. This point, however, has been subject to argument, and disagreement has arisen in the course of the licensing process as to whether a particular geologic feature was correctly or incorrectly interpreted to be a tectonte structure according to the intent of Appendix A. One interpretation of the definition found in Appendix A would be that the only features that may be considered tectonic l
structures (and therefore potential earthquake sources) are those whose physical characteristics are susceptible to mapping by direct methods of k investigation such as by boring or trenching. Such a narrow definition of 12 Enclosure "A"
( ,
a tectonic structure could be interpreted to exclude from consideration geophysical, geologic, and seismologic data that indirectly indicate the existence of buried structure but that do not define the structure as completely as surface mapping might. A narrow definition may result in an erroneous assessment of the presence or absence of particular structure or structural style and its influence on the distribution of earthquakes.
Patterns and rates of historic seismicity yield, in some cases, compelling evidence for.the existence or lack of existence of a structure capable of generating earthquakes. Other sources of reliable and potentially useful data which might be excluded by a restricted interpretation of Appendix A include photoinagery, magnetic, gravity, and heat flow measurements, geodetic surveys, and microcarthquake activity. Appendix A lacks explicit
(
guidance in this respect and therefore can result in licensing delays.
As noted above, what constitutes a reasonable correlation between seismicity and a tectonic structure is not defined in Appendix A and no guidance is given. The degree of correlation between earthquakes and structures may vary from a demonstrated causal relationship, to a close i
i spatial proximity of earthquakes with structures, to an entirely interpre-j tive relationship between the two. According to Appendix A, a " reasonable correlation" between earthquakes and structure must be determined. The staff interprets " reasonable" to require that a sound scientific basis be established to correlate particular earthquakes with tectonic structure or to establish within the strength of seismicity data that an unidentified causative structure exists. The scientific basis may be a complex series k Because Appendix A offers no of geological and seismological arguments.
13 Enclosure "A"
l .
i i
i
( i guidance, application of this interpretation has been controversial in 4
several instances. Narrower interpretations of " reasonable correlation" l have been raised which would require that the epicenter of the earthquake ;
i l be very accurately known, that it fall on a well-mapped and physically well-defined geologic structure, and that a causative relationship be e demonstrated. Such precision is rarely obtained at the present time. A slightly less narrow interpretation might permit the general association f ;
of a certain earthquake with a specific structure if its epicenter were !
near that structure, but not on it, and if the structure were well enough -
J known that a mechanism for generating earthquakes could be accepted. The broader interpretation, which the staff favors, would consider the correla-n tion of particular earthquakes with zones of crustal weakness that are not
(
necessarily specifically defined by known structures but are inferred on the l basis of geophysical data, geologic data, tectonic history, and seismicity, I and for which credible causative mechanisms may be established. The above :
interpretations, as well as others falling within the range mentioned, are all scientifically acceptable as methods of correlation, but the degree of 4
! conservatism is different for each case. Appendix A is deterministic and i r
' does not specify the degree of conservatism to be applied (i.e., in terms of explicitly defining conservatism through specifying acceptable proba-t
! bilities of earthquake recurrence, or specifying a quantitative rationale
! for the margins of safety associated with deterministic procedures); there- t l
I I
fore, the acceptance of a correlation becomes one of professional judgment i
based on available information. Because of differences in professional k -
! l l
i 14 Enclosure "A"
.~
(
. views, controversy leading to litigation has arisen over whether a correla-
- tion exists and whether it is acceptable.
A further problem with Appendix A is the lack of guidance on the -
assessment of seismic zones. It is becoming widely accepted among earth scientists that zones and clusters of seismicity in the eastern United States can be very useful in evaluating areas of present-day crustal instability and potentially high earthquake hazard. Many of these zones and clusters are clearly and persistently anomalous with respect to regional l background seismicity, broad-scale geologic structure, and known tectonic history. In several cases anomalous seismicity can be related to geologic ,
and geophysical data which also suggest local instability relative to
! f surrounding regions. These kinds of ar*malous seismicity data have been
-l \'
used in the same sense as other remote sensing data such as aeromagnetics, gravity, and heat flow data, to reasonably correlate large historical
, earthquakes with geologic structure. The present regulation provides no specific guidance on the use of seismicity as a means of indirectly f
identifying tectonic structur.es and in assessing seismic potential of a i
region.
At present no regulatory guidance exists as to the use of micro-earthquake surveys and stress measurements in the identification and assessment of seismically active structures. During the last several years, it has been recognized that microcarthquake and stress measurement ,j i
I data are becoming valuable in identifying and assessing zones of crustal weakness and instability. Because regulatory requirements do not mention
(
i 1
15 Enclosure "A"
the use of these data, questions arise as to when and how they should be used in performing investigations and specifically what weight should be given to them.
Related problems that arise after a correlation between an earthquake and structure has been accepted concern the size of an earthquake that may A
be generated. Guidance given in Appendix A for assessir.g the size of an earthquake that may be generated by a structure is basically limited to assessing capable faults. Appendix A lacks specific guidance in assessing tectonic structures that appear to be correlated with earthquakes but with which no capable faults have been identified. Generally, in the West, the seismic potential of seismically active structures is determined by con-sidering historical and instrumental earthquake frequency and size, along
( with the inferred potential derived from observations of fault length, dis-placement, and regional geologic history. In the East it is not clear how seismic potential should be assessed. This is because of the paucity of data on large earthquakes and the general absence of recent surface displace-ment in the eastern United States. It is questionable whether the types of assessments used in the West are applicable to assessing structures in the East, given the significant differences between the East and West with regard to such factors as rates of tectonic activity and tectonic settings; although some distinctions between the East and West were explicitly recognized in earlier drafts of Appendix A, these were dropped in the final ,
l revision.
i k
l 16 Enclosure "A" ,
l l
Additionally, Appendix A does not provide guidance for assessing seismically active structures in the nearfield. There have been several cases where this problem has become important. Broadly defined, the near-field is that area in such proximity to an earthquake source such that elastic waves generated by an earthquake are different in terms of fre-P-
quency content and prominent wave type than these waves arriving at a more distant site. The extent of the area is dependent on source dimensions, i .
attenuation, earthquake depth, and magnitude. Very few seismic records are available for earthquakes occurring in this area. Thus, the question arises as to how to evaluate nearfield effects given these variables and 1
the lack of instrumental data.
I
~
s 2.3 Capable Fault The term " capable fault" defined in Appendix A was unique to the regulation, i.e., it was not previously used in the earth science profes-sion. It was established as a measure of the likelihood that a fault could cause surface rupture and/or localize earthquake activity. The term has since gained world-wide use in the geologic and seismologic profession as a more precise definition for " active fault." Four basic elements are used in Appendix A to establish whether or not a fault is a " capable
- f aul t." These are (a) movement on a fault within the past 35,000 years or l
) multiple movements within the past 500,000 years, (b) a correlation with
" macro-seismicity," (c) a relationship to a known " capable fault," and, I for non-capability, (d) a structural association with geologically old
, ( structures.
17 Enclosure "A"
(
The capable fault concept is derived from observations of highly active faults located in the western United States where there is rela-tively high, ongoing tectonic activity represented by rugged topography, high rates of crustal deformation, and large and frequent earthquakes.
Although it was developed with western geology in mind, Appendix A appifes A
this concept uniformly across the entire United States, including the area east of the Rockies where rates of tectonic activity are relatively low.
In an effort to quantify in rule language, for licensing, a complex scientific concept, the concept does not permit reasonable accommodation of new work relevant to assessing fault hazards. The types of work that are relevant to this problem and that are becoming increasingly more widely 4
accepted include probabilistic analyses, calculations of recurrence rates
'( for earthquakes and fault movement, microearthquake monitoring, stress analyses and strain measurements. The question has been raised whether the present definition of capable fault should be modified to include the above methods.
The characteristics of most recent fault movement defining a capable fault were chosen to provide some measure of the hazard posed by surface faulting. They are not based on a rigorous assessment of deformational activity of faults as manifested by a certain level of earthquake activity related to rates of fault movement. They were, however, chosen and accepted as being conservative, based on empirical knowledge of numerous active faults with histories of surface displacement and large earthquakes.
Because the numerical age values are specifically stated in Appendix A,
( the determination of fault capability may appear to be straightforward.
18 Enclosure "A"
1
(
In practice, however, earth scientists have not always been able to acquire
" absolute" age data (radiometric age dates, etc.) to meet these criteria
! and often such assessments involve considerable professional judgment and l indirect or relative methods of dating (the use of rates of denudation, regional geologic history, geomorphology, etc.). In the absence of defini-tive guidance in Appendix A on the extent of investigation needed to adequately assess capability of a fault and the level of certainty needed to conclude a fault is capable, disagreement among geological experts has arisen.
Difficulty arises in applying the recurrent movement criterion in the definition of capable fault. For faults with extensive amounts of offset
- (tens of feet) and for minor offset (less than several inches), the
( ,
implementation of Appendix A is generally easily accomplished, i.e. large total offsets imply multiple movements and small total offsets usually imply a single movement. However, for intermediate amounts of offset bet-ween these two extremes, a determination of whether a single or recurrent movement has occurred is difficult to ascertain.
As indicated, the fault movement criteria were not originally based on a quantified consideration of rates of fault displacement; however, the
- numerical values assigned imply certain rates of earthquake activity.
l There is an inconsistency in Appendix A in that the rate of activity of a fault defined by the age of last movement criteria does not necessarily correspond to an explicit rate of activity as may be inferred from the
- seismicity element of the capable fault criteria.
k 1
19 Enclosure "A" 1
(
The term macro-seismicity is unique to Appendix A and is used in Appendix A as if it were a clearly defined term in the earth sciences. The term is undefined in Appendix A and is not a generally recognized term.
Macro-seismicity means either large (with respect to earthquake size and/or rate of earthquake activity) or long (in terms of persistency) f-earthquake activity. The staff has interpreted this to imply profound deep-seated tectonic activity. In current staff practice, macro-seismicity is considered to be a level of seismicity that implies significant, sus-tained, and coherent tectonic activity representative of major deforma-tional movement within the earth's crust. Originally, a specific earthquake
. magnitude was intended as a threshold in defining macro-seismicity; this is not stated or implied in Appendix A.
k In the definition of capable fault, the requirement concerning i
sacro-seismicity states:
" macro-seismicity (shall be) instrumentally determined with records of sufficient precision to demonstrate a direct relation-ship with the fault."
l In this regard, Appendix A provides no direction for establishing such a direct relationship. Appendix A provides no guidance as to what consti-
, tutes " records of sufficient precision" and only speaks to the use of instrumentally determined earthquakes without mentioning the use of his-torical earthquakes in such an assessment.
