ML062640130

Vermont Yankee Hearing - Entergy Exhibit 19, LER 1991-014-00, Vermont Yankee, Reactor Scram Due to Loss of 345KV Switchyard Caused by Defective Off-Site Carrier Equipment

ML062640130
Person / Time
Site:	Vermont Yankee
Issue date:	07/11/1991
From:	Reid D Vermont Yankee
To:	Document Control Desk, NRC/SECY
Byrdsong A T
References
50-271-OLA, Entergy-Licensee-19, RAS 12269, VYV #91-148 LER 91-014-00, NUDOCS 9107160385
Download: ML062640130 (6)

Text

DOCKETED Y'BTIONT YANKEE USNRC

'NUCLEAR POWER CORPORATION 116 SEP 19 PH1 3:36

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On 06/15/91 at 2224 bourws during aomlkd operation with Reactor pOvur at 1001, & Reactor krm occurred due to & Turbine Control Valve ast Closure on Generator Lzad Reject resulting ftro a lost of the 3451V North vitchyard Sue. Uhe event vwa initiated during a thundeittots ix which a liehtnin strike occurred on the W' phue of the 311 transtais1o. line betten Vemont Yankei and Northfield. Mhe fault resulted in the opening -f all 345K Air Trip breakers (ATO).

Ouring the event, a subeeqfuent Reactor Sram aid corretpoding Primary Contatimnt Isolation Signauls (M ) I() Groups 3 vere received due to Lev Reactor later level. The Reactor vat sU bilised In Not Standby using the gain Condenser, Conden"te, and Feedvater systems.

At 2100 hours0.0243 days <br />0.583 hours <br />0.00347 weeks <br />7.9905e-4 months <br /> on 06/16/91, after Reactor depretsuriSation was completed, Shutdovn Cooling ust the "Do R 0""1 on the OP loop was lnaitated. the reactor reached Cold Sbutdova at 0M bouts on 06117/91. The reactor vaz returned to critical at 1413 hours0.0164 days <br />0.393 hours <br />0.00234 weeks <br />5.376465e-4 months <br /> on 06120/91.

The Root Cause of this event is a defective (shorted) transistor in offuite (Scoble Fond) hotective Belaying Stem Carrier equipment. the need to perform additional testing of Carrier systems is be hag evaluated.

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On 06/15/91 at 2224t22 hours, during normal operation with Reactor power at 100%1, a Reactor scram occurred as a result of Turbine Control Valve fast Closure on Generator Load Reject due to a loss of the 34SKV North Svitchyard hs. The event was initiated during a line thunderstorm SWhich a lIghtning strike occurred on the 03' phase of the 381 transmission betveen Vemont Yankee and aorthfield, Ka. The fault resulted in the opening of the 81-IT and 381 Air

-Trip-lreskers-(?TUs).---n-umanticlpated trip of the 379 Scobie line on Carrier Overreach also occured coincident vith the fault resulting in trips of the 379 and 79-40 Alas. The cunulative effect of the breaker openings left only the Coolidge (340) Line connected to345KV Vermont Yankee.

This line subsequently tripped on overload, opening the 1 AlT. Vith all AlMs open, all load paths for Vermont Yankee', output were shad which resulted in a Generator Load Reject and subsequent plant scram.

Following the Generator Load Reject and Turbine Control Valve Fast Closure, plant buses remained connected to the Vain Generator via the Aux Transfomer for approximately 30 seconds it vhich point the Turbine tripped from a "Lo" Scram Alr Reader Pressure Tim Delayed Signal.

Durinu the first 10 seconds of this interval, plant buses experienced voltage oscillations While H ai Generator voltage output attempted to regulate ouring the transition from 100%

to approximately 5K load. the voltage oscillations experienced resulted in the following major system responsess a Primary Containment Isolation System (?CIS)(*JI) Groups Us, 21, 3A, SA and 53 eore received due to low 120VAC Instrument bus voltarv resulting in the closure of Group 5 Isolation valves as required.

- *A* and 034 Station Air Compressors tripped due to low 12OVAC Instrument bus voltage. both air compressors were restarted at 2233 hours0.0258 days <br />0.62 hours <br />0.00369 weeks <br />8.496565e-4 months <br />.

- Reactor lecirculation Units (11Us) 2 and 4 Tripped due to dropout of a 120VAC Dryvell Cooling and Control loom Air Conditioning Blocking relay from low voltage. Both nus vert restarted at 2233 hours0.0258 days <br />0.62 hours <br />0.00369 weeks <br />8.496565e-4 months <br />.

0'5' and 'C' Reactor Feedvater 1omps Tripped on Low Suction Pressure resulting from transients in the Condensate System which were caused by the undervoltage conditions. Feed flov was restored vithin 10 seconds.

OAP and "5' lecirc Pump Breakers opened due to Low Lube Oil Pressure. The loss of Lube Oil vas a result of blown control circuit fuses.

