ML20042E981

LER 90-002-01:on 900128,ESF Sys Actuations Occurred Due to Reactor Vessel Level Fluctuations After Manual Scram.Caused by Failure of O-ring on Fluid Inlet Port to Servo Valve for Hydraulically Operated Valve.Valve replaced.W/900430 Ltr

ML20042E981
Person / Time
Site:	Peach Bottom
Issue date:	04/30/1990
From:	Fulvio A PECO ENERGY CO., (FORMERLY PHILADELPHIA ELECTRIC
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-90-002, LER-90-2, NUDOCS 9005070060
Download: ML20042E981 (4)

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, -I PHILADEI.PHIA ELECTRIC COMPANY PEACH IKTi~IDM / SMIC POWER STATION R. D ., Box 208

- X1ta, Pet.aaylvania 17314 rexn monow-Tur etwasi or txc.:tteucs r"7) 4$6-7014 1

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i April 30, 1990

. Docket No. 50-278 Document Control Desk U. S. Nuclear Regulatory Coirmission Washingtoia, DC 20555 {

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SUBJECT:

-Licensee Event Report 1

Peach Bottom Atomic Power Station - Unit 3 i This LER concerns Engineered Safeguards Feature (ESF) actuations-due to.

reactor vessel level fluctuations following a manual scram. This LER has been revised to provide the results of.a failure analysis of a servo valve 0-ring.

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Reference:

Docket No. 50-278 i Report Number: 3-90-002 Revision Number: 01 Event Date: 01/28/90 3 Report Date: 04/30/90 1

Facility: '

Peach Bottom Atomic Power Station l

RD 1 Box 208, Delta, PA 17314 This LER is being submitted pursuant to the requirements of 10 CFR I 50.73(a)(2)(v).

3 Sincerely, I I

P ent Ma r.c: J. J. Lyash, USNRC Senior Resident Inspector T. T. Martin, USNRC, Region I f/

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On January 28, 1990 the Unit 3 reactor was manually scrammed due to a leak of Electrohydraulic Control System (EHC) fluid at the number one main turbine control valve. Reactor coolant level fluctuations following the scram resulted in three Group 11 and III Primary Containment Isolation System (PCIS) actuations. The cause of the EHC fluid leak was failure of an 0-ring on the fluid inlet port to tiie servo valve which controls the number one main turbine control valve. The reactor level fluctuatlons were aggravated by the need for rapid depressurization via the main turbine bypass valves, inability to-restart the 'C' reactor feedwater pump (RFP) and the subsequent batch feeding of reactor coolant via the High Pressure Coolant Injection (HPCI) and Reactor Core Isolation Cooling (RCIC) systems. Inability to resec the 'C' RFP was due to a RFP turbine hydraulic control problem. The cause of this problem was the lock nut on the RFP interlock dump valve setting adjustment bolt becoming unsecured due to system vibration. No actual safety consequences occurred as a result of this event. The EHC fluid leak was stopped and the leaking servo j valve was replaced. Investigation into the servo valve 0-ring failure could not

! determine a plausible root cause. This failure is considered an isolated occurrence l and not systematically recurring in nature. The RFP interlock dump valve lock nuts on both units will be mechanically secured. No previous similar LERs were identified.

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! Requirements for the Report J

This report is required pursuant to 10 CFR 50.73(a)(2)(iv) due to automatic and '

manual actuation of various Engineered Safety feature systems.

Unit Status'at Time of Event Unit 3 was in the Run Mode at 100 percent power. .

Description o'f-the Event On January 28,'1990 at 0854 hours0.00988 days <br />0.237 hours <br />0.00141 weeks <br />3.24947e-4 months <br /> the Unit 3 reactor power wastlowered'to 50 percent-and manually scrammed due to a leak of Electrohydraulic Control. System (EHC) fluid at.

the number one main turbine control valve.. Reactor water level then decreased to the

, minus eight. inch' level resulting in.an anticipated Group.II and III Primary Ccatainrent Isolation System (PCIS) actuation.. The main turbine was manually  ;

tripped, feactor level quickly recovered and at 0858 hours0.00993 days <br />0.238 hours <br />0.00142 weeks <br />3.26469e-4 months <br /> reached the plus forty

• five incn ievel.resulting in automatic' trip of the 'C reactor-feedwater pump (RFP).