According to Appendix A, if a fault is structurally related to a capable fault, it also must be considered capable. The only guidance on
( structural relationships provided in Appendix A is that movement on one 20 Enclosure "A"
I
'\( structure could reasonably be expected to be accompanied by movement on the other. Two types of relationships are possible: first, where there is a direct physical connection to a capable fault; second, where there is a genetic relationship between faults properly oriented in the same stress field. For either situation, there may be cases where movement on one P-fault could reasonably be expected to be accompanied by movement on the i
other. The requirement has been interpreted as only involving a direct connection to a known capable fault. The direct physical connection of faults is often extremely difficult to show and the data required to define these conditions are not specified in Appendix A. Since Appendix A is unclear in this regard, professional judgment must be used in making such determinations. Moreover, this clearly goes to the question of level
( of conservatism. .
Also in need of clarification is one of the attributes used in defin-ing a " fault" in Appendix A, i.e. the inclusion of ". . . any associated monoclinal flexure or other similar geologic structural feature." It is not clear how this characteristic should be used in assessing the length of faulting, the earthquake generating potential, or the potential for surface displacement of the fault.
I Appendix A further states in the "notwithstanding" clause in the last paragraph of the definition of capable fault that a fault that can be demonstrated to be structurally associated with other structural features that are geologically old is not capable. It appears that this statement was intended to apply mainly to the eastern U. S., but the concept may k
21 Enclosure "A" l
i .
\
( have a more general application. The statement implies that faults that can be shown to have formed in response to a tectonic regime that has ceased to exist, or has been substantially modified, need not be considered capable even if the fault exhibits one of the characteristics of capability.
It also could be interpreted to mean that faults in regions that have not f.
experienced known Quaternary or younger tectonism should nevertheless be con-sidered capable if such faults exhibit the characteristics of capability.
i Also, given the general observation of the antiquity of faults in the eastern U. S. and the significant differences between eastern and western U. S. tectonic settings, it is questionable whether it is the intent of i
Appendix A to require extensive investigations of faults in the eastern U. S.
( Movements or deformations of the Earth's crust can be of either a profound deep-seated nature (tectonic) or of a more superficial nature (non-tectonic). The latter include near-surface stress release, ice-shove features, growth faults, etc. The movement criteria in the definition of capable fault were intended to deal with tectonic deformation. It has been previously argued in a petition for rulemaking that Appendix A is not clear with regard to differentiating the types of fault movement. The petition was denied because the staff considered Appendix A clear on this point. The issue is included in this enclosure because it was raised in a number of public comments.
l A fundamental issue inherent in Appendix A is the concept of design-ing for surface displacement. Sections V (b) and VI (b) discuss the need
- to design for surface faulting. In order to accomplish and evaluate such
(
22 Enclosure "A"
(
. designs the geoscientist must provide the engineer with an assessment of the precise location and expected amount of surface displacement near or beneath a facility. Such a determination cannot be accomplished with a high level of certainty with our present understanding of fault behavior.
Although Appendix A does not include an explicit prohibition on use of a site which would require designing for surface displacement, the extensive investigations and analysis required by Appendix A would in effect result in such a prohibition. Present engineering and environmental practice contained in Regulatory Guide 4.7, " General Site Suitability Criteria for Nuclear Power Stations," states that sites located within 5 miles of a capable fault are generally not suitable and that sites that include capable faults are not suitable for nuclear power stations. The suggestion has been made that
( '
Appendix A state an explicit prohibition on siting near capable faults.
2.4 Specification of Safe Shutdown Earthauake (SSE) and Operatina Basis Earthquake (08E) Vibratory Ground Motion This section treats the methodology for specifying vibratory ground motion from earthquakes. The overall procedure involves (1) taking an earthquake of some size (magnitude or epicentral intensity), (2) assuming that event to occur at some defined location relative to the site, (3) determining an acceleration level at the site representative of this earthquake, and (4) specifying design ground motion corresponding to that acceleration level and representative of the postulated earthquake descrip-tion. Appendix A calls for the specification of two earthquakes for i
j 23 Enclosure "A"
(
design. The SSE is an earthquake based upon evaluation of the maximum earthquake potential of a region. The 08E is an earthquake that could reasonably be expected to affect a plant during its operating . life time.
Several issuas have been identified covering a wide range of topics of varying significance in the overall problem of specifying vibratory ground motion for use in engineering design. Other closely related technical issues are discussed in sections 2.1 through 2.5 of Enclosure B.
Appendix A calls for specification of earthquake size in terms of magnitude or epicentral intensity. Appendix A contains the additional requirement that the magnitude be specified on a Richter scale. In cer-tain parts of the country, other magnitude scales have traditionally been used to indicate earthquake size. In such cases, the Appendix A require-( . ment can impose an unnecessary constraint since a method for converting from the scale traditionally used to the Richter scale is not always available.
Alternatively, Appendix A permits that earthquake size may be expres-sed in terms of intensity on the Modified Mercalli Scale. Prior to 1934, l
nearly all earthquakes were rated according to intensity because instru-mental data were not available. Some larger earthquakes (post-1927) and a few great earthquakes (post-1900) have instrumental data. The classifica-tion of earthquakes on an intensity scale is highly subjective. In parti-cular, older events for which reports are limited may depend critically on the skills, objectivity, and biases of one or two observers. Questions frequently arise about the sizes and locations of some of these historical
( earthquakes. Such questions impact on considerations of the seismic 24 Enclosure "A"
d
- (
. design for particular nuclear power plants. As a result, the staff, applicants and the U.S. Geological Survey on a case-by-case basis have had
- to review the original data sources of earthquakes to reassess earthquake intensities and locations. There is needed for a reevaluation of earth-quake intensities and locations of generic scope to establish a more l accurate data base.
After establishing the sizes of earthquakes, the next step in the Appendix A methodology is to provide a representation of ground motion l from a series of earthquakes postulated to occur according to various sets of conditions. Thus, in establishing the SSE, Appendix A requires that
- earthquakes equal in size to the largest historical earthquakes associated with tectonic structures or with tectonic provinces be postulated to occur on those structures or in those provinces at the points of closest approach to the site. For the tectonic province in which the site is located, the i point of closest approach is at the site itself. If this were taken literally, the site would always be in the nearfield of the postulated i earthquake and special considerations would need to be given to nearfield effects. In practice, Appendix A has been interpreted to mean that the maximum intensity historically reported'in the province, in which the site is located, should be placed at the site, but not treated as nearfield.
This interpretation is implied in Regulatory Guide 1.60, " Design Response 3
Spectra for Seismic Design of Nuclear Power Plants." This interpretation hinges on the low probability that the plant will be in the nearfield of a randomly occurring earthquake of the postulated size, and on extensive
~
i.
1 25 Enclosure "A"
J 4( investigations in the site vicinity to identify potential earthquake sources. This practice does not appear consistent with a literal inter-4 pretation of Appendix A.
Ground motion in the Appendix A methodology is represented by an acceleration level in combination with response spectra (currently defined l'
by Regulatory Guide 1.60, " Design Response Spectra for Seismic Design of Nuclear Power Plants"). Two issues arise directly from Appendix A require-ments in this area. First, Appendix A specifies a minimum acceleration
! level for the SSE. Appendix A currently sets this level at 0.lg, but higher levels were considered in its development. The ACRS has recently 1
(last several years) questioned whether this minimum level should be raised. The reasons put forth for such an increase are: (1) it would
( provide additional conservatism in consideration of uncertainties in the data base and would simplify case review; and (2) it would help alleviate problems with backfitting arising from the trend in recent years toward higher SSE acceleration levels in the eastern U. S. because the design levels would be higher. The second issue arises from the requirement that ground motion be represented by response spectra corresponding to the accelerations at the foundation levels of plant structures. Difficulty l arises here as to what is meant by foundation level. The term foundation level may either imply some elevation below the ground surface or the strata ,
upon which the plant is founded whether it be at the surface or below ground.
The latter interpretation was intended; however, the present wording in Appen-
- dix A is not clear. Also, according to some investigators, difficulties arise here because nearly all the available data on ground motion are from
(
i .
26 Enclosure "A"
(
, measurements made at or near ground surface. In addition, some finite ele-ment method techniques developed to analyze variation of motion with depth and soil-structure interaction effects produce physically unrealistic results when the input motion is specified at foundation level at depth. The source of the discrepancy is the specification of a generalized motion at depth in the soil (i.e. , foundation level below ground surface) where it could not naturally occur. To avoid this problem in practice, Appendix A has been interpreted to require that the generalized motion be specified at the ground j
surface and the motion at depth is derived according to the techniques noted earl.ier. However, it is unclear whether this practice is consistent with a literal interpretation of Appendix A.
The discussion thus far has focused primarily on problems in specify-( ing the SSE. Problems also arise from requirements for the OBE, which have been found to be ambiguous, internally inconsistent, or contradictory (See Section 2.2 of Enclosure B for discussion of OBE engineering issues, and Enclosure F, memo from Minogue to Commissioner Mason, dated October 8, 1976, which provides further information on the OBE concept). The diffi-culty here arises from the deffi.ition of the OBE provided in Appendix A, its interpretation by different scientific and engineering disciplines, and the procedures described for determining the OBE acceleration level.
l I
The OBE is defined as an earthquake reasonably expected to affect the site during the plant's operating life. To some disciplines (geology / seismology),
this implies a probabilistic assessment over the 40 year lifespan of a plant.
l k
27 Enclosure "A" t
4 l i l l
To engineering disciplines an earthquake expected during the life of a faci- l lity implies an event whose likelihood is great enough that (economic con- 1 siderations would dictate that) a structure must be designed to accommodate it.
For structures involving substantial capital investment, this is an event in the range of 300 to 500 years. Elsewhere in Appendix A the maximum I
A acceleration corresponding to the 08E is required to.be at least half that of the SSE, tying the 08E to the deterministic methodology of the SSE.
j Based on earthquake data, for most of the U. S. an acceleration level of one-half that of the SSE does not correspond to an event reasonably expected during a 40 year period (i.e. nominal operating life), but rather to an 4
l .
earthquake having a much longer return period (in the range of 300 to 1,000 years for most plants). Alternatively, in some seismically active 4
( areas of the U. S. an acceleration level of one-half the SSE may not repre-sent a conservative estimate of an expected event because of the higher frequency of occurrence of earthquakes. To better meet the definition (as f
opposed to the requirement just noted) of the OBE as specified in Appendix A, the staff has accepted OBE acceleration values of less than half those of
- the SSE for a few sites. Such exceptions are permitted within the scope of i
Appendix A when supporting data to justify the departure are provided. In such cases, the staff has required probabilistic analyses of earthquake hazard to justify departures. However, it is unclear whether the allowance of such departures was the intent of Appendix A.
One additional aspect of the CBE issue is Appendix A requires that, '
h if ground motion in excess of that corresponding to the 08E occurs, the I
4 plant be shutdown and inspected. There are several problems in applying 28 Enclosure "A"
l this requirement. First, there is an overriding question, as to the determination of what constitutes exceedance of an OBE. Exceedance of the OBE can be defined in several ways, for example, sxceedance of the freefield OBE acceleration level, exceedance of design response spectra at different elevations in a plant, exceedance of response spectra at a single frequency or several frequencies by a certain amount. There is a need to establish definitive guidance in this area. Second, no NRC specific criteria for inspection in the event of an OBE have been developed. Third, given the increased use of probabilistic analysis in determining the OBE, OBE acceleration values could be set at any level even below the .05g mini-mum set forth in Appendix A (OBE = x .lg SSE minimum = .05g).
