ML20248G593

Regulatory Analysis for USI A-40, Seismic Design Criteria. Final Report

ML20248G593
Person / Time
Issue date:	09/30/1989
From:	Nilesh Chokshi, Shaukat S NRC OFFICE OF NUCLEAR REGULATORY RESEARCH (RES)
To:
References
REF-GTECI-A-40, REF-GTECI-SC, TASK-A-40, TASK-OR NUREG-1233, NUDOCS 8910100308
Download: ML20248G593 (27)

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Regulatory Analysis for L SI A LO,

" Seismic Design Criteria" Final Report U.S. Nuclear Regulatory Commission 1

OITice of Nuclear Regulatory Research S. K. Shaukat, N. C. Chokshi l

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AVAILABILITY NOTICE

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NUREG-1233 Regulatory Analysis for USI A-40,

" Seismic Design Criteria" Final Report Manuscript Completed: August 1989 Date Published: September 1989 S. K. Shaukat, N. C. Chokshi Division of Safety Issue Resolution Division of Systems Research  !

Office of Nuclear Regulatory Research i U.S. Nuclear Regulatory Commission i Washington, DC 20555 l

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J ABSTRACT

'Ihis report presents the regulatory analysis conducted by tory analysis discusses the impact of the proposed changes the NRC staff for the resolution of Unresolved Safety Is- in Standard Review Plan (NUREG-0800) Sections 2.5.2, sue (USI) A-40," Seismic Design Criteria." The regula- 3.7.1,3.7.2, and 3.7.3 as the resolution.

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CONTENTS Page Eii Abstract . . . . . . . ....... ... .. ... . .. .. .. . - ' - - -

Vii Abbreviations ..... . ...... .. ...... ... .. . .. .- - - ' '

ix Acknowledgements . . . . ..... . . . .... .. ... . . . .. ... .. .. . .. .

1 INTRODUCTION . . . . .. ..... .. . ... . ..... . . .. . .. ... . . .. . .. I 1

1.1 Background . . . . . . . . . ... .... .... .. . . . ... ........ .. . .. ..... .

1.2 Resolution of Public Comments . ... . . . ... . .... . .. ... . . . I 1.3 Value/ Impact Analysis . . . . . . . . . . . ..... .. ... .... .. .. ... . ... . . . I 1.4 Recommendations and Implementation . . . . . ........ .. . . . .. . 2 1.4.1 Future Plants (CP, PDA, FDA. and CP/OL Applications) . .. . . .... ... . .. ... 2 1.4.2 Existmg. Plants . . . . . . . . . . . . . . . ... .. ....... ... .. . . ... . .. 2 1.5 Conclusion . . . . .. ...... . ... .. .... ... ...... .. ... . .. .. . ... . . . .. 3 2 VALUE/IMPAC1' ANALYSIS . , ... . ...... ..... ... . ....... . . ... . ... 4 2.1 Value/ Impact Analysis for Proposed Revision to SRP Sections 3.7.1. 3.7.2, and 3.7.3. .. .. .... . .. 4 2.1.1 The Proposed Actions . . ... ....... .. . . .. . ..... . .. ... ... . 4 2.1.1.1 Summary of Issue . . .... . ... .. .... .... . . ... . .. . . 4 2.1.1.2 Recommendations . . . . . . ......... . .. . . . . . . . .... .. .. .. 5 2.1.1.3 Need for the Proposed Actions . .. ........ . .. .... .. .. .. .. 5 2.1.1.4 Ilasis for the Pioposed Actions .. ...... . ... ..... .. ...... . .. 5 2.1.1.5 Value/ impact Assessment of Proposed Actions . . . . . . . ... . ... . . 14 2.1.2 Technical Approach . . .. .... .... . ... .. .. .. . ... .. . . . 16 2.1.2.1 Technical Options ..... . ... . ....... .. .... . . .. .... . . 16 2.1.2.2 Discussion and Comparison of Technical Options . . . . . . . . . . . . ... .. . .. 16 2.1.2.3 Decision on Technical Approach . . . . .. . . .. .... .. .. .. ... 16 2.1.3 Plan for implementation . . . . . . ... . . . . . ... . . .. . . . . 16 2.1.4 Statutory Considerations . . . . . . . . . . .. ....... .. . .. ... . ... .. .. 16 2.1.4.1 NRC Authority .. . .. ..... .. ..... . .. ... ... ... . . 16 2.1.4.2 Need for National Environmental Policy Act Statement . . . .... . .. . . . 16 2.1.5 Summary . . . ... .. . .. . .... .. .. .. ... .. .... ..... . 16 2.2 Value/ Impact Analysis for Proposed Revision to SRP Section 2.5.2 . . . . . ..... .. . ....... . 17 2.2.1 He Proposed Action . . ... .. ..... .. ... ... ...... . . . .. . ... .. . 17 ,

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2.2.1.1 Jemmary of issue . . . . . . . . . . . . ... . ... .. .... . .. . .. .. ... .. 17 2.2.1.2 Need for the Proposed Action . . . . . . . . . ...... ........ .. . ..... 17 j 2.2.1.3 Value/ Impact Assessment of the Proposed Action . . ....... .. ... 17 j 2.2.2 Technical Approach .. . ... ... . . .. ... .. .. . . . .. ... ....... .. ..... .. 18 l 2.2.3 Plan for Implementation. . . . . . . . ... . ... .............- . ... ... 18 l I

2.2.4 Statutory Considerations . . . ... ... ..... . . ..... ... .. .. . . . 18

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CONTENTS Page 2.2.4.1 NRC Authority . . .. .. . ... .. . . . . . . . .. .... .... 18 2.2.4.2 Need for National Environmental Policy Act Statement ..... .. . ... . .. 18 2.2.5 Summary . .. .. . .. .. . . .. .. .. . . . . . . .. . . . . 18 3 REFERENCES.. .. . . . . . ..... .. . . .... . . .. .... . . .. 18 Tabics 1 Summary of proposed changes to SRP Sections 3.7.1,3.7.2, and 3.7.3 . . .. .... . .. . . .. .. 6 2 Auxiliary building shear wall .. . ... ... . . . ... .. ... . ... .. . 10 3 Industrv and NRC impact resulting from proposed changes in SRP Sectionr 3,7.1. 3.7.2, and 3.7.3 . . .. 15 i

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ABBREVIATIONS ACRS Advisory Committee on Reactor Safe- OBE operating basis carthquake guards OL operating licente ASCE American Society of Civil Engineers PDA preliminary design approval BNL Brookhaven National 12boratory PRA probabilistic risk assessment CHI Code of Federal Regulations PSD power spectral density CP construction permit RG regulatory guide CRGR Committee to Review Generic Require- RWST refueling water storage tank ments CST condensate stor ge tank SCST secondary condensate storage tank

1. * "" b EPRI Electric Power Research Institute SQUG Seismic Qualificatica Utility Group EQ et.rthquake SRP standard review plan FDA final design approval SSE safe-shutdown carthquake SSI s i!-structuce interaction IE Office of Inspection and Enforcement

'(NRC) SSMRP Seismic Saf ety Margins Research Program ISAP Integrated Safety Assessment Program SSRAP Senior Seismic Review Advisory Panel SSRS site-specific response spectrum LANL les Alamos National Laboratory

! ? NL 12wrence Livermore Nationallaboratory TAP task action plan LOCA loss-of-coolant accident 'I?C Taiwan Power Company l

NRC Nucicar Regulatory Cammissian USI unresolved safety issue J l

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ACKNOWLEDGEMENTS The technical findings relevant to the Unresolved Safety eas of design time-history and methods of seismic analysis isstre (US1) A-40, "Scismic Design Criteria," presented of above-ground tanks. In addition, the authors acknowl-in this report are the results of combined efforts of staff of edge the technical support providad by Dr. A. J. Philip-the Nuclear Regulatory Commission (NRC) and pacopoulos of Brookhaven National laboratory to Ic- '

1.awrcr.cc Livermore National I.aboratory (LLNL). The solve the public comments on the proposed resolution authors acknowledge the valued technical contributions package. 'lhe authors also express their appreciation to P.

nade to this study by Dr. D. W. Coats, Jr. and Dr. D. A. Sobel and L. Reiter of NRC for contributing to Section Lappa, of LLNI, who provided input concerning the ar- 2.2 of this report.

