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LER 97-001-00:on 970305,loss of Offsite Power on Unit 3 During Refueling Outage Resulted from Shorted Component. Replaced Relays Involved in Event W/Less Sensitive Relays
	ML18038B855
Person / Time
Site:	Browns Ferry
Issue date:	04/04/1997
From:	Austin S; TENNESSEE VALLEY AUTHORITY
To:	;
Shared Package
ML18038B854	List: ML18038B853; ML18038B855;
References
	LER-97-001-04, LER-97-1-4, NUDOCS 9704150185
	Download: ML18038B855 (16)
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NRC FORM 366 U.s. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB No. 3160-0104 (4 96), EXPIRES 04/30/98 ESTlMATED BURDEN PER RESPONSE To COMPLY WITH THIS MANDATORY INFORMATION COLLECTION REQUEST: 60.0 LICENSEE EVENT REPORT (LER) HRS. REPORTED LESSONS LEARNED ARE INCORPORATED INTO THE UCENSING PROCESS AND FED BACK TO INDUSTRY. FORWARD COMMENTS REGARDING BURDEN (see reverse for required number of ESTIMATE TO THE INFORMATION AND RECORDS digits/characters for each block) MANAGEMENT BRANCH rl-6 '33), U.S. NUCLEAR REGUlATORY COMMISSION, WASHINGTON. DC 20666 FACIUTY NAME (tl DOCKET NUMSElt tsl PAOE la)

Browns Ferry Unit 3 05000296 1 OF 8 TITLE (4I Loss of Offsite Power on Unit 3 During Refueling Outage Resulting From a Shorted Component EVENT DATE (5)

MONTH DAY YEAR LER NUMBER (6)

SEQUENTIAL NUMBER REVISION NUMBER MONTH REPORT DATE (7)

'AY YEAR NA'ocKET FAautv NAME OTHER FACILITIES INVOLVED (6)

NUMOErr FAaUTY NAME DOCKET NVMBErt 03 05 97 97 001 00 04 04 97 OPERATIN(3 THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR 6: (Check one or more) (11)

'N MODE (9) 20.2201 (b) 20.2203(a) (2)(v) 50 73(a) (2) 0) 50.73(a)(2)(viii)

Z0.2203(a)(1) 20.2203(a)(3)(a 50.73(a) (2) gr) 50.73(a)(2) (x)

POWER LEVEL (10) 000 20.2Z03(a)(2)(i) 20.2203(a)(3)(ii) 50.73(a)(2) (iig ,73.71 20.2203(a)(2) gi) 20.2203(a)(4) X, 50.73(a)(2)(iv) X OTHER 20.2203(a)(2) (iii) 50.36(c)(l) 50.73(a)(2) (v) specify In Abstract below or In NRC Form 366A 20.2203(a)(2) (iv) 50.36(c) (2) 50.73(a)(2)(vii)

LICENSEE CONTACT FOR THIS LER (12)

TELEPNONE NLrMSErt Cr>qua Ar~ Coast Steven W Austin, Licensing Engineer (205) 729-2070 COMPLETE ONE LINE FOR EACH COMPONENT FAILURE DESCRIBED IN,THIS REPORT (13)

CAUSE SYSTEM COMPONENT MANUFACTURER i::,~j:;.";.':";.:;;:;.>:::j;;"".) CAUSE SYSTEM COMPONENT MANUFACTURER To NPRDS To NPRDS SUPPLEMENTAL REPORT. EXPECTED (14) 'MONTH DAY YEAR EXPECTED YES SUBMISSION (If yes, complete EXPECTED SUBMISSION DATE). X No DATE (16)

ABSTRACT (Limit to 1400 spaces, l.e., approximately 15 single-spaced typewritten lines) (16)

