ML18033B189

LER 90-001-00:on 900126,reactor Protection Sys Bus 3A Deenergized Resulting in Actuation of Plant Esfs.Caused by Voltage Transient & Inadequate Design of Setpoints for Circuit Protectors.Bus 3A realigned.W/900226 Ltr

ML18033B189
Person / Time
Site:	Browns Ferry
Issue date:	02/26/1990
From:	Bynum J, Hsieh C TENNESSEE VALLEY AUTHORITY
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-90-001-03, LER-90-1-3, NUDOCS 9003080248
Download: ML18033B189 (12)

Text

ACCELERATED UTION DEMON~TION SYSIKM REGULATORY INFORMATION DISTRIBUTION SYSTEM (RZDS)

ACCESSION NBR:9003080248 DOC.DATE: 90/02/26 NOTARIZED: NO DOCKET FACIL:50-296 Browns Ferry Nuclear Power Station, Unit 3, Tennessee 05000296 AUTH. NAME AUTHOR AFFILIATION HSIEH,C.S. Tennessee 'Valley Authority BYNUM,J.R. Tennessee Valley Authority RECIP.NAME RECIPIENT AFFILIATION

SUBJECT:

LER 90-001-00:on 900126,deenergization of RPS bus by alternate supply circuit protector operations. D W/8 DISTRIBUTION CODE: ZE22T COPIES RECEIVED: LTR ENCL SIZE:

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

NOTES:1 Copy each to: S. Black,D.M.Crutchfield,B.D.Liaw, 05000296 R.Pierson,B.Wilson A D

RECIPIENT COPIES RECIPIENT COPIES ID CODE/NAME LTTR ENCL ID CODE/NAME LTTR ENCL D LA ROSS,T. 1,1 1 1 PD 1 1 INTERNAL: ACNW 2 2 ACRS 2 2 AEOD/DOA 1 1 AEOD/DS P/TPAB 1 1 AEOD/ROAB/DS P 2 2 DEDRO 1 1 NRR/DET/ECMB 9H 1 1' NRR/DET/EME B9H3 1 1 NRR/DET/ESGB 8D 1 NRR/DLPQ/LHFB11 1 1 NRR/DLPQ/LPEB10 1 1 NRR/DOEA/OEAB11 1 1 NRR/DREP/PRPB1 1 2 2 NRR/DST/SELB,8D 1 1 NRR/DST/SICB 7E .1 1 NRR/Dg~&+LB8Dl 1 1 NRR/DST/SRXB 8E RES/DSIR/EIB 1

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1 EXTERNAL: EG&G WILLIAMS,,S 4 4 L ST LOBBY WARD'RC 1 1 R LPDR 1 1 PDR 1 1 NSIC MAYS,G 1 1 NSIC MURPHY,G.A 1 1 NUDOCS FULL TXT 1 1 D

NOTES: 5 5 D

D NOTE TO ALL "RIDS" RECIPIENTS:

PLEASE'HELP US TO REDUCE WASTE! CONTACT THE DOCUMENT CONTROL DESK, ROOM Pl-37 (EXT. 20079) TO ELIMINATEYOUR NAME FROM DISHUBUTION LISTS FOR DOCUME!~ YOU DON'T NEED!

FULL TEXT CONVERSION REQUIRED TOTAL NUMBER, OF COPIES REQUIRED: LTTR 42 ENCL 42

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TENNESSEE VALLEY AUTHORITY Browns Ferry Nuclear Plant Post Office Box 2000 Decatur, Alabama 35609-2000 FEB RS 1M U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, D.C. 20555

Dear Sir:

TVA BROWNS FERRY NUCLEAR PLANT (BFN) UNIT 3 .DOCKET NO. 50-296 FACILITY OPERATING LICENSE DPR-68 REPORTABLE OCCURRENCE REPORT BFRO-50-296/90001 The enclosed report provides details concerning the deenergizations of a reactor protection system bus by motor generator circuit protector operations caused by the inadequate design of the protector setpoints. This report is, submitted in accordance with 10 CFR 50.73(a)(2)(iv).

Very truly yours, Vic Presi'dent Nuclear Power Production Encl'osures cc (Enclosures):

Regional Administration U.S. Nuclear Regulatory Commission Office of Inspection and Enforcement Region II 101 Narietta Street, Suite 2900 Atlanta, Georgia 30323 INPO Records Center Suite 1500 1100 Circle 75 Parkway Atlanta, Georgia 30339 NRC Resident Inspector, BFN 9pp22+

9>psp80248 0500p296 9PD<- ~DOCK o ptic:

An Equal Opportunity Employer

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NRC (6-89)

Form 366 FACILITY NAHE (1) i NUCLEAR REGULATORY COHHI SS ION LICENSEE EVENT REPORT (LER)

Approved OHB No. 3150-0104 Expires 4/30/92

[DOCKET NUHBER (2) IU~AMl)

R WN F RY T 1 F 4 TITLE (4) DEENERGIZATION OF REACTOR PROTECTION SYSTEH BUS BY ALTERNATE SUPPLY T T N BY NAD T D N TP N V NT AT T FA I I I I I SEQUENTIAL I IREVISIONI I I I FACILITY NAHES lDOCKET NUHBER(S)