The PCTS actuation was reset. At.0900 hours0.0104 days <br />0.25 hours <br />0.00149 weeks <br />3.4245e-4 months <br /> the turbine bypass valves were manually.

opened appt m imately 25 percent to provide depressurization_of the reactor. The 'C' ,

RFP could not-be restarted so the Reactor Core Isolation-Cooling System (RCIC) was '

manually started to control reactor-level.. However by this' time, 0909: hours, reactor -

level dropped to the minus three inch level resulting in a second Group 11 and III-PCIS actuation. The High Pressure Coolant Injection System (HPCI) was then utilized intermittently to assist in controlling reactor level.- The PCIS actuation was again reset. At 0933 hours0.0108 days <br />0.259 hours <br />0.00154 weeks <br />3.550065e-4 months <br /> reactor level decreased to the minus-2 inch level resulting in

) a third Group II and III PCIS actuation. By 0936 hours0.0108 days <br />0.26 hours <br />0.00155 weeks <br />3.56148e-4 months <br /> the Main Steam Isolation l -Valves were manually closed, reactor level had recovered HPCI and the EHC pump were t l- secured.

1 Cause of the Event The EHC fluid leak was caused by failure of an 0-ring on the fluid inlet port to the .l l servo valve for hydraulically operated valve H0-5275A which controls the number one I

! .l main turbine control valve. Investigation into the servo valve 0-ring failure could l

! l not determine a plausible root cause. This failure is censidered an isolated '

, l occurr m e and not systematically recurring in nature.

The initial Group II and III PCIS actuation was anticipated on low reactor level as a I result of the manual scram. The second PCIS actuation occurred on low reactor level due to the failure of the 'C' RFP to restart. This was caused by drift of the setting of the interlock dump valve which did not allow sufficient-oil pressure to be ~

maintained to open.the RFP turbine control valve. The root cause'of the setting-drift af the interlock dump valve is attributed to the lock nut on the setpoint

, adjustment bolt becoming unsecured due to system vibration. 1

1 The third low level PCIS actuation was due to the rapid level fluctuations created.by the combination of attempting rapid depressurization via the turbine bypass valves while batch feeding reactor coolant inventory with HPCI. Rapid depressurization was necessary due to.the potential for the-turbine bypass valves to fail closed on loss of EHC fluid.

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o ll ol3 oF ol3 l Analysis of the Event No actual safety consequences occurred as a result of this event.

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A Group 11 and 111 PCIS actuation s'gnal occurs when reactor coolant reat hes the zero inch level (172 inches above the top of active fuel). The initial low vel PCIS actuation im.nediately following the scram is not unexpected and is the result of void collapse upon insertion of the control rods. During this event reactor level fluctuated within a range of plus fifty five inches to minus 8 inches. A minus 8 inch level corresponds to 164 inches of reactor coolant ab'm the top of active fuel.

Closure of the main turbine bypass valves due to loss of EHC fluid would have been bounded by the Final Safety Analysis Report abr.ermal trrnsient analyses for a turbine trip without bypass or isolation of all main sts 7, lines. Therefore, the fuel was not S danger of becoming uncovered at any time du:1.a this event.

Corrective Actions ,

l The EHC fluid leak was stopped and the leaking EHC servo val'!e was replaced. A walkdown of the EHC fluid piping was performed and no other leaks were identified.

The A, B and C RFP interlock dump valves lock nuts were inspected in Unit 2 and 3.  !

The Unit 3 'B' RFP interlock dump valve lock nut was also found to be unsecured. The Unit 3 RFP interlock dump valves were readjusted and tested to verify proper setting.

The Unit 2 RFP interlock dump valve lock nuts were found to be properly secured. The l RFP inter 1cck dump valve lock nuts on both units will be mechanically secured to prevent inadvertent movement of the adjustment bolt.

I Previous Similar Events

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Hn a avious similar Licensee Event Reports have been identified.

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