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' January 31~, 1992

10CFR50.73 ,

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Docket No $0 461' ,

Document Control Desk!'M car T'ep!e t ;r - Cr c.t1::; ion '

Washington, D.C.. 20555-|~

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Subjecti -Clinton' Power Station --Unit 1-kicensee Event Report No. 92 001-00

Dear Sir:

'Please find enclosed Licensee Event Report No. 92 001 00:Jnternal Fault In The- "B" Phase Main Power Transformer Resulted in A

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Turbine Generator Trio And An Automatic Retietor SCRAM. This report isbeing;subtnitted.in accordance with the requiressents of 10CFR50,73.

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F. A. Span enbe , III|*

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On January 4,1992, with the plant in POWER OPERATION at 99 percentreactor power, the "B" phase main power transformer (MPT 1B) failed'due=to an internal fault, The transformer failure resulted in a turbine

generator trip-and an automatic reactor SCRAM. The automatic reactorscram occurred due to the turbine control valve fast closure. Vishin

. seconds of the SCRAM, the turbine driven reactor feed pump (TDRFP) IB'

tripped. Additionally, after the reactor SCRAM was reset, a SCRAM-discharge volume (SDV) drain valve failed to reopen and a SDV vent valve

L only opened to an intermediate position, The cause of the SCRAM was an|- internal fault in MPT 1B, The cause of the TDRFP 1B trip was attributed; to a worn thrust bearing, and the cause of the SDV vent and drain valvesj' failing to reopen was. attributed to air-leakage past the seat of a three-

way solenoid. valve, Corrective actions for this event include replacingI

the failed MPT with a spare MPT, restoring the TDRFP 1B thrust. bearingclearance to original manufacturer specifications, and rebuilding the

| three-way solenoid valve,

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DESCRIPTION-0F-EVENT

On January _4, 1992, at 0217 hours, with the plant at 99 percent reactorpower-in Mode 1 (POWER OPERATION), an-internal fault in the "B" phasemain power transformer (MPT IB) [XFMR) occurred result'.pg in a turbinegenerator [TO) trip'and a fast closure of the turbine control valveswhich caused an automatic reactor [RCT) SCRAM. Within seconds of thereactor SCFAM,. turbine driven reactor feed pump (TDRFP) [P) IB tripped. !

At :218 hours, control room operators reset the SCRAM logic.!

IBy 0220 hours, main control room personnel noted on the sequence ofevents [IQ)-printout that the MPT 18 sudden pressure trip relay had

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actuated. Control room operators also verified that the SUDDEN PRESSURE'

MPT 1B annunciator was alarming.

-By 0225 hours, the opera.cors had determined that the automatic deluS6system for.MPT 1B and MPT 10 had initiated. Operators were dispatched !

dnd did not= observe any_ fire on MPT 1B or on MPT IC.

:At 0310 hours; non-licensed operators isolated the deluge system for MPTIB and MPT IC to facilitate access to the lockout relays [86] and to,

allow the fire pumps to be restored to standby.

|By0323 hours,controlroomoperatorsnotedthatSCRAMDischarSeVolume(SDV)--drain valve [V) IC11-F011 had not reopened and that SDV vont valve-IC11-F010 indicated an intermediate position.

-By _0325 hours, a non-11cer :sd operator had restored the horizontal firepump (0FP03P) to standby status.

;By 0335 hours, a non licensed operator had restored the "B" G rc pump(0FP01PB)'to standby status.

At 0436 hours,-'a non-licensed operator manually opened valve 1C11-F001 todrain the SDV.

-At 0441 hours,-a non-licensed operator returned the handwheel for valve' -11011-F011 to:the neutral position,-and valve ICll-F01_1 immediatelyreturned to the closed position. Since the SCRAM logic had been_ reset, ivalve 1011-F011-should have remained open.

-.By-0535 hours; theLfire protection deluge system for MPT 1B and MPT IChad been restored to standby condition.

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No other automatic or manually initiated safety system responses werenecessary to place the plant in a safe, stable condition, No other-equipment or components were inoperable at the start of this event such.that their inoprable condition contributed to this event.

CAUSE OF THE EVENT

The cause of the evene is due to an internal fault in MPT 1B. IllinoisPower (IP) is evaluating if the failed transformer:should be shipped to arepair shop t. have a failure analysis performed to determine, ifpossible,. the cause of the internal fault. IP will submit a supplementalreport if the cause of the failure is identified and if the cause of thefailure significantly changes the information provided in this report.

The.following paragraphs discuss the equipment problems that occurredsubsequent to the reactor SCRAM:

TDRFP 1B Trin

Troubleshooting conducted on:the TDRFP 1B indicated that the mostprobable cause af the TDRFP trip was due: to -a worn thrust bearingon the feed pump. A check of the feed pump thrust bearingclearance indicated that-the clearance was larger-than the-r ,

. clearance specified by the manufacturer. In addition, thes*

L . increased feedwater demand resulted in an increased thrust on the,

TDRFP thrust bearing. This increased thrust, coupled with a larger-than specified thrust beatin5 clearance,.is suspected to havecaused.the. thrust bearing wear trip to actuate,

The increased feedvater demand resulted from-the set point set down-logic initiating. .The set-point set down logic initiated due tothe reactor water low level 3 condition, which normally occurs-after a reactor SCRAM. The set-point set-drwn. logic causes:a-

: larger demand for feedwater for about- ten seconds, followed ty-a' demand decrease. This-logic is designed to minimize the reactorlevel decrease that occurs after a reactor SCRAM due to thecollapse of voids,

:Egilure-of the SDV Vent Valve and Drain Valve To Open

Subsequent to the; event, troubleshooting of the SDV drain and ventvalves identified the following deficiencies (refer to Figure 1):* Air leaked out of the vent ports of the three way solenoid

~ -valves 1C11 F009 and 1C11-F182 when they were energized.