It would not be practical to permit the OBE acceleration level used in plant design
( to be so low that the ground motion often would be exceeded. Criteria for identifying the permissible risk here do not exist. Such low OBE values could result in large areas having OBE values exceeded during an earth-quake and, because of the requirements for shutdown, could cause blackouts.
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29 Enclosure "A" l . _ . - - - - - - - - - --
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ENCLOSURE B
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ENCLOSURE B ENGINEERING DESIGN ISSUES RELATED TO VIBRATORY GROUND M
- 1. INTRODUCTION f-In addition to the issues discussed in Enclosure A that arise in the application of Appendix A, there is another category of technical issues that relate to seismic design methodology. Included in this category are issues that derive from the interface with engineering design requirements of " ground motion" as determined in accordance with Appendix A methodology.
The issues identified in this enclosure, in general, represent areas that are either not dealt with or afforded very limited discussion in Appendix A.
5 They arise mainly from efforts by the NRC staff to provide infomation and procedures that supplement the regulation. These issues involve matters for which the state of the art is rapidly advancing and where the supporting, data base is continually 1:aing expanded by acquisition of new information.
They frequently require the exercise of engineering judgment. Suchjudg- ;
ments are intimately tied with issues of conservatism and consistency in l
review. I
{
The issues identified here have been the source of frequent and costly '
impacts on the licensing process, in terms of staff resources, engineering costs, and adverse safety impact in other areas, such as that caused by excessive stiffness of some systems. Some of the impacts of these issues are similar to those identified for issues discussed in Enclosure A. The k
1 Enclosure "B"
(
acquisition and assessment of new geological and seismological data during the review process and, in particular, between the Construction Permit and Operating License phases of review have produced significant delays in licensing actions and, in some cases, costly reanalysis or changes in design.
In other cases, licensing delays result from extended litigation and debate over the appropriateness of a methodology that provides an interface between ground motion and engineering design.
- 2. ENGINEERING REQUIREMENTS IN THE REGULATION Certain engineering design aspects of nuclear power plants are briefly treated in Appendix A. This treatment was placed in this site evaluation-related Appendix to contribute to an understanding of the ultimate use of the siting assessments covered, and was not intended as a definitive
( treatment of the engineering aspects of seismic design. Several of tM engineering concepts addressed in the regulation have been the subject of controversy because of their limited discussion.
Questions have been raised as to whether a regulation primarily intended for seismic and geologic siting evaluations can, or should, dis-i l cuss engineering considerations. Also, the question has been raised whether the regulation should address the hydrologic aspects of siting. The hydro-l logic review procedures have been supported by a series of regulatory guides which provide details in this area.
l 2.1 Specification of Vibratory Ground Motion
, Appendix A describes procedures for determining maximum vibratory
! ground motion at a site for use as an engineering design basis. These
(
! 2 Enclosure "B"
I I procedures were developed when relatively simple seismic design methods were standard practice. Now, complex methods are used in place of the earlier practices. These techniques have not, however, eliminated the controversy that is often associated with assigning a design basis for vibratory ground motion, and several questions remain to be addressed.
f-Specific areas in which difficulty arises are discussed below.
2.1.1 Site Specific vs Generalized Response Spectra Appendix A requires the development of response spectra for seismic design. Appendix A does not, however, provide a detailed procedure for deriving the response spectra. Regulatory guides and the Standard Review Plan (SRP) provide a supplement to Appendix A that is needed to complete the determination of vibratory ground motion. The staff has developed
( Regulatory Guide 1.60, which specifies broad-band spectra to be used.
These spectra represent the normalized mean plus-one-standard-deviation responses of records from 33 earthquakes of various magnitudes, recorded at various distances, and on varying site conditions. The staff at one time attempted to develop a site-specific method to derive response spectra (Agbabian-Jacobsen Associates, 1970, "A Study of Earthquake Input Motions for Seismic Design") but was unsuccessful. Difficulty was encountered l
because of limited data and in obtaining general acceptance.
Thus, following current practice, the SSE and OBE seismic design bases at a site are described by the Regulatory Guide 1.60 spectra appro-priately scaled to represent the earthquake hazard at the site consistent with Appendix A criteria. Because Regulatory Guide 1.60 is a smoothed
( spectrum that contains energy at all frequencies, it gives unrealistically 3 Enclosure "B" i
high values of motion at certain frequencies when used as input at the foundation level in some soil-structure interaction analyses. To avcid this, the input must be controlled at the free surface or a site specific spectrum must be used.
To alleviate the above problems, the staff encourages the use of site-dependent spectra because of improved analytical methods and the increased number of strong motion earthquake records. Such site-dependent analyses are stressed in a proposed revision of the SRP. In practice, site-dependent spectra are needed for certain analyses used to investigate liquefaction and, in some cases, in the design of embankment dams.
An additional question is whether there is enough data to develop site-dependent spectra for sites in the East. However, the same question
( ( can be raised regarding the applicability of Regulatory Guide 1.60 in the eastern U. S. since the response spectra do not include any eastern earth-quakes.
2.1.2 Variation of Ground Motion with Depth Appendix A requires that response spectra corresponding to the maxi-mum vibratory accelerations at the elevations of the foundations be defined.
Many methods have been proposed to achieve this requirement. Appendix A does not provide detailed guidance in this matter; therefore, regulatory guides and the SRP have attempted to complete the needed guidance.
Whether the vibratory ground motion for soil sites is specified at foundation levels or the ground surface, consideration of the variation of k
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4 Enclosure "B"
l 1
motion with depth is needed for defining design input for finite element soil-structure interaction models, liquefaction studies, and response ana- l lyses for earth structures. The present methods for considering the varia ,
tion of ground motion with depth include computer modeling techniques with varying degrees of complexity. The one dimensional shear beam analysis (SHAKE), which is frequently used, has a number of limitations: (1) it treats all wave types as vertically propagating shear waves, thus neglecting the effects of other seismic wave types that are included in the ground motion; (2) because it is an. equivalent linear elastic method, it is not applicable when large strains occur; (3) for some soil profiles, results can be unrealistic when generalized broad-band response spectra such as Regulatory Guide 1.60 are input at depth, and (4) the analysis is
( ( limited in that it does not account for certain geologic variations such as nonhorizontal layering and topography.
Some finite element methods have the advantage of permitting consider-ation of the effects of additional seismic wave types, such as surface waves and nonvertically incident waves. Usually these methods require specification of input motion at the base or at the side of the soil model.
However, since nearly all earthquake data have been recorded at or near the surface, there is uncertainty in the form of the base mot'in to be used for such analyses.
- It is the general view of the staff that use of site-dependent methods to estimate variation in ground motion with depth should be encouraged where data permits. Use of such procedures raises the question as to when does the data permit such analyses.
(
5 Enclosure "B"
2.1.3 Specification of Time History Appendix A requires specifying seismic input in terms of response spectra corresponding to the maximum vibratory accelerations to be expected ,
at a site. In addition to response spectra, a time history of vibratory ground motion is frequently needed to perform various design analyses, b
Any number of time histories may be developed that satisfy Regulatory Guide 1.60 response spectra requirements within any given tolerance. Some of these may be more conservative than others because the frequency of motfor of the actual accelerogram may be distributed so as to result in canceling modes of vibration that are of significance in power plant design. The question has been raised as to the need for explicit regulatory guidance in the use of time histories.
( 2.1.4 Duration of Shaking The duration of strong earthquake motion is important in characteriz-ing vibratory ground motion. It is a measure of the number of stress cycles that are applied to the structure and soil medium. Appendix A requires that the duration of shaking caused by earthquakes be given consideration in design. There is some lack of consistency in present practice in the treatment of duration. That is, the length of time and number of cycles of strong ground motion used is different for different analyses, e.g., in performing liquefaction analysis, structural over-turning analyses, and fatigue analysis.
The problem associated with duration of shaking is to define it in a complete and consistent manner. Several definitions of duration have been suggested by various investigators, but each involves uncertainties
( .
1 6 Enclosure "B"
( and limitations. The definition of duration could strongly influence trends from studies based on statistical analyses of strong action.
- 2. 2 OBE Use in Engineerina Additional questions, other than geologic-seismic ones, have been raised about the way vibratory ground motion representing the OBE is used in engineering design. The regulation requires that the effects of the OBE vibratory ground motion be considered in combination with normal operat-ing loads. In practice, loads arising from the OBE are combined with loads from other severe natural events. For example, the OBE is combined with the load from the standard project flood to evaluate seismically induced dam failure.
Viewed as applied to engineering design, the design basis for the SSE is specified in Appendix A to assure that the plant design adequately pro-tects the public health and safety in the event of an extreme earthquake.
The plant may well not be operational as a power plant following an SSE.
The OBE is established as the most severe earthquake following which the plant can safely be operated without special inspections. Engineering codes and design practice apply these two earthquake levels differently. Cate-gory I structures, systems and components, must maintain their safety func-tion for earthquake levels up to the and including the SSE. On the other hand, the engineering design objective with the OBE is that the plant is capable of being safe in operation after experiencing an event less than or equal to the 08E.
k 7 Enclosure "B"
I (
As these events are applied to design,.in use of the ASME Boiler and Pressure Vessel Code, the SSE is normally applied as a faulted condition, meaning that stress levels allowed by the code which would result in per-manent general deformation are permitted except when deformation would lead to loss of safety function. The OBE, on the otherhand, is considered as b
an Upset Condition or Design Condition in application of the code and is used in conjunction with lower allowable stress levels at which no general deformation would occur (elastic regime). In addition, to differences of allowable stresses for the OBE and SSE, there are other differences in design analysis methods in the application of Faulted and Upset Conditions.
Many designers see the SSE as being the basic seismic design basis with the OBE playing more the role of a cross-check basis using different analysis
( ,
procedures and different limits to assure the adequacy of the margin pro-vided by the SSE design over a wide range. Viewed from this perspective (which is the perspective of the engineering parts of Appendix A) the OBE is more an engineering safety factor applied to design analysis rather than being seen as a seismic event.
Because of the way some loads are combined, the associated damping used, and the stress levels allowed in current engineering design practice, situations occur where the loads arising from the OBE in combination with other loads are higher than loads for the SSE. In such cases, the deter-mination of the OBE acceleration level becomes significant and the SSE loses significance in engineering design. The problem has been exacerbated by the arbitrary Appendix A requirement that the OBE acceleration level be half
(
8 Enclosure "B"
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( < I
, that of the SSE. Because the OBE acceleration level is used in a number of engineering analyses in different and complex ways, the significance of this parameter to the overall safety margin of a nuclear power plant is difficult to assess and has not been determined. The question has been raised as to whether the OBE is needed at all or alternatively whether the separate uses of the OBE should be differentiated. There is a need for a detailed consideration of all uses of the OBE acceleration in engineering design, margins of safety that may be affected, and the extent to which geosciences and/or engineering assessments should affect determination of the OBE acceleration level. It is important to note that the data base is sufficiently large to permit the determination of the OBE probabilistically in many cases (See discussion in Section 2.4 enclosure A).