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REGULATORY ANALYSIS FOR USI A-40," SEISMIC DESIGN CRITERIA" l In January 1986, the staff decided to modify the proposed I INTRODUCTION regelatory guidance for SSI in the SRP and to revise the I

SRP to include the current state of knowledge in the li-censirig criteria. To accomplish this, NRC and Brook- l l

1.1 Background haven National laboratory (BNL) sponsored an SSI workshop that was held in Bethesda, Marylar i an l Unresolved Safety issue (USI) A-40 was m.it. hliy formu- June 16-18,1986. At tnis worksbop, current technology lated in 1977 to identify and quantify conservatisint mher- and procedures regarding SSIin the context of USI A-40 ent m scismic design enteria for nuclear power plants.

were discussed (NUREG!CP-0054). Participants in the Task Action Plan (l'AP) A-40 consisted of specific tech- workshop were utility persont.el, NRC staff, and other re- I rucal studies that concentrated on providing short.tenn searchers and consultants. There was also significant for-improvements m the current scismic design enteria. cign participation. The workshop participants, asked to f lawrence Livermore National Laboratory (LLNL), un- specifie.dly address the proposed SRP changes and to su g- l der a techm, cal assistance contract to the U.S. Nuclear gest impro ements in the arca of SSI, reached a reason-Regulatoiy Commission (NRC), provided a techmcal able cons;.nsus ir each of the technical areas. This con-overview of the results which were documented in sensus formed the preliminary basis fer the proposed i NUREG/CR-1161, " Recommended Revisions to Nu revision in the FSI atca.

clear Regulatory Commission Seismic Design Critena,,,

published in May 1980. NUREG/CR-1161 summarires In order to account for insights gained and discussions all technical work accomplished under USI A-4G an$ ,

made at another SSI workshop sponsored by the Electric

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makes specific recommendations for changes m scismic Power Research Institute '(UPRI)/NRC/Iaiwan Power de:,ign requirements. Company (TPC) held in Palc Alto, California in Decem- ,

ber 1987 on the Ixtunglarge-scale soil-structure interac- l tion project, specific questions were forniulated on SSI to In the spring of 1982, the staff reevaluated the status and  :

be sen1 out for public comment. These Lotung-related scope of certain unresolved safety issues, including A-40.

S31 questions along with tbc draft versions of the applica-As a result, the procedure for resolving USI A-40 was ble SRP sectioim were reviewed by the Coramittee to Re-modified. The changes resulted in: (1) accepting view Generic Requirements (CRGR)and weit published NUREG/CR-116i as the technical findings of l>SI A-40 for public comment as a FederalRegister Notice on June h and (2) NRC technical staff preparing appropriate revi.

E986.

sions to the Standard Review Plan i (SRP)

(NUREG-0800) based on recommendat ons made in NUREGICR-1161. The staff, therefore, did not prepare 1.2 RcSolution of Public Comments a separate report to present USI A-40 technical findings. The ccmments received from the nuclear industry through the end of October 1988 were reviewed. BNL A group of NRC staffinembers from appropriate techai- was engaged to address public comments and to recom-cal areas reviewed the recommendations of NUREG/ mend &mges in the applicable SRP scctious.The resolu-CR-1161 and proposed certain changes to the affected tion of public comments is documented iin NUREGI SRP sections. The suff completed the first draft of iD CR-5347, " Recommendations for Resolution of Public revision to the SRP in ently 1983. LLNLwas the contrac- Comments on USI A-40, Seismic Desigr. Criteria."

tor selected to prepare a value/ impact assessment for the proposed revision. In August 1984, NUREG/CR-3480, One public comment dcalt with decoupling the relation-

"Value/ Impact Asacssment for Scirmic Design Critrila" ship between the operating basis can hquake (ODE) and was published.The staff also performed additional tech- the safe-shutdown carthquake (SSE)/Dds comment was nical work in 1984 had 1985 to develop acceptance critcria consider 64 to be c utside the scope of USI A-40, and B NL 1 for use of a single tunc-history for seismicdesign. In addi- did not specifically cvaluate it. 'Ihc decoupling of the tion, the staff 8;30nsored a study at Las Alamos National OBE and SSE may have merit for future plants but would laboratory (LANL) m 1984 to rurvey the past perform- require a change in 10 CFR Part 100, Appendix A.This ance of above-ground tanks in scir.mic events and to rec- issac .is part of Generic issue 119.3, "Decoupling OBE ommend procedures for design. NUREG/CR-4776,"Re- From SSE "

sponse of Sc)smic Category I Tanks to Earthquake Excitation? was published in February 1987 outlining re- 1.3 Value/ Impact Arialysis suitsof thisstudy.The staff also reevalua6cd the proposed acceptance criteria for son. structure interaction (SSI) The value/ impact analyses included deterministic analysir e.nalyses, qualitative assessments oased on engineering i NUREG-1233

Regulatj;y Arealysis for USl A-40 experience and judgment, and quantitative ptobabilisti'e (b) consideration of highet mode respcases in risk assessment (PRA) analyses where these were possi- plant design; and bic. Because of the lArge uncertaitses, the PR A analyses i (uiscussed in Ihis repor() were noi ronclusive.'Ihe recom- (e) considerate n of wall flexibility in the design of l

mendations made cre, therefore, based priruarily on large above-ground tanks.

de'erministic and quahtative arguments.

(3) Possible cost saving will be achieved by eliminating ne analyses per formed el the staf;f'S qualitative assem or providing alternatives which remove unquantifi-ments led to the conclusion that for operating reactorand able excessive conservatism such as:

operating license (013 applicants, the proposed changes to the SR P would have httle effect on risk. as plants have (a) an option to use multiple time-histories for generally been, and willbe, seismically u pgraded by plant- the plant design; specific actions such as implementation of the Systematic

!! valuation Propram (SE1), the yiroposed imp (n) an option to use direct generation techniques tion of USl A-46, and NRC Hullet n programs.ymenta- For new for developing the floor response spectra; and construction permit (CP), preliminary design approval (PDA), final design approva! 07D A) and comEned h^ (c) elimination of enveloping requirements of re-cense (CP/OL) applists, no significant increases in sults from two different SSI analyses and al-costs are envisioned tu bplement the revised SRP sec- lowing some reduction in control motion with tions and such implementation will lead to more ur Form depth of embednient.

safety margins; therefore, the staff preposes that all new applicants be required to comply with the revised SRP (4) Greater confidence can be placed on seismic ade-scenony. quacy of nuclear phvf ts by incorporating the knowl-edge gabed as a result of new technological ad-The analyses also indicat6d that safety-related above-ground tanks which were designed using the Housner method (which assumes rdgid tank walls might have less (a) considcratkm of higher mode responses; and scismic capncity than tanks designed more recently with (b) devdopment of methods to account for the flexible-wallassumptian). Above-ground tanksaSe eva u- ~

wall flexibility in tank design.

aied below (Section 1.4.2).

On the basis of the findings of the value/ impact analysis ,

lA Recommendations and and the fact that operating reactors have been seismically J Implementation upgraded by the Systematic livzluation Program, by I plant-specific actions (e.g., IE Eulletin 80-11, " Masonry Wall Design," May 8,1980), or by both, the staff recom-L4.1 Future PInnts (CP, PDA, FDA, and mends that the proposed SRP changes should not be CP/OL Applications) b ckfitted but should be applMd to new CP, PDA, FDA, ne staff recommends tha'. the changes in the pror.csed and CP/OL applicanis who docket their applications aft er revision to SRP Sections 2.5.2, 3.7.1, 3.7.2, and 3.7.3 be the revised SRP sections hwe been issued.

issus d. The staff expcet3 that the adoption of the two-posed revision to SRP sections will achieve the following: 1.4 *' ExistinE Plants For cermin opeMng plants, safety-rdated, above-(1) Current staff practices will be reflected. These in.

clu:le: ground Duid tanks were identified as 2 petential backfit requirement ittm. Screening criteria for reviewing tanks required for safe shutdown are currently being developed (a) considermica of site-specific spectra to juuge by the Seismic Qualifirstion Utility Group (SQUG) the adequacy of seismic inputs; and within the framework of attivities related to the imple-mentation of USI A-46,"Scismic Qualification of Equip-(b) emphasis on neotectonics and scismicity tode- ment in Operating Plants." The NRC staff and the Senior fine tectonic provinces.

