ML062640120

Vermont Yankee Hearing - Entergy Exhibit 18, LER 91-009-00, Vermont Yankee, Reactor Scram Due to Loss of Normal Off-site Power (LNP) Caused by Inadequate Procedure Guideline

ML062640120
Person / Time
Site:	Vermont Yankee
Issue date:	06/06/1991
From:	Reid D Vermont Yankee
To:	Document Control Desk, NRC/SECY
Byrdsong A T
References
50-271-OLA, Entergy-Licensee-18, RAS 12268, VYV # 91-135 LER 91-009-00
Download: ML062640120 (12)

Text

I VE~RMONT Y'ANKlit NucLEAR P0%VF-R Co11woIIA-r-10N

-- I

• I. *~ *~*

~ j i..'.

F \0 t DOCKET H.UMBER 9% *.~

PROD. &UTILFC50-7 L.

. 4 June 6, 1991 VYV 0 91-135 C-. C,,

EP C-)

~.0 U.S. Nuclear Regulatory Commission > * -o .0-Document Control Desk C.)

Washington, D.C. 20555 C,)

IR-V

REFERENCE:

Operating License DPR-28 Docket No. 50-271 Peportable Occurrence No. LER 91-09

Dear Sirs:

An defined by 10 CFR 50.73, we are reporting the attached Reportable Occurrence as LER 91-09.

This report was originally scheduled for submittal on 05/23/91. However, a two week extension was granted on 05/22/91 by R. Barkley, Acting Section Chief, Reactor Projects 3A (via T. Hiltz, NRC Resident Engineer at Vermont Yankee).

Very truly yours, VERMONT YANKEE NUCLEAR POWER CORPORATION 4t. Donald A. Reid Plant Manager cc: Regional Administrator USNRC Region I 47S Allendale Road King of Prussia, PA 19406 &&D"aaeOIn.UM~~J kilaum~CY e5Wýýd&

• ,. J f"°

• b 7

-- elf .- "e,-v x Fr[*

NRC Form 366 U.S. NUCLEAR REGULATORY COMMISSION APPROVED OHS NO.3160-0104 (6-09) .EXPIRES 4/30/92 ESTIMATED BURDEN PER RESPONSE TO COMPLY WIlTH THIS INFORMATION COLLECTION REQUEST:

50.0 HRS. FORWARD COMMENTS REGARDING LICENSEE EVENT REPORT (LER) BURDEN ESTIMATE TO THE RECORDS AND REPORT MANAGEMENT BRANCH (P-530). U.S. NUCLEAR REGULATORY COMMISSION, WASHINGTON, DC 20555, AND TO THE PAPERWORK REDUCTION PROJECT (3160-0104). OVFICE OF MANAGEMENT AND BUDGET, WASHINGTON, DC 20603.

FACILITY NAME (") IDOCKET NO. (') I PAGE *;)

VERMONT YANKEE NUCLEAR POWER STATION .0 I 1 10o 10 2 17 1 0 O1 IO 0 TITLE (')

Reactor Scram Due to Loss of Normal Off-site Power (LNP) Caused By Inadequate Procedure Guideline EVENT DATE Is) LER NUMBER ( .) REPORT DATE (') OTHER FACILITIES INVOLVED (_)

DAY YEAR YEAR _SEO. 0 REVII MONT DAY I YEAR FACILITY NAMES DOCKET NO.(S) j H 0In1 d d II 042391 iL 10j0019 101005 213 111j OPERATING THIS REPORT IS SUBMITTED PURSUANT TO REQ'MTS OF 10CFR.6: 3' ONE OR MORE (1"1 NODE ) N - 20.402(b) L 20.405(c) X 50.73(a)(2)(iv) _ 73.71(b)

POWER 11 - 20.405(a)(1)(i) 50.36(c)(1)-- 5O.73(a)(2)(v) T3.Rifc)

LEVEL(10) 20.405(a)(1)(ii) 50.36(c)(2) 50.73(a)(2)(viti) OTHER:

PO.ER. 20.405(a)(1)(iii) 50.73(a)(2)i) 50.73(a)(2)(viii)(A)

......... 20.405(a)(1)(iv) - 50.73(a)(2)(ii) 5O.73(a)(2)(viii)(B)

............. 20.405(a)(1)tv) 50.73(a)(2)(iii) I_50.73(a)(2)_(_)

LICENSEE CONTACT FOR THIS LER (18)

NAAM TELEPHONE NO.

CODE DONALD A. REID, PLANT MANAGER 100I 21g A l COMPLETE ONE LINE FOR EACH COMPONENT FAILURE DESCRIBED IN THIS REPO T 0ai)

CAUSE SYST COMPNT MFR REPORTABLE ..... CAUSE SYST COMPNT MFR REPORTABLE ......

- TO NPRD . TO NPRDS ......

X FIE Ad E1 Q3 N .... N/A j !11 IIL ..