( 2. 3 Consideration of Aftershocks Aftershocks are smaller earthquakes following a major event. After-shock effects are required to be considered by Appendix A and it is per-missible to allow strain limits in excess of the yield strain for the SSE loading. In practice, however, structural stresses due to the SSE are not allowed to exceed yield stresses except in localized areas; therefore, after-shock effects are not taken into account. Should the SSE stresses be allowed to go beyond yield, considerations regarding low cycle fatigue and ductility demands during the aftershock must be properly accounted for in the design of systems and components. However, some plants have been designed to undergo a certain degree of inelastic deformation of structures. Such local yielding is not allowed to place structures, systems, or components in danger k
9 Enclosure "B"
of failure. This has been felt to be a safe practice since earthquakes approaching the SSE in severity are not likely to occur because of the low frequency of their observed occurrence.
2.4 Consideration of Potential Damage from Earthouakes Less than the SSE Appendix A emphasizes design of safety-related structures, systems, and components for only two levels of vibratory ground motion: the OBE and SSE (with the exception of aftershocks). Appendix A does not consider the probability of intermediate levels of shaking. In areas where the frequency of occurrence of strong motion is high, the plant site may experience a number of strong earthquakes approaching the OBE or between the OBE and SSE during its lifetime. At present, limited consideration is given in design to the number of earthquake events (in fatigue analysis) the plant
( might experience and the finite probability of yielding due to these events.
In attempting to design for such earthquakes, the same pitfalls discussed in consideration of aftershocks exist. A compromise is required between design for a broad spectrum of unlikely events and optimum design for normal operation. Design for a single limiting event (the SSE) and inspection and evaluation for earthquakes in excess of some specified limit (the OBE),
when and if they occur, may be the most sound regulatory approach.
2.5 Use of Probability for Considering Combinations of Loads Appendix A requires consideration be given to seismic and other con-current loads in the design of safety related structures, systems and com-ponents. Appendix A is stated deterministically and does not give any guidance concerning consideration of the probability of occurrence and failure as a result of the applied loads. At present, loads are combined 10 Enclosure "B"
{
(
, according to regulatory guides, ASME, and ACI codes. The staff considers the application of these load factors conservative. However, they lack a rigourous probabilistic basis.
t
- 2. 6 Need for Seismic Scram '
Appendix A notes that consideration is being given to the need for instrumentation to automatically shutdown (scram) a plant in the event of an earthquake that exceeds a predetermined intensity. The question of whether to have seismic scram instrumentation at commercial nuclear power plants has been a long standing dispute between the ACRS and the staff, and is an ACRS generic issue. The staff sponsored assessments of seismic scram (Lawrence Livermore Laboratory, UCRL-51619, " Evaluation of the Use M Seismic Scram Systems for Power Reactors" and Lawrence Livermore Labora-( tory, UCRL-52156, " Advisability of Seismic Scram") which confirmed the staff view thet such instrume .ation is not advisable. Subsequently, the ACRS was notified (memo from E. Case to M. Sender, dated May 19, 1977) that the staff considers the generic matter resolved, does not intend to require seismic scram instrumentation and does not plan to expend further effort or resources on additional studies. However, the ACRS has noted that the LLL study dealt with low-level earthquake intensity scram and has requested the staff to explore high-level earthquake intensity scram, such as that in use in Japan.
The issue remains an unresolved ACRS generic issue pending a visit to Japan in the spring of 1979 by members of the staff and ACRS.
k 11 Enclosure "B"
o
- 3. ISSUES REGARDING CONSERVATISM 3.1 Deterministic vs Probabilistic Approach The tectonic province concept as used in Appendix A can be thought of l
as having a combination of both deterministic and probabilistic character-istics.
It is deterministic in the sense that the distribution and size 1
of future earthquakes may be predicted from a given set of observed and i
interpreted conditions, i.e., for areas containing consistent geological features, there is a consistency of earthquake potential. From this assump-tion that earthquake activity is consistent over a region, it follows that the frequency of earthquakes to be expected can be determined based on the number of events in a given region during a given interval of time; this is a probabilistic concept. In the development of the tectonic province
( concept, the use of probabilistic analysis was not emphasized. Probabilis-tic approaches were not considered to be sufficiently reliable because; (1) the historic record of earthquake data is short, necessitating extrapolation beyond the data base to determine low probability events; (2) the data base is inhomogeneous, i.e., the data base varies in completeness both spatially l and temporally; (3) knowledge is lacking to identify earthquake source regions,apreliminarystepinsuchileterminations;and(4)informationto reliably estimate the maximum earthquakes in such regions (also a prerequi-site) is deficient. Because of these limitations, SSE level earthquakes for design estimated probabilistically would have large associated uncertainty.
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k 12 Enclosure "B"
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Today, a number of arguments have been brought forth in favor of using a probabilistic analysis: (1) the present Appendix A approach suffers from the same shortcomings mentioned above; (2) unlike a probabilistic approach, which allows for a quantified and consistent treatment of uncertainty, the present approach does not lend itself to such treatment; (3) the present approach does not lend itself to consideration of backfitting, which requires I
assessment of significance; (4) the staff is frequently called on to make probabilistic determinations to assess adequacy at hearings and before the ACRS; and (5) probabilistic approaches have been adopted and are becoming more widely adopted to determine earthquake hazard to establish the seis-mic design for all types of structures (e.g., LNG facilities, general struc-i
- , tures covered by the ATC building code).
Given these considerations, the issue has been raised as to whether, j the Appendix A methodology should be changed to emphasize probabilistic techniques for assessing earthquake hazard.
3.2 Empirical Relations Between Earthquake Size and Ground Motion Parameters As discussed in Enclosure A, Appendix A requires that ground motion from earthquakes, postulated to occur according to the methodology defined in the regulation, be specified in terms of an acceleration level. However, specific procedures relating infomation on earthquake size and distance
, to acceleration are not contained in Appendix A. Numerous empirical rela-tions between earthquake size (magnitude or intensity), distance, and acce-leration level have been published. The data show wide scattering. Pub-lished relationships have been derived using different data sets, data
, ( from differing geologic regions, and varying procedures for data reduction.
l 13 Enclosure "B"
l
( , It can be expected that as additional data become available the state of :
the art will continue to advance and still different relationships will be found. As the data base has evolved, different relationships have been developed and used to assess the acceleration levels at nuclear power plant sites. As a result, new plants assigned the same size earthquake as old P-plants have been designed for higher acceleration levels. An issue which has arisen is whether plants already designed and/or built should be reassessed in consideration of new data and new relationships relating earth-quake size to acceleration level.
To ensure some level of consistency in more recent reviews, the staff has adopted specific relationships between earthquake size,' distance, and I
acceleration level. Thus, in its Standard Review Plan, the staff has
( adopted as l'icensing practice the mean value of the intensity-acceleration relationship developed by Trifunac and Brady, where size is expressed as intensity, and the Schnabel and Seed relationship, where size is expressed as magnitude. Issues have been raised over whether these relationships are appropriate and, in particular, whether mean values derived from these relationships represent the proper level of conservatism.
A more fundamental issue has been raised as to whether the Appendix A methodology places too much emphasis on a single vibratory ground motion parameter: acceleration level. The totality of vibratory ground motion from an earthquake cannot be specified in terms of any one parameter. While Appendix A recognizes this in its requirement that ground motion correspond-ing to the SSE and 08E be represented by response spectra, these response 14 Enclosure "B"
I
(
. spectra and the required investigative procedures are keyed to acceleration level. Questions have been raised over whether more complete descriptions of the ground motions from earthquakes, postulated to occur according to
^
Appendix A, should be provided for use in assessing the engineering design.
3.3 General Lack of Definition of Overall Seismic Desian Conservatism An overriding issue is the lack of definition of some level of conser-vatism proper for seismic design. A major purpose of Appendix A is to set forth criteria for investigators to determine the vibratory ground motion at a site to use in seismic design; that is, to determine the input into the seismic design methodology. However, Appendix A does not define an explicit level of conservatism appropriate to this input. Rather, it defines a deterministic methodology to arrive at this input implicit in which is the premise that if the procedures are followed an acceptable level of conservatism will result. The lack of an explicit definition of an appropriate level of conservatism for the seismic input has led to diffi-culties in the licensing process. It places an undue burden on individual reviewers to define what is accaptable, which can lead to nonuniform appli-cation of Appendix A and unwarranted inconsistencies between' sites.
The staff has on occasion been pressed in hearings to assess levels of conservatism associated with design earthquakes (i.e., using probabil-istic analysis and defining the probability of exceeding the design earth-quake). Such an assessment is not called for in Appendix A; nevertheless the level of conservatism becomes a focus of debate.
15 Enclosure "B"
m
(' Because of the lack of overall definition of an appropriate level of seismic design conservatism, elements in the seismic design chain are reviewed in isolation from other elements, such as the siting and various engineering areas. Questions have been raised as to whether this results in compounding or counter productive conservatisms; whether uncertainties
). .
in one element are compensated i.n design margins of another element, such as whether the use of means of empirical relations (e.g., intensity-acceleration relations) in assessing earthquake vibratory ground motion are adequately compensated (given significant uncertainity in such rela-tions) by engineering safety margins; and whether increasing conservatism in one element might in fact reduce margins in another area such that over-all conservatism decreases, as may result from stiffening of structures to resist seismic loads where they would better remain flexible to withstand thermal and other stresses. A major research effort sponsored by the Office of Nuclear Regulatory Research is in progress to assess the conservatisms in I overall seismic design.
16 Enclosure "B"
y O 9
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kr ENCLOSURE C O
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. ENCLOSURE C l BROAD POLICY AND TECHNICAL ISSUES BEARING ON THE ,
IMPLEMENTATION AND REVISION OF APPENDIX A
- 1. INTRODUCTION g
The issues identified in this enclosure cover a wide range of topics that are related in various ways to the implementation of Appendix A.
These issues have been raised within the NRC and by representatives of other government agencies, industry and the public. The issues discussed below have impacts on present and future siting policy to varying degrees.
Some issues are obviously vitally important, such as whether Appendix A should be revised and, if so, to what extent. Others, such as issues pertaining to the relationship between NRC siting policy and other national policy, are important in terms of considerations regarding revision to Appendix A, but are not central to this stage of the staff's reassessment (i.e., identification of issues arising in the application of Appendix A).
- 2. IMPEDIMENTS TO IMPLEMENTING APPENDIX A IN THE LEGAL CONTEXT In addition to the difficulties that arise for technical and scienti-l l fic reasons, significant impacts on applicants and the staff have occurred
( because Appendix A is difficult to implement in a legal context. The development of Appendix A was itself precipitated by the Malibu hearings in the mid-sixties. In this licensing action, a hearing board and the Atomic Emergy Commission had difficulties bringing the case to conclusion because of problems in assessing the magnitude of hazard associated with k
1 Enclosure "C"
, faulting in the absence of professional standards in this area. Appendix A represents a d2 tailed codification of technical subject matter auch of which is not exacting and requires technical judgment and latitude in its application. Difficult tradeoffs are required between the need to avoid case-by-case litigation of recurring issues and the need for review h
flexibility in this fast moving technical area.