On March 5, 1997, at 1040 hours0.012 days 0.289 hours 0.00172 weeks 3.9572e-4 months Central Standard Time, (CST) Unit 3 received engineered safety feature system actuations due to a loss of offsite power. The loss of power was the result of the loss of both-the Athens and Trinity 161 KV power lines. Emergency Diesel Generators 3A, 3C, and 3D automatically started and tied to their respective shutdown boards. At 1122 hours0.013 days 0.312 hours 0.00186 weeks 4.26921e-4 months CST, BFN declared a Notification of Unusual Event (NUE) for Unit 3 due to a'loss of offsite power greater than 15 minutes and notified NRC in accordance with 10 CFR 50,72(a)(3). At 1136 hours0.0131 days 0.316 hours 0.00188 weeks 4.32248e-4 months CST, following the restoration of the offsite power to Unit 3, BFN terminated the NUE, and in accordance with 10 CFR 50,72 (c)(1)(iii) notified.NRC. The root cause of this event was the sensitivity of the auxiliary tripping relays. TVA has replaced the relays involved in the event with less sensitive relays. TVA is currently replacing other Westinghouse AR type relays, in similar applications with less sensitive relays. This event is being reported accordance with 10 CFR 50.73 (a)(2)(iv), as any event or condition that resulted in manual or automatic actuation of any engineered safety feature including the reactor protection system.

9704150185 970I/)04 PDR -

ADQCK 0500029b S PDR NRC FORM 366 (4-96)

~I JJRC FORM 3CCA U.S. JJIJCLEAR REOULATORY COMJISSION

'(44r5)

LICENSEE EVENT REPORT (LER)

TEXT CONTINUATION FACII(ZTY NAME LER NUMBER NUMBER NUMBER Browns Ferzy Unit 3 05000296 2 of 8 97 001 00 more space reqrer ~ use s oos copes orm (17)

z. PLMT CONDZTZONS Unit 1 was shutdown and defueled; Unit 2 was at 100- percent, power

,(3290 megawatts thermal) and'naffected by the event. Unit 3 was shutdown in a scheduled refueling outage with the reactor head removed'. The refueling cavity was flooded and the fuel gates were removed. No fuel movement was ongoing at,,the time of the event and the control rods [AA] were fully inserted. The Unit 3 500 KV [EL]

Transformer Bank was out of service for planned. maintenance, so relying on the 1'61'V [EL], power system foz offsite power.

it was ZZ. DESCRZPTZON OF EVENT A Event On March 5, 1997, at 1040 hours0.012 days 0.289 hours 0.00172 weeks 3.9572e-4 months Central Standard Time (CST), Unit 3 received engineered safety feature system. actuations (ESF) [JE]

due to a loss of offsite power. The loss of power was the zesult of the loss of both the Athens and Trinity 161'V power lines.

This was followed by the start of Emergency Diesel Generators (EDGs),[E J] .

At approximately 1039 hours0.012 days 0.289 hours 0.00172 weeks 3.953395e-4 months CST, following initial installation of an Emergency, Bearing Oil Pump [P][SI] foz,the 3B Reactor Feed Pump [SJ], as part of a Feedwater System upgrade, the pump was successfully electrically bumped to verify correct pump rotation.

Following successful verification of puny rotation, the pump was given a start signal. ~Within a few seconds it appazent that an electrical fault had occurred within the pump became visually motor '[MO], and it was shutdown from a local panel.

At 1040 hours0.012 days 0.289 hours 0.00172 weeks 3.9572e-4 months CST, due to the fault in the pump motor, the Athens and Trinity 161 KV power lines tripped resulting in a 1'oss of offsite power to Unit 3. Emergency Diesel Generators 3A, 3C, and 3D automatically started and: tied to their respective shutdown boards. Emergency Diesel Generator 3B and its respective shutdown board was,under administrative hold for planned maintenance activities and did not respond to the event.

Additionally, the loss of offsite power resulted in the Unit 3 reactor scram si'gnal and the initiation of'everal unplanned This included de-enezgization of the Unit 3 Reactor .ESF'ctuations.

Protection System (RPS) [JC]. The loss of RPS resulted in actuation or isolation of the 'Primary 'Containment Isolation System [JE] (PCIS) systems/components.

