NRC Generic Letter 1992-02

NRC Generic Letter 1992-002: Resolution of Generic Issue 79, Unanalyzed Reactor Vessel (PWR) Thermal Stress During Natural Convection Cooldown.

ML031200650
Person / Time
Site:	Beaver Valley, Millstone, Hatch, Monticello, Calvert Cliffs, Dresden, Davis Besse, Peach Bottom, Browns Ferry, Salem, Oconee, Mcguire, Nine Mile Point, Palisades, Palo Verde, Perry, Indian Point, Fermi, Kewaunee, Catawba, Harris, Wolf Creek, Saint Lucie, Point Beach, Oyster Creek, Watts Bar, Hope Creek, Grand Gulf, Cooper, Sequoyah, Byron, Pilgrim, Arkansas Nuclear, Three Mile Island, Braidwood, Susquehanna, Summer, Prairie Island, Columbia, Seabrook, Brunswick, Surry, Limerick, North Anna, Turkey Point, River Bend, Vermont Yankee, Crystal River, Haddam Neck, Ginna, Diablo Canyon, Callaway, Vogtle, Waterford, Duane Arnold, Farley, Robinson, Clinton, South Texas, San Onofre, Cook, Comanche Peak, Yankee Rowe, Maine Yankee, Quad Cities, Humboldt Bay, La Crosse, Big Rock Point, Rancho Seco, Zion, Midland, Bellefonte, Fort Calhoun, FitzPatrick, McGuire, LaSalle, Fort Saint Vrain, Washington Public Power Supply System, Shoreham, Satsop, Trojan, Atlantic Nuclear Power Plant, Clinch River
Issue date:	03/06/1992
From:	Partlow J G Office of Nuclear Reactor Regulation
To:
References
GL-92-002, NUDOCS 9203030209
Download: ML031200650 (10)

soUNITED STATES0 WNUCLEAR REGULATORY COMMISSIONC tWASHINGTON, D. C. 20555March 6, 1992TO: ALL HOLDERS OF OPERATING LICENSES OR CONSTRUCTION PERMITS FORPRESSURIZED WATER REACTORS (PWRs)SUBJECT: RESOLUTION OF GENERIC ISSUE 79, "UNANALYZED REACTOR VESSEL(PWR) THERMAL STRESS DURING NATURAL CONVECTION COOLDOWN"(GENERIC LETTER 92-02)The U.S. Nuclear Regulatory Commission (NRC) is providing this letter toinform addressees of (1) the NRC's resolution of Generic Issue 79, "UnanalyzedReactor Vessel (PWR) Thermal Stress During Natural Convection Cooldowns and (2)the conclusions reached by the staff as the result of the evaluations performedto resolve this generic issue. No new requirements are being established andno specific action or written response is required.BackgroundOn May 5, 1981, the NRC issued Generic Letter (GL) 81-21, "NaturalCirculation Cooldown," in response to a natural circulation cooldown (NCC)event that occurred at the St. Lucie Plant, Unit 1, on June 11, 1980. Thatevent caused a void (steam bubble) to form in the reactor vessel head. InGL 81-21, addressed to all operating PWR power reactor licensees andapplicants for operating licenses (except for St. Lucie, Unit 1), the NRCrequested. that addressees determine whether operator training and plantprocedures were adequate to effect a controlled NCC from operating conditionsto cold shutdown. The NRC requested addressees to demonstrate theircapability by test or analysis or both in accordance with Section 50.54(f) ofTitle 10 of the Code of Federal Regulations (10 CFR 50.54(f)).By letter of March 18, 1983, the Babcock & Wilcox Company (B&W) notified theNRC that large axial temperature gradients across the RV closure region maycause thermal stresses, beyond those considered in the original design ofPWR vessels, to develop in the reactor vessel (RV) flanges and studs. Thiscondition could be outside the design basis of the PWR RVs. During an NCCevent, the upper head of a PWR vessel is likely to remain at a highertemperature than the cylindrical portion of the vessel because there islittle or no mixing of the fluid in this region with the remainder of thefluid in the reactor vessel. Further, a steam bubble may develop in the topof the vessel as the reactor coolant system is depressurized. The NRCdetermined that this concern could be a generic safety issue and designated itas Generic Issue 79 (GI-79).DiscussionB&W performed a detailed analysis of the B&W 177 Fuel Assembly Reactor Vessel(B&W 177) and submitted it to the NRC by letter of October 15, 1984. The NRCused an independent confirmatory analysis performed by the Brookhaven NationalLaboratory (BNL) in May 1989, to evaluate the B&W submittal regarding the92033020