X FIK I11 Wi23*3 N ..... N/A I I SUPPLEMENTAL REPORT EXPECTED (1) EXPECTED MO DA YR SUBMISSION X7 YES (If yes, complete EXPECTED SUBMISSION DATE) NO DATE {ss) a 8 30 1 ABSTRACT (Limit to 1400 spaces, i.e., approx. fifteen single-space typewritten lines) (*6)

On 04/23/91 at 1448 hours0.0168 days <br />0.402 hours <br />0.00239 weeks <br />5.50964e-4 months <br />, during normal operation with Reactor power at loo,* a Reactor Scram occurred as a result of a Generator/Turbine trip on Generator Load Reject due to the receipt of a 345KV Breaker Failure Signal. The Failure Signal was the result of Breaker Failure Interlock (BFI) signals that occurred simultaneously in the 345KV and 116KV Breaker control circuitry during the restoration of a battery bank to Switchyard Bus OC 4A.

The cumulative effects of both (BFI) signals resulted in a total loss of 345KV and 115KV off-site power. An Unusual Event was declared at 1507 hours0.0174 days <br />0.419 hours <br />0.00249 weeks <br />5.734135e-4 months <br />. Both Emergency Diesel Generators provided power for essential safety related systems during the LNP until approximately 0430 hours0.00498 days <br />0.119 hours <br />7.109788e-4 weeks <br />1.63615e-4 months <br /> on 14/24/91 at which point off-site 345KV power was restored and backfed through the Station Auxiliary Transformer. During the evert, Torus Water volume exceeded the Technical Specification limit of 70,000 cubic ft. The Unusual Event was terminated at 1950 hours0.0226 days <br />0.542 hours <br />0.00322 weeks <br />7.41975e-4 months <br /> on 04/24/91. The reactor reached Cold Shutdown at 0357 hours0.00413 days <br />0.0992 hours <br />5.902778e-4 weeks <br />1.358385e-4 months <br /> on 04/26/91 and was returned to critical at 0300 hours0.00347 days <br />0.0833 hours <br />4.960317e-4 weeks <br />1.1415e-4 months <br /> on 04/30/91. The Root Cause of this event is failure of the repair department personnel to recognize the consequences of operating a DC bus without a connected battery bank. Corrective Actions to prevent reoccurence are presently being finalized and will be presented in a supplemental report.

'WURC Form 366A U.S. NUCLEAR REGULATORY CCMMISSION APPROVED OHS NO.3160-0104 IS-S9)EXPIRES 4/30/92 I ESTIMATED BURDEN PER RESPONSE TO COMPLY WITH THIS INFORMATION COLLECTION REQUEST:

50.0 HRS. FORWARD COIE MIS REGARDING LICENSEE EVENT REPORT (LER) BURDEN ESTIMATE TO THE RECORDS AND REPORTS TEXT CONTINUATION MANAGEMENT BRANCH (P-630), U.S. NUCLEAR REGULATORY CoGISSION, WASHINGTON, DC 20555, AND TO THE PAPERWORK REDUCTION PROJECT (3160-0104), OFFICE OF MANAGEMENT AND BUDGET, N.SHI0GTON._DC.20603.

UTILITY NAME (') DOCKET NO. (') 'ER sEo. S (I !REve PAGE (S)

LER NUMBER

_VERNNOT YANKEE NUCLEAR POWER STATION Of 51 Of7Il2 1 - 0O19I BIB*2IOF _9 TEXT (If more space is required, use additional NRC Form 366A) (i)

DESCRIPTION OF EVENT On 04/23/91 at 1448 hours0.0168 days <br />0.402 hours <br />0.00239 weeks <br />5.50964e-4 months <br />, during normal operation with Reactor power at lOOt, a Reactor scram occurred as a result of a Generator/Turbine trip on Generator Load Reject due to the receipt of a 345KV Breaker Failure Signal. The 346KV Breaker Failure Signal was received as a result of Breaker Failure Interlock (SFI) signals that occurred simultaneously in the 345KV Breaker 81-1T and 116 KY Breaker K-1 control circuitry.

The (BFI) signal from 116KV Breaker K-I initiated the following automatic system responses:

- Opening of 116KV Breaker K-186

- Opening of 346KV Breakers 379 and 381 The loss of 381 and 319 breakers removed all power sources to the Auto Transformer which in conjunction with the K186 trip resulted in a total loss of IISKV power.

The (BFI) signal from 345KV Breaker S1-IT initiated the following automatic system responses:

- Generation of 345KV Breaker Failure Signal

- Opening of 346KV Breakers 381 and IT

- Lockout of Main Generator 860P and 66GB relays, causing the wain Generator and Exciter Field breakers to open The Generator Primary and Backup Lockout relays initiated the following automatic system responses:

- Main Turbine Trip

- Opening of 34SKV Breaker 81-IT and Northfield Line trip at Northfield

- Attempted Fast Transfer of 4KV Buses 1 and 2 to the Startup Transformers but IISKV power was unavailable The cimulative effects of both (BFI) signals resulted in a total loss of 345KV and 115KV off-site power. However, an additional off-site power source was available through the Vernon Hydro Station Tie line. The 4KV Hydro station output, which is designated as a delayed access off-site power source, was available throughout the event.

