ML17311A939

LER 95-007-00:on 950512,determined That Bench Settings of air-operated Letdown & Containment Isolation Valves Adversely Affected Ability of Valves to Perform 10CFR50,App R Function.Affected Isolation Valves modified.W/950609 Ltr

ML17311A939
Person / Time
Site:	Palo Verde
Issue date:	06/09/1995
From:	Grabo B, James M. Levine ARIZONA PUBLIC SERVICE CO. (FORMERLY ARIZONA NUCLEAR
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
192-00934-JML-B, 192-934-JML-B, LER-95-007, LER-95-7, NUDOCS 9506130452
Download: ML17311A939 (19)

Text

PRIG RITY (ACCELER:%TED RIDS PROCI'.SSIiC~

REGULATORY INFORMATION DISTRIBUTION SYSTEM (RIDS)

ACCESSION NBR:9506130452 DOC.DATE: .95/06/09 NOTARIZED: NO DOCKET FACIL:STN-50-528 Palo Verde Nuclear Station, Unit 1, Arizona Publi 05000528 AUTH. NAME AUTHOR AFFILIATION GRABO,B.A. Arizona Public Service Co. (formerly Arizona Nuclear Power LEVINE,J.M. Arizona Public Service Co. (formerly Arizona Nuclear Power P RECIP.NAME RECIPIENT AFFILIATION

SUBJECT:

LER 95-007-00:on 950512,determined that bench settings of air-operated'etdown & containment isolation valves adversely affected ability of valves to perform 10CFR50,App R function. Affected isolation valves modified.W/950609 ltr.

DISTRIBUTION CODE: IE22T COPIES RECEIVED:LTR ) ENCL I SIZE:

TITLE: 50.73/50.9 Licensee Event Report (LER), Incident Rpt, etc.

NOTES:STANDARDIZED PLANT 05000528 RECIPIENT COPIES RECIPIENT COPIES ID CODE/NAME'D4-2 LTTR ENCL ID CODE/NAME LTTR ENCL PD 1 1 HOLIAN, B 1 1 TRAN,L 1 1 INTERNAL: ACRS 1 1 2 2 AEOD/SPD/RRAB NRR/DE/ECGB 1 1 @i% ~m~s 1 1 1 1 NRR/DE/EELB 1 1 NRR/DE/EMEB 1 1 NRR/DISP/PIPB 1 1 NRR/DOPS/OECB 1 1 NRR/DRCH/HHFB 1 1 NRR/DRCH/HICB 1 1 NRR/DRCH/HOLB 1' NRR/DSSA/SPLB 1 1 NRR/DSSA/SPSB/B 1 1 NRR/DSSA/SRXB'GN4 1 1 RES/DSIR/EIB 1 1 FILE 01 1 1 EXTERNAL: L ST LOBBY WARD 1 1 LITCO BRYCE,J H 2 2 NOAC MURPHY i G A 1' NOAC POORE,W. 1 1 NRC PDR 1 1 NUDOCS FULL TXT 1 1 U

N NOTE TO ALL"RIDS" RECIPIENTS:

PLEASE IIELP US TO REDUCE iVASTE! CONTACT I'HE DOCI.'MENT CONTROL DESK, ROONI PI-37 (ENT. 504-ZOS3 ) TO ELDIINATEYOI:R NAXIE FROil DISTRIBUTION, LIS'I'S I:OR DOCUXIEN'I'S 5'Ol')ON "I'I I:DI FULL TEXT 'CONVERSION REQUIRED TOTAL NUMBER OF COPIES REQUIRED: LTTR 28 ENCL 28

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Arizona Public Service Company

'PALO VERDE NUCLEAR GENERATING STATION

'P.O. BOX 52034 ~ PHOENIX. ARIZONA 85072-2034 JAMES M. LEVINE 192-00934-JML'/BAG/RAS VICE PRESIDENT

'NUCLEAR PRODUCTION June 9, 1995 U. S. Nuclear Regulatory Commission ATI N: Document Control Desk Mail Station P1-37 Washington, DC 20555-0001

Subject:

Palo'.Verde Nuclear, Generating Station (PVNGS)

'Units 1, 2, and 3 Docket Nos. STN 50-528/529/530 License Nos. NPFQ1/51'/74 L'icensee Event Report 95-007-00 Attached please find Licensee. Event Report (LER) 95-007-00 prepared and,submitted pursuant to 10CFR50.73. This L'ER reports a determination by APS that low bench settings for letdown/containment isolation air operated valves adversely affected the ability of the valves to perform their '10 CFR 50 Appendix R safety function to isolate letdown. In accordance with 10CFR50.73(d), a copy of this LER is being forwarded.to the Regional Administrator, NRC Region IV.