Generic Letter 92-02 -2 -March 6, 1992stresses in the reactor vessel and the reactor vessel closure studs. The NRCstaff also performed a detailed fracture mechanics evaluation of the nozzleshell course and the reactor vessel closure studs. The staff discussed theseanalyses in NUREG-1374, "An Evaluation of PWR Reactor Vessel Thermal StressDuring Natural Convection Cooldown," May 1991, which is enclosed. The NRCconcluded that the B&W 177 meets.the currently applicable regulatory designstress and fracture prevention criteria for NCC transient conditions up to andincluding those used by the NRC and its contractor in these analyses, as shownin Figure 3 of NUREG-1374.In 10 CFR 50.73(a)(2)ii(A) and (B), the NRC requires the licensee to submit alicensee event report for any event that resulted in .the nuclear power plantbeing in an unanalyzed condition that significantly compromised plant safetyor in a condition that was outside the design basis of the plant. Theanalysis noted above considers a B&W 177 to be in an analyzed condition andwithin its design basis for NCC events that are bounded by the NCC transientprofile shown in Figure 3 of NUREG-1374.The detailed analyses by B&W, NRC, and BNL indicated clearly the extremelycomplex nature of this type of analysis. This analysis included numerousthermal-hydraulic and mechanical modeling assumptions which, althoughconsidered to be conservative, were not confirmed by specifically measureddata. Calculated stress results for the B&W 177 were as high 'as 98-percentof allowable values in the RV-studs specified in the American.Society ofMechanical Engineers (ASME) Code. While the Code allowable value includesmargins, differences between the stresses calculated, by B&W and thosecalculated by BNL, indicated that an RV could be in arrunanalyzed conditionfor certain NCC events, particularly for events complicated by-other factorssuch as an atmospheric dump valve that is stuck open.The limitations of the analysis, as stated above, prevented the staff frommaking a definitive conclusion regarding compliance with the applicableregulatory criteria of B&W 177s that might experience an NCC that isoutside the bounds of the analysis assumptions, or for B&W non-177s and otherPWR vessels that may experience a significant NCC event in the future.However, the staff reviewed the results of the analyses and the qualitativeextrapolation of those results and concluded the following:1. The B&W 177 is considered analyzed for NCC events that are boundedby the NCC transient profile shown in Figure 3 of NUREG-1374.2. It is extremely unlikely that a single .NCC event will cause thefailure of any U.S. PWR RV, even if a cooldown rate of 100 0F per hour isexceeded.3. An NCC event that does not exceed a total cooldown of 100 IF,independent of rate, would not be expected to compromise the safetyof any U.S. PWR RY. However, it may result in' the RV being outsideits documented design basis.

Generic Letter 92- 02 -3 -March 6, 19924. Exposure of U.S. PWR RVs to certain NCC transients, particularlytransients complicated by other factors such as a stuck-open atmosphericdump valve, may result in a condition that is outside the documenteddesign basis of the RV.The NRC staff has further concluded that (1) NCC events of the type analyzed,which result in the plant being brought to a cold shutdown condition occurinfrequently and (2) the actual severity of a specific NCC event willdetermine the need for (if any) and the extent of actions that may be requiredof any licensee following certain NCC events that may place a reactorvessel in an unanalyzed condition Qr outside its documented design basis.Therefore, no requirement for generic or plant-specific actions was deemednecessary for safety reasons.Backfit-ViscussionThe NRC is establishing no new requirements in this generic letter and isrequiring no specific action. Existing regulations address any calculationsthat may be required to be performed after an NCC event. Therefore, the NRCis not imposing a backfit.This generic letter contains ho requirements for collecting information andtherefore is not subject to the requirements of the Paperwork Reduction Act of1980 (44 U.S.C. 3501 et Mse.).Although no response to this letter is required, if you have any questionsregarding this matter,'please contact the technical contact listed below.Si ncerelJlans G. PartlowAs ociate Director for ProjectsOffice of Nuclear Reactor RegulationEnclosure:NUREG-1374