Prior to the event, the plant was in the process of completing the replacement of Switchyard Battery Bank 4A in accordance with a Maintenance Department guideline. All work with the exception of restoring the connection of the battery bank to the DC 4A bus, was completed without incident. While performing the final sequence of actions necessary to reconnect the battery bank to DC Bus 4A, a DC voltage transient occurred on the bus which initiated the event.

NRC Form 366A (6-89)

W Form S36A U.S. NUCLEAR REGULATORY COMMISSION APPROVED HiS NO.3160-0104 EXPIRES 4/30/92 ESTIMATED BURDEN PER RESPONSE TO COMPLY WITH THIS INFORMATION COLLECTION REQUEST:

60.0 HRS. FORWARD COMMENTS REGARDING LICENSEE EVENT REPORT (LER) BURDEN ESTIMATE TO THE RECORDS AND REPORT TEXT CONTINUATION MANAGEMENT BRANCH (P-630), U.S. NUCLEAR REGULATORY COMMISSION, WASHINGTON, DC 20566, AND TO THE PAPERWORK REDUCTION PROJECT (3160-0104), OFFICE OF MANAGEMENT AND BUDGET, WASHINGTON. DC_20603.

UTILITY NAME (1) DOCKET NO. (') LER NUMBER ('I PAGE (I)

YEAR Rg. .. REVSI ERMONT YANKEE NUCLEAR POWER STATION 0* d 0d2A 71 9 111- 0I1 1g 1 " 0 1 a1 dL TEXT (If more space is required, use additional NRC Form 366A) (")

tESCRIPTION OF EVENT (cont.)

During the first second of the event (1448:29 hours), as a result of the inablility to reenergize 4KV buses I and 2 from Fast Transfer to the Startup transformers, all

.station-ads-fed-Arom these buses were-)ost. Major systemresponses to the loss of the power included the trip of Reactor Protection System (RPS)(*JC) "A" and "B" NO sets and receipt of Primary Containment Isolation Signals (PCIS)(*JM) Groups 1, 2. 3 and S resulting in the required closure of PCIS Groups 1, 2. and 3 isolation valves. (Motor operated valve closures within these Groups occurred after Emergency Diesel Generator power was supplied to the respective buses).

The loss of all power on 4KV Buses I thru 4 initiated the opening of Tie breakers 3T1 and 4T2 to provide isolation of Safety Buses 3 and 4 which, in the event of normal power loss, are aligned with the station Emergency Diesel Generators. An autostart of both diesels followed which reenergized Bus 3 and Bus 4 at 1448:45 hours. Both diesels remained in operation without incident until approximately 0430 hours0.00498 days <br />0.119 hours <br />7.109788e-4 weeks <br />1.63615e-4 months <br /> on 04/24/91 at which time off-site 345KV power was restored and backfed through the Station Auxiliary Transformer.

In response to the Scram, Operation personnel entered Emergency Operating Procedure OE 3100, "Scram Procedure" which governs reactor operation in a post-scram environment.

Immediate actions initiated at 1450 hours0.0168 days <br />0.403 hours <br />0.0024 weeks <br />5.51725e-4 months <br /> by Operations personnel to stabilize Reactor pressure and level included the manual lifting of Safety Relief Valve (SRV)-A. the manual Initiation of High Pressure Coolant Injection System (1PCI)(*BJ). and startup of both RHR loops in the Torus Cooling mode. Both RPS MG sets were successfully restarted and RPS buses reenergized at 1515 hours0.0175 days <br />0.421 hours <br />0.0025 weeks <br />5.764575e-4 months <br />. The initial scram was reset at 1533 hours0.0177 days <br />0.426 hours <br />0.00253 weeks <br />5.833065e-4 months <br />.

During the period from 14S0 hours on 04/23/91 to 1346 hours0.0156 days <br />0.374 hours <br />0.00223 weeks <br />5.12153e-4 months <br /> on 04/24/91, the combination of HPCI and Reactor Core Isolation Cooling (RCIC) (*BN) systems and SRV's were manually employed in accordance with procedure CE 3100 to Control Reactor pressure level.