~ PCIS Group 1, .Main Steam Line Drain Isolation Valve [SB] [ISV]

close signal

~ PCIS Group 2, Shutdown cooling mode of Residual Heat Removal

[BO] system; Drywell floor drain isolation valve, Drywell equipment drain sump isolation valve [WP]

NRC FORM 366A (4W5)

NRC FORMSCCA U.S. NUCLEAR REOULATORY COIVMSSION (405)

LICENSEE EVENT REPORT (LER)

TEXT CONTZNUATZON FACILITY NAME LER NUMBER NUMBER NUMBER Browns Ferry Unit 3 05000296 3 of 8 97 001 00 more space requ<r, use s copes orm (17)

~ PCZS Group 3, Reactor Water Cleanup [CE]

~ PCZS Group 6, Primary Containment Purge and Ventilation [JM]';

Unit 3 Reactor Zone Ventilation [VB]; Refuel Zone Ventilation

[VA]; Standby Gas Treatment (SGT),[BH] system; Control Room Emergency Ventilation (CREV)[VZ]

~ PCIS Group 8, Transverse Incoze Probe [IG] withdrawal signal Additionally, the Spent Fuel Pool Cooling System. [DA] Pumps tripped on undervoltage.

The affected systems were returned to pze-event alignment by 1138 hours0.0132 days 0.316 hours 0.00188 weeks 4.33009e-4 months CST. All systems responded as expected during the loss of offsite power and subsequent reactor scram signal.

This event is reportable in accordance with 10 CFR 50.73 (a)(2)(iv), as any event or condition that resulted in manual or automatic actuation of any engineered safety feature including the reactor protection system.

B. Zno rable Structures, nents, or stems that Contributed to e Event:

During the event, the Unit 3 Main Bank 500 KV Transformer was out of service for planned maintenance, and Unit 3 was relying on the 161 KV power lines for offsite power. Had the Unit 3 Main Bank 500 KV Transformer been in service during the event, the loss of the 161 KV power lines would not have resulted in the loss of offsite power to Unit 3.

C. Dates and roximate Times of Ha or Occurrences:

March 5, 1997 at 1040 CST Unit 3 experienced a loss of offsite power resulting in a full xeactor scram signal.

March 5, 1997 at 1122 CST TVA declared a Notice of Unusual Event (NUE) on Unit 3 due to a loss of offsite power. TVA made a one hour notification in accordance with 10 CFR 50.72(a)(3).

March 5, 1997 at 1131 CST TVA made a 4 hour nonemezgency notification to NRC in accordance with 10 CFR 50.72(b)(2)(ii) ~

NRC FORM 386A (405)

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'RC FORM 366A U.S. NUCLEAR REOULA')ORY COMWSSION (44r5)

LICENSEE EVENT REPORT (LER)

TEXT CONTINUATION FACZLZTX KAME NUM88R NVM88R Browns Ferry Unit 3 05000296 4 of 8 97 001 00 more space is requir, use s orrs copies oim 1 )

March 5, 1997 at 1136 CST Following successful restoration of offsite power, TVA terminated the NUE. Follow-up notification for termination of the NUE was made in accordance with 10 CFR 50.72 (c) (1) (iii) .

D. Other S stems or Secon Functions Aff'ected:

None.

E. Method of Discove The Unit 3 operator received numerous main control room alarms indicating the loss of the 161 KV Athens and Trinity power lines.

This was followed by indications that a ful'l reactor scram signal had been generated, and EDGs 3A, 3C, and 3D had started and tied to their respective shutdown boards.

orator Actions:

Operator actions taken during- this event were as expected.

Operations responded .to the loss of offsite power to Unit 3 using the applicable portions of Abnormal Operating Instzuctions 0-AOI-57-1A; Loss of Offsite Power (161 And 500 KV)/Station Blackout, 3-AOI-99-1; Loss Of Power to One RPS Bus, 3-AOI-100-1; Reactor Scram and Emergency Plan -Implementing Procedure (EPIP), EPIP-1, Emergency Classification Procedure.

G. Safet S stem Re nses:

The safety systems listed in section IIA of this report 'responded to the loss of offsite power as designed.

CAUSE OF THE EVENT A. Immediate Cause:

The loss of offsite power and subsequent scram were initiated by a failure of a conductor in the Emergency Bearing Oil Pump for the 3B Reactor Feed Pump. The conductor came in contact with the rotor in the motor thus causing a ground fault in a 250 volt Non-1E direct current (VDC) [EI] circuit.