Technical Contact:

J. D. Page, RES(301) 492-3941 Generic Letter 92-02-3 -March 6, .3924. Exposure of U.S. PWR RVs to certain NCC transients, particularlytransients complicated by other factors such as a stuck-open atmosphericcump valve, may result in a concition that is outside the documenteddesign basis of the RV.The NRC staff has further concluded that (1) NCC events of the type analyzed,which result in the plant being brought to a cold shutdown conditionoccurinfrequently and (2) the actual severity of a specific NCC event willdetermine the need for (if any) and the extent of actions that may be requiredof ariy licensee following certtirn hLC even-ts that may place a reactorvessel in an unpnaliyzec ctrditiGr cr outside its documertec cesigr Lisis.Therefore, no requirement for generic or plant-specific actions was deemednecessary for safety reasons.

Backfit Discussion

The NRC is establishing no new requirements in this generic letter and isrec!L-;rg ne spucific action. Existing reglatiuns address any calculationsthat may be required to be performed after an NCC event. Therefore, the NRCis not imposing a backfit.This generic letter contains no requirements for collecting information andtherefore is not subject to the requirements of the Paperwork Reduction Act of1980 (44 U.S.C. 3501 et seg.).Although no responis tc i;l i Air is required, if you have any questiorsregarding this ratter, please eoffact the technical contact listed below.Sincerely,James G. PartlowAssociate Director for ProjectsOffice of Nuclear Reactor RegulationEnclosure: DISTRIBUTION:NUREG-1374 Seie atiimdasheet

Technical Contact:

• See previous concurrenceJ. D. Page, RES(301) 492-39&1MF :FD1-1:LA TP-------- 7BE --------l ~ l t-----------------------E-- : I-------______ ------___ ___- ----------NAME :CVogan _ :NConicella:smm:*RBaer :*JRichardson :*RACapra------~ :--- ~----------:--------:----- ----------DATE * \/;x/921 :1 /3/92f :10/04/91 :1,/04/°1 :10' 24'91OFC :DPW:1A:DRPE~TT A:--ECR76-:C-- AT--- -- -- -- -- .-------------- :- ------- ---- --- ---- --- -- -*A T-- -- ---- -NAME :*ELeeds :*MBoyle :*JMain :*CBerlinger :*WRussellDATE :10/29/91 :10/29/91 :1 101 /91 :11/12/91 :11/13/91* ....-UKh :ADP 'D------------------ ------- ----- --------------..................................................................................... -- -----=I -- --------------------... ..NAME :JPartlow : :---- ----- ---------------------- -- ----------------DATE D e3amGR92 * :1-____ lt rIAL IgoURU UFD, Oocument Na me: GI 79 GENERIC LTR TAC 81364 Gener c Le-tter 91--3 -4. E osure of U.S. PWR RVs to certain NCC transients, particularlytransients complicated by other factors such as a stuck-open atmosphericdump 'valve, may result in a condition that is outside the documenteddesign basis of the RY.The NRC staf has further concluded that (1) NCC events of the type analyzed,which result the plant being brought to a cold shutdown condition occurinfrequently an (2) the actual severity of a specific NCC event willdetermine the ne for (if any) and the extent of actions that may be requiredof any licensee fo lowing certain NCC events that may place a reactorvessel in an unanal zed condition or outside its documented design basis.Therefore, no requir ent for generic or plant-specific actions was deemednecessary for safety asons.