Seismic Review Achrisory Panel (SSRAP), which advises the SQUG, will review the screening criteria. The NRC (2) Potential sources of nonconservatisms will be climi- stnff anticipates that the irrpiementation of the tank nated. These include:

anchorage review guide ind.will be sufficient to resolve this concern for the 72 planis ibat are subjem to the (a) provision of additional requiremectna deter- requirements of USI A-46. Pages S9 to 95 of NUREG/

mine the adequacy of a single time. history for use in plant design; CR-3480 report on the results f a survey of tank suppliers conducted by lawrence Livermore National NURl!G-1233 2

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I laboratory. It should be noted that the survey indicates ing a seismic event could lead to unacceptatic con-that most tank suppliers have used flexible-wall analysis sequences. The recently completed seismic margin for a number of jcars. Ilowever, if during the implemen- study on the Maine Yankee Atomic Power Station tation of USI A46, a plant does not meet the screening identified the RWSTas a component that controllad criteria or guidelines, or if the screening criteria are not seismic capacity of the plant. Consequently, the efficiently definitive to judge tank adequacy for a par- RWSTat the Maine Yankee plant has now been up-ticular plant, then the need for a plant-specific backfit will graded to increase the capacity of the plant. Simi-be evaluated on a case-by-case basis. No separat e action is larly, seismic risk studies conducted in conjunction thereforeproposed within the resolution of USI A-40 for with USl A-45, " Decay Heat Removal," have also identified the RWST as a risk-sensitive component the plants covered by the requirements of USI A-46.

for a number of planto The remainder of the plants fall into two groups:

(1) plaats that were subject to licensing resiew by the staff (5) SEP scismic review also identiried use of rigid-wall ,

after about 1984 and (2) plants that were reviewed by the criteria for design of free-standing tanks as a poten- I staff during the period between the late 1970s and 1984. tial safety problem. Most of the field-crected tanks For more recent phmts (group 1), the NRC staff con. required modifications. Four potential failure firmed in its licensing review that flexible-wall analysis tnodes were identified- (a) overstressing anchor was used and clearly no further action is needed. For the bolts because of tension caused by tank overturning plants in group 2, a survey of the NRC staff reviewers re- moment, (b) weld failure at tank wall and anchor vealed that tanks for many of these plants were designed bolt chair, (c) buckling of side walls, and (d) failure of reinforced-concrete foundations. 'Ihese t;mks were using flexible. wall analysis or wall rigidity was not a con-sideration (reinforced-concrete tanks). However, the designed assuming the tank wall is rigid. Reanalysis status is uncertain regarding analysis technique used for considering flexibili'ty of the tank walls identified the four Gites listed below. These four plants (6 units) will be potential failure modes.

handled by issuing a request-feFi nformation letter under 10 CFR 50.54(f). (6) Surveyors of damages in past earthquakes (NUREG/CR-4776) have repeatedly pointed out e Watts Bar 1 & 2 the susceptibility of large, above-ground, vertical tanks under carthquake loads. Experience has also

+ Callaway 1 & ; confirmed that tankt have failed by the four failure i e Wolf Creek 1 modes mentioned above (items Sa-5d). Addit oaal

. Ilarris I failure modes, such as damage to pipin;; and othcr connecting systems, foundation damage, and buck-The technical f.;ases for star f's recommendation ef further Ung of roof and floor plates have also been obser ved, consideration of above-ground tanks are the foll6 wing:

(7) On the basis of experience gained in the SEP review (1) The uppading of tanks is required to satisfy Geveral and historical survey of tank performance in actual Design Criterion 2 (10 CFR Part 50, Appendix A) earthquakes, it is anticipated that only the large i

which, in part, stes that " structures, rystems, and free-standing mctal tanis in the yard (refueling components important to safety shall be designed to water storage tanks and condensate storage tanks) withstand the effects M natural phenomena such es are of concern for failure modes Sa-5d.

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l earthquakes. .without loss of capability to perform their safety functions."

l (2) Although ti c PR A analysis results are incimelusive, On the baus of recommendations made in NUREG/

the probabi;lity of tank failure decreases sipificantly CR-1161, consensus achieved at the NRC/BNL-if the proposed design criteria are used. sponsored SSI workshop, insights gained at the INung; SSI workshop, and resolution of public comments, the (3) Tne ficxible-wall modcl is mon appropriate as it staff has developed the final revision to SRP Sec ions 1 rcpresents a more realistic situatica.The use of the 2.5.2,3.7.1,3.7.2, and 3.7.3. This revision represents an Housnct method of analysis (rigid-wall a 3urnption) improvement in the analytical methods used in the scis-could result it; tank 6mpns that underpredict ihe mic design of nuclear power plants and will, in some cases, design forces by a fac?or of 2.0 to 2.5. reouce new plant costs by eliminating excessive conserva-tism. In part, some of the changes in SRP Section 2.5.2 )

(4) The safety importance of safety-related tanks (par- reflect current industry practice and staff procedures that l

icularly the refueling water storage tank (RWST) of have evolved since the Standard Review P4an was issued i pressuined water reaciors)is high. The failure dar- in 1975. In addition, a number of editorihl changes and 3 NUREG-1233 i _ _ _ m ,

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Regulatory Analysis for USl A-40 clarifications were made. The staff also assessed the need 2.1 r quiring a review of safety-rclated, above. ground Value/ Impact Analysis for Pro-posed Revision to SRP Sections 3.7.1, 3.7.2, and 3.7.3 The staff has concluded that the proposed actions satisfy 2.1.1 The Proposed Actions the modified objective of USI A-40 to rcSect the current 2.1.1.1 Summary of Issue state of the art in seismic design in the licensing process.

The assessment of the effect of Ihe proposed changes on Structures, systems, and components important to the plant safety indicates that the changes ara warranted and sa:eth of nuclear power plants are regmred to withstand will contribute to a more uniform and consistent licensing the effects of natural phenomena such as earthquakes.

process. All the proposed changes in the SRP that consti- Broad requirerpents for earthquake resistance are mdi-tute the resolution of USI A-40 are to apply to new CP, cated in Title 10 of the Code of Federal Regulations (10 PDA, FDA, and CP/OL app?icants only. CFR) Parts 50 and 100. Detailed guidance as to the ac-ceptable ways of meetmg these requirements is gn en m various regulatory guides. Safety analysis reports for each plant are reviewedin accordance with the review and ac-ceptance criteria described in the Standard Review Plan.

2 VALUE/ IMPACT ANALYSIS Early nuclear power plants were designed without spe-cific scismic design requirements. In the early 1970s, the requirement for seismic resistance was mer. horsed in the The staff has developed specific propos:ds for changing regulatory guidance in the form of proposed changes to r0gulation.ne state of knowledge hhs advaaced rapidly, Standard Review Plan (SRP) Sections 242,3.7.1,3.7.2, and there ere generations of mulet.r power plants that and 3.7.3 (NUREG-0800). The proposed changes in have various levels of seismic design requirements. Also, regulatory requirements are supported by deterministic the complex process of seismk design involved many en-arguments, qualitative assessmen9, and by proba5ilistic gineering disciplines-seismic, teotechnical, structural, risk assessment (PR A) analyses where these are feasible. mechanical, electrical, and nuclear. Each discipline in the design y rocess control'ed the design parameters in its do-main. As the total Geismic design process evolved, to questions emerged:

The proposed changes represent an improvement in the analytical methods used in the seismic design of nuclear (1) How adequate are the plants m carh,er generations power plants.These rnethods will provide greater conf;. with respect to present safety requirements?

dence in the seismic adequacy of structures, syrtes, and cornpener.ts that are required to withstand the dfects of (2) What is the margin of safety m. the overall seismic earthquakes in future plants. In some cases, these im- design process?

proved methods will reduce new plant capita! costs by chmmating excessive conservatism. 'nte analytical meth- USI A--40 was initiated to address these questions. NRC ods used by the mdastry and the staff have evolved since Task Action Plan A-40(TAP A-40) stated the objectives the Standard Review Plan was issued in 1975. Herefore, as: "to investigate selected areas of the seismic design se-some of the proposed changes reflect crrrent industry de- 9.uence to determine their conservatism for all types of sign practice and the associated staff review procedures. sites, to investigate alternate approacher to parts of the Also, editorial changes and clarificati6ns were made. Fur- design sequence, to quantify the overa:1 conservatism of ther improvements were made througn the resolution of the design sequence, and to modify the NRC criteria in public comments. Significant improvements resuhed in the Standard Review Plan if changes are found to be justi-the SSI area and design time-histery requirements. fled. He seismic designs of some of the older reactors were reviewed within the framework of the Comnussion's Systcmatic Evaluation Program (SEP). Techt.ical work The staff prepaNd the value/impaci assessment for the completed as part of TAP A-40 contributed significantly proposed revisim to SRP Section 2.5.2. Lawrence Liver- to those reviews. Criteria in the Standard Review Plan serve as the srJety baseline for seismic re-review, even more National Laboratory (LLNL) assisted the staff in preparing the value/ impact assessment for SRP Sections though specific compliance is not required.

i 3.7.1,3.7.2, and 3.7.3. nc value/ impact analyses for SRP Studies under the USI A-40 program included the follow-Sections 3,7.1,3.7.2, and 3.7.3 are presented in Section ing: (1) quantification of consenatism m seismicdesign, 2.1 of this report.The value/ impact tinalysis for SRP Sec-(2) elasto-plastic seismic analysis methods, (3) site-tion 2.5,2 is presented in Section 2.2 of this report. specific response spectra, (4) nonlinear structural e

NURiiG-1233 4

Reg'ilatory Analysk for USI A et0 l

1 dynamrc-analysis procedures, and (5) soil-structure iater- same extent, does not affect industry design or 9ction. Technical findings of USI A-40 are presented in analysis procedures.