The first use of RCIC system began at 1645 hours0.019 days <br />0.457 hours <br />0.00272 weeks <br />6.259225e-4 months <br /> on 04/23/91. During the above 23 hour2.662037e-4 days <br />0.00639 hours <br />3.80291e-5 weeks <br />8.7515e-6 months <br /> period, several additional events transpired. The following is a summary and discussion of those events:

• Energy Information Identification System (EIIS) component Identifier NRF Form 366A (6-89J

J-M fom 366A U.S. NUCLEAR REGULATORY COMMISSION APPROVED OMS NO.3160-0104

• EXPIRES 4/30/92 ESTIMATED BURDEN PER RESPONSE TO COMPLY WITH THIS INFORMATION COLLECTION REQUEST:

50.0 MRS. FORWARD COMENTS REGARDING LICENSEE EVENT REPORT (LER) BURDEN ESTIMATE TO THE RECORDS AND REPORTS TEXT CONTINUATION MANAGEMENT BRANCH (P-630). U.S. NUCLEAR REGULATORY COMMISSION, WASHINGTON, DC 20555, AND TO THE PAPERWORK REDUCTION PROJECT (3160-0104), OFFICE OF MANAGEMENT

_ANDBUDGET -WASHINGTON, VC_20603.

UTILITY MAME (1) I DOCKET NO. (,) I LER NUMBER (*) 1 PAGE (6)

ý-VERMONT YANKEE NUCLEAR POMER STATION d1ofddd 1 1I1 1Il -t10loI -lol ld lo CIO TEXT (If more space is required, use additional NRC Form 366A) (,)

DESCRIPTION OF EVENT (cont.)

A. Reactor Scrams on "Lo" Reactor Water Lev'1 were experienced at 1634 hours0.0189 days <br />0.454 hours <br />0.0027 weeks <br />6.21737e-4 months <br /> and 2112 hours0.0244 days <br />0.587 hours <br />0.00349 weeks <br />8.03616e-4 months <br /> on 04/23/91.

___ The first Scram occurred due to low Reactor water level duri r! the process of securing HPCI and transferring to RCIC. -Prior to the scram, reactor pressure and level had been steadily decreasing during the first 30 minutes of HPCI operation which prompted a change in cooling systems by Operations personnel. During the process of securing HPCI, Reactor Water level continued to decline to the 132 inch "Lo" level setpoint which initiated the Reactor scram. PCIS - Groups 2, 3. and 5 isolations which would normally initiate on "Lo" Reactor water level were already present from the initial Scram at 1448 hours0.0168 days <br />0.402 hours <br />0.00239 weeks <br />5.50964e-4 months <br />. After receiving the Scram, Operations personnel completed the transfer to RCIC for level and pressure control. Reactor pressure and level recovered after RCIC initiation. The Scram and PCIS Groups 2, 3, and 6 isolations were subsequently reset at 1648 hours0.0191 days <br />0.458 hours <br />0.00272 weeks <br />6.27064e-4 months <br />.

The second Scram resulted as a momentary drop in water level was experienced due to level shrink resulting from an increase in Reactor pressure experienced after cycling SRV-D. Water level dropped to approximately 112 inches during the pressure surge. The initiation of PCIS Groups 2, 3, and 5 logic occurred coincident with the level drop as required. The scram was subsequently reset at 2127 hours0.0246 days <br />0.591 hours <br />0.00352 weeks <br />8.093235e-4 months <br />. PCIS Groups 2 and 6 logic were reset at 2128 hours0.0246 days <br />0.591 hours <br />0.00352 weeks <br />8.09704e-4 months <br /> and Group 3 logic later reset at 2154 hours0.0249 days <br />0.598 hours <br />0.00356 weeks <br />8.19597e-4 months <br />.

B. Emergency Operating Procedure OE 3104, "Torus Temperature and Level Control Procedure",

was entered at 1633 hours0.0189 days <br />0.454 hours <br />0.0027 weeks <br />6.213565e-4 months <br /> and 2112 hours0.0244 days <br />0.587 hours <br />0.00349 weeks <br />8.03616e-4 months <br /> on 04/23/91 due to Torus water volume exceeding the Technical Specification limit of 70,000 cubic ft.

In both occurrences, actions were taken in accordance with OE 3104 to reduce Torus water volume. Water reduction actions undertaken after the first entry into OE 3104 were successful and Torus water volume was reduced and maintained below 10,000 cubic ft. Later in the event, at 2112 hours0.0244 days <br />0.587 hours <br />0.00349 weeks <br />8.03616e-4 months <br />. Torus water volume was not able to be maintained below 70,000 cubic ft. This resulted in the entry into the Technical Specification, "Required Cold Shutdown in 24 Hour" requirement. Due to the volume limitations of Torus water being processed through Radwaste. the Torus volume remained above 70,000 cubic ft. until 1926 hours0.0223 days <br />0.535 hours <br />0.00318 weeks <br />7.32843e-4 months <br /> on 04/24/91. The Technical Specification cold shutdown requirement and OE 3104 were excited at this time.

C. RCIC tripped on overspeed at 1904 hours0.022 days <br />0.529 hours <br />0.00315 weeks <br />7.24472e-4 months <br /> on 04/23/91. The overspeed trip was reset at 1912 hours0.0221 days <br />0.531 hours <br />0.00316 weeks <br />7.27516e-4 months <br /> and operation of the system resumed.

