ML18036A824

LER 92-006-00:on 920728,automatic Main Turbine Trip & Reactor Scram Occurred from Indicated High Water Level Spike on Two of Three Level Channels.Caused by Inaccurate Evaluation & Diagnosis.Event to Be reviewed.W/920827 Ltr

ML18036A824
Person / Time
Site:	Browns Ferry
Issue date:	08/27/1992
From:	Shingleton E, Zeringue O TENNESSEE VALLEY AUTHORITY
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-92-006-02, LER-92-6-2, NUDOCS 9209020016
Download: ML18036A824 (18)

Text

ACCEI ERATED DI RIBUTION DEMONS RATION SYSTEM REGULATE Y INFORNATION DISTRIBUTION YSTEN (RIDS)

ACCESSION NBR: 9209020016 DOC. DATE: 92/08/27 NOTARIZED: NO DOCKET ¹ FACXL: 50-260 Broens Ferry Nuclear Poeer Station. Unit 2. Tennessee 05000260 AUTH. NAME AUTHOR AFF I L I ATIQN SHINQLETQNI E. B. Tennessee Val 1 ecI Authority ZERINQUE. O. J. Tennessee Val 1 ecI Auth oriicI RECIP. NANE RECXPIENT AFFILIATION

SUBJECT:

LER 92-006-00: on 9207282 automatic main turbine trip h reactor scram occurred from indicated high eater level spike on tLUo of three level channel s. Caused be inaccurate evaluation Zc diagnosis. Event to be revieeed. W/920827 ltr.

DISTRIBUTION CODE: IE22T COPIES RECEIVED: LTR Q ENCL ( SIZE:

TITLE: 50. 73/50. 9 Licensee Event Report (LER)z Xncident Rpt. etc.

NOTES:

REC XP IENT COPIES RECIPIENT COPIES XD CODE/NAME LTTR ENCL ID CODE/NANE - LTTR ENCL SANDERS> M. HEBDON> F 1 ROSSi T. 1 1 lNTERNAL: ACNW 2 2 ACRS 2 2 AEOD/DOA 1 1 AEOD/DSP/TPAB 1 AEOD/ROAD/DSP 2 2 NRR/DET/ENWEB 7E 1 1 NRR/DLPG/LHFB10 1 1 NRR/DLPG/LPEB 10 NRR/DOEA/OEAB 1 NRR/DREP /PRPB11 2 2 NRR/DST/SELB 1 NRR/DST/SICB8H3 1 8D'RE/J%T/SPLBSDZ 1 1 NRR/DST/SRXB 8E 1 1 RES PZXE 02 1 1 RES/DSIR/EIB 1 RGN2 FILE 01 1 1 EXTERNAL: EQLecG BRYCEI J. H L ST LOBBY WARD 1 NRC PDR 1 1 NSIC NURPHYi G. A 1 NSIC POQRE> W. 1 1 NUDOCS FULL TXT 1 1 NOTE TO ALL "RIDS" RECIPIENTS:

PLEASE HELP US TO REDUCE WASTE! CONTACT THE DOCUMENT CONTROL DESK.

ROOM PI-S7 (EXT. 504-'2065) TO ELIMINATEYOUR NAME FROM DISTRIBUTION LISTS FOR DOCUMENTS YOU DON'T INEED!

FULL TEXT CONVERSION REQUIRED TOTAL NUNBER OF COP lES REQUIRED: LTTR 31 ENCL 31

~ i Tennessee Valley Authority, Post Office Box 2000. Decatur.'Alabama 35609 O. J. 'Ike'eringue Vice President, Browns Ferry Operations

.AVG 27 1992 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington,,D.O. 20555

Dear Sir:

TVA BROWNS 'FERRY NUCLEAR PLANT (BFN) UNIT 2 DOCKET NO. 50-260 FACILITY OPERATING LICENSE DPR-52 LICENSEE EVENT REPORT LER-50-260/92006 The enclosed report provides. details concerning a Unit 2 reactor scram .on indicated high reactor water level that occurred on July 28, 1992. The indicated high water level was caused by a signal spike that occurred when a relay was replaced, in the contr'ol circuit of the Feedwater Level Control System during troubleshooting.

This report is submitted in accordance with 10 CFR 50.73(a)(2)(iv).