If you have any questions, please contact Burton A. Grabo, Section Leader, Nuclear Regulatory Affairs, at (602) 393-6492.

Sincerely; JML/BAG/RAS/pv Attachment cc:. L. J. Callan (all with attachment)

K. E. Perkins K. E. Johnston INPO Records Center Bo~~~o"52 q5obo9 3y',

pDR AOOCK ~5ooo528 0

PDR

11 LICENSEE EVENT REPORT (LER)

ACILITYNAME (I) DOCKET NUMBER (2) PAGE (3)

Palo Verde Unit 1 0 5'0 0 0 5 2 8 1 oF0 8 LE (4)

Adverse Affect of Low Bench Set on Fisher Air 0 crated Letdown/Containment Isolation Valves EVENT DATE S LER NUMBER 6 REPORT DATE OTHER FACILITIES INVOLVED 8 MONTH YEAR <<<<e'i: SEOVENTIAL;~c><<RE(43aON MONTH FACN/IYNAMES NVMBERS NUMBER .(.". NUMBER Palo Verde Unit 2 0 5 0 0 0 5 2 9 0 5 1 2 9 5 9 5 0 0 7 0 0 0 6 0 9 9 5 Palo Verde Unit3 0 5 0 0 0 5 3 0 REPORT IS SUBMITTEDPURSUANT TO THE REOUIREMENTS OF 10 CFR C (Check one or more ot the following) (11) 20.402(ti) 20.405(c) 50.73(s)(2)(n) 73.71(b) 20 405(a)(1) RL38(c)(t) 50.73(a)(2)(<<) 73.71(c)

LEYEL(to) P 0 P (1)(i) 0'0.405(a) 50.38(c)(2) 50.73(a)(2)((4<<)

X OTHER(~

20.405(s)(1)(ia) 9L73(a)P)0( 5073(a)(2)( ")(A) bebw snd ln T(oL NRC Fo(m 20.405(a)(t)(n) 50.73(a)(2)(<<<<) 50.73(a)(2) (rrii)(8) 20.405(a)(t)(<<) 9173(a)(2)(m) '0.73(a)(2)(i0 T.S. 6.9.3 UCENSEE CONTACT FOR THIS LER (1 2)

E E NUMBER Burton A. Grabo, Section Leader, Nuclear Regulatory Affairs 6 0 2 3 9 3 - 6 4 9 2 COMPLETE ONE UNE FOR EACH COMPONENT FAILURE DESCRIBED IN THIS REPORT (1 3)

MANUFAC- REPORTABLE ~g<<<<)<<<<::.':.@<<(,<i%!::i"'AUSE SYSTEM COMPONENT MANUFAC. REPORTABLE 'i:c<<<<;.<<!g;I<< .',, <<,;iy TURER TURER TO NPRDS C B I S V F 1 3 0 SUPPLEMENTAL REPORT EXPECTED (14)

((<<<<<<<<e <<<<<<0 e<<<<<<<< i<< ~ i~ <<<<e<<e<<c<< ~

YES 0I Tea, complete EXPECTED SUBMISSION DATE)

<<<<<<<<i (1()

DATE (15)

On May 12, 1995, Unit 1 was in Mode 5 (COLD SHUTDOWN) during, a scheduled refueling outage, and Units 2 and 3 were in Mode 1 (POWER OPERATION), operating at approximately 100 percent power, when APS Engineering personnel determined that the bench settings of the air-operated letdown and containment isolation valves adversely affected the ability of the valves to perform their 10 CFR 50 Appendix R safety function to isolate letdown. APS Engineering determined that during postulated fires in fire zones outside of the control zoom, a condition could exist in which the letdown line would, not be effectively isolated in accordance with the existing Pre-Fire Strategies, and as required by 10 CFR 50 Appendix R.

Preliminary evaluation indicates the cause of the event was attributed to the failure of the valve manufacturer to accurately calculate packing friction when determining bench settings.

Letdown isolation valves in Unit 1 have been modified to provide the necessary seating force, and compensatory measures have been implemented in Units 2 and 3 which provide direction for isolating letdown during various fire scenarios.

There have been no previous similar events reported pursuant to 10 CFR 50.73.

