ML20070T557

Responds to NRC Request for Browns Ferry Unit 2 Dominant Core Damage Sequence Success Criteria.Success Criteria for 10 Core Melt Scenarios,Encl

ML20070T557
Person / Time
Site:	Browns Ferry
Issue date:	04/02/1991
From:	Wallace E TENNESSEE VALLEY AUTHORITY
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
NUDOCS 9104040275
Download: ML20070T557 (21)

Text

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,_,_,~,_..-,._m APR 02 (991 U.S. Nuc19ar Reguletory Cocuales'un ATTN: Document Control Deck Washington, D.C. 20$$$

centlement In the Patter of ) Docket Nos. 50-259 Tennoncee Valley Authority ) $0-260

) $0-296 BROWHS IERRY NUCLEAR PLANT (BFN) - PROBAU1LISTIC RISK ASSESCHENT (pkA)

SUCCESS CRITERIA FOR DOMINANT SEQUENCES This letter is in responso to the NRC'c requent for Browns Ferry Unit 2 dominant core damage coquence cucceso critoria (Reference 1). TVA has enclooed the cucconc etitoria for the 10 core melt scenartoo which were identiflod by the latent (September 1987) draft UFN pkA as having the highest probability of occurrence. Each PRA ocenario concidern the succeso or failure of a series of systems, equipment, or operator actions. The initiating event and the subsequent failure of cafety functiono lo sunnarized at the beginning of. cach scenario. Safety functions which are assumed to perform adequately are annotated an succcesco in the right margin. Safety functions which are assumed not to perform adequately are annotated as failures, The September 1987 draft BFW pRA was oci t tnally baned on the denign on BFN Unit 1, circa 1984/1985, Many conservatismo and apuumptions woro incorporated which, if more rigorounty evaluated, could result in more realistic models and would tend to decreaun the calculated overall core damage frequency. The draft pRA ovaluaten events which are beyond the design bases of the plant. Therefore, direct comparison of the succeso criteria may not correspond to the way these accident scenarloo are treated in the current UFN Final Safety Analysis Report, Safo Shutdown Analysis, or Emergency Operating procedures.

The ten BFN cominant ooquences described in the enclosure were previously provided to URC in Reference 2. The purpose of that submittal was to provide information requested by the NRC Staff to support the concluulonc l l that the draft BFN pRA will concorvatively reficct the configuration of Unit 2 at the time of restart and that BFN is not an outlier with reop ct to severe accident characteristics when compared to plants of similar 2 type and vintage. NRC cubsequently audited the draft UFN pRA, including 9104040275 910402 PDR P ADOCK 05000259

. . eon gt> i k

APR 0 Z 1991 U.S. Nuclear Ragulatory Conuniusion these dominant sequences and their success criteria, and provided restart and post-restart conunente to TVA for resolution (References 3 and 4).

TVA responded to the NpC's restart comment in Referenco 5. In Reference 6, NRC stated that the restart concerns were rdequately resolved and concurred with TVA's conclusion that the Brownn Ferry fuellity was not an outlier with respect to core melt frequency and that the draft. UFN pHA results were acceptable for th,. restart of Unit 2.

NRC stated in Reference 4 that the posterectart comments were provided for TVA's use in the continued refinement of the draf L BFN pRA and in TVA's efforto to respond to Generic Letter 88-20, Individual plant Examination for Severo Accident vulnerabilition. In response to Generic Letter 88-20 (keferente 7), TVA committed to compicto a Level 1 pRA and containment analycle by September 1, 1992 for Brownu Ferry. This updated UPN pRA will natisfy the criteria of Generic Letter 88-20 and NUREG 1335.

This updated pRA 10 incorporating ituproved pkA methodologica ao well as changen to the UPN plant configuration and operating proceduroc and will more accurately reflect UFN Unit 2. In the procorn of completing this update, new dominant sequences may be identified which may or may not include the current oequences.

In Reference B. NRC requented TVA provido a dependency matrix which showed the reliance of safety related equipment and equipment itnport. ant to cafety on normal and emergency power supplico und support systems.