Sincerely,

0. J. Zeringue Enclosure cc: See page 2 9209020016 920827 PDR ADOCK'5000260 PDR

~<8 vU U~

]pi U.S. Nuclear Regulatory Commission

.NG 27 1992 cc (Enclosure):

INPO Records Center

,Suite 1500 1100 Circle,75 Parkway Atl'anta, Georgia 30339 Paul Krippner American Nuclear Insurers Town Center, 'Suite 300S 29 South Main Street West Hartford, Connecticut 06107 NRC Resident Inspector Browns Ferry Nuclear Plant Route 12, P.O. Box 637 Athens, Alabama 35609-2000 Regional Administrator U.S. Nuclear Regulatory Commission Region II 101 Marietta Street, Suite 2900 Atlanta, Georgia 30323

'Thierry M. Ross U.S. 'Nuclear Regulatory Commission One White Flint, North 11555 Rockville Pike Rockville, Maryland'0852

i5 0 1 NRC Form 366 . NUCLEAR REQRATORY GN%6SSION Approved OMB No. 3150-0104 (6-09) Expires 4/30/92 LICENSEE EVENT REPORT (LER)

FACILITY NAME (1) )DOCKET .NUMBER (2) I TITLE (4) Reactor Scram on Indicated High Reactor Water Level Caused by Signal Spike During Feedwater Level I I I I I SEQUENTIAL I IREVISIONI I I I FACILITY NAMES IDOCKET NUHBER(S)

A I I I I I I I I I I I OPERATING I ITHIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR 5:

MODE I I w 120.402(b) l20.405(c) I)L 150 73(a)(2)(iv) I73 71(b)

POWER I )20.405(a)( l)(i ) i50.36(c)(l) (50.73(a)(2)(v) )73.71(c)

LEVEL I [20.405(a)(l)(ii) i50.36(c)(2) (50.73(a)(2)(vii) (OTHER (Specify in 1 )20.405(a)( l)(iii) ]50.73(a)(2)(i) )50.73(a)(2)(viii)(A) [ Abstract below and in

[20.405(a)( l)(iv) (50.73(a)(2)(ii) (50.73(a)(2)(viii)(B) [ Text, NRC Form 366A)

T T T NAHE N H AREA CODE 1'

HP I I IREPORTABLEI I I I IREPORTABLEI N N P T 14 EXPECTED I I SUB HIS SION I I I m 0 ABSTRACT (Limit to 1400 spaces, i.e., approximately fifteen single-space typewritten lines) (16)

On July 28, 1992, at 1932 hours0.0224 days <br />0.537 hours <br />0.00319 weeks <br />7.35126e-4 months <br />, during troubleshooting on the Unit 2 Feedwater Level Control (FWLC) System, Browns Ferry Unit 2 experienced an automatic main turbine trip and reactor scram from an indicated high water level spike on two of three level channels. Engineered safety feature actuations included Primary Containment Isolation System Group 2, 3, 6, and 8 logic and all trains of Standby Gas Treatment and Control Room Emergency Ventilation.

The level spike was induced by the replacement of a relay in the control circuit of the FWLC system. Subsequent investigation determined that the relay replacement was not required. The root cause of this event was an inaccurate evaluation and diagnosis of prior feedwater system trouble symptoms, and failure to anticipate the FWLC circuit response to the relay replacement.

Administrative controls will be provided to ensure high risk troubleshooting activities on certain plant systems receive an independent technical review in addition to that already done. TVA will review this event with appropriate personnel.

NRC Form 366(6-89)

i NRC Form 366A U.S. NUCLEAR REGULATORY COMMISSION Approved OMB No. 3150-0104 (6-09) Expires 4/30/92 LICENSEE EVENT REPORT '(LER)

TEXT 'CONTINUATION FACILITY NAME (1) IOOCKET NUMBER (2)

I ( SEQUENTIAL ( ) REVISION/ / / )

Browns Ferry Unit 2 I I I I I TEXT (If more space is required, use additional NRC Form 366A's) (17)

I. PLANT CONDITIONS Unit 2 was at approximately 100K power. Browns Ferry Units 1 and 3 were defueled. Troubleshooting was in progress- on the Feedwater Level Control system (FWLC). The FNLC controller and the 2A and 2B feedpumps were in manual.

II- DESCRIPTION OF EVENT A. ~gyt:

On July 28, 1992, at 1932 hours0.0224 days <br />0.537 hours <br />0.00319 weeks <br />7.35126e-4 months <br />, during troubleshooting on the Unit 2 FWLC system [JB], Browns Ferry Unit 2 experienced an automatic main turbine trip and a reactor scram from an indicated high water level spike on two of three level channels. The replacement of a'FMLC system control relay, which reestablished the circuit loop integrity, caused a full scale signal spike. This current spike caused the actuation of two level switches, thus, completing the necessary two out of three logic for main turbine trip and reactor feedpump (RFP) trip.