0 I

LICENSEE EVENT REPORT (LER) TEXT CONTINUAT(ON ACIUTYNAME DOCKET NUMBER LER NUMBER PAGE YEAR SEQUENTIAL ~~ EVISIO NUMBER NUMBER PALO VERDE UNIT 1 0 5 0 0 0 5 2 8 9 5 0 0 7 0 0 0 2 OF 0 8 1 ~ EVENT CLASSIFICATION This LER 528/529/530/95-007 is being submitted pursuant to Technical Specification (TS) 6.9.3 which states "Violations to the requirements,of the fire protection program described in the Final Safety Analysis Report which would have adversely affected the ability to achieve and maintain safe shutdown in the event of a fire. shall be reported in accordance with 10 CFR, 50.73."

In addition, this event also meets the reporting requirements of 10 CFR 50.73 (a)(2)(vii), which states: [Licensees shall report] "Any event where a single cause or condition caused at least one independent train or channel to become inoperable in multiple systems of two independent trains or channels to become inoperable in a single system designed to: (A) Shut down the reactor and maintain it in a safe shutdown condition; (B) Remove residual heat; (C)

Control the release of radioactive material; or (D) Mitigate the consequences of an accident."

Specifically, it was determined that the bench settings of the air-operated letdown and containment isolation valves (BD)(CB)(ISV) adversely affected the ability of the valves to perform their 10 CFR 50 Appendix R safety function to isolate letdown.

2. EVENT DESCRIPTION On May 12, 1995, Unit 1 was in Mode 5 (COLD SHUTDOWN) with reactor coolant system (RCS) (AB) temperature at 95 degrees Fahrenheit (F) and at atmospheric pressure, and Units 2 and 3 were in Mode 1 (POWER OPERATION), operating at approximately 100 percent power, when APS Engineering personnel (utility non-licensed) determined that the bench settings of the air-operated letdown and containment isolation valves adversely affected the ability of the valves to perform their 10 CFR 50 Appendix R safety function to isolate letdown. APS Engineering determined that during postulated fires in fire zones outside of the contzol room (NA) a condition could exist in which the letdown line would not be effectively isolated in accordance with the existing Pre-Fire Strategies, and as required by 10 CFR 50 Appendix R.

The following Units 1, 2 and 3 isolation valves were affected by this event:

CHB-UV-515 Upstream Containment Isolation Valve, CHA-UV-516 Downstream Containment Isolation Valve, and CHB-UV-523 Outside of Containment Letdown Isolation Valve.

On April 14, 1995, during Local Leak Rate Testing (LLRT) the as-found leakage rate foz Unit 1 air-operated valve (AOV) CHB-UV-523 was quantified at 24, 631 standard cubic centimeters per minute (sccm), while acceptance criteria is

</= 500 sccm. The previous LLRT which had been performed on September 27, 1993, had resulted in a leakage rate of only 22 sccm for this same valve.

Subsequent diagnostic testing using a Fisher "Flow Scanner" indicated the valve could not achieve adequate seating force with the vendor recommended bench setting of 22-38 pounds per square inch gauge (psig).

Ib LICENSEE EVENT REPORT (LER) TEXT CONTINUATION ACIIllVNAME DOCKET NUMBER LER NUMBER PAGE YEAR SEOUENTIAL 'g)g EVISIO gi.'UMBER .;.,p'UMBER PALO VERDE UNIT 1 0 5 0 0 0 5 2 8 9 5 0 0 7 0 0 0 3 OF 0,8 EXT On April 20, 1995, APS Engineering personnel performed a preliminazy calculation to determine whether the bench setting recommended by Fisher Valves for the letdown/containment AOVs was adequate to achieve seat leakage requirements. The preliminary calculation indicated the existing bench set setpoints were too low and none of these valves could meet seat leakage requirements at full system pressure of 2485 psig. Another preliminary calculation was performed to determine the bench set pressure required to achieve shut-off at 2485 psig system pressure. The calculation revealed the low bench setpoint would have to be 38 psig and the upper setpoint would have to be 58 psig to achieve shut-off at 2485 psig.

APS Engineering personnel contacted, Fisher Valves and zequested a preliminary calculation to determine if the 667 DBQ/60 actuator could be set up to the 38-58 psig bench set without compromising the valve spring (in terms of coil interference). Fisher Valve's evaluation indicated the model 667 DBQ/60 actuatoz could not withstand a 38-58 psig bench set without compromising the associated spring.

On May 11, 1995, APS Engineering personnel met to determine the possible consequences of the 667 DBQ/60 actuators'nability to achieve shutoff at a system pressure of 2485 psig, and the affect to the operating Units 2 and 3.