- 3. INTERFACE OF ISSUES WITH OTHER NRC POLICY 3.1 Lack of Policy Statements Concernino Early Site Reviews, Limited Work Authorization, and Alternative Site Reviews At present, no policy has been established regarding the requirements
, for geologic and seismic infortnation needed for issuance of a limited work I
authorization and for alternative site review, and only limited guidance
}( has been given regarding information required for an early site review.
With regard to early site reviews, more detailed guidance is needed con-carning the options available to an applicant in terms of level of detail required for preliminary geologic and seismologic investigations. The question has been raised whether the NRC should provide an applicant the option of accepting a conservative and preapproved seismic design value l rather than conduct the type of extensive regional analyses presently required. Such an option would reinforce the need for publication of an NRC seismic zoning map and more explicit policy concerning'early deter-minations of vibratory ground motion at a site.
(
2 Enclosure "C"
3.2 Seismic Desian of Fuel Cycle Facilities The consequences of failure of a fuel cycle facility are considered less than that of a nuclear power plant. Therefore, less stringent seismic and geologic siting and design requirements are considered appropriate.
Use of Appendix A requirements and conservatisms for fuel cycle facilities implies either that undue conservatism is being applied in designing fuel cycle facilities or puts in question the level of conservatism adequate for nuclear power plants. Because Appendix A lacks an explicit level of conservatism based on a rigorous assessment of consequences, requirements for fuel cycle facilities cannot be readily determined based on scaling down requirements in Appendix A. The lack of an explicit level of conser-vatism based on the consequences of failure as a result of earthquakes,
( for instance, does not permit a quantitative determination as to the appropriate level of conservatism to be applied for fuel cycle facilities with respect to these consequences. Regulations and regulatory guides presently being developed for fuel cycle facilities are turning to probabil-istic analysis procedures to determine design earthquakes (e.g., for independent spent fuel storage facilities). This allows for the specifica-tion of the level of conservatism required. Such methods are being widely accepted (e.g., ATC-seismic codes for large buildings, LNG regulations) and maps are available to facilitate determining the earthquake potential at a site and seismic design input. As discussed previously, the use of probabilistic procedures in determining seismic input for nuclear power plants is an issue in itself. The use of such analysis for fuel cycle k
3 Enclosure "C"
( -
facilities while. excluding its use for nuclear power plants can be inter-i preted as an apparent inconsistency in policy. However, probabilistic analysis is being used for fuel cycle facilities because of the lower risk associated with such facilities compared to nuclear power plants which allows for designing for lower earthquake levels in the range where the P-data can be assessed with greater confidence.
3.3 Consideration of Seismic Design of Nonradiological Safety Structures.
Systems and Components Requirements in Appendix A only address structures, systems and components that are considered safety related. It has been suggested that j the NRC should require some level of earthquake resistant design for i
non-safety related systems for the following reasons: (1) because of the complexity'of interaction between safety and non-safety structures, systems, and components, elements of uncertainty may be introduced as to the overall i
- adequacy of design; (2) there are some systems, components, and structures associated with nuclear facilities that pose nonradiological risk to the public health and safety. As an example, the failure of an ultimate heat sink dam could result in loss of life as a result of flooding. Present j practice is to classify as safety related only those dans that may lead to radiological hazards. Under present NRC regulation, requirements are l absent for the seismic design of systems, structures, and components not classified as related to radiological safety, but which may pose non-radiological hazards. The overall issue has been extensively discussed in the past, e.g., SECY-76-399, SECY-77-222 SECY-78-358 and is presently under assessment as a separate topical issue outside the staff assessment
{
l 4 Enclosure "C"
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of Appendix A. The issue is mentioned here only as it relates to the l seismic area.
- 4. ISSUES PERTAINING TO NATIONAL POLICIES AND PRACTICES 4.1 Consistency of MRC Seismic and Geologic Sitina Policy and Practice with Other National Policies and Practices f-l 4.1.1 Earthquake Hazards Reduction Act of 1977 (EHRA)
Several aspects of EHRA have significance with respect to our present seismic and geologic siting policy and practice and particularly to con-siderations for revision of Appendix A. The Act calls for increased earthquake research on and the development of new methods and procedures for design and construction to mitigate earthquake hazards. The Act specifically gives priority to the development of methods and procedures
( for earthquake-resistant design and construction for nuclear power plants.
Authorizations to be appropriated under the Act are considerable; therefore significant advances in technological knowledge in this area are expected.
As such, this places a significant priority on ensuring that present NRC policy and practice as well as regulatory requirements be amenable to readily assimilating information developed from research carried out under the Act.
As previously discussed, one significant issue identified is that, because Appendix A is a regulation and is detailed, procedures and methods contained in the regulation cannot be readily modified to assimilate advances in the state of the art. -
Another objective of the Act bearing on revision to Appendix A, is r
that the Act gives priority to the development and implementation of 5 Enclosure "C"
(
', earthquake prediction. Appendix A was developed prior to many recent advances in earthquake prediction concepts and methodology. At present no NRC policy exists concerning what action, if any, may be required if an earthquake prediction is made for the area of a nuclear power plant.
4.1.2 Presidential Directives b
4.1.2.1 Executive Order - Improvina Government Regulations On March 23, 1978, the President signed an executive order entitled:
" Improving Government Regulations." Several aspects of the executive order bear on problems previously identified in the application of Appendix A and on procedural considerations that should be made in deliberating on possible revisions to the regulation.
The latter will be treated in the next phase of the staff effort as noted in Enclosure D. This discussion k is limited to the aspects of the executive order bearing on currently 1
identified problems.
First, the executive order calls for the language of a regulation to be simple and clear as possible, that is, understandable to those subject to its provisions. Appendix A is a technical regulation directed at technical experts but written and structured in a very complicated manner difficult for the technical experts to follow.
Second, the executive order calls for regulations that do not impose unnecessary burdens on those affected by it (i.e., individuals, the public, private organizations, States, and local government). As noted previously, issues have been raised concerning the requirements of Appendix A placing 6 Enclosure "C"
( an undue burden on applicants through excessive conservatism associated with some requirements and lack of clarity in some requirements, and ulti-mately on the public.
4.1.2.2 National Energy Policy The National Energy Plan bears both directly and indirectly on NRC seismic and geologic policy and practice. The Plan bears indirectly on the seismic and geologic siting area in that it calls for overall improve-ment in the licensing process (i.e., establishing reasonable and objective criteria for licensing, reduction in extensive licensing procedures where standard design is involved, and overall reduction in delays and licensing time). '
The Plan bears directly on seismic and geologic siting policy and
'k practice in that it requests the Commission to develop fim siting criteria with clear guidelines to prevent future siting in potentially hazardous locations. The President in his energy address on April 20, 1977, specifically stated that new plants should not be located near earthquake fault zones. This policy is consistent with general NRC staff site suit-ability practice. However, Appendix A does not contain any explicit pro-hibition for construction on a capable fault.
4.1.3 Draft Congressional Legislation Two bills introduced before the 95th Congress have bearing on our present siting policy and practice in this area: HR882, Nuclear Energy Reappraisal Act and HR11704, Nuclear Siting and Licensing Act. Although both bills were not reported out of committee, nevertheless, they illustrate past and present Congressional concerns that have a bearing on Appendix A and the
(
7 Enclosure "C"
( geological / seismological licensing review.
HR882 called for an assessment of the NRC licensing process, which has permitted nuclear power plants to be built over geologic faults. As noted previously, regulatory criteria i prohibiting siting near hazardous faults has been identified as an issue.
HRil704 pertained to this area of siting in that it called for the establish-A ment of an early review permit, emphasis on standardized design, and a combined construction permit and operating license to reduce licensing time. Thus, one of the important goals in modifying present policy and practice, and regulatory requirements is the reduction of licensing time.
Specific issues revolving around this have been identified (e.g., establish-ing detailed early site review policy in the geologic and seismic siting area).
k 4.1.4 Comparison of NRC and Other Federal Acency Critical Facility Seismic and Geologic Sitina Policy and Practice The staff has reviewed seismic and geologic siting criteria in use or under development by other Federal agencies to determine differences in approaches to provide a broader perspective on issues related to potential revisions to Appendix A. It should be noted that comparison of policy and practice cannot be readily made because different structures are involved, which require somewhat different siting and design approaches.
The staff has examined those earth science criteria for critical structures listed on the enclosed Table. Major differences in criteria j are:
(1) The definition of a hazardous fault differs in tenas of terminology used, the ages used to define a fault as a hazardous one, the use of
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- 3. 0 8 Enclosure "C"
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. seismicity levels in defining a hazardous fault, the inclusion in criteria of prohibitions on siting near hazardous faults, and the reliance on probabilistic assessments to assess potential future fault movements.
(2) The procedures used to define seismic input for design differs in terms of the degree of reliance on maps to identify seismic source regions, use of probabilistic procedures, the procedures used to define regions to assess regional earthquake potential and the use of differing ground motion parameters for design input.
Generally, it appears the criteria are moving towards seismic and geologic assessments that require probabilistic analysis and the definition of acceptable levels of risk (dependent on the hazard associated with a par-ticular facility) in quantitative terms.
- 5. EXTENT AND NATURE OF REVISIONS TO APPENDIX A The foregoing discussions in this and previous enclosures have identi-fied numerous issues that arise directly and indirectly from the applica-tion of Appendix A in its present form. Many issues raised deal with fundamental problems identified when Appendix A was in early stages of development (a decade ago), and during the initial public comment period when Appendix A was issued as a proposed rule in 1971. Because of the fundamental nature of difficulties it is clear at this time some form of l revision to Appendix A is warranted. A number of options have been l considered:
- a. Minor revisions (word changes, expansion for clarity) to present i k regulation;
- 9 Enclosure "C"
( b. Substantial expansion of regulation (add detail to sections that are difficult to interpret because of their general nature);
- c. Simplifying the regulation (deleting sections) and simultaneously 1 providing more detailed information in regulatory guides;
- d. Rescinding Appendix A and rely entirely on regulatory guides to provide detailed guidance on the subjects covered as has been done successfully in the hydrology area.
The staff consensus is that option C is the most desirable way to proceed. Accordingly, the next stage of the staff effort will be directed toward: (1) revising the regulation in a more simplified form; (2) supple-menting the revised Appendix A with a series of regulatory guides to be issued concurrently with the revised regulation; (3) assessing through a value/
( impact analysis the resolution of issues identified in this paper; and (4) developing a program for continuous updating of regulatory guidance.
