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Docket No. 50-425 y

U. S. Nuclear _ Regislatory ConsissionATTN:: Document Control DeskWashington, D. C. 20555

Gentlemen:-

V0GTLE ELECTRIC GENERATING PLANTLICENSEE EVENT REPORT

FAILURE OF TEMPERATURE CHANNEL CIRCUIT CARDDURING NI SURVEILLANCE CAUSES REACTOR TRIP

In accordance with 10 CFR 50.73, Georgia Power Company hereby submits theenclosed report related to an event which occurred on February 23. 1991.

Sincerely,

. . t

.. ..

C. K. McCoy

CKM/NJS/ gab-

Enclosure:.-LER 50-425/1991-006

xc: Georoia Power CompanyMr. W. B. Shi > mar. -

Mr.~P. D. Rusiton-r

Mr. S. H. Chesnut-NORMS ,

L S.' Nuclear Reaulatory CommissionMr. S. O. Ebneter, Regional Administrator-

|Mr.fD. S. Hood, Licensing Project Manager, NRRMr. B.: R. Bonser, Senior Resident Inspector, Vogtle

.

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On 2-23 91, with the overpower delta T (OPDT) and overtemperature delta T (OTDT)bistables for reactor protection channel IV tripped for surveillance testingpurposes, Reactor Coolant Loop 1 narrow range average temperature (T evg) failedlow. Personnel performing the surveillance on protection channel IV werepromptly directed to back out of their procedure and restore the channel toservice. However, before any action could be taken, the OPDT and OTDT bistablesfor protection channel I tripped, and a reactor trip occurred at 1138 CST onOTDT since the required 2 out of 4 logic was satisfied. With the exception of asteam dump to condenser valve that failed open, no equipment malfunctions orabnormalities occurred following the reactor trip and by 1158 CST the plant hadbeen stabilized in Hot Standby.

A subsequent review of computer data indicated that after failing low, Loop 1T-avg returned to near normal immediately prior to the reactor trip. Utilizingthe plant simulator, it was demonstrated that if the signal from the Loop 1T hot narrow range temperature instrument failed low slowly and then returned tonormal, the channel I OPDT and OTDT bistatics would trip due to the apparentsudden increase in temperature, T oubleshooting of the Loop 1 T-hot instrumentchannel found that a power supply on a Westinghouse 7300 Series RTD Amplifier(NRA) card had failed.

The NRA card was replaced and the channel IV surveillance was completed beforereturning the reactor to power.

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LICENSEE EVENT REPORT . (LER)TEXT CONTINUATION--

FACI'LITIhAM[ (1) DDCK[1 NUMMR (!) L[R WUMMR ($) l' AG( (3)YEAR 5[Q HUM E[V

V071H Ilf.CIRIC CDERATING PIRfr 1 NIT 2 05000425 91 006 00 2 DF 4ILAT

A. REQUIREMENT FOR REPORT

This report is required per 10 CPR 50.73 (a)(2)(iv) because the eventresuited in an unphnned automatic actuation of the Reactor ProtectionSystem MPS).

B - UNIT STATUS AT TIME OF EVENT

At the time of the ev.nt, thit 2 was operating in Mode 1 (Power Operation)at 100% of rated thermal power. A channel calibration was in progress for

_ power range nuclear instrumentation channel IV (2N44). In accordance withprocedure requirements for the channel calibration, the overpower delta.T(OPDTU and overtemperature delta d (OTDT) bistables for RPS protectionchannel IV had been tripped at 1115 CST.

C. LESCRIPTION OF EVENT

on 2 23 91, at approximately 1137 CST, the Unit 2 Reactor Operator (RO)observed Reactor Coolant Loop 1 Low Average Temperature (T avg) and Loop 1low Low T avg bistable status lights illuminate on the status light board inthe Control Room.- The RO immediately checked the Loop 1 T avg indicator andobserved'it to be indicating low. Also, the Loop 1 OTDT setpoint indicatorwas observed to be indicating high. Recognizing that a failure of a Loop 1narrow range temperature instrument had occurred, the Control Room operatorspromptly entered abnormal operating procedure 18001 C, " Primary Systems

Ji Instrumentation Malfunction." The Unit Shift Supervisor also directed' Instrumentation and Controls (160) personnel performing the channelcalibration on 2N44- to back out-of their procedure and restore protectionchannel IV to servine. Hewever, before any action could be taken,-!