The APS Engineering team determined .through calculations that two valves acting in series are capable of closing against the differential pressure associated with a break of the letdown line. Further, the APS Engineering team determined that all safety functions for these valves were capable of being performed except for the isolation of the letdown line during a postulated fire in certain fire zones out-side containment where only a single valve may be available, assuming worst case single active failures.

There were no safety system responses as a result of this event and none were necessary.

3. ASSESSMENT OF THE SAFETY CONSEQUENCES AND IMPLICATIONS OF THIS EVENT APS Engineering personnel determined the following safety functions are performed by the affected containment/letdown isolation valves: 1)

Containment Isolation (CHA-UV-516 and CHB-UV-523); 2) Mitigation of UFSAR Chapter 15 Letdown Line Break Outside Containment (NH) Event (CHB-UV-515 and CHA-UV-516); 3) Isolation of a high energy line break (HELB) of the letdown line in the Auxiliary Building (NF)(CHB-UV-515 and CHA-UV-516); 4) isolation of the letdown line for Reactor Coolant inventory control for 10 CFR 50 Appendix R fire scenarios (CHB-UV-515, CHA-UV-516 and CHB-UV-523); and 5)

Isolation of the letdown line upon receipt of a Safety Injection Actuation Signal (BP/BQ)(JE) (CHB-UV-515 and CHA-UV-516).

Based upon the safety functions performed by the valves, APS Engineering personnel assessed the impact of low bench set and determined isolation could be provided by a series combination of the isolation valves. The net system leak rate was then determined as a function of RCS pressure, based upon .the two valve isolation. These analyses assumed that foz a letdown line break discharging to the Auxiliary Building atmosphere, two of the three valves in series were operable to provide isolation.

0 LICENSEE EVENT REPORT (LER) TEXT CONTINUATION ACIUTYNAME DOCKET NUMBER LER NUMBER PAGE SEQUENTIAL -,:+ EVISIO NUMSER NUMBER PALO VERDE UNIT 1

'0 5 0 '0 0 5 2 8 9 5 0'0 7 0 0 0 4 OF 0 8 Additional evaluations were completed to assess the consequences of single valve isolation, considering low bench set. These analyses provide necessary input for parallel Auxiliary Building HELB and 10 CFR 100 evaluations.

Two Valve Isolation Evaluation:

Two valves in series will function to provide isolation at a higher relative pressure differential than that achieved by a single valve.

The effect of the downstream valve of the two valve pair is to impose a higher back-pressure on the upstream valve. The corresponding reduced pressure differential across the first valve results in the valve closing and a larger seat force after the valve plug is engaged in the seat. The analyses show that series isolation valves will provide adequate seat force to restrict the net leakage thzough the letdown line to less than 1 gpm under pressure differential conditions consistent with the design rating of the letdown line.

Single Valve Isolation Evaluation:

This condition is applicable for a postulated letdown line break upstream of the outboard containment isolation valve with concurrent single failure of either of the other two inboard isolation valves. The analyses show that a single isolation valve with a bench set of 22 psig will close at a differential pressure of 1878 psig. Therefore, assuming atmospheric conditions immediately downstream of the single isolation valve, RCS pressures above 1878 psig will result in flow through the valve. A conservative mass flow rate was established for this condition and subsequently used as input in the 10 CFR 100 and HELB analyses.

Consequences of a HELB at the Containment Penetration:

A terminal end break at the containment penetration upstream of CHB-UV-523 in conjunction with a single failure of CHB-UV-515 to close results in the single valve isolation scenario. An evaluation using the conservative mass flow rate determined for attempted single valve isolation at 22 psig bench set resulted in acceptable off site dose consequences significantly below those required by 10 CFR 100.

The current design basis HELB analysis assumes ten minutes at the full break blow down rate prior to the operator remote manually closing the letdown isolation valve. The scenario considered the inadequate isolation provided by the low bench set of the valves and assumes 15 additional minutes at a lower flow rate for the operators to assess the inadequate isolation. At this time, the operators commence cooldown, and 1.93 minutes latex the RCS has depressurized to 1878 psig, zesulting in complete closure of the isolation, valve and termination of the letdown line blow down 'to the Auxiliary Building.