Backfit Discussion

The NRC is establishing no ew requirements in this generic letter and isrequiring no specific action Existing regulations address any calculationsthat may be required to be pe ormed after an NCC event. Therefore, the NRCis not imposing a backfit. \This generic letter contains no r uirements for collecting information andtherefore is not subject to the re, irements of the Paperwork Reduction Act of1980 (44 U.S.C. 3501 et seq.).Although no response to this letter is equired, if you have any questionsregarding this matter, please contact th technical contact listed below.S cerely,Jam G. PartlowAssoc ate Director for ProjectsOffice f Nuclear Reactor RegulationEnclosure: DISTRIBUTION:!IUREG-1374 See attached sheet

Technical Contact:

• See previous concurrenceJ. D. Page, RES(301) 492-3941OFC :P6I_-1:LA :PDI-1:PM Hi :R S :DET \ :PDI-1:D--- -- --- -- --:-- -- ---V--- : -- ------:- ---- ------NAME :CVogan it :NConicella:smm:*RBaer :*JRichardso :*RACapraDATE :'/ /91 'N :ll/ /91 : / /91 :10/04/91 ./ /91UP : UKFW:TIA : UKL: I A : I tCH ED :UGS C tAU.., .-- -- .-- ------.-- ----------- --: --_ 1 -_:-NAME :*ELeeds :*MBoyle :*JMain :CBerlinger :WRus 11DATE : / /91 : / /91 : / /91 : iI/iV/91 3/9-FC- : :D:\NAME :JPartlow : :DATE : / /91. :: -: ..OF An C Nm A 7% 0L LTiRD WY83Docu--nt Name: GI 79 GENERIC LTR TAC 81364I )

Generic Letter 91-\(4) Exposure of U.S. PWR RVs to certain NCC transients, particularlytransients complicated by other factors (e.g., stuck-open atmosphericdump valve), may result in a condition that is outside the documenteddesign basis of the RV.The NRC sta f has further concluded that (1) NCC events of the type analyzed(i.e., NCC events -that result in the plant being brought to a cold shutdowncondition) have a low frequency of occurrence, and (2) the actual severity of aspecific NCC event will determine the need for (if any) and the extent ofactions that may e required of any specific licensee following certain NCCevents that may pl ce a reactor vessel in an unanalyzed condition or outsideits documented design basis. Therefore, no requirement for generic orplant-specific actio i was deemed necessary for safety reasons.

Backfit Discussion

No new requirements are beng established in this generic letter, and nospecific action is required Any calculations that may be required to'beperformed subsequent to an N event are covered by existing regulations.Therefore, no backfit is being imposed.This generic letter contains no formation collection requirements andtherefore is not subject to the re uirements of the Paperwork Reduction Act of1980 (44 U.S.C. 3501 et seq.).Although no response to this letter is required, if you have any questionsregarding this matter, please contact t technical contact listed below.Si erely,James G. PartlowAssoci te Director for ProjectsOffice Nuclear Reactor RegulationEnclosure: DISTRIBUTI :NUREG-1374 ee a ac e sheet

Technical Contact:

J. D. Page, RES *See previous ncurrence(301) 492-39410FC :PDI-1:LA :PDI-1:PM :R S :DET----------- ------ --------- :------------NAME :CVogan Ad :NConicella:sum:*RBaer :*JRichardson RACapra'KQc :DATE : !/..K/91 :1iO//9m :10/04/91 :10/21/91 :ot 4/91OFC :DRPWtA DREA :TECH ED :OGCB :A0---------9F---------------- :-------------- :-------------- ------X---NAME :ELeeds :MBoyle : :JMain :CBerlinger :WRusselDATE :10//Il :1P/ Ie91 : / /91 : / /91 : / /91NAME :JPartlow : :DATE * / /91 : :UtLLIAL KLLUKU LUFTn^.mtman+ Mn..n 9 70 r.I:NFDTrW I TR TA " i 16 Generic Letter 9'---3 -\ \(4) Exposure of U.S. PWR RVs to certain NCC transients, particularlytransients complicated by other factors (e.g., stuck-open atmospheric\ dump valve), may result in a condition that is outside the documenteddesign basis of the RV.The NRC staff has further concluded that (1) NCC events of the type analyzed(i.e., NCC events that result in the plant being brought to a cold-shVtdowncondition) have a low frequency of occurrence, and (2) the actual severity of aspecific NCC event will determine the need for (if any) and the extent ofactions that may be required of any specific licensee following certain *NCCevents that may place a reactor vessel in an uranalyzed condition or outsideits documented desigr basis. Therefore, no requirement for generic orplant-specific actions was deemed necessary for safety reasons.