TVA developed thin information baced on current plant design and provided PRA based dependency matrices to NRC % Reference 9.

In cununary, NRC han concurred with TVA that the Browna Ferry f acility was not an outlier with roepect to core moit. frequency and that the draf L BFN PRA results were acceptable for the restart of Unit 2. The draft. BFN pHA is conservative and in in the proceno of being updated to more accurately refIcet the pie.ht..

There are no commitmentn contained in this letter. If you have any questiono please contact patrick P. Carter, Manager of Site Licensing, at.

(205) 729-3570.

Vnry truly youro.

TENNESSEE VALhEY AUTil0RITY N W(7)}% y E. C. h face,/Hanage Nuclear Licencing and Regulatory Affalta 1 En"tiSuro cc: See page 3 I

U

.

• l 5

A(PR 021991 U.S. Nuclear Regulatory Commission cc (Enclosurec):

Ms. S. C. Black, Deputy Director Project Directorate 11-4 U.S. Nucicar Regulatory Commission One White Flint, North 11555 Rockville Pike, Rockville, Maryinnd 20852 NRC Resident inspector Browns Ferry Nuclear Plant Route 12, Box 637 Atheno, Alabama 35609-2000 a Mr. Thierry M. Ross, Project Manager U.S. Nuclear Regulatory Commission One White 711nt, North 11555 Rockville Pike Rockville Maryland 20852 l Mr. B. A. Wilson, Project Chief U.S. Nucicar Regulatory Comission Region 11 101 Harletta Street, NW, Suite 2900 Atlante, Georgia 30323 4AE

l  ;

! REFERENCES 1

d 1) WRC letter, dated December 26, 1990 Success Criteria for Dominant Soquinces Leading to Core Damage at Browns Ferry Nucicar l Plant Unit 2 a

2) TVA letter, dated August 25, 1988, Probabilistle Risk Assessment -

Summary Report ,

1

3) NRC letter, dated March 29, 1989, Volumo 3, Section 111.1. 0

(Probabilistic Risk Assessment) of the Nucicar Performanco Plan -

browns Ferry Nuclear Plant, Unit 2

4) NRC letter, dated July 10, 1989, Addittorial NRC Staf f Audit insights Pertaining to the Browns Ferry Risk Review - Generic l Letter 88-20

5) TVA letter, dated June 15, 1989, Nucioar Regulatory Commiselon Audit of TVA's Probabilistic Risk Assessment

6) NRC letter, dated January 23, 1991, NUREC-1232, Volume 3 Supplement 2 - Browns Ferry, Unit 2

' 7) TVA letter, dated October 30, 1989, Proposed Program in Response to Generic Letter 88 Individual Plant Examinailon for Severe Accident Vulnerabilities

8) NRC letter, dated June 6, 1990, intorrelated Safety System Dependencios

9) TVA lotter, dated October 1, 1990, Interrelated Safety System Dependenclos ,

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1

. Page 1 of 17 ENCLOSURE BROWNS FERRY NUCLEAR PLANT i

SUCCESS CRITERIA SDRU INFORMATION -

SCENARIO 1 Loss of feedwater followed by failure of HPCI and RCIC, and failure of manual ADS blowdown (all support available)

Suberiticality Automatic scram on low reactor water level (success)

Vessel isolation j EHC System (all stop valves or all control (success) ,

valves and bypass valves)

RCS Overpressure Protectiog_

(not required)

Coolant Makeup (hilh_ pressure)

HPCI or (failurc)

RCIC Depressurization Manual ADS blowdown by opening four of (failure) six SRVs i

' Coolant flakaup (low pressurel (Not effective -' vessel at high pressure)

' Containment Protection .

Initiate torus cooling per procedures (see decay heat removal function below).

Page 2 of 17 ENCLOSURE BROWNS FERRY NUCLEAR PLANT SUCCESSCRITERIA

SUMMARY

INFORMATION l

l SCENARIO: 1 (continued)

' Decay ifeat Removal (torus _coolin_g]

One of four RilR pumps and one of two associated  ;

RilRSW pumps and .