" Recommended P.evisions to Nuclear Regulatory Com-mission Seismic Des;gn Criteria"f NUREG/CR-1161). In (3) Although some cider si!es were designed to seismic criteda less rigorous than present requirenlents,Sig-that report the ccmtractor reviev'ed all technical work nificant upgra fing has b~een or will be achieved by performed under the A-40 pregam, as well as other per.

tineni inforrnation, and developed specific recomn enda. the SEP, the imp}ementation of the USI A-46 reso-tions for Standard Review Pian changes. During staff re. lation, and by stTff bulletins and information notices viety of the cc6 tractor's recommendations, a censultant (e.g., IE Bulichn 80-11, "Masorny Walt Design").

was hired to develop acceptance criter.a f or neview of sin-(4) The Standard Review Plan serves as a safety gle time 4! story applications (NUREU/CR-3M) These baseline in re-review programs such as the SEP and criteria took the form of a " target" power spectral density f r disposition of plant-specific concerns in the l (PSD) function, as included in the proposed revision to ,

cvent a scismic design consideration is identified.

SRP Section 3.7.1, that was published for public com, ment. 'l his criterion el PSD was further improved by two In summary, the staff proposes to asue revised SRP Sec- 1 membeu of the panel who were selecteo to address pul'- tions 3.7.1, 3.7.2, and 3.7.3 for implementation on CP, I lic comments. Considerable effort ww devoted (see PD A. FDA, and CP'OL applications docketed after the NUREG/CR-5347) to devek? the miniinum PSD re- revision is issued.

quirement e;hich i; nUw contained in the final SRP Sec-tica 3.7.1, Appendix A. 2.1.1.3 Need for the Proposed Actions As the resolution of I'S1 A-40 progressed, industry and The SRP ghanges r.re needed to upgrade seismic design staff 'practicts were 1tvised to include consideration of requirements and to reflect current staff review practice.

the improved procedures; therefore, the Siandard Re-view Plan does not adequately reflect the latest state of 2.1.1.4 11 asis for the Proposed Actions -

the technology. For this reason, many of the proposed Consideranon of the NUREG/CR-1161 recommenda-changes have no impact because they serve to update the tiens and resolution of public comments resulted in a to-SRP to reflect current understandmg and practice. tai of 25 proposed changes in SRP Sections 3.7.1,3.7.2, and 3.7.3. Of the 25 proposed changes numbered in Ta-2.1.1.2 Ruommendations ble 1, the first 24 were rcccmmended in NUi1EG/

CR-1161 and further discussed in NUREG/CR-3480; The staff Las completed its review of the recommenda- the 25th was the result of resolving public commeni  ;

tions made in NUREG/CR-Il61. The staff accepted  !

many of tbe recommendations,and rejected a few of the (NUREG/CR-5347). It was determined that 14 of the 25 changes represent options or clarifications of existing rev recommendatias for various reasor,s. In response to quiremyats. Although these changes do not result in any public comments, liNI. and a panel of consultants made appreciam trnpact, they are proposed because they re- ,

additional recommendations in NUREG/CR-5347. ,

flect current industry practice and the state of the art. For  !

Those recommendations that the NRC staff accepted are

?hese cases, no explicit estimate of change m risk or cost includcd in the proposed revision to SRP Sections 3.7.1, was deemed necessary.The renaining 11 of the proposed 3.7.2, and 3.7.3. Therefore, the staff proposes to revise changes could have a potential benefit or impact and a and issue these SP P sections, valut/ impact assessment was performed.The assessment was qualitative in some cases in which it was not practical The str.ff recornmends that the revised Standard Review to conduct PR A analyses. The 11 proposed changes were Plan sections be implemented only on new CP, PDA, gr uped into 6 areas for th6 purpose of conductag the FDA, and CP/OL applications docketed after the effec- analyses or discussion as suramarized below.The change tive date of the revision.The primary reasons for this rec-tiumbers (from Table 1) are shown for each area. More commendation are: than one change number for an area incans that both the review procedures and acceptance criteria sections w'ere (1) The intent of the USl A-40 study was to survey the changed in the SRP, Change numbers that do not eppear rtate of the art and to upgrade the Standard Review in the list below represent either options pr edibrial Plan to reflect current understanding of seismic de-ch:mges.

rJgn principles and procedures. The investigations described in NUREG/CR-1161 and results of the Area 1 Design Time-Histoiy-Change No. 6 staff's regulatory analysis led to the conclusion tint backfitting is not required. Area 2 Seismic Analysis Methodt and Combination of Modal Responses-Change Nos.13,14 and 17 (2) In adution, the proposed revision,in some cases, re-Area 3,Tanks-Change Methods of Seismic Nos. 20,Analy3, . and 24 sis of Above-ground flects current industry design practices and thus, to 5 NUREG-1233

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Regulatorv Analysis for USI A-40 Area 4 Soil-Structure Interaction-Change No.15 spectrum is still a primaty requirement controlling design input, and 1he minimum PSD requirement is a secondary Area 5, Development of Hoor Response Spectra and Ef. check to prevent a severe deficiency of power over any i fccts of Parameter Vanations on Floor Response frequency range.The PSD requirement does not add ad- J Spectra-Change Nos.16 and 18 ditional conservatism beyond that contaitted in the design Area 6, Andysis Procedure for Damping-Change response spectrum.

No. 25 ..

Therefore, only a small additional cost wotdd be antici-pated to implement this requirement, that is, the cost to Only areas 1,2, and 3 (above) were reviewed in detail to compare PSDs and, perhaps, the cost of regenerating arti-assess potential valuelimpact. PRA analyses were con- ficial time-histories to meet the PSO criteria.The cost of ducted enly for areas 2 and 3. Proposed changes in area 4 regenerating artifkial time-histories would regrresent a lyive been substantially revised since NUREG/CR-3480 smal!, one-time anesis cost. Additional rewew time as-was published, and the value/ impact found in Section sociated with the P3D reqMrement would be approxi-3.4.3 of that report is no longer applicable. He revised matdy 3 person. weeks. Estimated analysis and revbw position is more consistent with the recommendations costs associated with this preposed new requirt. ment are made in NUREG/CR-1161 and, additionally, reflects the small and considered insignificant.

recommendations and findir.gs resulting from NRC/

DNL-sponsored SSI workshop held in June 1986 This change is caly a secondary requirement; the primary -

(NUREG/CP-0054). The revised position also took ad- requirement is the same as in the current SRP version.

vantage of insights pained at another SSI workshop spon- Therefore, the proposed change coes not warrant backfi/

sored by EPRl/NRC/raiwan Power Company (TPC) hela considerations. However, the need to qualify the use of a in December 1987 on the Lotung large-scale soil- single, artificial time-history, with respect to a proposed structure interaction project, which ixluded forced vi- target PSD function, will identify potential nonconser- J bration tests as well as actual carthqceke response data. vatism in energy content of the time-history, therefore a Purther improvements were made during the course of forwardfi/ for this proposed change is recommended.

] i resolving public comments throup,h recommendations mah by 11NL and a panel of consultants (NUREG/

CR-5347). The proposed changes in areas 5 and 6 were Arca 2 determined to result in insignificant or no impact. Each of He proposed change in area 2,"Scismic Analysis Meth-mese areas is discussed m detail below. ods and Combination of Modal Responses," requires that specia( consideration be given to the responses associated Arca i with high-frequeney modes when the rerporac spectrum methrd of analysis is used. To essess th.e impact of this The proposed requirements in trea 1, " Design Time- change on seismic risk, the PRA analysis performed on Ilistory," specify 6 hat smgle, artificially generated time- the Zien Nuclear Phnt, as part of the Seismic Safety Mar-histories meet not only the existing requirements con- gins Research Program (SSMRP) phase ll study, was used cerning the enveloping of target response spectra,1:U+. as a base case. His proposed change would affect all also snat power spectral densuy (PSD) ftnyctions gener. Category I structures in the plant. It was not feasible to ated from these time-hi, tories rnatch a mmimum PSD re-change the entire SSMRP model to represent the effects qmrement. cf the proposed change. Instead, one representative structure was selected to demonstrate the effect on risk.  !