Since the plant was operating at greater than 30K power, direct actuation.

of the reactor protection system [JB] for reactor scram and recirculation pump trip occurred when the main turbine tripped. The main turbine bypass valves and four steam relief valves (SRVs) actuated to reduce and maintain reactor pressure following .the trip. Additionally, the Alternate Rod Insertion (ARI) logic actuated on high reactor pressure. Primary Containment Isolation System (PCIS) [JM] Group 2, 3, 6, and 8 logic actuated as expected for this event.

All trains of Standby Gas Treatment [BH] and Control Room Emergency Vent'ilation [VI] started as expected'. The reactor was brought to a hot shutdown condition pending investigation of the FNLC system. Unit Operators (UO) (Utility, licensed) reset Engineered Safety Feature (ESF) logic and restored ESF systems to their normal standby alignment by 1936 hours0.0224 days <br />0.538 hours <br />0.0032 weeks <br />7.36648e-4 months <br /> on July 28, 1992.

This event is reported in accordance with 10 CFR 50.73(a)(2)(iv) as an event or condition that resulted in a,manual or automatic actuation of an ESF.

B. t t t t t t t t th None.

NRC Form 366(6-89)

0 NRC Form U.S. NUCLEAR REGULATORY COMMISSION Approved OMB No. 3150-0104 366A'6-09)

Expires 4/30/9Z LICENSEE EVENT REPORT (LER)

TEXT CONTINUATION FACILITY NAME (1). IDOCKET NUMBER (Z)

I I t I SEQUENTIAL I I REVI SION I I I I I Browns Ferry Uni t 2 -I I I I I TEXT (If more space'is.required, use additional NRC Form 366A's) (17)

C- t July 28, 1992 at 1932 CDST Main and feedpump turbines tri.p, reactor scram with recirculation pump trips. PCISs 2, 3, 6 and 8 actuated as expected.

July 28, 1992 by 1936 CDST ESF logi.c reset and systems restored to standby alignment.

July 28, 1992 at 2316 CDST A four hour report was made to NRC in accordance with CFR 50.72(b)(2)(ii).

'0 D. t None.

E. th The reactor scram and ESF actuations were immediately recognized by the Unit Operator upon receipt of indications in the Control Room.

F.

Operati.ons personnel promptly responded to the turbine trip/reactor scram and maintained complete control of the unit during the shutdown sequence.

G. t t ESF actuations occurred as designed on a high reactor water level scram.

PCIS Group 2 (shutdown cooling mode of residual heat removal [BO]), 3 (reactor water cleanup [CE]), 6 (primary containment purge and vent [JM]),

and 8 (Traversing In-core Probe [IG]) logic actuated as expected for this event. All trains of Standby Gas Treatment and Control Room Emergency Ventilation systems started as expected. Additionally, the ARI logic actuated on high reac'tor pressure.

The plant response to the transient was as expected, except the Traversing In-core Probe (TIP) ball valves were found in the open position, the supply breaker to the 480 Volt Reactor Motor Operated Valve (RMOV) Board 2C tripped, and'he SRV tailpipe temperature chart recorder paper jammed.

The TIP ball valves were found in the open position during scram recovery actions due to a local TIP isolation logic reset pushbutton swi.tch being stuck in the reset position.

NRC Form 366(6-89)

IS II NRC Form 366A U.S. NUCLEAR REGULATORY COHHISSION Approved OH8 No. 3150-0104 (6-89) Expires 4/30/92 LICENSEE EVENT REPORT (LER) .

TEXT CONTINUATION FACILITY NAME (1) (DOCKET NUHBER (2)

I I I I SEQUENTIAL I I REVI SION I I I I I Browns Ferry Unit 2 I I I I I TEXT (If more space is required, use additional NRC Form 366A's) (17)

II? CAUSE OF THE EVENT The turbine trip/reactor scram on high reactor water level was caused by a current spike that occurred when a relay was replaced in the FWLC circuit during troubleshooting activities. Installation of the replacement relay reestablished the circuit loop integrity and caused a full scale signal spike that actuated the electronic level switches in the loop. The proportional amplifiers in the circuit were saturated when the circuit was opened and caused the current spike when the circuit was closed. This completed the necessary two out of three logic for main turbine trip and reactor feedpump trip.

