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 STATES 0 WNUCLEAR REGULATORY

COMMISSION

C tWASHINGTON, D. C. 20555 March 6, 1992 TO: ALL HOLDERS OF OPERATING

LICENSES OR CONSTRUCTION

PERMITS FOR PRESSURIZED

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 to inform addressees of (1) the NRC's resolution of Generic Issue 79, "Unanalyzed Reactor Vessel (PWR) Thermal Stress During Natural Convection Cooldowns and (2)the conclusions reached by the staff as the result of the evaluations performed to resolve this generic issue. No new requirements are being established and no specific action or written response is required.Background On May 5, 1981, the NRC issued Generic Letter (GL) 81-21, "Natural Circulation Cooldown," in response to a natural circulation cooldown (NCC)event that occurred at the St. Lucie Plant, Unit 1, on June 11, 1980. That event caused a void (steam bubble) to form in the reactor vessel head. In GL 81-21, addressed to all operating PWR power reactor licensees and applicants for operating licenses (except for St. Lucie, Unit 1), the NRC requested.

that addressees determine whether operator training and plant procedures were adequate to effect a controlled NCC from operating conditions to cold shutdown.

The NRC requested addressees to demonstrate their capability by test or analysis or both in accordance with Section 50.54(f) of Title 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 the NRC that large axial temperature gradients across the RV closure region may cause thermal stresses, beyond those considered in the original design of PWR vessels, to develop in the reactor vessel (RV) flanges and studs. This condition could be outside the design basis of the PWR RVs. During an NCC event, the upper head of a PWR vessel is likely to remain at a higher temperature than the cylindrical portion of the vessel because there is little or no mixing of the fluid in this region with the remainder of the fluid in the reactor vessel. Further, a steam bubble may develop in the top of the vessel as the reactor coolant system is depressurized.

The NRC determined that this concern could be a generic safety issue and designated it as Generic Issue 79 (GI-79).Discussion B&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 NRC used an independent confirmatory analysis performed by the Brookhaven National Laboratory (BNL) in May 1989, to evaluate the B&W submittal regarding the 92033020

Generic Letter 92-02 -2 -March 6, 1992 stresses in the reactor vessel and the reactor vessel closure studs. The NRC staff also performed a detailed fracture mechanics evaluation of the nozzle shell course and the reactor vessel closure studs. The staff discussed these analyses in NUREG-1374, "An Evaluation of PWR Reactor Vessel Thermal Stress During Natural Convection Cooldown," May 1991, which is enclosed.

The NRC concluded that the B&W 177 meets.the currently applicable regulatory design stress and fracture prevention criteria for NCC transient conditions up to and including those used by the NRC and its contractor in these analyses, as shown in Figure 3 of NUREG-1374.

In 10 CFR 50.73(a)(2)ii(A)

and (B), the NRC requires the licensee to submit a licensee event report for any event that resulted in .the nuclear power plant being in an unanalyzed condition that significantly compromised plant safety or in a condition that was outside the design basis of the plant. The analysis noted above considers a B&W 177 to be in an analyzed condition and within its design basis for NCC events that are bounded by the NCC transient profile shown in Figure 3 of NUREG-1374.

The detailed analyses by B&W, NRC, and BNL indicated clearly the extremely complex nature of this type of analysis.

This analysis included numerous thermal-hydraulic and mechanical modeling assumptions which, although considered to be conservative, were not confirmed by specifically measured data. Calculated stress results for the B&W 177 were as high 'as 98-percent of allowable values in the RV-studs specified in the American.Society of Mechanical Engineers (ASME) Code. While the Code allowable value includes margins, differences between the stresses calculated, by B&W and those calculated by BNL, indicated that an RV could be in arrunanalyzed condition for certain NCC events, particularly for events complicated by-other factors such as an atmospheric dump valve that is stuck open.The limitations of the analysis, as stated above, prevented the staff from making a definitive conclusion regarding compliance with the applicable regulatory criteria of B&W 177s that might experience an NCC that is outside the bounds of the analysis assumptions, or for B&W non-177s and other PWR vessels that may experience a significant NCC event in the future.However, the staff reviewed the results of the analyses and the qualitative extrapolation of those results and concluded the following:

1. The B&W 177 is considered analyzed for NCC events that are bounded by the NCC transient profile shown in Figure 3 of NUREG-1374.

2. It is extremely unlikely that a single .NCC event will cause the failure of any U.S. PWR RV, even if a cooldown rate of 100 0 F per hour is exceeded.3. An NCC event that does not exceed a total cooldown of 100 IF, independent of rate, would not be expected to compromise the safety of any U.S. PWR RY. However, it may result in' the RV being outside its documented design basis.