A number of corrtments were received on the require- The structure selected was a critical shear wallin the Zion I ment outlined in the proposed revision to the applicable auxiliary building. On the basis of recent studies cited in I SRP sections issued for public comment. These com- the contractor's repart, NUREG/CR-3480, it was deter-ments ranged from requests for clarification to those ex- mined that,if the proposed methods of mode combina-pressing strong reservations regarding the target PSD tion were applied, the wall stresses might increase by as function (Kanai-Tajimi form) int;1uded in the proposed much as 33% Therefore, the shear wall strength was in-SRP Section 3.7.1 for public comment. In orde.r to resolve creased by 33% from tue wall strength used in the base- i these comments, two n:cmbers of the consultant panel case SSMRP study. The reduction in total risk w ts exam- I were asked to develop a minimum PSD requirement for ined.

Regulatory Guide (RG) L60 spectra.The procedure for  ;

developing this requirement is described in Appendix A Table 2 summarizes results of the probabilistic risk assess-  !

to the proposed 6RP Section 3.7.1. This requirencnt is ment (PR A) analysi:. Results of this analysis indicate that such that the time-tdstoty based upda this minimum PSD by increasing the wall strength (because of the proposed function should produce a responso spectrum that lies requirement). the probability of failure of the shear wall close to, but generaily falls below, the RG 1.60 response decreases r,ignifictmtly. This decrease is an imposed i spectrum.Thus, the enveloping requirement of response change that results from ' adjustments in the fragility 9 NUREG-1233

Regulatory Analysis for USI A-40 curve. The resulting change in core-melt probability is considered. 'Ihe calculatetl change in release was very small, even for the highest earthquake (EQ) level negligible for the entire range of EQ level considered. I f

I Table 2 Auxiliary building shear wall EQ lesel P falla P. fail, P.CMo PCM t Mrem o Mremi i I

.0ti .10g 0. O_ 3.7E-8 3.7E-8 23E-3 2.9E-3

.10 .20p 1.33 E-9 1.94 E-12 1.8E-8 1.8E-8 1.5 E-3 1.5E-3

.20 .32g 6.96E-6 2.09E-8 24E-5 2.f;E-5 5.3E + 0 5.3E + 0 i

.32-A2g 7.43 E-4 5.03 H-6 1.2E-6 1.20-6 3.7E + 0 3.7E + 0 l

.4?. .53g L76E-2 1.59E-3 5.2E-7 5 .2 11 - 7 1.2E + 0 1,2E + 0 '

.53 .69g 6,76E-2 7.22E-3 2.0E-7 1.9E-7 5.9E-1 5.9E-1 l -

I 110 level a carthquake acceleration race for which vakes are applicable; P-fail = condaional probabili:/ of shear wall collapse; 4 P-Cat = annuahzed pmbabihty of core melt due to an carthqMe of this level, '

Mrrm = total sk in amn-remicar from au carthgaake within the given levd The remainog variables have a subscript of either "0" or "1". A "o" subscript column represents the oQinal vrlue before lhe component streigth was modified. A "1" srbscript column represents the pcumodification '

value.

The cohimn "P-f ail" reptesents Ihe conduional component Lilue e probabihty. By coraditional,it is meant the I probabihty does not ?ncluh 114 frsabihty of occurrence of Ln eart hquake in the specified range. The cokima P-CM" represents the annual probabihty of cuir-melt. The cohimn "Mrciu" represents the total contribu-tion to r isk,in man-temiycat , from the specified carthquau level. l!olh the cere-mcit art man-rem valuepre unconditionallhat is. they 40 hclude the annual probabihty of occurrence of an earthquake in the rpecifsd i range.

l Although the shear wall of the auxiliary building is the construction changes in future plants are anticipated as a j most significant component that is relevant to this tash,it rtsult of the proposed revision. In general, seismic shear i is not a major contributor to risk. From Table 2 it can be stresses in reinforced-concrete walls are well below allors -

seen that, even before the wall is strengthened, the condi- able stresses and a 33% increase in calculated stresses tional failure probability is negligible at the two lowest would not affect well design in most cases. It is also be-EQ levels. Other structural fail.ttres exist which dJminatC. IIcved that the maximum increase in the base-of-wall They are uplift of the containment basemat r.nd collapse overturning moments of approximately 10% would not of the roof of the service water cribhouse, lead to (ty appreciabk ch'mges in wall reiVorcement.

Plants different from Zion may not be subject to these other structural failures. For those phmts, the failure of e An additional two staff-weeks of effut might be required wall similar to the auxiliary building's shear wa'l at Zion to review changes in analysis resuiting from the adoption could be a dominant contributor to risk. Tac severing of of thin proposed new requirement. The total cost in-electrical and fluid lines and the impacting of debris on creases associrded with this requirement are not expecf ed adjacent equipment can be important common-mode fail- to exceed $5000 (estimated in 1985) for new planta.

ures of vital safety systems. Thus, strengthening vital walls should be considered an important seismic safety improvement. The staff recommends that this requirement should be apphed to new applications because, as discussed above, The impact of the proposed SRP changes would be to thit, requirement, in general, will not improve the safety eliminate this possible, but generally unlikely, source of of existing structures and the reanalysis of existing struc-l nonconservatism in design.The change would make clear tures would be very costly. Therefore, the proposed l the cause of Ihis nonconservatism and would eliminate change does not warrant backfit considerations. However, l

the necd for the use of approximate methods, which have the proposed change would climinate a possible source of been used in the past to correct this deficiency. One: com- nonconservatism in design at a very moderate increase in puter programs are modified the added analpicai costs cost. On the basis of these considerations, aforwardf;f is and engineerir.g efforts would be small. Furthermore, no recommended for new applications.

NUREG-1233 10

Regulatory Analysis for USI A-40 Aria 3 bolt chair,(c) buckling of side wall, (d) failure of re- .

inf reed-concrete foundations. These tanks were j For area 3. " Methods of Seismic Analysis of Above-designed assuming a rigid tank wall. Reanalysis con. q ground Tanks," the proposed revision to SRP s ections re- ,

sidenng flexibilky of the tank walls resulted in the quires that dynamic etfects and tank-wall flexibility be identification of thtse potential failure modes.

considered in the analysis of above. ground tanks. As db-cussed in NUREG-3480, the regulatory analysis for (5) Surveyors of eart.hquake damage (NUREG/

above-ground tanks was perfonned by conducting the risk CR-4776)have repeatedly pointed out the suscepti-analysis usmg the Zion SSMRP model. Initially, the bility of larg% above-ground, vertical tanks under analysis was performed on the sec(mdary condenrate stor- earthquake loads. Experience has also confirmed cge tank (SCS P). Subsequently, additional studies were that tanks have failed by the four failure rnodes men-made on the refueling water storage tank (RWST) using tioned above (items 4a-4d). Additional failure i various assumptions. Ilowever, the resnits of these stud- nodes such as damage to piping and other connect-ies were inconclusive -in terms of estiaiating changes En ing systers, foundation . damage, and buckling of man-rem rrieases and depended heavily on asst.mpuans roof and floor plates have al;o been observed.

made regardm, g post-core-melt phenomena (contam-rnent failure mode and release category assignments). On the basis of this discussion, the sthff rscommends that the proposed changes in tank design along with some Notwithstanding the incor.clusive nature of the value/im- clarifications based on resolution of public comments by pact study reported in NUREG/CR-3480, some recent the conMitant panel should be incorporated in the Stam seismic risk and margin studies support the preposed revi- dard Review Plan and should be applied to new applica-sion to SRP sections. Also,9bservation of the perform- tions (forward fit). As discussed in the introduction, the ance of tanks in carthquakesoffers further support as fol- review of the safety-related, above-ground tanks neces-lows: sary for safe shutdown in operating plants will be con-ducted under the implementation of the USI A-46 pro- <

(1) Although the PR A analysis results are inconclusive, gram and other reviews. No separate action is, therefore, the probability of tank failure decreases significantly proposed under the resolution of USI A-40.

if the proposed design criteria are used.