Energy Information Identification System (EIIS) Component Identifier NRC Form 366A (6-89)

-C'1 Form 366A U.S. NUCLEAR REGULATORY COMMISSION APPROVED OHS 0O.3160-0104 (64s) eEXPIRES 4/30/92 ESTIMATED BURDEN PER RESPONSE TO COMPLY' WITH THIS INFORMATION COLLECTION REQUEST:

60.0 HRS. FORWARD COMMENTS REGARDING LICENSEE EVENT REPORT (LER) BURDEN ESTIMATE TO THE RECORDS AND REPORTS TEXT CONTINUATION MANAGEMENT BRANCH (P-30), U.S. NUCLEAR REGULATORY COMMISSION, WASHINGTON, DC 20566, AND TO THE PAPERWORK REDUCTION PROJECT (3160-0104), OFFICE OF MANAG EMENT

'UTILITY NAMIE ( ) DOCETAND BUDGET, WASHINGTON DC 20603.

'UII)OCKET NO. LER NUMBER PAGE_

N AYEAE P SEC. REV# o VERMOlNT YANKEE NUCLEAR POIE R STATION 4 510 4 t;_ 1 0: n_ ! I 9-f:Iode Fo TEXT (If more space is required, use additional NRC Form 366A) (,)

DE C=IPTION OF EVENT (cont.)

The ts ip is attributed to an operator error in the adjustment of the RCIC Flow Controller prior to switching from the MANUAL to AUTO mode.

D. The "A" Station Air Compressor tripped at 1WS42 _ours QnQO4123/1idue-to-_nadequate Service Water cooling flow. A reserve diesel air compressor was subsequently connected to the outlet of the "D" Station air compressor and became operable at 1759 hours0.0204 days <br />0.489 hours <br />0.00291 weeks <br />6.692995e-4 months <br />.

The rematning "B" Station Air compressor also tripped at 1731 hours0.02 days <br />0.481 hours <br />0.00286 weeks <br />6.586455e-4 months <br /> on thermal overload due to inadequate Service Water cooling flow and was subsequently restarted at 1736 hours0.0201 days <br />0.482 hours <br />0.00287 weeks <br />6.60548e-4 months <br />. The "C" and "O" station Air compressors were unavailable due to the LNP. The five (6) minute interval in which all Station Air compressors were out of service resulted in a 16 psig. Instrument Air header pressure drop. In response to the "B" Station Air Compressor Trip, Operations personnel entered procedure ON 3146, "Low Instrument/Scram Air Header Pressure", and initiated imediate efforts to restart the "B" Station Air Compressor. No air supplied equipment malfunctions were experienced during this interval. The reduced Service Water flow to the Station Air compressors and other plant equipment is being reported separately as Licensee Event Report (LER) 91-12.

At 1925 hours0.0223 days <br />0.535 hours <br />0.00318 weeks <br />7.324625e-4 months <br /> on 04/23/91, 116KV Breaker KISB was manually closed which restored power to the Startup transformers via the Keene (K18S) line. 4 KV bus breakers 13 and 23 were subsequently closed to reenergize Buses I and 2 which power the normal station loads. Because of the fact that testing was continuing in the Switchyard with only one breaker closed, the decision was made to leave the emergency diesels connected to 4MV buses 3 and 4. This would ensure that power to 4KV buses 3 and 4 would not be interrupted if another LNP occurred.

At 1950 khurs on 04/24/91, based on normal off-site power having been restored and Torus water volume having been reduced below 70,000 cubic ft., the Unusual Event was terminated. At 0207 hours0.0024 days <br />0.0575 hours <br />3.422619e-4 weeks <br />7.87635e-5 months <br /> on 04/26/91, Shutdown Cooling using the "0" RHR pum on the "B" loop was initiated. The reactor reached cold shutdown at 035? hours.

The reactor was returned to critical at 0300 hours0.00347 days <br />0.0833 hours <br />4.960317e-4 weeks <br />1.1415e-4 months <br /> on 04/30/91.

Investigations into the cause of the event, along with troubleshooting, testing, and repair efforts were initiated imeidiately after the start of the event. A Switchyard response team was formed with specific directives to:

- recover off-site power

- stabilize the switchyard

- gather technical information related to the evcnt

- begin root cause analysis research NRC Form 356A (6-89)

zTh.-

RC Fore 365A U.S. NUCLEAR REGULATORY COHPIISSION APPROVED OHS M0.3150-0104 l(-49)" *EXPIRES 4/30/02 ESTIMATED BURDEN PER RESPONSE TO COMPLY WITH THIS INFORMATION COLLECTION REQUEST:

50.0 MRS. FORWARD COMMENTS REGARDING LICENSEE EVENT REPORT (LER) BURDEN ESTIHATE TO THE RECORDS AND REPORTE TEXT CONTINUATION KANAGEMENT BRANCH (P-630), U.S. NUCLEAR REGULATORY COMMISSION, WASHINGTON, DC 20555, AND TO THE PAPERWORK REDUCTION PROJECT (3160-0104). OFFICE OF MANAGEMENT E ...... __ _ .... I AND BUDGET, ASHINGITON, DC 20603.