- 'A and 0B" Advanced Off Gas (LOG) Recombiners tripped due to low 120VAC Instrument bus voltage. This resulted in the blovout of a Steam Jet Air glector (SJAI) Rupture Disc.

In addition to the (low voltage) received ICS signals, a decreasing 127 inch 'L0' Reactor Water level was experienced 7 seconds into the event, at 2224:29 hours, generating a Reactor Scram and remaining ?CIS Group 23 and 3B isolation signals resulting in the required Group 2 and 3 Isolations. The water level reached a low of 122 inches and is attributed to void collapse from the initial Scram.

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Approximately 10 seconds into the event, at 2224.32 hours3.703704e-4 days <br />0.00889 hours <br />5.291005e-5 weeks <br />1.2176e-5 months <br />, the 381 ATO reclosed which reenergised the Auto Transformer. The 379 AT5 reclosed 12 seconds later at 2224t44 hours.

• Coincident vith the turbine trip at 2224t50 hours, a Generator Lockout vas initiated which resulted in fast Transfer of plant buses to the Startup Transformers. Vith reliable 1151V power available froo the Auto Transformer, 4KV and 480V Sus voltages remained stable from this point on.

In response to the Scram Operations personnel entered Emergency Operating Procedure 02 3100 'Scram Procedure' which governs reactor operation in a post-scram environment. Operators noted during the Scrams that approximately 25 of the Control Rods lacked VFull In* indication the associated rod display was blank). Reactor power was verified to be less than 2Z, by verage Power Range Nonitor (AIMI) doanscalse indication. This condition prompted the entry lato berg ncP rating Procedure 01-3101 leactor Pressure Vessel (IV) Control Procedure in which Scram as Initiated 2226 ht 22 hours and sub2equently reset at 2228 hours0.0258 days <br />0.619 hours <br />0.00368 weeks <br />8.47754e-4 months <br />. Un resetting of the Scrams all rods indicated 0008 and 01-3101 vas exited. The loss of indication for a portion of the Control lods is attributed to a known phenomena called rod overtravel in which a loss of position indication can occur if a control rod inserts sllhtly past the full

'In position resulting in a mlsalipnaent of the corresponding position indication switches.

• .. During the event, Reactor pressure and level were maintained using the Main Condenser, Coneonsate, and Peedvater systems. At 2100 hours0.0243 days <br />0.583 hours <br />0.00347 weeks <br />7.9905e-4 months <br /> on 06/16/91, Shutdown Cooling was initiated using the *DO INK pump on the S' loop. The reactor reached Cold Shutdovn at 0500 hours0.00579 days <br />0.139 hours <br />8.267196e-4 weeks <br />1.9025e-4 months <br /> on 06/17/91. The reactor was returned to critical at 1413 hours0.0164 days <br />0.393 hours <br />0.00234 weeks <br />5.376465e-4 months <br /> on 06/20/91.

The loot Cause of this event Is a defective (shorted) transistor in offiite (Scobie Pond)

Protective Rel System Carrier equipment. The lightning strike which occurred on the mBe phase of the 381 .Tr.asmission line between VT and Northfield, Ka. would normally have only resulted In an Isolation of the 381 line. However, the defective component in the Scoble Pond Carrier equipment caused a subsequent loss of the 379 line. This routed the full Generator output through the 340 (Coolidge) line. The Coolidge line cannot handle full generator output and tripped out on overload vhich resulted in a loss of the 345KV yard and caused the Reactor 1to Scrm on Generator Load Reject.

After the plant Scram, an extensive testing and troublshooting effort was performed by Vermont Yankee and Nev legland Pover Service Co. (REPSCO) to determine the cause of the Scobie Line Carrier trip. It was found that the the equipment on the V1 end operated as deslgned and sent a Carrier block signal to Scobie to prevent tripping. Although the signal was received at Scobie Pond, the trip sigal- was not blocked. A failed transistor in the Carrier equipment logic section prevented the blocking signal from reaching the tripping logic. Since the trippling logic did not see a blocking signal it caused the Scobie line to trip at Scobie Pond and Vermont Yankee.

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1. Lightning strike on the B phase of the Northfield line vas the contributing cause to the event.

Fi 'A.- 4 m The events had ainimal adverse safety implications.

1. The Reactor Protective System operated as designed and scrammed the reactor on Generator Load Reject resulting from the loss of 345KV pover.

2. Past transfer to an off-site source occurred as designed upon receipt of a Generator Lockout.

3. All other safety systems responded as expected.

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Imediate corrective actions included recovering from the reactor scrams, troubleshooting and repair of the ScobLe Pond equipment, and reactor stabilization utilizing appropriate plant procedures.

w~mzm VT Haintensnce Department and VELCO Svitchyard Engineers vill evaluate testing requirements for Svitchyard Carrier systems.

The above Long Term Corrective Action viii be completed by 11/01/91.

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

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