Alignment to torus cooling mode

• These functions do not af fect the f requency of core darnage f or sequence 1.

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_ . _ _ _ _ . . . . . , . _ . . . ~ - _ . - - _ , . . . _ _ , . , _ . -_ _ - .

Page 3 of 17 ENCLOSURE BROWNS FERRY hTC1. EAR Pl. ANT SUCCESS CRITERIA SDKM INFOL9.II05 SCENARIO: 2 Sn.all LOCA with failure of torus cooling (nil support available)

Suberiticality Autamatic scram on high drywell pressure or (success) low reactor water level RCS Overpressure Protection (not required)

Rx Overfill Protection All three MFWS pumps trip on high reactor water (succesa) level Coolant Makeup _(hi_gh pressure)

IIPCI or (success)

RCIC

'Depressurization (No additional depressurir.ation required)

Coolant Make_up_(low pressure)

One loop (both pumps in loop) of core spray (success)

Containment Protection Initiate torus coci;ng per procedures (see decay heat remaval function below)

. . . _ . . _ _ _ . _ . _ _ _ _ _ . . . _ _ _ . . _ . _ _ _ - . _ _ . _ . _ - ._ ~.__.__m..__...__. __

j . .

1 j Page 4 of 17

. ENCLOSURE BROWNS FERRY NUCLEAR PIANT j Sl100ESSCRITERIAStMRYINFORMATION 1

SCENARIO: 2 (continued)

Decay Heat Retuoval (torus _. cooling]

i One of four RHR pumps and one of two associated RHRSW purnpa and (fallure)

Aligntnent to torus cooling mode

• The subject pipe break is assumed to be of sufficient size to assist EPCI or RCIC operation in reducing ptessure such that the low pressure injection systems can provide core cooling.

Page 5 of 17 ENCLOSURE PROWNS FERRY NUC'. EAR 11 ANT SECCESS CRI 3 iA S H E Y IhiO M IION

'SCLNknIOb; and 9 Closure of all MSIVs followed by failure of IIPCI, RCIC, and manual ADS blowdown (all support available)

};beriticality Automatic scram on MSIV position or high (success) neutron flux RCS Overpressure Protection Nine of thirteen SRVs lift (success)

Vessel Isolation All SRVs rescat (success)

Coolant Makeup _(high_ pressure) l(PCI or (failure)

RCIC Depres suri za t ic_tj Manual ADS blowdown by opening four of (failure) six SRVs

• • Coolant Makeup (low pressure)

Not effective - vessel at high pressure

• • Containment Protection Initia e torus cooling per procedures (see decay heat removal function below) i i.A

. Page 6 of 17 ENCLOSURE BROWNS l'ERRY NUCLEAR PLANT SUCCESS CRITERIA S M ARY N ORMATION

' SCENARIOS: 3 and 9 (continued)

'* Decay Heat Removal (torus coolinAl ,

One of four Ri!R pumps and one of two associated RHRSW pumps and Alignment to torus cooling mode +

• The only difference between scenario 3 and scenario 9 is that in the former the Condensate System is availabic and in the latter it is not. The availability of the condensate is not directly relevant in identifying this as a potential core danage scenario.

• • These functions do not affect the frequency of core damage for '

scenarios 3 and 9.

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Page 7 of 17 LNCLOSURE LROWNS FERRY NUCLEAR PLANT SUCCESSCRITERIASM1ARYINFORMION SpENARIO: 4 Transient with subsequent one to three stuck open relief valves followed by failure of torus cooling (all support available)

Suberiticality Automatic scram on low reactor water level (success)

• RCS Overgressure Protection (Not required. The stuck open relief valve depressurizes the reactor vessel.)