UTILiTY NAME (D) DOCKET NO. () ' ER NUM ER (*) PAGE (0)

YEAR SEO.a REVSI I I VERMONT YANKEE NUCLEAR POWER STATION d061dd d 1 12 91 - " oI e1 - oIod GIOFId I TMXT (If more space is required. use additional NRC Form 366A) (")

DESCRIPTION OF EVENT (cont,)

The recovery of off-site power began with the attempt to restore 11SKV power from the Switchyard via 11SKV Breaker KISS and the Startup transformers. This was determined to be the easiest path In obtaining an off-site power source due to the need to close only

_one-breakerl-.owederthe -KI -Oreaker-SF I-s4gnat - remained locked -in-due-to-a -failed zener diode on the associated trip card and prevented the closure of KISS. At 1925 hours0.0223 days <br />0.535 hours <br />0.00318 weeks <br />7.324625e-4 months <br />, the BF1 signal from the KI to the K1S6 Breaker was blocked allowing reclosure of KISS and subsequent restoration of power to 4KV buses I and 2. The KI SF? trip card was subsequently replaced with an identical card from a spare breaker. The 4 hour4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> effort to close the 91186 breaker was a direct result of the length of time required for New England Power Service Co. (NEPSCO) relay technicians to travel to Vermont Yankee from Providence, Rhode Island.

After 116 KiV poer as established through the Keene KISS line, efforts to close Breaker KI continued in order to establish a more reliable source of 11SKV power through the Auto Transformer. Hotever, due to communication problems between VY and the New England Switching Authority (RENVEC) concerning priorities over breaker testing, a three hour delay occurred before 2S1KV power was made available through the Auto Transformer. While Vermont Yankee was attempting to close the I1 breaker, REWVEC was pursuing efforts to establish connections between the ring bus and thme Northfield line by reclosing the S1-IT breaker.

In a parallel effort, at 1900 hours0.022 days <br />0.528 hours <br />0.00314 weeks <br />7.2295e-4 months <br />, Operation orders were given to complete backfeeding of the plant from the 345 yard through the Rain Transformer. The effort to backfeed was possible due to the availability of the Coolidge and Scobie lines.

The Northfield line was unavailable due to the S1-IT SFI signal. Again, the backfeed effort was hampered by Communication problems with RENVEC, personnel delays, and equipment malfunctions. Backfee-ing was completed at 0410 hours0.00475 days <br />0.114 hours <br />6.779101e-4 weeks <br />1.56005e-4 months <br /> on O4/24/91.

Vermont Yankee Technical Specification requirements for Off-Site Power were met during the Sackfeeding effort by the availability of one off-site transmission line (Keene KISS line in service) and a delayed access power source (Vernon Nydro Station).

In conjunction with the above efforts. Maintenance department personnel with the help of technicians SUpplied by NEPSCO and the battery charger vendor, performed preventative and corrective maintenance on the four battery chargers related to DC Bus "A and 6A. Significant repairs and testing were performed on the affected units.

Additional testing and repair% were initiated to the Stuck Breaker Failure Unit (S8FU) Logic trip cards for the 81-IT, 361 and KI breakers. The cards for 381 and KI breakers tere found to have failed zener diodes. The 81-IT (SBFU) relay was found to be functioning properly.

NRC Form 366A (6-69)

ItC Fo* 3S66A U.S. NUCLEAR REGULATORY COMIISSION APPROVED ONS NO.3150-0104 S.EXPIRES 4/30/92 ESTIMATED BURDEN PER RESPONSE TO COMPLY WITH THIS INFORMATION COLLECTION REQUEST:

60.0 HRS. FORWARD COMMENTS REGARDING LICENSEE EVENT REPT (LER) BURDEN ESTIMATE TO THE RECORDS AND REPORTE TEXT CONTINUATION MANAGEMENT BRANCH (P-530). U.S. NUCLEAR REGULATORY COMMISSION, WASHINGTON, OC 20566, AND TO THE PAPERWORK REDUCTION PROJECT (3160-0104). OFFICE OF MANAGEMENT

._____AND BUDGET, WASHINGTON. OC 20603.

PAGE ('2 UTILITY NAME (') DOCKET NO.

,YEAR1 ('3 LER NUMBER SEO. 8 (a) REVS

-V 01WT YANKEE NUCLEAR POWER STATION Of 11 dd 2 911-101019 - 1o010 7 OF 010 TEXT (If more space is, required. use additional NRC Form 366A) (")

DESCRIPTION OF EVENT (cont.)

Discussions with the manufacturer indicated that the zener diodes are no longer employed on newer revision trip cards and have recommended the removal of the zener diodes based on their vulnerability to voltage transients. Based on this recommendation.

the Maintenance Dept. has removed the zener diodes from these units in accordance with written-direc-tion frtF~Indor.