Area 4 (2) The flexible-wall model is more appropriate as it The proposed revision in area 4," Soil Structure Interac-represents a more reahstic situauon. The use of the tiom" included in the proposed Msolution for public com-Housner method of analysis (r gidavall assumption) ment was based,11 part, on a workshop sponsored by could result in tank designs that underpredict the NRC/ENL a June 2986 (NUREG/CP-0054). The objec-desmn forces bv a factor of 2.0 to 2.5. tives of this wo7kshop were:

(3) 'Ihe safety importance el safety-related tank, n) To examine the SSI rclated licensing concerns auG l particularly the condcasate storage tank (CST) and sarious procedures and alternatives jointly by:he the refueling water storage tank (RWST)] of re:gulators, practitioners researchers, titility repre-pressurized-water reactors is high. Failure of such sentativeS and other inte-ested groups in the light of tanks during a seismic event could lead to unaccept- the recent analytical and experimental aevelop-abtimmsequences. The receyitly completed scismic rrents.

margin study on the Maine Yankee Atone Power Station identified the RWST as a component that (2) To examine the areas of greater uncertainties and l controlled seismic capacity of the plant. Conse- means to address them.

quently, the RWSTat Maine Yankee has naw been -

tyraded toicercase the capacity of the plant. Simi- (3) To review the licensing criteria in the SSI area and larly, scismic risk studies conducted in conjunction discuss suggestions to improve the licensing procces.

with USI A-45," Decay IIcat Removal," have also identified the RWST as a risk-sensitive component Two broad alternative acceptance crit cria for SSI analyses for a number of plants. were given in the proposed revision for public comment.

(4) SEP scismic reviews also identified use of rigid-wall Altematiw I critena for design of free-standing tanks as a poten- Alternative 1 was similar to the Laff acceptance criteria tial safety problem. Most of the field-crected tanks published in the current version of the SRP section with required modifications. Four potential failure some modifications in the proposed revision.

modes were identified: (a) overstressing anchor bolts because of tension caused by tank overturning As discussed earlier, the control motion was specified at moment, (b) weld failure at tank wall and anchor the ground surface in the free field-cither at the surface 11 NUREG-1233

_ _ _ _ _ _ - _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ._ _ _ ._. ._ l

RfTulatory Aulysis for USI A-40 of a rock ontc.op (rcal or hypothetical)in the vicinity of (1) Iotung data iricate that, for that site, the decon-the site, or at the ground surface (finished grace) at the volution procedure did not predict consistent results plant site. Appropriate variation in soil properties must to reflect observed variation of motion along the be considered so that the effects of the resulting dips in depth. Therefore, for proposed acceptance criteria the response spectra (at the natural frequencies of soil in SRP Section 17.2.11 (primarily, for Alternate 1),

columne at the foundation level of aa embedded stroc- should a limuation be included, such as no more tare are s.mootted out in the design floor spectra. The than 40% reduction be allowed from the su face n;o-mynitude and the form of the reduction in the spwtra at tion (e.g., ASCE Standard 4-86 contains such a limi-i the fondation level should be supported by the Ody of tatica) to the deconvolved snotion at the foundetion the data appheable to the site. In a case of a rite with shal-level in the free field for certain site conditions?

low overburden Wonsisting of one layer or severallayers) os crlying a hard str atcm, .my reduction in the surface mo- (2) A number of post-test correlation studies of the tion may not be a@ropriate. I stung facility found that the calculated soil damp-ing values had to be reduced to match the observed For ernbedded structures. it must be noted that a transla- results.1 herefore, t.hould a 1.mitation be placed on tional motion specified ouIherurface of the soilproducc3 the soil dampingvalues used in the SSI analysis, par-translatjor: and rotation of the massicss foundation, and ticularly when a simplified nalf-space approach is the SSI mrthod should account for tM rotational effect. used?

For cases in which reducoon in the transnational motion is not considered, it is generally not ne%:ssay to consider Si 7 i io E b in rotational effects of the mass! css foundation. 'Iht .nput mlues of soil used to determine the soil pwperties motion at the base of the discre!c soil model or toil col- (c.g., shear modulus ar.d dampmg) t' sed in the SSI umn should produce the specified des (gn spectra at the analysis?

ft t surface of the soit profile in the free ficid (finished E ' (4) Should tim requiwment of enveloping results of the two methc0 of SSI be retained in proposed alter-nate 1 of tne acceptance criteria in SRP Section Al/cmatire 2 3.7.2J1,in light of theicniWion that may ha placed For sites at which the detailed siteepecific investigations " "" " "

t were performed, alternative 2 allowed an optio1 to em-p!oy a suitable state-of-the art apprnach to puform the SSI analysis wA . it resorting to tne enveloping mquir*

The resolution of public(omments on the proposed SRP ra:nts discusseu sor alternatisc 1. Ilowever, greater ef- sections and Lotung questions resulted in major revision, fons were required m ad-Jressmg uncertainties und pei- -

in the SJ1 syca, to the proposed SRP version for public forming sensitivity studie3 corrmers. The final proposed version includes major changes as follows:

The proposed 0 vision for public comment abo included .

elimination of enveloping requirement of results specific guidehnes ao aco unt im such items as non. from two different methods of analyses linearity, uriatirou of soil properties, layering effects, strain dependence of matenal properties, modeling of a location for application of control motion and limita-soil. structure system using various approaches, and spa. tion on reduction of cor,troi rnotion with embedment tial variation of ground motion.

clarification and more guidance on soil properties Subscouent to the NRC/lWL-sponsored SSI workshop, variation EPRI/NRC/FPC sponsored another SSI workshop at Palo Alto, Californ'ia in December 1987 to disc'1ss the With respect to the analysis method, the revised S. tP sections analytical prMi.;tions of actuJ recorded scismic response contain thefallowing:

of a quarter +cale-tmxlel co6tainment stnwture with a A complete soil-sn uctorc interaction analysis must prop-simuiated sicain generator loop inside (at Lotung facility crly account for all effects due to kinematic and inmti:d in Taiwan, see EPRI NP-6154, March 1989). IPind pre-interaction for surface or embedded structures. Any dictions using several analyt; cal techniques pointed out analysis method based on either a direct approach or a some limitations of the various approaches. In light of this substructure approach can be used without enveloping information the reviewers of the U51 A-40 proposed requirements of results from two different analyses, prce resolution package were asked during the public com- vided the following conditions are met:

ment period to comme:Ji on the following quc stions since the proposed staff revision did not include findings dis- a. The structure, foundatien, and soil are properly cussed at this satter workshop.

modeled to ensore that the results of analyscs are NUR EG-1233 12

Regulatory Analysis for USI A-40 within the range of applicability of the particular ' If the accompanying rotational componenis of motion are method emph)yed; and ignored, no reduction is permitted in the horizontal com-poner;t of motion at the foundation level.

b. The input me tion at the basc of a discitte soil model IVith respect ro clarification and more guidance, thefallowing or sod column should prodo e the specified design 8"idelines c,re provided for selection and variation of soil spectra at the free surface of soi' profile in tht fice . ..

field (firished grade). ProPunevn de E anh.

Unless the s;te is well investigated, the variation in soil

. properties should be considered by performing SSI analy-It is noted that there g enough confidence m. the current ses using three sets of values (defined in terms of shear methcds used to perform the SSI analysis to capture the moduli and soil hysteretic damping ratio). These three basic ph6nomenon and provide adegaate design informa' analyses should be performed using average (or best esti

• tion; however, the confidence m the ability to implement mate)value, twice the average yalue and half the average these inethodologies is t,ncertain. Therefore, m order t ensure proper implementation, the fo' lowing considera- value af the low strain shear medulus (Ga defined at

.10 *% peak shear strain). The same shear modulus deg-tions should be addressed m performing SSI analysts:

radation (G/ Gmx ) and hysteretic darnping (D) curves as function of peak shear strain can be used fer each ofIhese

a. Perform s ensitivity studies to identify important pa- three analyses. Final values of shear modulus and damp-raneters (e.g., bonding and debondini; of side walls, ing ratio used for each of the aralyaes are to be compat-ncnsymmatry of embedment, beation of bounda- ible with the strain levels expected in the free firld consis-ries) and to assist in judging ihe adequacy of the final tent with carthquake Rvels. In no case should the lower results. These sensitivity sindies can be performed bound shear modidus be less than that Wlue consistent i

by the use of well founded and properly substanti- with standard foundation analysis, that yields foundation ,

uted iimple models to give better insight; settlement under static loads exceeding design allow-ables.The upper bound shenr modulus should not be less

b. Through the use of some appropriate bancbmark than the best-estimate shear modulus . defined at low problems, the user should demonstrate its capability strain and as determined from the geophysical testing to properly implement any SSI methodologies; and program. In to case should the matetial soil damping, as expressed by the hysteretic damping ratio D (defined iu ASCE Stmidard 4-86), exceed 15%

c. Perform enough paramtric studies with the proper varinhn of parameters (e.g., soil properties) to a+ The staffs revised position now is much less prescriptive dress the uncertainties (as applicable to the given and removes unnecessary conservatism contained in the site). present version of SRP Cection 3.7.2.

The staff performed a quCtative regulatory analysis IVith respect to the control metion, the rcrised SRP sections based on past experience and seismic PRA results to as-contain thefollowing: sess the impact of the proposed changes in the SSI area.

