ML20024G718

LER 91-004-00:on 910321,RCIC Turbine Main Steam Supply Line Outboard Containment Isolation Valve Isolated.Caused by Negative Trip Setpoint of Trip Unit Being Reached.Caution Added to RCIC Sys Operating procedures.W/910419 Ltr

ML20024G718
Person / Time
Site:	River Bend
Issue date:	04/19/1991
From:	England L, Odell W GULF STATES UTILITIES CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-91-004, RBG-34-869, NUDOCS 9104260116
Download: ML20024G718 (4)

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5 G U Ea F STATES UTIIsETIES COMPANY

r. nom m .w,s easi cma soum sir .au, un s m m r 44Ea CCOI $M f h 49 346 F$1 April 19, 1991 RDG- 34,869 File Nos. G9.5, G9.25.1.3 U. S. Nuclear Pogulatory Occmission Documnt Control Desk Washington, D.C. 20555 i Gentlennn:

River Bend Station - Unit 1 Docket No. 50-458 Please find enclosed Licensee Event Ibport No.91-004 for River Bend Station - Unit 1. This report is subnitted pursuant to 10CFI60.73.

Sincerely, f

T 1, /'

W. H. Odell Manager - Oversight River Bend Nuclear Group cc: U.S. Nuclear Regulatory Ocmnission 611 Ryan Plaza Drive, Suite 1000 Arlington, TX 76011 NRC Resident Inspector P.O. Box 1051 St. Francisville, IA 70775 INPO Records Center 1100 Circle 75 Parkway Atlanta, GA 30339-3064 Mr. C.R. Oberg Public Utility Cmmission of Texas 7800 Shoal Creek Blvd., Suite 400 North Austin, TX 78757 9104260116 910419 PDR ADOCK 05000458 ggy S PDR /

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I i l ABSTR ACT ftet so #d00 spoor s e , esarossmerocy Presea sead>* sosee typewntma haw 118l At 1420.on March.21, 1991, with the reactor in Operational Condition 1 (Power Operation) and the reactor core isolation cooling (RCIC) system isolated-for maintenance, an unplanned-engineered. safety feature (ESP) actuation occurred when the RCIC turbine main steam supply line outboard containment isolation valve, 1E51*MOVF064, isolated. This

.reportL is submitted pursuant to 10CFR50.73 (a) (2) (iv) since this event constitutes an ESF-actuation.

It.is postulated that after the RCIC system was isolated for maintenance by shutting 1E51*MOVF063, cooling in the steam supply line caused steam to condense and flow into the variable leg instrument line1which resulted in negative differential pressure surges in flow transmitter 1E51*PDTN084A. The negative trip setpoint of the trip unit was reached which caused the ESP-actuation. This is the probable root cause of the event. A caution has been added to RCIC system '

, operating procedures that both RCIC turbine main steam supply line containment isolation valves 1E51*MOVF063 and IE51*MOVF064 should be isolated when the system is removed from service for maintenance to

-prevent unexpected ESP actuations.

The ESF actuation occurred per design. The RCIC system was inoperable

-and isolated for maintenance per the action statement of Technical Specification 3.7.3. Therefore, the health and safety of the public was not adversely affected.

NXC Form 364 56498

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0l0 0l2 or 0l3 rezen.- - . es am.unn REPORTED CONDITION At 1420 on March 21, 1991, with the reactor in Operational Cendition 1

_(Power Operation) and the reactor core isolation cooling (RCIC) system

(*BN*) isolated-for maintenance, an unplanned engineered safety feature (ESP) actuation occurred when &D isolation signal was-received-which caused the RCIC turbine main stene supply line outboard containment isolation valve, IE51*MOVF064, (*ISV*) to stroke closed.

This report is submitted pursuant to 10CFR50.73 (a) (2) (iv) since this event ~ constitutes an ESF actuation.

INVESTIGATION At the time of the event, the RCIC turbine main steam supply line inboard containment isolation valve, 1E51*MOVF063 (*ISV*) was closed and the RCIC system (*BN*) was isolated. A limiting condition for operation (LCO)-was' issued for planned system maintenance with a tagging clearance-in place. The RCIC turbine main steam supply line outboard containment isolation valve, 1E51*MOVF064 (*ISV*), had been left open to minimize cycling of the valve since it had a bonnet stem

' packing leak. Note that operating procedures for the RCIC system did not specify if one or both containment isolation valves should be shut during? system maintenance.

At 1420'on March 21, 1991, approximately seven hours after the system had been isolated, a Division I isolation trip signal was received, causing the RCIC turbine main steam supply line outboard isolation valve, 1051*MOVF064 (*ISV*), to stroke closed. The trip signal was reported to have been received from the RCIC steam supply line differential pressure high trip unit 1E31*ESN684 (*PDS*) (Rosemount trip unit model 510).

It is postulated that after the RCIC system was isolated for-maintenance by shutting IE51*MOVF063, cooling in the steam supply line caused steam to condense and flow into the variable leg instrument line which' caused pressure oscillations resulting in negative differential pressure surges in flow transmitter 1E51*PDTN084A (*FT*). .

-The negative trip setpoint of the trip unit was reached which caused the ESF actuation. This is the probable root cause of the event.

Upon completion of RCIC maintenance activities, a check of both trip units revealed both to be indicating 0" of water, which is the normal no flow condition, goo =~ ms .u s w ees o.ea< m<ii

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0l0 01 3 0F 0 13 ras a . m ~ .nc ama.,,m A review of previous LERs Sau identified one event hav!ng limited similarity. LER 90-046 reported an isolation of valve IE51*MOVF063

(*ISV*). The isolation was caused by the RCIC equipment room differential temperature switch IE31*N603D. The cause of the actuation of this switch is indeterminate. As a precaution, the temperature switch was replaced. The similarity between LER 90-046 and the current event le limited because the isolation signals originated from different trip units. '

CORRECTIVE ACTION A caution has been added to RCIC system operating procedures that both RCIC turbine main steam supply line containment isolation valves 1E51*MOVr063 and 1E51*MOVF064 should be isolated when the system is removed from service for maintenance to prevent unexpected ESP actuations.

SAFETY ASSESSMENT The ESP actuation occurred per design. The PCIC system was inoperabic and isolated for maintenance per the action statement of Technical Specification 3.7.3. Therefore, the health and safety of the public was not adversely affected.

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