After response team efforts were completed, a Root Cause/Corrective Action Report (CAR) was drafted on the event from a Switchyard perspective. In the draft report, the following conclusions were reached:

- The voltage transient on the OC 4A bus occurred when battery charger 4A-SA was disconnected from the DC-6A bus which rendered bus DC 4A susceptible to voltage spikes due to the absence of a battery bank.

- The specific cause of the zener diode failures which resulted in the 81-IT and KI breaker (BFI) signals is attributed to the voltage transient which occurred on Bus DC U.

- A portion of the additional problems found with DC Bus 4A and SA battery chargers which ranged from shorted diodes/SCRs and blown surge suppressor fuses, were concluded to be pre-existing and were responsible for the voltage transient.

CAUSE OF EVENT The Root Cause of this event is the failure of the repair department personnel to recognize the consequences of operating a DC bus without a connected battery bank.

The Maintenance Guideline, an internal Maintenance Department document prepared by the department Electrical Engineering staff, was inadequate in that it did not take into consideration all battery charger failure modes when floating a DC bus without a battery bu*. The consequences of losing battery charger power while the bus is energized without a battery connected were considered during the revision of the Guideline, but not the potential of the battery chargers to fail high or induce a high voltage spike on the bus, both which have the potential to damage electronic circuitry.

The previous revision of the Guideline called for the two DC buses (4A & SA) to be cross-connected and fed jointly by the 4A/SA battery charger during the maintenance on the batteries. Following cross-connection, the Guideline required opening of the battery breakers. This evolution was successfully accomplished and the required work on the IRC Form 36)A (6-69)

IRC Form 366A U.S. NUCLEAR REGULATORY COMMISSION APPROVED ONS NO.3160-0104 EXPIRES 4/30/92 ESTIMATED BURDEN PER RESPONSE TO COMPLY WITH THIS INFORMATION COLLECTION REQUEST:

50.0 HRS. FORWARD COMMENTS REGARDING LICENSEE EVENT suEPORT (LER) BURDEN ESTIMATE TO THE RECORDS AND REPORTI FEXT CONTINUATION MANAGEMENT BRANCH (P-630). U.S. NUCLEAR REGULATORY COMMISSION, WASHINGTON. DC 20566, AND TO THE PAPERWORK REDUCTION PROJECT (3160-0104), OFFICE OF MANAGEMENT AND BUDGET, WASHINGTON. DC 20603.

UTILITY NAME (') DOCKET NO. "() SEQ. 0 I V'PAGE YER LER NUMBER IE V' (2)

VEftONT YANKEE NUCLEAR POWER STATION Ofd ddd01- SIJ-1 0 1019 L 0- dS 1OF TEXT (If more space is required, use additional NRC Form 366A) (")

&ME OF EVENT (cont.)

batteries was completed without incident. Recovery of the battery required the closure of the battery output breaker first, essentially paralleling the two battery banks until the 4A/BA charger output breaker was opened. In June 1990. the Guideline was revised

-due-to-Operations-Department concer-n-w ith-par-alelng-batter es---The--new revision -required that the cross connection between bus 4A and SA provided by battery charger &A/BA be opened prior to the reclosure of the bus 4A battey breaker. This configuration rendered bus 4A without a battery and susceptible to voltage excursions from either the 4A or 4A/6A battery chargers.

CONTRIBUTING CAUSES

1. 345KV and 115KV breaker failure relays were susceptible to false initiation due to control voltage transients.

2. The switchyard battery chargers were in a degraded mode such that they created DC bus control voltage disturbance when the chargers were disconnected from associated batteries.

3. Lack of Switchyard battery charger and overall Switchyard preventative maintenance.

ANALYSIS OF EVENT The events had minimal adverse safety implications.

1. The plant responded to the reactor trip and LNP as designed. The Emergency Diesel Generators operated as designed and supplied power to Emergency plant buses until off-site power was restored.

2. The Reactor Protective Systen operated as designed and scrammed the reactor on Generator Load Reject resulting from the 345KV Breaker Failure Signal

3. An evaluation was performed by the Operations Department relevant to the loss of both "A" and "B" Station Air compressors. The analysis concluded that the 6 minute interval in which tle "B" Station Air compressor was out of service which resulted in a 16 psig. drop in the station air supply system did not significantly challenge any plant equipment.

4. All other safety systems responded as expected.

NRC Form 366A (6-89)

Form 36SA TC U.S. NUCLEAR REGULATORY COMMISSION APPROVED OHS NO.3160-0104

(.6-9) EXPIRES 4/30/92 ESTIMATED BURDEN PER RESPONSE TO COMPLY WITH THIS INFORMATION COLLECTION REQUEST:

50.0 HRS. FORWARD COMENTS REGARDING LICENSEE EVENT REOORT (LER) BURDEN ESTIMATE TO THE RECORDS AND REPORTS TEXT CONTINUATION MANAGEMENT BRANCH (P-630). U.S. NUCLEAR REGULATORY COMMISSION, WASHINGTON, DC 20565, AND TO THE PAPERWORK REDUCTION PROJECT (3160-0104), OFFICE OF MANAGEMENT I AND BUDGET, WASHINGTON, DC 20603.