Generic Letter 92- 02 -3 -March 6, 1992 4. Exposure of U.S. PWR RVs to certain NCC transients, particularly transients complicated by other factors such as a stuck-open atmospheric dump valve, may result in a condition that is outside the documented design 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 occur infrequently and (2) the actual severity of a specific NCC event will determine the need for (if any) and the extent of actions that may be required of any licensee following certain NCC events that may place a reactor vessel in an unanalyzed condition Qr outside its documented design basis.Therefore, no requirement for generic or plant-specific actions was deemed necessary for safety reasons.Backfit-Viscussion The NRC is establishing no new requirements in this generic letter and is requiring no specific action. Existing regulations address any calculations that may be required to be performed after an NCC event. Therefore, the NRC is not imposing a backfit.This generic letter contains ho requirements for collecting information and therefore is not subject to the requirements of the Paperwork Reduction Act of 1980 (44 U.S.C. 3501 et Mse.).Although no response to this letter is required, if you have any questions regarding this matter,'please contact the technical contact listed below.Si ncerel Jlans G. Partlow As ociate Director for Projects Office of Nuclear Reactor Regulation Enclosure:

NUREG-1374

Technical Contact:

J. D. Page, RES (301) 492-3941 Generic Letter 92-02-3 -March 6, .392 4. Exposure of U.S. PWR RVs to certain NCC transients, particularly transients complicated by other factors such as a stuck-open atmospheric cump valve, may result in a concition that is outside the documented design 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 conditionoccur infrequently and (2) the actual severity of a specific NCC event will determine the need for (if any) and the extent of actions that may be required of ariy licensee following certtirn hLC even-ts that may place a reactor vessel in an unpnaliyzec ctrditiGr cr outside its documertec cesigr Lisis.Therefore, no requirement for generic or plant-specific actions was deemed necessary for safety reasons.

Backfit Discussion

The NRC is establishing no new requirements in this generic letter and is rec!L-;rg ne spucific action. Existing reglatiuns address any calculations that may be required to be performed after an NCC event. Therefore, the NRC is not imposing a backfit.This generic letter contains no requirements for collecting information and therefore is not subject to the requirements of the Paperwork Reduction Act of 1980 (44 U.S.C. 3501 et seg.).Although no responis tc i;l i Air is required, if you have any questiors regarding this ratter, please eoffact the technical contact listed below.Sincerely, James G. Partlow Associate Director for Projects Office of Nuclear Reactor Regulation Enclosure:

DISTRIBUTION:

NUREG-1374 Seie atiimdasheet

Technical Contact:

• See previous concurrence J. D. Page, RES (301) 492-39&1 MF :FD1-1:LA

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DATE D e3amGR92 * :1-____ lt rIAL IgoURU UF D, 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, particularly transients complicated by other factors such as a stuck-open atmospheric dump 'valve, may result in a condition that is outside the documented design 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 occur infrequently an (2) the actual severity of a specific NCC event will determine the ne for (if any) and the extent of actions that may be required of any licensee fo lowing certain NCC events that may place a reactor vessel in an unanal zed condition or outside its documented design basis.Therefore, no requir ent for generic or plant-specific actions was deemed necessary for safety asons.

Backfit Discussion

The NRC is establishing no ew requirements in this generic letter and is requiring no specific action Existing regulations address any calculations that may be required to be pe ormed after an NCC event. Therefore, the NRC is not imposing a backfit. \This generic letter contains no r uirements for collecting information and therefore is not subject to the re, irements of the Paperwork Reduction Act of 1980 (44 U.S.C. 3501 et seq.).Although no response to this letter is equired, if you have any questions regarding this matter, please contact th technical contact listed below.S cerely, Jam G. Partlow Assoc ate Director for Projects Office f Nuclear Reactor Regulation Enclosure:

DISTRIBUTION:

!IUREG-1374 See attached sheet

Technical Contact:

• See previous concurrence J. D. Page, RES (301) 492-3941 OFC :P6I_-1:LA

PDI-1:PM

Hi :R S :DET \ :PDI-1:D--- -- --- -- --:-- -- ---V--- : -- ------:- ---- ------NAME :CVogan it :NConicella:smm:*RBaer

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WRus 11 DATE : / /91 : / /91 : / /91 : iI/iV/91 3/9-FC- : :D:\NAME :JPartlow

: DATE : / /91. :: -: ..OF An C Nm A 7% 0L LTiRD WY83 Docu--nt Name: GI 79 GENERIC LTR TAC 81364 I )

Generic Letter 91-\(4) Exposure of U.S. PWR RVs to certain NCC transients, particularly transients complicated by other factors (e.g., stuck-open atmospheric dump valve), may result in a condition that is outside the documented design 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 shutdown condition)

have a low frequency of occurrence, and (2) the actual severity of a specific NCC event will determine the need for (if any) and the extent of actions that may e required of any specific licensee following certain NCC events that may pl ce a reactor vessel in an unanalyzed condition or outside its documented design basis. Therefore, no requirement for generic or plant-specific actio i was deemed necessary for safety reasons.

Backfit Discussion

No new requirements are beng established in this generic letter, and no specific action is required Any calculations that may be required to'be performed subsequent to an N event are covered by existing regulations.