Rx Overfill Protection -

All three MFWS pumps trip on high reactor water (success) level Coolant Makeup (high pressure) ,

HPCI (success)

Depressurization (No additional depressurization required) i Coolant Makeup (low pressure)

One loop (both pumps in loop) of core spray (success)

Containment Protection Initiate-torus cooling per procedures (see decay heat removal function below)

Decay Heat Removal (torus cooling)

One of'four RHR pumps and one of two associated RHRSW pumps and (failure)

Alignment to torus cooling mode l

1

*The initiator was a transient in which nine of thirteen SRVs successfully lif ted, but one to three (of thirteen) f ailed to subsequently rescat.

.-- . . _ _ . _ - . _ _ . , _ . _ . _ _ _ _ _ - ~ _ _ _ . . . _ _ . _ _ .- .-._ _

. Page 8 of 17 ENCLOSURE BROWNS FERRY NUCLEAR Pl. ANT SUCCESSCRIIERIASMARYINf0EATION SCENARIO: $ Loss of offsite power followed by failure of IIPCI and RCIC, Manual ADS blowdown is I successful but LPCI. core spray, and torus l cooling fail. (With exception of 4-kV shutdown board A all support is available)

Suberitical{ty Automatic scram on turbine stop valvo position (success) or high neutron finx RCS Overgressure Protection Nine of thirteen SRVs lift (success)

Vessel Isolation All SRVs resent (success)

Coolant Makeup (high_ pressure)

IIPCI or (f ailure )

RCIC

! Depressurisation Manual ADS blowdown by opening four of (auccess) i- six SRVs Coolant Makeup (low pressure)

One loop (both pumps in loop) of core spray ,

or (failur0)

• 0ne of four RilR pumps containment Protection Initiate torus cooling per procedures (see decay heat removal function below)

Page 9 of 17 ENCLOSURE BROWNS FERRY NUCLEAR PLANT SUCCESSCRITERIASLERYINf0RETION SCENARIO: 5 (continued) pe_c.ar Heat Removal (torus cooling)

'Two of four RHR pumps and one of two RHRSW pumps associated with each RHR pump and (failure)

Alignment to the torus cooling mode

• Note: The model strongly couples these two functions.

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. . i Page 10 of 17 ENCLOSURE ,

l BROWNS FERRY NUCI. EAR Pl. ANT SUCCEf1CRITEilA

SUMMARY

INFORMATION SCENARIO: 6 Failure of the turbine pressure regulator closed followed by failure of HPCI, RCIC, and i.anual ADS blowdown (all support available).

Suberiticality Automatic scram on turbine stop valve position (success) or high neutron flux RCS Overpressure Protection >

Nine of-thirteen SRVs lift (success)

Vessel Isolation All SRVs reseat (success)

Rx Overfill Protection All three MFWS pumps trip on high reactor (success) water level Coolant Makeup (high pressure)

HPCI or (failure) l -RCIC j Depressurization Manual ADS blowdown by opening four of (failure) six SRVs

' Coolant' Makeup (low pressure)

(Not effective - vessel at high pressure) l

' Containment Protection

, Initiate torus cooling per procedures.

1 (see decay heat removal function below)

. - _ _ - _ . _ _ ~ . . _ . _ _ - _ - - . _ - . . - _ . _ _ . _ . - _ . . - . . _ . . . . ~ . _ . . _ , . _

._____ ____. ___. m .. - __ . . _ _ . , _ _ _ . . _ _ . _ . . _ _ _ _ _ _ . _ . _ . _ _ _ - - _ . .

Page 11 of 17 ENCLOSURE BROWNS FERRY NUCLEAR PLANT SUCCESSCRITERIASMARYINFORMATION SCENARIO: 6 (continued) ,

' Decay Heat Removal _(torus cooling)

One of four RHR pumps and one of two associated RHRSW pumps and Alignment to the torus cooling mode

• These functions do not affect the frequency of core damage j for scenarlo 6.

l 1

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. Page 12 of 17 ENCLOSURE BROWNS l'ERRY NUCLEAR PLANT ,

SUCCESS CRITERIA SMARY INF0DMION SCENARIO: 7 Loss of condenser vacuum followed by f ailure of IIPCI, RCIC, and manual ADS blowdown (all support available)