0 41 LICENSEE EVENT REPORT (LER) TEXT CONTINUATION AGILITYNAME DOCKET NUMBER LER NUMBER PAGE

') SEQUENTIAL REYISIO NUMBER NUMBER PALO VERDE UNIT'1 0 5 0 0.0 5 2 8 9 5 0 0 7 0 0 0 5 oF0 8 EXT Radiological Consequences:

The radiological consequences of this event aze documented in calculation 13-NC-ZY-249, Revision 0. The calculation assumes a total RCS flow for the event prior to complete isolation at 27 minutes following event initiation of 14,700 pounds per minute. The analysis concludes that the total RCS release to the Auxiliary,Building during the 27 minutes prior to complete isolation would result in a dose of 7.2 Rem at the exclusion area boundary . I The analysis is based on a specific activity of l microcurie,per cubic centimeter I-131 equivalent in the RCS. The specific activity is the maximum allowable per TS 3/4.4.7. Standard Review Plan 15.6.2 states that the consequences of this event are acceptable if the resulting dose does not exceed a small fraction (10 percent) of 10 CFR 100 guidelines. The 10 CFR 100 guideline is 300 Rem. Ten percent of this is 30 Rem. Therefore the additional leakage would not result in exceeding the applicable criteria.

Auxiliary Building Environment:

In addition, an analysis was conducted to determine the effects of .this additional RCS release on the Auxiliary Building environment to ensure no safety related equipment was adversely affected. The results of this analysis indicate that the additional, lower maximum mass flow rate during the 16.93 minutes following the initial 10 minute release currently postulated did not result in a more adverse environment. The flow rate was low enough that the building began to cool following partial isolation at 10 minutes, although at a slower rate than with total line bzeak isolation.

Appendix R Fire Hazards Analysis:

To ensure letdown isolation as assumed in the fire hazards analysis, two,valves must be shown to be available for a fire in any analysis area. The current control room fire strategy calls for closing CHB-UV-515 and'ecuring the nuclear cooling water pumps, which will cause CHB-UV-523 to close, from outside the control zoom. Therefore, two valves are closed and the isolation function can be performed with the current bench sets. The fire strategy for fires outside the control room calls" for closing either CHB-UV-515 or CHA-UV-516, dependent on the analysis area and corresponding A or B train circuits potentially affected.

Therefore, only one valve can be currently assumed to close for analysis areas outside the control zoom.

LICENSEE EVENT REPORT (LER) TEXT CONTINUATION ACIUTYNAME DOCKET NUMBER LER ,PAGE

++'VISIO NUMBER'.;g.",

SEOUENTIAL NUMBER;,<A'UMBER PALO VERDE UNIT 1 0 5 0 0 0 5 2 8 9 5 0 0 7 0 0 0 6 OF 0 8 Compensatory measures, as. described in the pre-fire strategies manual,

'have been implemented which prescribe certain operator actions that must be performed in the event of a fire outside 'the control room to ensure two letdown isolation valves are closed. Essentially the action is to open the disconnect switch, which fails the valve closed, at the specified auxiliary relay cabinet for the valve whose circuits are potentially affected. Otherwise the operator can manually close/ensure close the subject letdown valves from the control room for analysis areas where the letdown valve circuits aze not affected by fi're.

The analyses and compensatozy measures described above demonstrate the vendor recommended low bench set for the letdown and containment isolation valves on the letdown line does not represent a deficiency in design which could result in the loss of a safety function. Calculations have determined that two valves acting in series are capable of closing against the differential pressure associated with a break of the letdown line. Accordingly, all safety functions are capable of being performed except for isolation of the letdown line during a postulated fire in certain fire zones outside. containment where only a single valve may be available assuming worst case single active failuzes.

Compensatory measures have been implemented in accordance with 10 CFR 50 Appendix R which prescribe certain operator actions that must be performed in the event of a fire outside the control room. The actions will ensure two letdown isolation valves are closed, thereby ensuring performance of this safety function.

This event did not result in any challenges to the fission product barriers or result in any releases of radioactive materials. There, were no adverse safety consequences or implications as a result of the event. This event did not adversely affect the safe operation of the plant or the 'health and safety of the public.

4 ~ CAUSE OF THE EVENT A preliminary evaluation indicates the cause of the event was attributed'o a vendor oversight in that valve packing resistance was not fully considered when the bench settings of the AOVs were determined (SALP Cause Code B:

Design, Manufacturing, Installation Error). An investigation of the event is underway in accordance with the APS Corrective Action Program.

evaluation results differ from this determination or if information is If the developed which would affect the readers understanding or perception of the event, a supplement to this report wil'1 be submitted to describe the final root cause of failure.