Enclosure D describes more fully the subsequent stage of the staff effort.
l( -
10 Enclosure "C"
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LIST OF EARTH SCIENCE CRITERIA FOR CRITICAL STRUCTURES AGENCY FACILITY TITLE USE NRC Nuclear Seismic and Geologic Siting Criteria for Nuclear Power Regulation for licensing Power Plants Plants; 10 CFR Part 100, Appendix A - November 1973 C0E Dans Earthquake Design and Analysis for Corps of Engineer Guidance for staff and Dans; Reg #1110-2-1806 - April 1977 contractors DOT LNG Liquefied Natural Gas Facilities (LNG), Federal Proposed draft regulation Facilities Safety Standards; 49 CFR Part 193 - April 1977 for licensing VA Hospital Earthquake Resistant Design Requirements for VA Guidance for staff and Facilities Hospital Facilities; Handbook H-08 Hay 1977 contractors BR Dans US8A Design Earthquake Selection Procedures; Guidance for staff and
- November 1977 contractors EPA Hazardous Standards Applicable to Owners and Operators of Working draft regulation Waste Hazardous Waste Treatment, Storage and Disposal for licensing Facilities Facilities; 40 CFR Part 250 - February 1978 ATC Buildings Tentative Provisions for the Development of Seismic Tentative provisions (NSF/ISS) Regulations for Buildings - June 1978
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e a ENCLOSURE D 9
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ENCLOSURE D SUtetARY OF SUBSEQUENT STAGE IN THE ASSESSMENT OF CURRENT SEISMIC AND GF0 LOGIC SITING CRITERIA, POLICY, AND PRACTICE A. PRELIMINARY VALUE-IMPACT STATEMENT (PVIS)
- 1. Objectives The next phase of the staff assessment is a preliminary value-impact analysis of issues identified to structure the revised
,- Appendix A and associated guides and then follow-on guides. The objectives of this phase include:
- a. Using value-impact assessment to consider the resolu-tion of issues identified. This will involve considera-tion of alternative ways of resolving technical and procedural issues through an assessment of values and impacts, i.e.,
considerations of tradeoffs in meeting objectives. During this stage, the specific recommendations made by the staff, ACRS and their consultants, and public comments will be addressed.
- b. Using the PVIS as the working document to explore resolution of issues prior to actually revising Appendix A in order to: (1) establish the intent as to how the regulation
( should be revised; (2) establish a common reference of 1 Enclosure "D"
intent to review changes to the regulation; (3) avoid missing substantive issues that need resolution; and (4) minimize the review of word changes to the agulation (i.e., to minimize the number of drafts and work required to derive an acceptable revised regulation).
b c. Using the PVIS to establish the framework for the supple-mental regulatory guides,
- d. Documenting clearly what, how, and why decisions were reached.
- e. Forming the supplementary statement (statement of considera-tion) for revision of Appendix A and other policy and practice.
- 2. Preliminary Value-Impact Process It is the staff's intent to perform the analysis in accordance with guidance already established by OSD, NRR (NRR office Letter No. 16) and by the Commission (Secretary Memorandum January 23, 1978). Details of the analysis are contained in the above guidelines and will not be repeated here. Special considera-tions will be given to those areas listed below.
- a. Executive Order - Improving Government Regulations
- b. Congressional Bills and Directives
- c. Other Acts (EHRA NEPA, MSLP)
- d. Other Agency regulations (LNG, Dams, ATC)
- e. National Energy Plan
- f. Early site review k
2 Enclosure "D"
( g. Standardized design
- h. Backfitting
- i. Use of Research
- j. Impact on other facilities under NRC jurisdiction
- 3. Results of Preliminary Value-Impact Analysis The results of our analysis will include:
- a. Recommendations to the Commission on the resolution of issues;
- b. Recommendations to the Commission for rulemaking and establishment of regulatory guidance,
- c. Recommendations for obtaining further public input; 1
- d. Connon ground for all to assess the revised regulation.
( B. REVISION OF APPENDIX A TO 10 CFR PART 100 AND DEVELOPMENT 0F SUPPLE-MENTAL GUIDES On the basis of decisions reached in performing the preliminary value-impact analysis, a revised regulation and regulatory guides will be developed for promulgation for rulemaking and public comment. At the earliest, the revised regulation and supplemental regulatory guides could be promulgated in FY 1980.
k
> 3 Enclosure "0"
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ENCLOSURE E
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ENCLOSURE E BACKGROUNO AND SOURCES OF INFORMATION USED IN THIS PAPER P-I. BACKGROUND A. Historical Perspective Prior to 1973, when Appendix A " Seismic and Geologic Siting Criteria for Nuclear Power Plants," to 10 CFR Part 100, " Reactor Site Criteria" became effective, regulatory requirements in the geologic and seismic siting area were contained in 10 CFR Part 50, " General Design Criteria," and in 10 CFR Part 100 These regulations provided merely broad
( requirements in the seismic and geologic siting area. A need for more definitive regulatory criteria in the earth science area arose from the difficulties encountered in licensing review of the siting of several nuclear power plants in California in the early sixties. The types of difficulties encountered included: (1) the lack of standards to assess adverse seismic and geologic conditions at a site; (2) the need for guidance to appifcants as to the type of investigatory procedures to follow; (3) the absence of review procedures for the staff; (4) the lack of a framework to make legal determinations and to assess compliance; and (5) protracted delays and considerable debate in the licensing process over technical issues, k
1 Enclosure "E"
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As a means of resolving the above difficulties, work was initiated in 1966 to draft seismic and geologic siting criteria which lead to a semi-formal siting document developed in March 1969. In 1971, Appendix A was published for public comunent, and in late 1973 the regulation became effective. Appendix A in final form represented a compilation of pro-P-
cedures and methods developed primarily from experience gained during the review of early sites. It also reflected a synthesis of a broad spectrum of professional views and ideas.
In developing Appendix A, it was recognized that the criteria were based on limited available data and that revision would be appropriate as the state of the art improved and additional infomation became available.
Such a statement is expressed in the PURPOSE of the regulation.
( B. Review of the Application of Appendix A Extensive experience has been gained in the appifcation of Appendix A and difficulties in applying the regulation have arisen. Many of the problems Appendix A was intended to resolve were not resolved.
As a result of problems encountered, in 1976 the staff began a reevaluation of the regulation. As a means of ascertaining the extent of problem areas, the staff held several meetings and prepared a " straw man" revised draft of Appendix A. The " straw man" draft differed primarily from the regulation in the arrangement of sections and the incorporation of additional wording to increase clarity. This draft was circulated to staff for comment. Coments received were nume*ous and indicated a need for an in-depth reassessment.
k 2 Enclosure "E"
l
( . Concurrently with the staff review of Appendix A, a broader review was underway of overall siting policy. In Policy Paper SECY-76-286 dated May 25, 1976, the staff informed the Commission of this overall review. That paper also informed the Commission that the seismic and geologic siting criteria were under separate review. Subsequently, in Policy Paper SECY-76-286A dated December 14, 1976, the staff outlined topics in the seismic and geologic siting area being considered.
By letter dated January 27, 1977, the Secretary of the Commission requested a proposed policy statement on seismic requirements. During the preparation of the requested paper, the Secretary issued a new memorandum dated June 30, 1977, requesting the following: (1) that the staff address only present siting policy and practice; (2) that SD and NRR in a followup
( paper describe and analyze major issues not covered in other siting papers; and (3) that 50 and NRR prepare an alternative siting statement to present siting policy.
In response to the first directive, the staff prepared an Information Report (SECY-77-288A), dated August 18, 1977. That paper described current seismic and geologic siting policy and practice for nuclear power plants, its historical development, and outlined the staff's subsequent papers.
Thus, SECY-77-288A established the framework for this paper and subsequent papers.
Following the preparation of SECY-77-288A, issues pertaining to overal seismic and geologic siting policy and practice were solicited and compiled from technical and legal staff. Additionally, on December 15, 1977 and January 26-27, 1978, meetings were held with the Seismic Subcommittee of
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3 Enclosure "E"
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the Advisory Committee on Reactor Safeguards (ACRS) and their consultants.
The purpose of these meetings was to obtain their views on problem areas identified by the staff and to solicit any additional problems. Following the last meeting, the staff received reports from ACRS consultants. Also on January 19, 1978, the staff published a notice in the FEDERAL REGISTER requesting public comment on issues pertaining to Appendix A. The public comment period ended March 1, 1978. Eighteen public comments were received in response to the notice, and one public coment was received at the second ACRS meeting. Source documents used in the identification of issues are summarized below.
II. SOURCES OF INFORMATION A. Staff Sources Our review has consisted of discussions, meetings and solicita-tion of procedural and technical issues from the staff (i.e., geoscientists, hydrologists, and engineers in 050, NRR, RES, and I&E whose responsibili-ties fall under Appendix A) and from both the Regulations and Hearing Divisions of OELD. Formal coseents on issues were received from:
- 1. J. C. Stepp, Chief, Geosciences Branch, DSE, NRR, memo to L. 8eratan, dated 11/22/77. Geoscience staff recommenda-tions for revisions to Appendix A.
- 2. V. Stello, Jr. . Director, DOR, NRR, memo to L. Beratan, dated 12/6/77, request for engineering and hydrology input into the revision of Appendix A, 10 CFR Part 100.
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4 Enclosure "E"
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. 3. L. Hulman, Chief, Hydrology - Meteorology Branch, DSE, NAR, memo to L. Beratan, dated 12/14/77, proposed revision to 10 CFR Part 100, Appendix A " Seismic and Geologic Siting Criteria for Nuclear Power Plants."
- 4. I. Sibweil, Chief. ESB, DSS, NRR, dated 11/10/77 input into the revision of Appendix A,10 CFR Part 100.
- 5. Comments were also received from the above sources as well as other staff sources (NRR, IAE, 050, RES, OELD) during the revision of the straman draft, draft issue papers, and during several meetings of the staff. Additionally, infor-mation was obtained from the staff during the ACRS seismic subcommittee meetings.
( 8. Advisory Committee on Reactor Safeauards Source information examined from the ACRS includes: generic reports; reports on specific sites in which generic items were mentioned; comments by the seismic subcommittee and their consultants during and in response to three days of meetings on Appendix A. Specific documents include:
- 1. Site reports:
(a) Comments by J. C. Maxwell, consultant for ACRS, on Skagit, dated 8/30/77.
(b) ACRS reports on Perkins and Cherokee, including D. Okrent's remarks, dated 4/14/77.
(c) ACRS report on North Anna dated 1/17/77.
k 5 Enclosure "E"
( 2. Generic reports:
(a) ACRS generic report #4, on seismic scram, 4/16/76.
(b) Minutes and consultant reports at the 178th ACRS ,
Meeting, 2/6-8/75.
- 3. Information obtained during the Seismic Subcommittee meeting I-with the staff on Appendix A, 12/15/77, 1/27-28/78.
(a) Transcript of the above meeting (approximately six hundred pages).
(b) Consultant letter reports by:
A. H-5. Ang John D. Maxwell H. Bolton Seed
- ( .
Shailer S. Philbrick Merit P. White James T. Wilson (2 letters)
Zenon Zudans (c) Letter report by David Okrent.