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* Valve IC11-F011 immediately c;. ied and remained open when theinstrument air time delay valve (1C11-F159A) was manuallyopened about one-eighth of a turn.

A~small piece (less than 1/16 inch) of foreign material'

(belisved to be solder) was found between the backseat anc- - disc of valve IC11 F009.

* The bench settings for valve IC11-F010 were found to be 6 and30 pounds per square inch gauge (psig). Bench settings of 6and 30 psig means that the valve starts to open at 6 psig andis fully open or backseated at 30 psig. The correct openingbench settings for IC11-F010 are 18.5 and 60 psig.

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* The bench settings for valve IC11-F011 were found to be 20and 62 psig. The correct. bench settings for IC11 F011 are18.5 and 60 psig.

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- Maintena'nce noted that the SDV vent valve (IC11 F180)operated erratically when stroked repeatedly. The valve-stemwas found to be bent and galling was noted on the disc andstem. Additionally, the bench settings for IC11-F180 werefound to be 8 and 25 psig whereas the correct bench settingaare 3 and 15 psig.

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| Based on the deficiencies identified above, the cause of valveL IC11-F011 failing to open and valve IC11-F010.only opening to.an-

intermediate position 'is attributed to the air leakage past theseat of valve 1011-F009. In May of 1991, personnel discovered that

- valve 1C11LF011 took approximately 17 minutes to reopen, andMaintenaner Work Request MVR D17319 was written to address the slow ->

reopening time. However,-MVR=D17319 was not scheduled to be workeduntil'the next refueling outage, currently scheduled to begin March

c 1992. Based on the. findings noted above, the slow opening time ofL. . valve IC11 F011 noted in May 1991 is attributed to-the small-amounti of air leaking past the seat of valve IC11-F009 and out the vent

port. In May of 1991, valve 1C11 F1594 provided sufficient airvolume-to eventually pressurite the air header enough to open valve

s 1C11-F011', but the leakage past valve 1C11-F009 continued top increase. -Valve 1C11-F159A is a throttle valve which creates aL time delay for the closing'of valver IC11eF011-and 1011-F010. Inf January, due to the. increased leakage past the seat'of valve IC11--'

F009,-valve IC11 F159A at the throttled position-was not capable ofsupplying-enough air volume to pressurize the air header

: completely. . Therefore, due to the variance between the benchsettings of valves 1C11-F010 and 1C11-F011 and the reduced pressurein the air header, valve 1C11-F010 opened to an intermediate

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position and valve IC11 F011 failed to move from the closedposition.

CORRECTIVE ACTION

MPT IB was replaced with a spare MPT undar MVR D10238.

-The-following corrective actions address the equipment problems whichoccurred subsequent to the reactor SCRAM: +

TDRFPlB

The TDRFP 1B thrust bearing was machined and shims were added torestore the thrust clearance to be within original manufacturerspecifications per MVR D28271.

SDV Vent and Drain Valves

-Valves 1C11-F009 and 1C11-F182 have been rebuilt per preventive_

maintenance tasks-(PM) PMMRDM003 and FMMRDM004 In addition, thebench settings for valves 1C11-F010, IC11 F180, and 1C11 F011 werereset to their correct values, and valve IC11-F180 was rebuilt per,

MWR D17319.

The Nuclear Station Engineering Department (NSED) has performed a'

review to determine. if similar applications exist where anietstrument air throttle valve is used to perform a time delay forthe opening or closing of air operated valves. There were no other-applications ~found, _ Additionally, the five year PMs.to rebuildvalves 1C11-F009 and IC11-F182 are being evaluated to determine ifthey are adequate and if an increased frequency is needed. -NSEDwill also evaluate the need for a routine PM to check the. bench-settings on valves 1C11-F010, IC11-F011, IC11-F180,-and 1011-F181.Maintenance vill blow down the air lines which supply _ air to theSDV vent aid drain valves. These actions are scheduled to be-completed by May 30, 1992. Finally, to determine if systemenhancements are needed, !"'ED will evaluate alternative means ofproviding the opening an . . Losure delay for the SDV vent and drainvalves. - This action is % - ed to_ be completed by July 30, 1992.

Station procedure 9012.01 " SCRAM Discharge Volume Vent and Drain-Valve Operability Test" will be revised to ensure that the openingand closure times for valves 1C11-F011, IC11-F010, IC11-F180 and'IC11 F181 are recorded and verified against the design criteria.This action is scheduled to be completed by May 30, 1992.

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ANAINSIS OP EVENT ;

This event is reportabic under dia provisions of 10CTRSO.73(a)(2)(iv) dueto the automatic initiation of t waactor Protection System |JC).

Assessment of the safety consequer.cca and inoplications of this eventindicates that this event was not nuclear safety significant. This, i

transient van ecmpared to a cin.dar transient, " Generator Load Rejection ,

with Ilypass", described in Chapter 1$ of the Updated Safety Asalysis |Report and the Transicut Garety Analysis Design Report (CEZ 7355). Theevent was found (- be within the design brais of the plant.

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ADDITIONAL INFORMATIONi

The failed MPT in a Ceneral Electric suodel 390E648, single phase, 310/347MVA, forced oil and air, 60 liertz, with a low voltage side of 22,000 Vand a high voltage side of 34$,000 v.

LEE 88 028-00 discussed .4ruiti transfortner fault which resulted in a,

turbine generator trip and a reactor serarn.

LER 89 028 00 discussed the failure of a sudden press'tre relay on HPTwhich resulted in a turbine generator trip and a reactor SCRAM,

For further infortsation r.'gardin6 this event, contact. S. E. Rasor,Director Plant Maintenance, extension 3204.

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