The failure was the result of poor craftsmanship. An individual

[contract, electrician] terminating the power leads for the 3B Reactor Feed Pump Emergency Bearing Oil Pump did not exhibit due care in his work. After terminating the power feed conductors to the motor conductors, the individual replaced them into the motor housing.

NRC FORM 368A (4M)

0 NRC FORIVI 3QCA U.S. NUCLEAR REOULATORY COIVIVIISSION (44)5)

LICENSEE EVENT REPORT (LER)

TEXT CONTINUATION FACZLITX NAME DOCKET LER NUMBER NUM88R NUMSKR Browns Ferry Unit 3 05000296 5 of 8 97 -- 001 -- 00 more space is requfr ~ Use a eris copies This type of motor does not have a termination box, and the conductors are normally terminated and then placed back in the motor housing. In placing the conductors back in the housing, the electrician allowed one of the conductors to be placed into the rotor portion of the motor. 'Subsequently, when .the motor was started, the resulting contact of the conductor with the rotor severed the conductor causing a ground in the motor power circuit.

The motor involved in the event is a 7.5 horsepower 250 VDC motor. During the event, no breaker tripped or fuse cleared as a result of the ground fault. TVA determined analytically that a fault current of approximately 450 amps for 0.2 seconds occuzred.

Under these conditions, this is an expected response.

B. Root Cause:

The root cause of this event was the sensitivity of the auxiliary tripping relays [94]. These relays, (Westinghouse AR type) are 250 VDC fast acting relays used as tripping relays in the circuits involved in this event.

TVA's investigation into this event has determined that based on vendor information Westinghouse AR type relay can operate in approximately 2 milliseconds at current as low as 20 milliamperes. The relay can operate due to transient current flow caused by a ground on a circuit with long cable runs where capacitance has developed which can discharge through ground.

Initially personnel responding to the event found no indication of protective relay [94] operation. Furthermore, there was no indication that the protective relays that would normally operate due to a fault on the 161 KV system had operated. However, upon further investigation, lockout relays were found tripped at the cooling tower switchgear. Personnel found the ground switches in the 161 KV switchyard for both the Athens and the Trinity lines closed indicating the auxiliary tripping relays had operated and the main breakers open.

C. Contributin Factors:

Circuit capacitance of the field cables contributed in the event.

TVA believes that the relay actuation occurred because of the ground in the 3B Reactor Feed Pump Emergency Bearing Oil Pump motor resulting in a transient voltage to actuate the auxiliary relays. TVA developed an equivalent circuit of the schematic and modeled the circuit utilizing computer software. By simulating the event, TVA found that this type of relay would actuate with a ground at the 3B Reactor Feed Pump Emergency Bearing Oil Pump motor followed by separation of the motor conductor.

NRC FORM 366A (4M)

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FORM 3CSA ZACILZTZ HAMS Browns Ferry more space re Unit requrr, 3

use e LICENSEE'VENT oopee 05000296 OAll 1 )

'REPORT CONTINUATZON

'97 (LER)

NUMBER 001

'EXT NUCLEAR REOULATORY COIVMSSION.

NUMBER, 00 6 of 8 The computer simulation indicated that the current that caused relays to. operate was through the capacitance to ground in long.-

field'ables (approximately 2000-4500. feet) into the grounded 3B Reactor Feed Pump 'Emergency Bearing Oi:1 Pump motor conductor through the motor field'nductance (field, armature or both) and through the relay coil. The capacitance to ground discharged when the grounded. motor leads separated which resulted in .a voltage spike on the control'i:rcuit causing the operation of four AR type relays. TVA has determined -based on .field testing that the minimum operating voltage foz this type of'elay is 59 volts. DC.

Additionally, TVA has found that a,transient on one 250 non-, 1E.

VDC circuit has the potential to trip both the Athens and Tzinity 161 KV power lines. The motor that failed was powered from battery .board 4, the same battery board'hat suppl'ied'ontrol power to the Athens and Trinity protective relays. The failed motor conductor placed a ground on the battery board, resulting in a .transient which operated protective relays.

This operation resulted in the loss of both the Athens and.

Trinity. 161 KV power lines and operation of .two 500 KV power circuit breakers. The operation of the two 500 KV,.power circuit breakers had no impact on, plant operation.