C. Formal Public and Industry Comments.
Formal comments on problems in the application of Appendix A were obtained in response to a staff Federal Register notice published on 1/19/78. The public comment period ended 3/1/78. Additionally, one public comment was received during the ACRS seismic subcommittee meeting.
Also, at the request of the AIF, two days of meeting were held with NRR to discuss problems and recomendations for change. Specific sources follow:
k 6 Enclosure "E"
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- 1. Public comments in response to staff notice.
(1) Weston Geophysical Research, Inc.
(2) Arizona Public Service Co.
(3) Le8oeuf, Lamb, Leiby and MacRae (4) Pacific Gas and Electric Co. 4 (5) D'Appolonia Consulting Engineers, Inc.
(6) Lindvall, Richter, and Associates (7) California Division of Mines and Geology (8) E8ASCO Services, Inc.
. (9) Southern California Edison Co.
(10) Los Angles Dept. of Water and Power 4
(11) General Electric Co.
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(12) New York State Geological Survey (13) Law Engineering Testing Co.
(14) Stone & Webster (15) Dames & Moore (16) Sargent & Lundy Engineers
- (17) Commonwealth Edison
-(18) Nathan M. Newmark Consulting Engineering Services
- 3. Comment by AIF
! a. Letter from J. Ward to H. Denton, dated 6/15/76, l
summarizing comments presented at 5/12/76 meeting.
- ( b. Verbal comments received at 1/9/78 meeting.
l 7 Enclosure "E"
(
D. Comments by the USGS The staff has solicited formal comments on Appendix A from the USGS. Formal written comments are still pending; however, we have had several discussions with USGS staff on Appendix A, on 9/30/77 with James Devine and with members of the USGS Nuclear Advisory Group in formal V-and in informal session on 12/77. Comments received in these discussions have been considered in our compilation of issues.
E. Additional Sources Numerous other sources have been considered in our comoilation of issues. Included here are discussions with professional peers, review of papers presented at professional meetings, scientific publications discussing Appendix A, documents relating to NRC policy and practice,
( documents related to interfacing issues, and the Appendix A historical file.
8 Enclosure "E"
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ENCLOSURE F
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.00.Franklin Research Center A Division of The Franklin Institute Z. ZUDANS, PH.D-Senior Vke Presdent and Chief Operating Off>cer March 29, 1982 Dr. R. Savio Senior Staff Engineer U.S. Nuclear Regulatory Commission Advisory Committee on Reactor Safeguardt Washington, D.C. 20555 re: Seismic Deafgn Criteria
Dear Dr. Savio:
In summary, it appears to me that Dr. Jennings' proposal does not suggest significant changes to the NRC licensing process. Most of his recommendations (if not all) correspond to the NRC licensing practices at the present time (like for example, site specific seismic input, use of ductility, and fracture toughness). The only potentially different approach is the suggested 200 year return time for OBE and the 2080 year return time for the SSE, (although I reca14 various ACRS meetings, when return time was used as an argument).
Some specific comments related to Dr. Jennings' proposal are given in the enclosure.
Very truly yours, l
l eo Z n s' ces enior Vice President enc 1.
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- 2. Zudans T. Stilwall Comments on Seismic Design Criteria
References:
- 1) NRC Memorandum Feb. 16, 1982 (R. Savio to 2. Zudans, et' al.)
- 2) Letter of Oct. 5, 1981 (Paul E. Jennings to Commission,er Gilinsky)
You have asked me to comment.upon Dr. Jennings' letter of Reference 2.
Dr. Jennings offers his suggestions for seismic criteria for nuclear power plants. Many of them closely parallel those currently in use by the NRC.
I will attempt to comment on the major suggestions.
- 1. Response Spectra Paul Jennings appears to suggest that ground response spectra, site-specific both in magnitude and in shape, be developed for each~ plant.
I have no objection to this in principle but I doubt that a sufficient data base can be developed for each plant to assure probabilistic reliability of individual shapes for ground response spectra based exclusively on site-specific data.
Current NRC practice is to detelop site-specific amplitudes (based on local geological and seismological investigations at the site) and then apply them to a universal, standardized response spectrum (derived by Newmark, 81ume, and Kauper from an envelope of a number of well documented earthquake records) normalized to a Ig amplitude. .
It should be noted, however, that Appendix A to Part 10010CFR$0 requires site-specific response curves be developed for sites situated near known faults.
It also permits licensees to take exception to specific procedures (e.g., use of the response spectrum of ' Reg. Guide 1.60) if they can adequately document and justify a different position; thus, there is no absolute prohibition of use of site-specific response spectra - if adequate evidence is available to justify their use.
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'.2 Capable Fault Criteria . . a Dr. Jennings suggests a list of faults be prepared for each plant together .
with their activity rates and states that this sidesteFs the " capable fault" '- -
definition.
Similar information is currently required to be reported (see Rag.' Guide 1.70) by applicants. However, I fail to see that Dr. Jennings' suggestion'aidesteps e t.he need for some definition of which faults must be included on the,' list and which may be omitted from it. Would he, for example, include a fault which is known to have been very active in past geological times but bas been quiescent during the (geologically brief) time that spans human history?
- 3. Conservatism of the Design Response Spectra Paul Jennings suggests the design response spectra be based on envelopes of statistical data for 200 year (OBE) and 2000 year (SSE) earthquakes taken to enclose data one standard deviation from the mean, i.e., the expection would then be that roughly one third of actual earthquakes of the specified return intervals would exceed the design curve.
Current NRC criteria assure that the magnitude of.the SSE will be at least as large as any earthquake ever known to have occurred at the site or the
, equivalent effects at the site of the most severe' earthquake known to have occurred in its tectonic province (see 10CFR50; Part 100. Appendix A; Article V).
The OBE is taken as at least half this magnitude.
With respect to the SSE (which, because of its potential consequences to public health and safety, I regard as particularly significant) NRC's approach is certainly more conservative than Dr. Jennings' suggestion. Dr. Jennings suggestion - that the SSE be defined as the mean asp 11tude (plus one standard deviation) of earthquakes with an expected 2000 year return time - seems quite conservative if one considers only one plant to be in operation for forty years. However, I am by no means confident that it defines a level of acceptable risk for, say, one hundred plants (each replaced when its 40 year life is done) operating on the U.S.A. throughout the next century.
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- 4. Permissible Responses
- Paul Jennings suggestion that design criteria for 'the OBE restrict maximum, stresses to the elastic region; and that inelastic behavior be permitted (but structural integrity against collapse or catestrophic failure be maintained)-
under SSE, echoes in principle NRC current practice. ' -
- One of the major justifications for spec,1fying different design criteria in response to these distinct events is precisely that 'the SSE has a much lower probability of occurrence. However, to impose graded allowable exedrsions into the inelastic region based on site-specific estimates of frequency of, occurrence at individual sites (as Dr. Jennings appears to advocate) even if it were possible to determine what such individual limits should' be - would create a ' maze of regulatory difficulties.
- 5. Fracture Toughness Criteria Dr. Jennings mentions, in passing, the importance of ductility and toughness in assessing structural adequacy under reismic loading.
- This is an important area currently being at. dressed by the NRC. Although such considerations are indeed important in assessing earthquake resistance capability; the fracture-toughness of structures is much more severely taxed under postulated LOCA conditions (or undercooling transients).
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Comissioner Victor Gilinsky Nuclear Regulatory Comission Room H1113
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Dear Comissioner Gilinsky:
In response to a telephone request by Ed Abott, I will try to lay out in.a few pages how I believe the earthquake resistant design criteria should be developed for major projects siTelf as nuclear plants, if this could be done in the absence of regulatory or historical' constraints. The outline will necessarily be brief and the fine points will be glossed over. In preparing this letter I am relying on my general background plus recent experiences with the seismic design of offshore drilling platforms for the Santa Barbara Channel and with the Seismic Review Panel the California Public Utilities Comission has formed for the proposed LNG facility at Point Conception. The major differences between what the NRC does and what I recomend is that some oversimplified definitions and practices of the NRC are not used 'and the judgments are made differently.
- As the first step, seismological and geological studies are under-taken to determine the possible sources of ground motion and surface faulting (the latter is assumed to be avoided by site selection). One of the results of these studies would be a list of faults, with estimates of the geologic time of their last movement and their activity rates.
This gradation of importance sidesteps the " capable fault" definition and does not require equal treatment of a fault with abundant evidence of movement and one that may have moved a few centimeters in the last half-million years. The estimate of current activity rates, which typically would vary by orders of magnitude, would be useful for deter-mining both the upper level (SSE) and lower level (OBE) earthquakes. If the larger historic earthquakes near the site could not be tied to an identified fault, this fact would affect the conservatism used in applying the seismological and geological assessments.
The major result I would expect from the geoscientists would be, for each source, their estimate of the biggest earthquake expected, on the average, every 200 years, for example, and also that expected, on the averle, every few thousands of years (say 2000 years). The 200 ,
and the .10 define, in effect, the OBE and the SSE and the NRC should
( decide the numbers depending on its desired conservatism. An earthquake '
, with a return period of thousands of years approaches the " maximum credible," but represents a more meaningful way to address the problem. ,
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Comissioner Victor Gilinsky Page Two 5 October 1981 .
, Also, the geologist and seismologist should prpvide an estimate of the
' biggest earthquake that could occur anywhere near the site, but not *
. clearly identified with the faults they have found.
From these design earthquakes the earthquake engineer would develop -
the expected level of response spectra for shaking at the site. In most
, cases, one or two design earthquakes we'uld govern -- each in different parts of the frequency domain. The design spectra could then be set conservatively with respect to the average of the expected response spectra, with some tailoring for limitations of the methods used to estimate the response spectra. (Seesketch.)
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4 hg e rn ey One could justify using the average curve, since the design earthquakes are chosen conservatively, but more comon practice is to take another -
conservative step and to set the design spectra above the expected average of the response spectra, e.g., near the estimate of the mean plus one standard deviation. This process would be done for both the SSE and 08E.
In this step, attention is concentrated on that part of the frequency .
domain which is important for the project. The peak acceleration of the expected ground motion is usually of minor importance in this pro-( cedure since it is associated with a frequency higher than the important -
( modes of the structure.
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Comissioner Victor Gilinsky Page Three 5 October 1981 I Af'ter the design s etra for both levels of earthquakes are constructed for the required values of damping, the next step is to specify the allowable response of the structure in the two cases. For the lower level, only elastic response is usually permitted. For the upper level earthquake, the allowable response is larger and the amount pemitted should bear some relation to the probability of experiencing the motion as well as the
! . potential hazard of the structure. Generally, the more improbable the event, the more the structure should be allowed to respond beyond tht i elastic limit, as long as structural integrity against collapse or cata-
! ' strophic failure is retained. In the case of conventional steel-framed i
offshore drilling platforms, levels of response around twice the yield level are allowed, provided a collapse mechanism (plastic hinges in all legs) does not develop. Other types of structures can take more or less:
- the amount of reliable ductility is an engineering question that depends on the structure's fom, the materials of construction, the details of elements and joints, etc. This last stage, at which the allowable response is set, is also the point where the overall conservatism of the design is evaluated and any needed adjustments made. Note that this is in the hands of the earthquake engineers and designers, where it belongs.