The staff finds that the proposed SRP revision removes The control motion should be consistent with the proper- some orbitrary excessive conservatism from the current ties of the soil profile. I or profiles consisting of compe- Acquirements, which is likely to lead to lesser design ret tent soil or rock, with tclatively uniform variation of prop- quirements for the structur31/ mechanical components erties with depth, the control motion should be applied at and piping systems. Since the existing requirement of en-the free ground surface at the top of finished grade. For veloping results frem two different methods of SSI analy-profiles consisting of one or more thm, soil layers ove rlay- sis has been climinated, the burden on the industry is ing competent material, the control motion should oc ap- substantially reduced on the analytical efforts. Risk is not plied at an outcrop (real or hypothetical)at the top of the expected to change becaum the advancement in the state wmpetent materialin the viemity of the site. Vanation of of the art of the SSl is such that the results from two meth-amplitude and frequency content with depth may be con-ods, if appled properly, are not expected to be signifi-sidered for partially embedded structurea,. The spectml candy difftrent. Hence, the proposed changes in the SSI amplitude of the acceleration response spectra / horizon-area will provide a benefit with no change in risk.

tal component of motion)in the free field at the founda- I tion depth shallbe not less Ctan 60% of the corresponding The effects of other changes in the SSI area on structural design response spectra at the finish grade in the free components and on seismic risk estimates are considered '

field (ASCE Standard 4-86). When variation in soil prop- negligible for the following reasons:

erties [is} considercJ (as discussed below), the 60% limi-tation may be satisfied using envelope of the three spectra (1) A number of recent OL plants were designed to corresponding to the three soil properties. 1975 SRP requirements which specified the 13 NUREG-1233

Regulatory Analysis for USI A-40 free-facid motion at the free surface for SSI analysis, no backfit is recommended. However, the adoption of this During the OL review, the implementation of the change will result in smoother reviews and will save time current position of input motion at the foundation {

for NRC and the nuclear industry,

{

level in the free field resulted in a modification of '

some structural floor beams of seismic Category I structures at one plant. No hardware changes re- Area 6 suited at other plants. (Note that the staff's investi- For area 6, " Analysis Procedure for Damping," the pro-gation was limited to the safe-shutdown systems and posed change results from resolution of public comment structures which housed them, and allowance was for which qualitative arguments are made to determine made for tested strength values in some cases.) the impact. This change defines an acceptable limit on composite modal damping. The requirement states that (2) The design of structures or components is not gov- the use of composite roodal damping approach is accept-erned by the seismic loads alone. For example, the able provided the composite modal dampmg ts limited to thicknesses of exterior walls are more often con- h end M, @cn some oh suggd trolled by requirements for protecting against tor- method must be used.

nado missiles.

The current SSI evaluations are generally made in the fre-quency domain, taking into account the frequency de-(3) The scismic PRAs and safety-margin studies have pendence of the impedance functions; thus, the use of j generally shown cortsiderable margin in the seismic composite modal damping is not required. Its use only be-Category I structures and components beyond the j comes an issue when the simplified soil-spring (lumped- I design basis. Any small change in the margin is not parameter, frequency-independent) approach is used in I likely to have any meaningful impact on the seismi- the SSI analysis. Since in actual practice the use of the I cally induced cere melts or offsite consequences of composite modal damping approach is very limited, the radioactive release, as the uncertainties in both the impact of the preposed clarification on the industry is hazard ar.d fragihty estimates are still large. negligibic or Lonc.

In summary, the impact on the irtlustry is the removal of 2.1.1.5 Value/ impact Assessment of Proposed Actions arbitrary conservatism in the cuner t SSI analysis procc-dures, thus leading to lessen design requirements with Impact on Industry and NRC substantially reduced analytical efforts. This will benefit the mdustry without changmg risk. The impact on th Industry and NRC impact resulting from proposed staff is the increase in the review time, as the proposed changes in SRP Sections 3.7.1,3.7.2, and 3.73 is provided cevision allows for new developments and calls for more " able 3. NRC impact is liinited to additional review rigorous consideration of uncertainties.This change does time.

not warrant backfit considerations Impact on Other Government Agencies g7,, y Since the seismic design review and acceptance is carried out solely by NRC staff, no impact is projected on other For area 5, " Development of Floor Response Spectra and government agencies.

Effects of Parameter Variations on Floor Response Spec-tra," quantitative value/ impact assessments were not pre-pared for proposed changes. However, qualitative argu. Public Value ments were made in the contractor's report '1he following describes "value" to the public in a qualita-(NUREG/CR-3480). It should be noted that the pro- tive sense if the proposed revision to SRP sections is posed requirements relate to the justification for the use adopted.

of a single time-history in design and, as such, the value/

impact is the same as discussed for area 1 and the argu- e ment for no need to backfit this proposed change is the poss ble reduction in new plant capital costs by eliminating or providing alternatives which may re-same as for area 1. The proposed options for the use of move unquantifiable excessive conservatism multiple time-histories for the direct generation of floor spectra are like y to result in removing unnecessary con-e greater confidence in the scismic adequacy of nu-servatisms for pipmg systems and equipment.

clear power plant structures, systems, and compo-nents as the proposed revision reflects new techno-11 is, therefore. concluded that this change will not have logical advances and incorporates the knowledge significant impact on plani design or risks. For t his reason, gained NUREG-1233 14

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Regulatory Analysis for USI A-40 Overall Value//mpact of tire Proposed Actions (2) Option 2 (above) would require that all existing

.The value/ impact considerations were primarily determi-plants reanalyze their structures and make changes mstic and judgmental. The PR A analyses conducted were as necessary to comply with the new criteria. This-not conclusive because of limitations in modelmg and could potentially have significant cost impact on li-censees and applicants.

some anomahes m the predicted releases because of as-sumptions made about containment failure modes. Th estimated costs were developed by the staff and by LLNL (3) Option 3 (above) would resolve the safety question and its consultants. The proposed action will revise the of seismic Category I above-ground tanks, the only SRP sections to reflect current state-of-the-art and indus- item for which potentially significant concern was try practices. Revision to SRP sections will improve the identified. However, as discussed previously, the in-heensmg process by assurmg uniform review, reducmg tegrity of tanks is ensured when reviewed with the need for additional staff requests for mformation, screening criteria during implementation of USI eliminating some potential saurces of nonconservatisms, A-46.

and providing options that may lead to a reduction in un-necessary conservatism. Estimates indicate that this can 2.1.2.3 Decision on Technical Approach be achieved at very low cost.

Option 1 was selected because it satisfies the objective of upg ading the review guidance to reflect current technol-For the above-ground vertical tanks, the staff concludes ogy and industry practice.

that although the seismic design adequacy is a potential safety issue, the review of tanks is adequately covered by other programs, including the implementation of USI 2.1.3 Plan for Implementat. ion A-46, " Seismic Qualification of Equipment in OperatinB The staff recammends that CP, PDA, FDA, and CP/OL Plants?'

applicants who docket their applications after the pro-posed SRP sections are issued be requested to consider the seismic design practice for Category I structures, sys-2.1.2 Technical Approach tems, and components to 1be criteria given in the revised SRP sections.

.2.1.2.1 Technical Options The following options were considered: 2.1.4 Statutory Considerations 2.1.4.1 NRC Authority (1) Revise SRP Sections 3.7.1,3.7.2, and 3.7.3 and apply them only to new CP, PDA, FDA, and CP/OL appli. Since the changes are proposed in revision 1o SRP Sec-cations. tions 3.7.1 through 3.7.3, this action falls withm, the statu-tery authority of the NRC. Furthermore, the review of seismic design of Category I structures, systems, and com-(2) Revisc SRP Sections 3.7.1,3.7.2, and 3.7.3 and apply ponents is within the statutory authority of the NRC to them to alllicense applicants and backfit them to all determine safety of nuclear power plants.

holders of operating licenses.

2.1.4.2 Need for National Environmental Policy Act (3) Revise SR P Sections 3.7.1,3.7.2, and 3.7.3 and initi- Statement i

ate plant-specific review of seismic Category I tanks to determme if backfit is necessary. An analysis pursuant to the National Environmental Pol-cy Act is not necessary, because the proposed changes re-late only to a document (the SRP) which contains guid-2.1.2.2 Discussion and Comparison of Technical ance to the NR C staff for t aview and evaluation of reactor license applications.