,protection channel 1 OPDT, OTDT6 and turbine runback bistable status lights

y were observed to illuminate, and a reactor trip occurred at 1138 CST. Thefirst out annunciator: indicated that the trip had occurred on OTDT,

. i

on the reactor trip, all control rods were observed to fully insert end aFeedwater Isolation (WI) and an Auxiliary Feedwater (AW) actuation

,

occurred per design. After completing the immediate operator verifications ;'

required by emergency operating procedure 19000 C, "E 0 Reactor Trip orSafety Injection," emergency operating procedure 19001 C, "ES 0.1 ReactorTrip Response," was ' entered at 1140 CST. A step in procedure 19001.Crequires-the control' board controller for steam dump to the condenser to be

-placed in the steam pressure mode. When this step was performed, asteam dump to-condenser valve (2PV 507B) failed open. Operators respondedby taking the controller out of the-steam pressure mode, and.the steam dumpvalve closed. The steam dump valve was then manually isolated, and operatoraction was taken to close the "A" Train Main Steam Isolation Valves (MSIVs)to limit the ccoldown of the Reactor Coolant System. No additionalequipment malfunctions or abnormalities occurred, and at 1158 CST, unitoperating procedure 12006 C, " Unit Cooldown to Cold Shutdown," was enteredafter the plant had been stabilized in Mode 3 (Hot Standby).

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TEXT CONTINUATION-

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V07TLE IHCIRIC CENIRATIfC PLANT tNIT 2 05000425 91 006 00 3 or 41[U

D. CAUSE OF EVENT

The direct cause of the reactor trip was the trip of protection channel Ibistables while protection channel IV OPDT and OTDT bistables were trippedfor surveillance testing. This completed the required 2 out of 4 logic, anda reactor trip occurred on OTDT.

A subsequent review of computer data indicated that after failing low, Loop1 T avg had returned to a near normal value immediately prior to the reactortrip. Additionally, the computer data indicated that the Loop 1 OTDTsetpoint had decreassd immediately prior to the trip. Utilizing the plantsimulator, it was demonstrated that if the signal from the Loop 1 T hotnarrow range temperature instrument 2TE 411A failed low slowly and thenreturned to normal, the actuations and indications observed prior to thetrip would occur. Troubleshooting of the Loop 1 T hot instrument channelthen found that a power supply on a Westinghouse 7300 Series RTD Amplifier(NRA) card had failed and was therefore the root cause of the reactor trip.The return of Loop 1 T avg to a near normal value immediately prior to thereactor trip apparently caused the channel I OTDT bistable to trip due toboth an indicated increase in T-avg and the rate of change of T avs in thepoeitive direction.

Troubleshooting of the problem that caused steam dump valve 2PV 507B to failopen found that the positioner l'eedback arm was loose, causing the linkageto the positioner cam to be out of adjustment. This resulted in thepositioner integratin6 to maximum output with a zero input and caused2PV 507B to fail open once the steam pressure mode was selected. !

E. ANALYSIS OF EVENT

The reactor tripped as designed when the required logic was satisfied forthe OTDT reactor trip. On the reactor trip, all control rods fullyinserted, and a feedwater isolation and an auxiliary feedwater actuationoccurred per design. Appropriate operator action was taken to isolate the'

steam dump valve th*; failed open and to limit the cooldown of the ReactorCoolant System. based on these considerations, it is concluded that therewas no adverse effect on plant safety or on the health and safety of thepublic as a result of this event. j

F. CORRECTIVE ACTIONS

1._ The. Loop 1 T hot NRA card that failed has been replaced and an AnalogChannel Operational Test has been satisfactorily performed for the Loop1 Delta T/1 avg instrumentation.

2. The channel calibration for power range n telear instrumentation channelIV was completed prior to returning the rtactor to power.

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FACIL117 hAME (1) DDCKLT NUMB [R (t) LER huMBER (5) PAGE (3)H AR 5[Q HUM REV

V071H EECIRIC CENERATING PIAhi INIT 2 05000425 91 006 00 4 0F 4TLAI

3. The problem with the pos%ioner feedback arm on steam dump valve2PV 5078 has been corrected.

G. ADDITIONAL INPORMATION

1. Failed Components Identification

Westinghouse 7300 Series RTD Amplifier (NRA) CardPart No: 2837A15C01

Steam Dump to Condenser Valve 2PV-507BMasone11an 6 inch 40,000 Series Control Valve Equipped With SpringDiaphragm Actuator, Bailey AP Series Positioner, and Masonelian EX9000

-I/P Transducer

2. Similar Previous Events

-LER's 50 425/1989 024 and 50 424/1987 018 describe similar previousreactor trips which occurred after OPDT and OTDT bistables for aprotection channel had been tripped for maintenance or testin8 Purposes.In those events, a circuit card falluta in another protection channel

uccurred and caused OPDT and/or OTDT bistables for the additionalprotection channel to trip, which crapleted the required coincidentlogic for a reactor trip.

.3. Energy Industry Identification System Codes

Plant Protection System JC

Main / Reheat Steam System - SB

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