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N F N T OPERAT ING I ITHIS REPORT IS SUBHITTED PURSUANT TO THE REOUIREHENTS OF 10 CFR 5:

HODE r m r f f 11 win ll I20.402(b) [20.405(c) i&Iso 73(a)(2)(iv) I 173.71(b)

POHER I ) [20.405(a)( 1)(i) [50.36(c)( 1) I l50 73(a)(2)(v)

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LEVEL I (20.405(a)(l)(ii) I l50 36(c)(2) (50.73(a)(2)(vii) )OTHER (Specify in l l20 ~ 405(a)( l)(iii) l l50.73(a)(2)(i) ]50.73(a)(2)(viii)(A) Abstract below and in

)20.405(a)( l)(iv) (50.73(a)(2)(ii) I l50 73(a)(2)(viii)(B)

~ Text, NRC Form 366A)

.4 v .7 i( 7 T TFR NAHE I T

] AREA CODE lr Hi nin N F E TH IREPORTABLEI I I' IREPORTABLEI I l A Y HPN T T R T 0 T HAN R I I I I I I I I I I I I A P T 14 I I SUBHISSION I I I N T 1 ABSTRACT (Limit to 1400 spaces, i.e., approximately fifteen single-space typewritten lines) (16)

On January 26, 1990 at 0215 hours0.00249 days <br />0.0597 hours <br />3.554894e-4 weeks <br />8.18075e-5 months <br />, unit 3 reactor protector system (RPS) bus 3A was deenergized resulting in the actuation of plant engineered safety features, including standby gas treatment actuation, control room emergency ventilation actuation, and primary containment isolations. Investigations revealed that one of the associated circuit protectors, 3C1, tripped to separate the distribution bus from its power supply. This was caused by the operation of an internal relay which sensed an overvoltage in the alternate supply.

The cause of the voltage transient has not been determined. The root cause of the event is an inadequate design of the setpoints for the circuit protectors for the RPS. This resul'ts in the lack of an adequate operating margin for the RPS voltage regulation.

After the event, the power distribution for RPS bus 3A was realigned to the normal power supply, motor generator set 3A. As .the result of previous RPS deenergization events, appropriate corrective actions were in progress at the time of this event.

An evaluation of the circuit protector relays to determine to what extent temperature will affect the relay setpoints, and completion of the reevaluation of the circuit protector setpoints to determine appropriate design changes will be done. Additionally, design documents are being initiated to allow smaller deadband relays to be installed.

NRC .Form 366(6-89)

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'ttRC Form 366A 1 NUCLEAR REGULATORY COHHISS ION Approved OHB No. 3150-0104 (6-89) Expires 4/30/92 LICENSEE EVENT REPORT (LER)

.TEXT CONTINUATION FACILITY NAHE (1) iOOCKET NUHBER (2) t> HB PA I I I I SEQUENTIAL I I REVISIONI I I I I I Y

~RW~NF 1 F 4 TEXT (If more space is required, use additional NRC Form 366A's) (17)

On January 26, 1990 at 0215 hours0.00249 days <br />0.0597 hours <br />3.554894e-4 weeks <br />8.18075e-5 months <br />, unit 3 reactor protection system (RPS) [JCI bus 3A was deenergized resulting in the completion of the'ctuation logic for several plant engineered safety features (ESFs). ESFs affected by this event include standby gas treatment system [BH], control room emergency ventilation system [VI], and primary containment isolations [JM]. The affected components responded as expected.

Operations personnel initiated an investigation into the cause of the RPS bus deenergization and discovered that RPS circuit protector 3C1 was tripped. This removed the alternate power supply, 480 volt shutdown board 3B, from RPS bus 3A. On January 27, 1990, RPS bus 3A was realigned to the normal power supply, motor generator (NG) set 3A. The affected actuations/isolations were reset.

Visual inspection of the circuit protector found no abnormalities, and no sources of system voltage transients were identified that would have been probable to cause the circuit protector to actuate. The voltage supplied by the alternate supply transformer to the RPS bus was found to be normal.

Troubleshooting of the circuit protector revealed that the overvoltage relay was tripped. The circuit protector room temperature was less than 55 degrees F.

This colder than normal room temperature may have had an effect on the relay setpoints.

All three units were shutdown and defueled at the time of this event. No fuel handling or operations over spent fuel were in progress during this event. The unplanned actuations of the ESF systems are reportable in accordance with 10 CFR 50.73(a)(2)(iv)

As a result of previous RPS deenergizations, a task force to evaluate the circuit protector performance of the RPS had'been established in Harch of 1989.

This task force identified that a small margin exists between the allowable setpoints for the overvoltage/undervoltage trips and the accuracy range of the voltage regulation for the RPS. The resulting small operating margin is due to the conservatisms in the calculations used to determine these setpoints.