Ra tttauac=

Subsequent investigation determined that the relay replacement was not required. The root cause of this event was an inaccurate evaluation and diagnosis of prior feedwater system trouble symptoms and failure to anticipate the circuit response. 7here were no administrative controls that required independent technical review of the troubleshooting activity.

The FMLC system design was considered a contributor to the event. The current FWLC system is very sensitive to signal perturbations and to single failures. The signal disturbances induced by the troubleshooting activity were of short duration and the system design was such that both loops had to be worked on simultaneously.

IV. ANALYSIS OF THE EVENT This event, a reactor scram resulting from a trip of the main turbine from 100K power, is fully within the design basis of the plant. The Safety Analysis Report also assumes the failure of the main turbine bypass valves to open in conjunction with this event. The event'occurred from a plant initial condition that is practically the most severe for events of this type, and without assuming additional equipment failure, the stress to the plant was near maximum for this type of event. The plant response to the transient was as expected, except as described above. No degradation of plant operating or safety margins occurred. The event did represent a challenge to reactor safety systems, however, nuclear safety was not decreased by this event.

NRC Form 366(6-89)

NRC Form 366A 'U.S. NUCLEAR REGULATORY COHHISSION Approved OHB No. 3150-0104 (6-89) Expires 4/30/92 LICENSEE EVENT REPORT (LER)

TEXT CONTINUATION FACILITY NAHE (1) (DOCKET NUHBER (2)

I ( ( (SEQUENTIAL ( (REVISION( ( ( (

Browns Ferry Unit 2 I I I I TEXT (If more space is required, use additional NRC Form 366A's) (17)

V CORRECTIVE ACTIONS tv t

1. The original control relay was tested and found acceptable.

2. TVA verified that the removal and reinsertion of the relay causes a current spike.

3. The master FWLC controller was tested and proper operation was verified.

4. The TIP ball valve failure to close was investigated and a stuck reset pushbutton switch was identified and corrected.

5. The supply breaker for the 480 VAC RMOV Board 2C was replaced as a precautionary measure prior to restart. This problem had occurred

~

previously and was under 'investigation at the time of this event.

B. tv t t t R

1. Administrative controls will be provided to ensure high risk troubleshooting activities on certain plant systems have action plans reviewed by technical personnel independent of the original plan development.

2. TVA will review this. event with appropriate personnel.

3. As previously committed in LER 50=260/92004, TVA will evaluate implementation of the Scram Frequency Reduction Committee recommendation to design and install a digital feedwater control system.

Although not considered a part of the corrective action for the reportable event, TVA plans to pursue further corrective actions to eliminate the potential for TIP pushbutton switch jamming- and to correct the SRV tailpi.pe temperature recording chart paper jamming problem.

NRC Form 366(6-89)

il Ik iJ

NRC Form 366A U.S. NUCLEAR REGULATORY COHHISSION Approved OMB No. 3150-0104 (6-89) Expires 4/30/92 LICENSEE EVENT REPORT (LER)

TEXT CONTINUATION FACILITY NAME (1) )OOCKET NUNBER (2)

I I I I SEQUENTIAL I IREVISIONI I I I I Browns Ferry Unit 2 I I I I I TEXT (If more space is required, use additional NRC Form 366A's) (17)

VI. ADDITIGMAL INFORKLTIGN A.

None.

B.

The FWLC design has previously been identified as a major contributor to scram frequency at BFN. As a whole, the system is intolerant of upset and includes many places where a single component failure or maintenance activities can cause a serious plant transient or shutdown. Records collected by the BPtl Scram Frequency Reduction Team show a total of 13 reactor scrams from FMLC problems between 1978 and 1985. One additional scram occurred on Unit 2 from a master FMLC controller failure on April 27, 1992 (Reference LER.50-260/92004). Three of these events occurred from sensed or actual high water level, but none resulted from signal spiking during maintenance or troubleshooting.

VII. CONNI3HENTS

1. TVA will implement administrative controls to ensure high risk troubleshooting activities on certain plant systems have action plans reviewed by technical personnel independent of the original plan development. This will be completed by October 15, 1992.

2. TVA will review this event with appropriate personnel. This will be completed by October 30, 1992.

Energy Industry Identification System (EIIS) codes are identified in the text as [ZX].

NRC Form 366(6-89)

l5