'ANALYSZS OF THE EVENT Bzowns Ferry is analyzed in Chapter 14 of the Updated Final Safety Analysis Report for a loss of offsite power assuming a starting point of greater than 100'b reactor power. In this instance, the loss, of offsite power occurred while Unit 3 was in scheduled refueling outage.

Consequently, this event had minimal impact on Unit 3 and no impact on Unit .2 operati;on. Additionally, affected components functioned as designed. Thus, this event is bounded by the plant safety analysis and had no impact on the safety of the plant, its personnel, or the public.,

CORRECTZ'.~~ ACTZONS A. Zmmediate .Corrective Actions:

Offsite'ower was restored to Unit 3. Other affected systems were returned to their pre-event status. 'The motor was inspected and meggered to verify no internal damage. The motor conductors were zetezminated and the pump successfully operated.

Modifications .activities associated with DC pump motor iristallation was 'stopped. TVA Modifications Management met with the appropriate craft personnel to discuss the event, emphasizing the significance of the event, reinforcing safety and quality craftsmanship.

NRC FORM 3MA (~)

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HRC FORM366A U.S. NUCLEAR REGULATORY COMVlSSION (485)

LICENSEE EVENT REPORT. (LER)

'EXT CONTINUATION PACZLITZ 'NAME DOCKET PAGE NUMBER NUMBER Bzowns Ferry Unit 3 05000296 7'of 8 97 001 00 Illoce space Is foqos ~ Uso a as copes cell t ).

B. Corrective Actions to Prevent, Recurrencet TVA has replaced the relays, that were involved in the loss of offsite power with"less. sensitive relays. 'TVA is c'urrently replacing the Westinghouse AR type relays in similar appl'ications with less sensitive relays.

'TVA will evaluate the current 161 KV and 500 KV protective relay functions for possible design changes .

vz. ADDZTZONAL ZNFORMATZON A. Failed onents:

None.

B. Previous LERs on Similar Eventst LER 259/85003 di'scusses an event in which a perturbation on a non-1E 250 VDC power system actuated relays and resulting in start of EDGs. On February 5, 1985, during functional'esting of the protective rel'ays for the Unit '2'tation Service. Transformers

[XPT] and Main Generator [GEN], a voltage spike was generated when connecting test equipment which resulted'in the operati'on of high speed tripping relays. 'This resulted in the tripping of the both 161 KV lines (two '500 KV 'Power Circuit Breakers) and a brief ~

undervoltage condition on Shutdown Boards C and D. 'This resulted in Units 1 and 2 .EDGs C and, D starting. Because the undervoltage condition. was brief, the EDGs did not tie to the Shutdown Boards.

The cause of this event was attributed to the 250 VDC battery board .bus filter being, inoperable.. The investigation team was able to repeat the, event with the battery .board bus filter inoperable. However, with the 250 VDC battery board bus filter operable,, the voltage spike would not actuate the relays. The Westinghouse, AR type fast acting relays could have contributed in this event. An evaluation of the relays was performed 'to see This evaluation resulted if they need replacement or desensitized.

in a design change request that was not implemented. Based on the cause and, contributing factors of this event, TVA determined BFN could be safely operated without replacement of these type relays.

TVA does aot coasidez this cozzective action a Regulatozy Cctsaitmeat. TVA's cozzective actioa pzogzaa will tzack cocpletioa of the actioa.

NRC FORM 366A (495)

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l' NRC FORM 3QCA U.S. NUCLEAR REOULA'IQRY COWISSION

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'ICENSEE EVENT REPORT (LER)

TEXT CONTINUATION ZACZLZTZ ?RIME NUMBER NUMBER Bzowns Ferry Unit 3 05000296 8 of 8 97 '01 ~ 00 IIIOfO SPOCO IO fOqUlf ~ USO Il Ol4l OOPIOS Onn 17)

VZZ . COMHZTMENTS TVA will complete the replacement of the Westinghouse AR type relays in similar applications with less sensitive relays by June 1, 1997.

Energy Industry Identification System (EIIS) system and component codes are identified in'he text with brackets (e.g., [XX]).

NRC FORM 36BA (445)

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ML18038B855

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