The combination of the relay model used on unit 3 and the existing setpoints are such that the overvoltage and undervoltage relay deadbands overlap. Since decreasing voltage transients are judged to be more likely to occur than overvoltage transients, the operating voltage of the power supply had been left closer to the overvoltage setpoint of the circuit protector. This is necessary to ensure the undervoltage deadband would be cleared should the undervoltage relay ever be tripped during a transient. Small voltage transients can cause the operating voltage to exceed the circuit protector setpoint and result in the NRC Form 366(6-89)

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NRC Form 366A (6-89)

NUCLEAR REGULATORY CONNISSION LICENSEE EVENT REPORT (LER)

TEXT CONTINUATION i Approved ONB No. 3150-0104

.Expires 4/30/92 FACILITY NANE (1) iOOCKET NUNBER (2) PA I I I I SEQUENTIAL I I REVISIONI I I I I I Ne ~Ng~

+ MYRRH,I~FE N 1 n TEXT (If more space is required, use additional NRC Form 366A's) (17) deenergization of the RPS bus. To increase the margin between the circuit protector setpoints and the power supply operating voltage, the circuit protector relay setpoints are being reevaluated with the goal of widening them, and smaller deadband relays will be specified.

The systems affected during this event are designed to shut down the reactor, contain and process any radioactive releases, and to fulfill their safety functions upon loss of initiation logic power. The systems responded correctly to the loss of power; therefore, plant safety was not adversely affected. The plant's safe shutdown capabilities would not have been diminished had the unit been in power operation.

The cause of the voltage transient has not been determined. However, the root cause of the RPS bus deenergization has been previously identified in LERs 259/89024 and 259/84027. The root cause of this event is an inadequate design of the setpoints for the circuit protectors for the RPS. This results in the lack of an adequate operating margin for the RPS voltage regulation.

Contributing to this event was the wide deadband of the circuit protector relays. The operating voltage had been set closer to the overvoltage relay setpoint in order to ensure clearing of the undervoltage deadband. This leaves a small margin between the circuit protector overvoltage setpoint and the operating voltage.

Following the deenergization of RPS bus 3A, the affected components were verified to have actuated/isolated as required and investigation into the cause of the event was initiated'. After the closure of the modification on the RPS MG set 3A, RPS bus 3A was realigned to the normal power supply.

As the result of previous RPS deenergizations, an evaluation of the design basis for the RPS circuit protection setpoints had been performed and appropriate design changes initiated. However, a reevaluation of the circuit protector setpoints is being performed in an attempt to further increase the available operating margins. Additionally, design documents are being initiated to allow the circuit protector relays to be replaced with relays having smaller deadbands. This will help ensure the relays will reset when the supply voltage return to normal following a transient.

Additionally, an evaluation will be performed on the circuit protector relays to determine to what extent temperature will affect the setpoint of this type of relay. Appropriate corrective actions will be implemented as a result of 'this evaluation.

NRC Form 366(6-89)

41 I

NRC Form 366A (6-89)

.UCLEAR REGULATORY COMMISSION LICENSEE EVENT REPORT (LER)

TEXT CONTINUATION t Approved OMB No. 3150-0104 Expires 4/30/92 FACILITY NAME (1) iOOCKET NUMSER (2) PA I I I I SEQUENTIAL I I REVI SIONI I I I I I N J I I I MNFRYNT 4 TEXT (If more space is required, use additional NRC Form 366A's) (17)

On March 7, 1989> RPS channel 3A (unit 3) momentarily deenergized because the circuit protector had tripped (LER 296/89003). The root cause is the design of the circuit protectors on the alternate power supply being sensitive to minor electrical disturbances. Contributing to the event was the lack of priority given to the maintenance activity which allowed the RPS system to remain on its alternate supply unnecessarily long.

LER 259/89024 documents two RPS bus deenergization events that occurred on August 26, 1989 and September 20, 1989. It was determined that voltage fluctuations in the RPS output voltage caused the trips of the circuit protectors. As a result of the problems identified during the previous task force evaluation, the root cause of this LER was determined to be an inadequate design of the setpoints for the circuit protectors. Design changes have been initiated to replace the open design voltage adjustment potentiometers with ones having an enclosed design and add a time delay to the circuit protector actuation circuitry to reduce the number of spurious circuit protector trips.

On October 16, 1989, RPS channel 1B (unit 1) was deenergized because the circuit protector had tripped as the result of output voltage fluctuations (LER 259/89027). The immediate cause of this event had not been determined.

The root cause of this LER was determined to be an inadequate design of the setpoints to the circuit protectors. The voltage regulator was replaced to ensure that it was not causing the voltage fluctuations. An evaluation of the circuit protector setpoints was performed to determine appropriate changes.

1. Complete the reevaluation of the circuit protector setpoints and issue the appropriate design changes to allow changing these setpoints by March 31, 1990.

2. Specify replacement of relays with a smaller deadband by March 31, 1990.

3. Perform an evaluation of the circuit protector relays to determine to what extent temperature will affect the relay setpoint, and initiate appropriate corrective actions by March 31, 1990-Energy Industry Identification System (EIIS) Codes are identified in the text as

[xx].

NRC Form 366(6-89)