Therefore, no backfit is being imposed.This generic letter contains no formation collection requirements and therefore is not subject to the re uirements of the Paperwork Reduction Act of 1980 (44 U.S.C. 3501 et seq.).Although no response to this letter is required, if you have any questions regarding this matter, please contact t technical contact listed below.Si erely, James G. Partlow Associ te Director for Projects Office Nuclear Reactor Regulation Enclosure:

DISTRIBUTI

NUREG-1374 ee a ac e sheet

Technical Contact:

J. D. Page, RES *See previous ncurrence (301) 492-3941 0FC :PDI-1:LA

PDI-1:PM

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NAME :CVogan Ad :NConicella:sum:*RBaer

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DATE : !/..K/91 :1iO//9m :10/04/91

10/21/91

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X---NAME :ELeeds :MBoyle : :JMain :CBerlinger

WRussel DATE :10//Il :1P/ Ie91 : / /91 : / /91 : / /91 NAME :JPartlow

: DATE * / /91 : : UtLLIAL KLLUKU LUFT n^.mtman+

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I TR TA " i 16 Generic Letter 9'---3 -\ \(4) Exposure of U.S. PWR RVs to certain NCC transients, particularly transients complicated by other factors (e.g., stuck-open atmospheric

\ dump valve), may result in a condition that is outside the documented design 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-shVtdown condition)

have a low frequency of occurrence, and (2) the actual severity of a specific NCC event will determine the need for (if any) and the extent of actions that may be required of any specific licensee following certain *NCC events that may place a reactor vessel in an uranalyzed condition or outside its documented desigr basis. Therefore, no requirement for generic or plant-specific actions was deemed necessary for safety reasons.

Backfit Discussion

No new requirements are being established in this generic letter, and no specific action is required \ Any calculations that may be required-to be performed subsequent to an '\CCevent are covered by existing regulations.

Therefore, no backfit is being iimposed.This generic letter contains no information collection requirements and therefore is not subject to the requi1irements of the Paperwork Reduction Act of 1980 (44 U.S.C. 3501 et seq.).Although no response to this letter is r quired, if you have any questions regarding this matter, please contact the echnical contact listed below.Sinc ely, James G. Partlow Associate Director for Projects Office of clear Reactor Regulation Enclosure:

DISTRIBUTION

NUREG-1374 See attached s et

Technical Contact:

J. D. Page, RES *See previous con rrence (301) 492-3941 0FC--- D LAl-- :PD I-1 ----- *RES ---------T- Y I-AFME :CVogan :NConicella:smm:*RBaer

• JRichardson

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10/21/91.

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DATE :/ /91 :/ /91 :/ /91 :/ /91 / /91 OFC :ADP NAME :JPartlow

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natoIjent Name: GI 79 GENERIC LTR TAC 81364 Generic Letter 91--3 -.) Exposure of U.S. PWR RVs to certain NCC transients;

particularly transients complicated by other factors (e.g., stuck-open atmospheric dump valve), may result in a condition that is outside the documented esign 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 shutdown condition)

have low frequency of occurrence, and (2) the actual severity of a specific NCC eve will determine the need for (if any) and the extent pf actions that may b required of any specific licensee following certain NCC events that may pla a reactor vessel in an unanalyzed condition or outside its documented design asis. Therefore, no requirement for generic or plant-specific actions as deemed necessary for safety reasons.

Backfit Discussion

No new requirements are bein established in this generic letter, and vo specific action Is required.

ny calculations that may be required to Se performed 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 and therefore is not subject to the requ ements of the Paperwork Reduction Act of 1980 (44 U.S.C. 3501 et seq.).Although no response to this letter is r uired, if you have any questions regarding this matter, please contact the chnical contact listed below.Since ly, James G artlow Associate irector for Projects Office of clear Reactor Regulation Enclosure:

DISTRIBUTION:

NUREG-1374 See attached sh et

Technical Contact:

J. D. Page, RES (301) 492-3941 0EC :11Dl-MA :P M-lPM REtS MEtl 1-D NAME :CVogan on -:JRic' son :RAC ra DATE : /91 --- 2,/91-- C- -o ----- / ----DATE I :j912-/91

1m -/91 :/7~91:/ 1 ORC :DRPR:TA :DRPE:TA :TECH ED :OGCB :ADT\NAME :ELeeds :MBoyle :JMain :CBerlinger

WRussell DATE :/ /91 ://91 ://91 :/ /91 ://91 UFC :ADP NAME :JPartlow

: DATE * / /91 * * * : rirrswa no UIGIIAL KI -'1J WUI'.if -.. PT 7tn rr11rBTP ITD TAP 011CA

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

-6 DISTRIBUTION:

GENERIC LETTER 92-02 -March 6, 1992 Central Fles NRC PDR PDI-1 Reading S. Varga J. Calvo R. A. Capra N. Conicella C. Vogan J. Richardson, 7/D/26 E. Leeds, 13/H/24 M. Boyle, 16/H/3 C. Berlinger, 8/D/22 W. Russell, 12/G/18 J. Partlow, 12/G/18 R. Baer, NLS/302-