(- With the design spectra deterinined and the damping and allowable response fixed, the design criteria are complete. The next step is to see that they are implemented correctly. Most major projects employ three means for doing this. First, they have a consulting board (s) that helps resolve questions that arise. Second, the design is usually reviewed by an independent firm; this is often a condition for insurance. Third, there is, or should be, an active inspection process during construction.
A final coment: Once the design criteria are reasonably conserva-
, tive, the most generally effective way to increase the seismic capacity of structures is not to simply raise the criteria, but to concentrate on the detailing of members and joints to insure a tough, ductile structure.
This is often not a question of much cost, but involves instead choices of layout for the facility, the location of structural members, the
- choice of materials, the placement of reinforcing steel in critical areas, etc. Because of the many possibilities, these factors cannot be covered by codes or general criteria; this is where the quality of the
! engineering is important. $
I sense in drafting this letter that I have reached, if not i exceeded, the length of document desired, and I will now stop. If I can help further, or if you want to discuss these questions further, please do not hesitate to call.
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SINCERELY YOURS, Md -h; PAUL C. JENNINGS
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':E ACT00. S AFEGUARDS MEMORANDUM FOR: f.F.Fraley,ExecutiveDirector
/ Advisory Committee on Reactor Safeguards FROM: Gary G. Zech, Technical Assistant Technical Support Branch, HRR
SUBJECT:
SEISMIC DESIGN MARGIN EVALUATION OF SYSTEMS AND EQUIPMENT REQUIRED FOR SAFE SHUTDOWN - NORTH ANNA POWER STATION, UNITS 1 AND 2 (NA-l&2)
In its letter of January 17, 1977, to Chairman Rowden, and in a supplemental letter of February 17, 1977, to B. C. Rusche, the ACRS recommended that the NRC staff review the design of systems and equipment at NA-l&2 required to achieve safe shutdown and continued shutdown heat removal following a seismic event.
The staff conducted a series of plant site visits and meetings with the Virginia Electric and Power Company (licensee) to discuss specific systems and equipment regarding this subject. Also in response to the ACRS request for additional review, the staff presented a summary of its findings to the full ACRS Committee on March 9,1978. The staff findings concluded that the seismic design margin was adequate for the systems and equipment at NA-l&2.
However, during the March 9,1978 meeting the ACRS members raised further specific questions about seismic design margins. By letter dated March 14, 1978, you requested the staff to prepare a report summarizing the seismic design margin evaluations for NA-l&2.
Altnough this report to the ACRS was originally scheduled to be completed on November,1979, higher priority work during the past year has delayed the completion of this task.
By this memorandum, you are advised that we have contracted a consultant (0RNL) to complete this work and the report is now scheduled for completion in December,1980.
Gary G. h. Tec nical Assistant Technical Support Branch Office of Nuclear Reactor Regulation M:;; pnat TuomdM @
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RECEIVED . Battelle Columbus Laboratories
- 505 King Avenue 2m m e 31 . coi== bus. ohio 42202 Telephone (614) 424 6424 Telen 24 5454 .
U.S. NUCLEAR REG.COMM.
ADVISORY CCMMtiTEE ON .
REACTOR SAFEGUARD 5 February 27, 1980 .
Mr. E. Igne, Staff Engineer U.S. Nuclear Regulatory Comission DISTRIBUTED TO ACRs - ' ~ ~ ~ MEMBERS
~
Advisory Comittee on Reactor Safeguards ! -
1717 E Street, NW Washington, DC 20555
Dear Mr. Igne:
Coments on Meeting of ACRS, Subcomittee on -
Plant Arrangements, Afternoon at February 21, 1980 Enclosed are my coments on the subject meeting. ,
Yours very truly, . ,
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E. C. Rodabaugh Applied Solid Mechanics Section .
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cc: Myer Bender -
Oak Ridge National Laboratory Oak Ridge, Tennessee 37830 rvfgn ut u '
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. WATERWAYS EXPERIMENT STATION. CORPS OF ENGINEERS P. O. BOX 631
- VICKSBURG. MISSISSIPPI 3918o
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28 June 1979
. DISTRIBUTED TO ACRS MEMBERS RECEIVED ADVISORY COMMITTEE hUOh Mr. Bob Jackson REACTOR SAFEGUARDS U 5 Geology and Seismology Division , .
Directorate of Licensing j{j[ 1219/9 Nuclear Regulatory Com=ission pg Washington, D. C. 205h5 t. g ig gg,.,4 4 g V
Dear Bob:
The Corps of Engineers is in the beginning stages of generating a manual for seismic evaluations and we are at the same time reviewing our practices for comparability with those of experts, consulting firms, and other Government agencies. Thus, I would like to ask if your agency would participate with us in analyzing some hypothetical situations. We hope to produce a review of general practices and to see what agreement there may be in the specifying of earthquake motions. The results may be useful to all of us. The attached sheets (Inc11) list the problems that we wish to consider.
If you have any questions, please call me at (601) 636-3111, extension 3329 Sincerely, 1 Inc1 ELLIS L. KRINITZS?2 As star.ed l'"*7 -
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. QUESTICIMAIRE Following is a series of hypothetical problems :
Location A. Directly over the trace of the San Andreas fault, California.
Mc.ximum credible earthquake (MCE): Magnitude (M g ) = 8.3.
Location B. Five kilometers from the trace of the San Andress fault, California. MCE: Magnitude (Mg )-= 8.3.
Location C. Fifty kilometers distant from the trace of the San Andreas fault, California. MCE: Magnitude (Mg ) = 8.3.
Location D. One-hundred and fifty kilometers from the source area of the New Madrid earthquake events in central United States. MCE:
Magnitude (m b
) = 7.5 Location E. A floating earthquake postulated in eastern United States.
Active faults cannot be located. MCE: Magnitude (Mg ) = 6.5 ,
Location F. A floating earthquake postslated in western United States.
Active faults cannot be located. MCE: Magnitude (Mg ) = 6.5 ,
Location G. Damsite at a major reservoir. Induced seismicity has been interpreted. MCE for the induced event: Magnitude (Mg ) c 6.5 ,
Problem: For each of the above locations, please provide the following:
- a. Peak motions for forming a time history. Horizontal peak
. particle acceleration, velocity, displacement, and duration (bracketed
> .05,g). Give motions separately for rock site and soil site, if different.
- b. Acceleration for selecting smoothed response spectra. Give the acceleration at the site for use in entering the smoothed response spectra of Nuclear Regulatory Commission Regulatory Guide 1.60. Give motions separately for rock site and soil site, if different.
(General Instructions)
The motions may be either deterministic or probabilistic, accordir4 to your preference. If you choose a probabilistic approach, please state the probability of the maximum credible event.
Include references and equations, or copies of graphs, used in specifying all motions.
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If you prefer that earthquake motions be expressed by other means than those which are requested, please give us your preferences and your reasons. But please also give your interpretations o f the requested motions so-that we may have the comparability that we are after.
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[ JAN 3 1 1077
(.hp ..HE I Mr. Myer Bender, Chaiman Advisory Comittee on Reactor Safeguards V. S. Nuclear Regulatory Commission Washington, D. C. 20555
Dear Mr. Bender:
REQUEST FOR INTERPRETATION OF THE COMMITTEE COMMENTS ON NORTH ANNA POWER STATION, UNITS 1 AND 2 We have reviewed the Comittee's January 17, 1977 report on the North Anna Power Station, Units 1 and 2. We are uncertain as to (1) the Comittee's conclusion with respect to the staff's position on the Stafford fault zone, (2) the intent of the Comittee's coment with respect to the recomendation of the Committee's con-sultants relative to a minimum SSE for the Eastern United States,.
and (3) the intent of the Comittee's coment with respect to staff review of the seismic design capability of the plant.
With respect to the first matter, the Committee stated its position on every item addressed in its report, except for the matter of the Stafford fault zone. The Comittee merely states that " Consultants to the ACRS concur" with the staff interpretation, without including the Comittee's own position. We are viewing this as an oversight and are interpreting your report to mean that the Comittee also concurs with the staff interpretation.
With respect to the second matter, the Comittee's report states the following relative to the Comittee's consultants:
While they generally find the current design bases acceptable for the already constructed North Anna plants, they have recommended that, in view of the uncertainties of knowledge concerning the sources of earthquakes in the Eastern United States, a minimum l
safe shutdown earthquake (SSE) of 0.2g acceleration should be utilized for new plants for which construction permit applications are submitted in the future.
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Mr. Myer Bender JAN 3 1 F.?
We are uncertain as to the intent of the inclusion of the above comment in the Committee's report without the benefit of the Committee's view on the recommendation of its consultants. We have interpreted the comment to mean that the Committee, although not in complete agree-ment with its consultants' recommendation, nevertheless believes it-to be worthy of inclusion in any future staff considerations that may be directed to modification of current practices with respect to setting seismic design criteria. The recommended position would, of course, require a change in the Commission's regulations.
The term " Eastern United States," as used in the Committee's comment, is being interpreted to mean that part of the nation east of the Rocky Mountains rather than that part east of the Mississippi River or east of the Appalachian Mountains. In effect, this would set a minimum for the entire United States since, in general, seismic
. accelerations for areas west of the Rocky _ Mountains are already assumed to be above 0.2g.
With respect to the third matter, the Committee's report states the following:
The Applicant presented partial information ~ concerning the ,
calculated safety factors during safe shutdown earthquake conditions for some of the engineered safety features.
The Committee recommends that the NRC Staff review this.
aspect of the design in detail and assure itself that significant margins exist in all systems required to accomplish safe shutdown of the reactors and continued shutdown heat removal, given an SSE. The Committee believes that such an evaluation need not delay the start' of operation of North Anna 1 and 2. The Committee f wishes to be kept informed.
We are uncertain of the Committee's intent with respect to its recom-t mendation -that the staff review the design in detail and assure itself i that significant margins exist in all systems in the event of an SSE.
l We have interpreted the Committee's comment such that an appropriate audit by the staff of the licensee's design calculations will satis-factorily resolve the Canmittee's concern. Accordingly, we plan to l
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. 4 . i Mr. Myer Bender JM S1 W examine the stresses in selected components in essential systems for the safe shutdown earthquake with a determination of the importance of the SSE-induced stress (or OBE-induced stress if it is controlling) to the overall stress, and a qualitative assessment of the adequacy of the indicated margins. The components will be selected on the basis of drawing reviews, site visits, and judgment as those likely to envelope the response of all system components. ;
If the interpretations we have made of the Committee's coments dis-cussed above are counter to the intent of the Committee, we would appreciate prompt notification to this effect along with a more definitive description of the Committee's true intent. i Sincerely, ./ )
- / h- Atff5 Q Ben C. Rusche, Director Office of Nuclear Reactor Regulation
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