Options (1) Adoption of option 1 (above) would incorporate techmcal work on the A-40 task into the bcensmg 2.1.5 Summary guidance for 'uture plants. The changes recom- The staff concludes that the proposed actions satisfy the mended would have little impact on licensees or on objective of USI A-40 to reflect the current state of the the NRC for new plan;s before the structural design art in seismic design in the licensing process.The assess-has been initiated except for its use as guidance for ment of the effect of the proposed changes on plant safety seismic re-reviews such as SEP and ISAP (Inte- indicates that the changes are warranted and will contrib-grated Safety Assessment Program). ute to a more uniform and consistent licensing process.

NURiiG-1233 16

Regulatory Analysis for USI A-40 l

Value/ Impact Analysis for Pro. NURFG-0011; Watts Bar, NUREG-0847). the staff has 2.2

.. . found site-specific spectra to be a 1eahstic approach to as-posed Revision to SRP Sectiori sessing the adequacy of sare-shutdown carthquale (SSli) 2.5.2 spectra because this method uses state-of-the-art scis-mological information and data analysis. The develop-2.2.1 The Proposed Action ment of an SSRS typically involves the co!!cction of accel-eration time-histories from earthquakes of similar 2.2.1.1 Summary of Issue magnitudes (similar to the SSE for the site), recorded at appropriate distances, and encompassing geologic condi-Standard review plans (SRPs) are prepared for the guid- tions similar to the site being modeled. O'ne example ohm ance of NRC staff reviewers who perform safety reviews.

SSRS, developed by LLNL for the NRC, is contained in The proposed action is to revise SRP Section 2.5.2, %- N UREG/CR-1582. Volume 4 (Appendix A, Section 4.3).

bratory brourd Motion" (N UREG-0800), to update and "Scismic Hazard Analysis Application of Methodology, clarify procedures used m the staff's seismology review. Results and Sensitivity Studies." Site-specific response spectra allow for the direct estimation of the response The proposed revision to SRP Section 2.5.2 reflects cur- spectrum at all frequencies for specific magnitude, dis-rent staff practice in the review of CP and OL applica-

, tance, and recording site conditions, rather than the need tions. Ihis revision meorporates recent staff positions to develop a reference acceleration (g value) for a site-taken in conjunction with current r eviews, Advisory Com- independent standard spectrum. It has been the staff's n.ittec on Reactor Safeguards (ACRS) views. carlier sug- position that the appropriate representation of the gestions proposed by the NRC staff and the staff at reponse spectra as dcrived directly from the real time-I awrence Livermore National Laboratory (NUREG/ histories is the 84th percentile. The choice of the 84th CR-1161),and resolution of public comments (NUREG/ percentile is based upon (1) past staff practite and licens-CR-5347). Most licensees are aware of these positions ing decisions. (2) the level used in deriving the RG 1.60 and proposed requirements. 'Ihc proposed SRP revision spectral shape, and (3) accounting for uncertainty in pre-reflects state-of.the-art procedures in assessing seismic dicting response spectra.

hazards. Some of these changes constitute part of the resolution of USI A-40.The staff considered that instead of separating the changes that result from the A-40 study and those changes that are part of a routine SRP revision, 2.2.1.3 Value/ Impact Assessment of the Proposed it would be better to consider all SRP changes as one ac- Action tion.The objectives of USI A -40 are discussed in Section .

,Ihere will be no impact on industry because the proposed 2.1.1.1. The LLNL recommendations in NUREG/

revision to SRP Section 2.5.2 reflects current staff prac-CR-1161 that affect SRP Section 2.5.2 and have been ac-tice and most licensees are aware of these requirements.

ccpted by the NRC staff are included in the proposed revision to SRP Section 2.5.2. One purpose of the SRP is toimprove the nuclear power mdustry's understandmg of the staff review procesr.The proposed revision will reduce delays in the licensing pro-Although a value/ impact statement is needed as part of cm because information needed for the staff review can t he Committe to Review Generic Requirements (CRGR) be incorporated in the safety analysis reports at the time package for A-40, as discussed below, the staff finds that of docketing instead of later through staff questions and this proposed action will have no impact on licensees, applicant responses. Ihis implementation will not reduce the risk, but will improve the SRP description of current 2.2.1.2 Need for the Proposed Action staff practice in licensing.

In recent years, as the staff has considered pertinent new information and analyses, it has revised its procedures for In some OL reviews, the site-specific spectra have ex-reviewing the scismic design of nuclear facilities. In re- cceded the SSE and licensees were required to review the cent reviews, pertinent new information has been devel-structural aspects. However, applying RG 1.60 spectra oped through the time-consuming process of geestioning would, in some cases, have required even more analysis. l the applicant's safety analysis reports. A revision to the SRP is needed so that future safety analysis reports will include pertinent inforraation at the time of docketing. Since the seismic design review and acceptance is carried The major change contained in thi; revision, site-specific out solely by the NRC staff, no impact on other govern-spectra, is discussed next. ment agencies is projected. The "v due" to the public, if the proposed revision to the SRP is adopted, is greater The SR P revision suggests that, wherever possible, a site- confidence in the ground-motion input to seismic design specific response spectrum (SSRS) should be developed. because the proposed revision reflects state-of-the-art in recent site reviews (for exampic. Sequoyah, practices.

17 NUREG-1233

Regulatory Analysis for USl A-40 nic overall value/ impact evaluation of the proposed ac- 3 REFERENCES tion indicates that there is no cost impact to industry and no change in public risk.

ASCE Standard 4-86, " Seismic Analysis of Safety-Related Nuclear Structures and Commentary on Stan-i 2,2,2 Technical Approach dard for Seismi:: Analysis of Safety-Related Nuclear Structures," American Society of Civil Engineers, Sep-i The only alternative to revising the SRP is to maintain the temb h present SRP version. 'lhis would increase review time in U.S. Nuclear Reguhitory C,ommission, IE 13ulletm 80-11, those cases in which informanon needed for the staff re.

view must be obtained through staff ouestions and appli- " Masonry Wall Design," May 8,1980.

cant responses.

-- , NUREG-00ll," Safety Evaluation Report of the The proposed revision to SRP Section 2.5.2 reflects the curn nt state of the art and staff practice and is the recom.

mended approach. -- , NUREG-0800, " Standard Review Plan for the

' Review of Safety Analysis Reports for Nuclear Power Plants. LWR Edition," July 1981.

2.2.3 Plan for implementation

-- , NUREG-0847, " Safety Evaluation Report Re-The imp!cmentation plan is c,ontained in the proposed lated to the Operation of the Watts Bar Nuclear Plant, SRP revision. Umts 1 and 2. June 1982.

-- , NUREG/CP-0054, " Proceedings of the Work.

2.2.4 Statutory C.onsiderat,ons i shop on Soil-Struct ure 1nt emetion," Hethesda, Maryland, June 16-18,1986.

2.2.4.1 NRC Authority

-- , NUREG/CR-1161, " Recommended Revisions Since the changes are proposed in revision to SRP Section to Nuclear Regulatory Commission Seismic Design Crite-2.5.2, this action falls within the statutory authority of the ria " May 1980.

NRC. Furthermore, the review of seismic design of Cate-pory 1 structures, systems, and components is within the -- , NUREG/CR-1582. Volume 4, "Scismic Hazard statutory authority of the NRC to determine safety ade. Analysis Application of Methodology, Results and Sensi-quacy of nuclear power plants. tivity Studies " October 1981.

-- , NUREG/CR-3480, Walue/ Impact Assessment 2.2.4.2 Need for National Environmental Policy Act for Seismic Design Criteria " August 1984.

Statement An analysis pursuant to the National Environmental Pol-

, NUREG/CR-3509, " Power Spectral Density icy Act is not necessa:y, because the proposed changes re- Functions Compatible With NRC Regulatory Guide 1.60 Response Spectra," June 1988.

late only to a document (the SRP) which contains guid-ance to the NRC staf f for review and evaluation of reactor license applications. -- , NUREG/CR-3805, Volumes 3,4, and 5. "Engi-neering Characterization of Ground Motion," August 1986.

2.2.5 Sununary

-- , NUREG/CR-4776, " Response of Seismic Cate-The proposed revision to SRP Section 2.5.2 will update gmy I Tanks to Earthquake Excitation," February 1987.

and clarify procedures used in the staff scismology review.

These revised procedures will decrease licensing costs by - - , NUREG/CR-5347, " Recommendations for decreasing review time.There are no new requirements Resolution of I ublic Comments on USI A-40, Seismic in the proposed revision because the changes in the pro- Design Criteria," March 1989.

posed revision are current staff practice and known to most licensees. Furthermore, there is no change in public - - , WASH-1400 (NUREG-75/014), " Reactor risk. The staff recommends that the revised SRP Section Safety Study: An Assessment of Risks in U.S. Commer-2.5.2 be issued. cial Nuclear Power Plants," December 1975.

NURI G-1233 18

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10 SUPPLEVENT ARY NOTES

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