No unusual characteristics of the work location (e.g., noise, heat, poor lighting) directly contributed to this event. There were no procedural errors which contributed to this event. There were no personnel errors which contributed to this event.

LICENSEE EVENT REPORT (LER) TEXT CONTINUATION ACIUTYNAME DOCKET NUMBER LER NUMBER PAGE YEAR SEQUENTIAL ?...? EVISIO NUMBER cxp<j NUMBER PALO VERDE UNIT 1 0 5 0 0 0 5 2 8'9 5 0 0 7 0 0 0 7oFO 8

5. STRUCTURES SYSTEMS AND COMPONENTS The three Unit 1 isolation valves were not required to be operable when the condition was identified. Unit 2 and 3 isolation valves were, and have remained, in sezvi ce.

The failure mode of the isolation valves is the potential foz through-seat leakage at a differential pressure of 2485 psig. The mechanism of the failed component is the bench setup of the valve operator. The effect of through-seat leakage is that the valves cannot be solely relied upon to isolate letdown flow. No failures that rendered a train of a safety system inoperable were involved.

The low bench settings were discovered after the LLRT of Unit 1 CHB-UV-523 was quantified at 24, 631 sccm. Subsequent diagnostic testing using a Fisher "Flow Scanner" indicated the valve could not achieve adequate seating force

,with the vendor recommended bench setting of 22-38 pounds per square inch gauge (psig).

All of the valves affected by this event are 2" globe valves with Fisher model 667 DBQ/60 actuatozs. The following is a brief description of the valves'unctions.

CHB-UV-515 Upstream Containment Isolation Valve This air diaphragm open, spring-closed globe valve provides for letdown isolation, system protection, and emergency safety featuzes.

closed manually in the control room or on the remote shutdown panel, It may be or, upon receipt of a high-high regenerative heat exchanger (CB) outlet temperature of 450'F or a Safety Injection Actuation Signal (SIAS),

this valve is closed automatically. The, valve fails closed on loss of electrical power or air.

shutdown panel (ZU).

It also has a disconnect switch on the remote CHA-UV-516 Downstream Containment Isolation Valve This air diaphragm open, spring-closed globe isolation valve, provides for letdown isolation and emergency safety features. Zt may be closed manually in the control room or on the remote shutdown panel, or, upon receipt of a Containment Isolation Actuation Signal (CIAS)(BD)(JE) or SZAS, this valve is closed automatically. The valve fails closed on loss of electrical power or air.

CHB-UV-523 Outside Containment Letdown Isolation Valve This air diaphragm open, spring-closed globe valve provides letdown isolation and safety features. It may be closed, manually, or, upon receipt of a CIAS or a low nuclear cooling water (CC) flow of 39 gpm from the letdown heat exchanger, it is closed automatically. The valve fails closed on loss of electrical power or air.

IP LICENSEE-EVENT REPORT (LER) TEXT CONTINUATION ACIUTYNAME DOCKET NUMBER LER NUMBER PAGE YEAR ;<Pp SEQUENTIAL EVISIO NVMSER NVMSER PALO VERDE UNIT 1 0 5 0 0 0 5 2 8 9 5 0 0 7 0 0 0 8 OF 0 8 None of the valves'unctions were affected by the bench settings except for the isolation of the letdown line during a postulated fire in certain fire zones outside containment where only a single valve may have been available.

No safety systems were declared inoperable as a result of the event.

6. CORRECTIVE ACTIONS TO PREVENT RECURRENCE Unit 1:

The effected isolation valves in Unit 1 were modified by reducing the stroke length of the actuator, replacing the existing spring with a stiffer 3320 pound per foot spring, modifying the travel stops and increasing the bench settings. The new configuration will meet or exceed all requirements for letdown system isolation.

Units 2 .and 3:

Compensatory measures, as described in the pre-fire strategies manual, have been implemented which prescribe certain operator actions that must be performed in the event of a fire outside the control .room to ensure two letdown isolation valves are closed. Essentially, the action is to open the disconnect, switch, at the specified auxiliary relay cabinet foz the valve whose circuits aze potentially affected. This action causes the valve to fail closed. Otherwise the operator can manually cl'ose/ensure close the subject letdown valves from the control room for areas where the letdown valve circuits aze not affected by fire.

These compensatory measures will be effective until the remaining Unit 2 and 3 valves, can be modified.

7~ PREVIOUS SIMILAR EVENTS There have been no previous similar events reported pursuant to 10 CFR 50.73 in the last three years, that were attributed to the AOV bench settings.

It P