Suberiticality Automatic scram on turbine stop va'Ive position (success) or high neutron flux RCS Overpressure Protection Nine of thirteen SRVs lift (success)

Vessel Isolation All SRVs reseat (success)

-Coolant Makeup (high pressure)

HPCI or ( '81ure)

RCIC

.i Depressurization l

Manual ADS blowdown by opening four of (failure) i' six SRVs

' Coolant Makeup (low pressure)- ,

f L (Not effective - vessel at high pressure)

3 i

Page-13 of 17 t

ENCI4SURE BROWNS FERRY NUCLEAR FLANT SUCCESS CRITERIA S M Y INFORMATION SCENARIO: 7 (continued)

' Containment Protection Initiate torus cooling per procedures

, (see decay heat removal function below) j' ' Decay __IIeat Removal (torus cooling}

One of four RitR pumps and one of two associated RilRSW pumps and Alignment to the torus cooling mode

• These functions do not affect the frequency of core damage for scenario 7. ,

i i

Page 14 of 17 ENC LOSURE BROWNS FERRY NUCLEAR PLANT SUCCESS CRIIERIA S M INF0 M IION SCENARIO: 8 Loss of offsite power followed by failure of vessel isolation due to one to three SRVs stuck open. (initially power is not available to 4-kV shutdown board C, but is available to the other three unit 1/ unit 2 4-kV shutdown boards. EECW system failure results in loss of power at the 4-kV shutdown boards not initially failed. All common actuation signals are available).

Suberiticality Automatic scram on turbine stop valve position (success) or high neutron flux RCS_0verpressure Protection Nine of thirteen SRVs lift (success)

Vessel isolation All SRVs rescat (failure)

Coolant Makeup (high pressure) llPCI (success)

Depressurization (No additional depressurization needed)

• Coolant Makeup (low pressure)

One loop (both pumps in loop) of core spray or (failure)

One of four Rl!R pumps

• Containment Protection Initiate torus cooling per procedures (see decay heat removal function below)

Page 15 of 17 ENCLOSURE BROWNS FERRY NUCLEAR PLANT SUCCESS CRITERIA SDNH INF0BIION SCENARIO: 8 (continued)

' Decay Heat Removal _(torus cooling}

Two of four RHR pumps and one of two RHRSW pumps associated with each RHR pump and (failure)

Alignment to the torus cooling mode

• Failure of these functions are guaranteed since loss of EECW results in loss of the supporting diesel generators.

+

Page 16 of 17 ENCLOSURE BROWNS FERRY NUCLEAR PLN4T h

SUCCESSCRITERIA

SUMMARY

INFORMATION SCENARIO: 10 Transient with subsequent one to three stuck open relief valves followed by failure of high ar.d low pressure injection and toras cooling (EECW and electric power support available; no common actuation signals are available)

Suberiticality Automatic scram on low reactor water level (success)

• RCS Overpressure Protection (not required)

Rx Overfill Protection All three MFWS pumps trip on high reactor (cuccess) water level Coolant Makeup (high pressure {

HPCI (fal'ure)

Depressurization Manual ADS blowdown by opening four of (success) six SRVs Coolant Makeup (low pressure]

One loop (both pumps in loop) of core spray or (failure)

• • 0ne of four RHR pumps

= Containment Protection Initiate torus cooling per procedures (see decay heat removal function below).

4 Page 17 of 17 l

ENCLOSURE BROWNS FERRY NUCLEAR ?LANT l SUCCESSCRITERIASlERYINFORMATION  !

I l

SCENARIO: 10 (continued) I Decay _ Heat Removal _(torus cooling]

• • 0ne of four RHR pumps .tud one of two assoc!sted RHRSW pumps and (failure'-

Alignment to the torts cooling mode

• The initiator was a transient in snich nine of thirteen SRVs successfully lif ted, but one to three (of thirteen) f ailed to subsequently reseat.

• • Note: The model strongly_ couples these two functions. t

!