UTILITY NAME (D) DOCKET NO. (I) LER NUMBER (0) PAGE (5)

YEAR SEp. UI REV#

VEMONTYANKEE NUCLEAR POWER STATION 01 Q d(A 0f I9 1 " 0 10 19 -I0 oI dJ O.F TEXT (If more space is,required, use additional NRC Form 366A) (t,)

CORRECTIVE ACTIONS

" SHORT TERM CORRECTIVE ACTIONS

1. Immediate corrective actions included recovering from the reactor scram, restoration Qf1-off--site-

- *-, and Switchyard and reactor stabilization utilizing appropriate plant procedures.

2. The current revision of the Maintenance Dept. Guideline has been cancelled and the previous revision reinstated with an additional requir.ement that a review be performed prior to its use for dealing with any evolution requiring switchyard battery removal.

3. Review all other plant guidelines and Procedures pertaining to battery switching operations.

LONG TERN CORRECTIVE ACTIONS TER .. ...C'V .. ..

ACTION Long Term Corrective Actions are presently being addressed per our Root Cause/Corrective Action process. The Corrective Action Report is presently being finalized. In accordance with prior commitments made to the NRC at the AIT exit meeting held in King of Prussia on 05/14/91, a letter detailing plant Corrective Actions to be initiated in response to the event and NRC concerns will be forwarded to the NRC by 07/15/91. Based on information presented in the finalized Corrective Action Report, a supplement to this report will be forwarded to the Commission.

ADDITIONAL INFORMATION There have been no similar events of this type reported to the commission in the past five years.

ATTACHMENTS Sketches: a. Switchyard Distribution

b. Switchyard DC Bus System NRC Form 366A (6-89)

NC Form 3S6A U.S. NUCLEAR REGULATORY COMMISSION APPROVED OHS NO.3160-0104 (6-691

• EXPIRES 4/20/92 ESTIMATED BURDEN PER RESPONSE TO COMPLY WITH THIS INFORMATION COLLECTION REQUEST:

50.0 ARS. FORWARD COMMENTS RF.OARDING LICENSEE EVENT REPORT (LER) BURDEN ESTIMATE TO THE RECORDS AND REPORTS TEXT CONTINUATION MANAGEMENT BRANCH (P-530), U.S. NUCLEAR REGULATORY COMMISSION, WASHINGTON. DC 20566, AND TO THE PAPERWORK REDUCTION PROJECT (3160-0104), OFFICE OF MANASEMENT AND BUDGET,, WASHINGTON. DC 20503.

UTILITY ANKE 1'1 DOCKET NO. (') LEA NUMBER 1) 1PAEf (

.VE3SNT YJKEE NUCLEAR POWER STATION d-At dd ih 71 1 1I- 0I0 191- 1 1 OF II TEXT (If more spce is required, use additional NAC Form 366A) (1,)

ScMTM!?9go A - - Y I £

( B; ~Afl leia %6obM "9

F 4

I 6

_,L i

95¶o*5*1 F V 0 gA. d.

t l

LFL WC Form 36" (6-09)

W Form 36SA U.S. NUCLEAR REGULATORY COMINSSION APPROVED OHS NO.3150-O104 (S-S91 JEXPIRES 4/30/92 ESTIMATED BURDEN PER RESPONSE TO CONPLY' WITH THIS INFORMATION COLLECTION REQUESTs 50.0 HRS. FORWARD COMHENTS REGARDING LICENSEE EVENT REPORT (LER) BURDEIN ESTIMATE TO THE RECORDS AND REPORTI TEXT CONTINUATION MKAAEMENT BRANCH (P-530), U.S. NUCLEAR REGULATORY COPMISSION, WASHINGTON, DC 20555. AND TO THE PAPERWORK REDUCTION PROJECT (3160-0104). OFFICE OF MANAGEMENT

,__________ ANDBUDGET, WASHINGTON, DC 20603.

UTILITY NAM OCKET 00. (9)I D' L[* NU*IB_ 14 1, 1 AE Is)

M SYEAR SEQ.e_ REV# I F I VEIMONT YANKEE NCLEARP PER STATION 19 1lol- - I I OF Ii m - - -

TEXT (If more space is required, use additional NAC Form 366A) (19)

-cm

• !~21: CATEMZT PA--iEv e#%ASLr 40k F-,

I JL

-F-- .....

m

-a-4TRTA R I 12.5 1 Jbc '0o Ii

-k --

't3 0fasl Dc plic A 1-1 AT~

SwITC::H YA.D DC BUS SV'STttA WC Form 36 (*-fl)