Backfit Discussion

No new requirements are being established in this generic letter, and nospecific action is required \ Any calculations that may be required-to beperformed subsequent to an '\CCevent are covered by existing regulations.Therefore, no backfit is being iimposed.This generic letter contains no information collection requirements andtherefore is not subject to the requi1irements of the Paperwork Reduction Act of1980 (44 U.S.C. 3501 et seq.).Although no response to this letter is r quired, if you have any questionsregarding this matter, please contact the echnical contact listed below.Sinc ely,James G. PartlowAssociate Director for ProjectsOffice of clear Reactor RegulationEnclosure: DISTRIBUTIONNUREG-1374 See attached s et

Technical Contact:

J. D. Page, RES *See previous con rrence(301) 492-39410FC--- D LAl-- :PD I-1 ----- *RES ---------T- Y I-AFME :CVogan :NConicella:smm:*RBaer :*JRichardson :RA praDATE : / /91 : / /91 :10/04/91 :10/21/91. : / 1OFR :DRP-Vl-TA -- -- PE-M E--- ED -OGCB -ADT -NAME :ELeeds :FMBoyle :JMain .' :CBerlinger :WRussell______:-- -- --- -- ------------ --_--___-_ ......... ---- -- -- --------______-_---____DATE :/ /91 :/ /91 :/ /91 :/ /91 / /91OFC :ADPNAME :JPartlow : : -:DATE * / /91 * *U CALt-RORD-COPY.---------natoIjent Name: GI 79 GENERIC LTR TAC 81364 Generic Letter 91--3 -.) Exposure of U.S. PWR RVs to certain NCC transients; particularlytransients complicated by other factors (e.g., stuck-open atmosphericdump valve), may result in a condition that is outside the documentedesign basis of the RV.The NRC staf has further concluded that (1) NCC events of the type analyzed(i.e., NCC eve ts that result in the plant being brought to a cold shutdowncondition) have low frequency of occurrence, and (2) the actual severity of aspecific NCC eve will determine the need for (if any) and the extent pfactions that may b required of any specific licensee following certain NCCevents that may pla a reactor vessel in an unanalyzed condition or outsideits documented design asis. Therefore, no requirement for generic orplant-specific actions as deemed necessary for safety reasons.

Backfit Discussion

No new requirements are bein established in this generic letter, and vospecific action Is required. ny calculations that may be required to Seperformed subsequent to an NCC vent are covered by existing regulations.Therefore, no backfit is being i osed.This generic letter contains no inf mation collection requirements andtherefore is not subject to the requ ements of the Paperwork Reduction Act of1980 (44 U.S.C. 3501 et seq.).Although no response to this letter is r uired, if you have any questionsregarding this matter, please contact the chnical contact listed below.Since ly,James G artlowAssociate irector for ProjectsOffice of clear Reactor RegulationEnclosure: DISTRIBUTION:NUREG-1374 See attached sh et

Technical Contact:

J. D. Page, RES(301) 492-39410EC :11Dl-MA :P M-lPM REtS MEtl 1-DNAME :CVogan on -:JRic' son :RAC raDATE : /91 --- 2,/91-- C- -o ----- / ----DATE I :j912-/91 1m -/91 :/7~91:/ 1ORC :DRPR:TA :DRPE:TA :TECH ED :OGCB :ADT\NAME :ELeeds :MBoyle :JMain :CBerlinger :WRussellDATE :/ /91 ://91 ://91 :/ /91 ://91UFC :ADPNAME :JPartlow : :DATE * / /91 * * * :rirrswa noUIGIIAL KI -'1J WUI'.if -.. PT 7tn rr11rBTP ITD TAP 011CA

Document Name:GI 79 GENERIC LTR TAC 81364Requestor's ID:MICKENSAuthor's Name:Conicella N.Document Comments:GENERIC LETTER 91 -81364-

-6DISTRIBUTION: GENERIC LETTER 92-02 -March 6, 1992Central FlesNRC PDRPDI-1 ReadingS. VargaJ. CalvoR. A. CapraN. ConicellaC. VoganJ. Richardson, 7/D/26E. Leeds, 13/H/24M. Boyle, 16/H/3C. Berlinger, 8/D/22W. Russell, 12/G/18J. Partlow, 12/G/18R. Baer, NLS/302-