ML20086E418

AO 74-11:on 740705,08 & 09,both Plant Stack Monitoring Sys Failed.Caused by Direct Lightning Strikes to Ventilation Stacks During Electrical Storms.Radiation Exposure to Personnel Negligible

ML20086E418
Person / Time
Site:	Vermont Yankee
Issue date:	07/15/1974
From:	Riley B VERMONT YANKEE NUCLEAR POWER CORP.
To:	NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION I)
Shared Package
ML20086E426	List: ML20086E418 ML20086E428
References
AO-74-11, VYV-3330, NUDOCS 8312070149
Download: ML20086E418 (5)

Text

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n (o) U VERMONT YANFEE NUCLEAR POWER CORPORATION SEVEhTY SEVEN OROVE STRECT RuTLAso, Vua.stoxr osvon VYV-3330 -m P.,O. DOX 157 VERNON. VCRMONT oS354 July 15, 1974 N

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Director ff f, s .s i

Directorate of Regulatory Operations, Region I United States Atomic Energy Commission .

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REFERENCE:

Operating License DPR-28 L 2, 5 @4

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Docket No. 50-271 N /' V/ '

Abnormal Occurrence No. A0-74-11 (10 Day Report)

Dear Sir:

As defined by Technical Specifications for the Vermont Yankee iiueleat Euwet 5tativu, 5euluu 6.7, we nu icev&Line, Lhe folivo lu t, Abnormal Occurrence as A0-74-ll.

At 1324 on July 5,1974, during a severe electrical storm in which direct lightning strikes to the top of the plant ventilation stack were observed, both plant stack monitoring systems became inoperable. At approximately 2000 on July 8, 1974, and at 0900 and 1300 on July 9, 1974, both stack monitors were rendered inoperable again for a period of less than five minutes in each case in order to make necessary repairs on the monitors. These occurrences exceed the Limiting Condition of Operation of Technical Specifications section 3.9.B.1.a which requires that at least one of the two plant stack monitoring systems shall be operable at all times.

The plant was operating at approximately 80% power at ti.e time of the lightning strike. Approximately 30 seconds after the loss of the stack monitors another lightning strike resulted in a generator / turbine trip which caused a reactor scram. In addition to the loss of the stack conf toring systems, the perimeter fence r:onitor, the stack base fan, the process computer, miscellaneous annunciating and indicating circuits, five auxiliary equipnent transformers, the stack particulate tape transport, the No. 1 fan in the east cooling tower, and the instrumentation from the site meteorological tower were rendered inoperable.

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M 00FY SENT REGION # 674o

t_ 2 O O VERMONT YAMEE NUCLEAR POWER CORPORATION Directorate of Regulatory Operations July 15, 1974 Page 2 The A0G system was kept in service until approximately 0400 on July 6, 1974. In an effort to minimize the stack release rate, the turbine gland seal steam pressure was immediately reduced to <1 psig in accordance with revised operating procedure and the mechanical vacuum pump was not used until approximately 1700 on July 6, 1974. During the i period of time that the stack gas monitors were inoperable, grab gas samples were taken at the stack on a periodic basis. The attached table summarizes the results of these gas sampics. The maximum release rate l following the shutdown was 111,119 pCi/sec and this was observed at 1707 i on July 6, 1974. The Technical Specifications limit at this time was determined to be 1,330,000 pCi/sec based on an Ey of .06.

1 l At approximately 1030 on July 6, 1974, in an attempt to put the Steam l Jet Air Ejectors in service to reduce the reactor vessel pressure, the l Cor, trol Room Operator noticed that there was no flow indication through the system. An operator was immediately sent to investigate the air ejectors and he discovered steam coming from the air ejector room and rightly assumed l that the rupture disk in the off-gas line had ruptured. The air ejectors l

were then immediately secured and the rupture disk was replaced with a new one. It was postulated that the disk ruptured as a result of being flexed luward when the alt electors were shutdown nr 6466 nn in iv n. ann rnan flexed outward again when the air ejectors were placed ba'k c in service at 1030. Radiation exposure to personnel was negligible due to the fact that j no one entered the air ejector room until after the air ejectors were shutdown.

Repairing and testing of vital equipment and instrumentation that was damaged as a result of the lightning incident was completed and the plant j was started up on July 8, 1974.

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The stack particulate tape transport was repaired and returned to service during the af terncon of July 5,1974. The stack gas monitors were first declared fully operable at 1815 on July 6,1974. Prior to that time, difficulty was encountered while source calibrating the monitors due to the high activity background in the area of the gas detectors. Subsequent to declaring the monitors operable on July 6, it was determined that the monitors l wer'e still not performing satisfactorily. This was evidenced by the higher than normal readings en the monitors for the plant conditions and the discrepancy between the monitors and the release rate determined from gas grab samples taken at the stack. Stack Gas I was thoroughly inspected, repaired and further calibration was completed by 1945 on July 8. In order to repair the monitor, it was necessary to remove it from the instrument chasis. In order to do this, the electrical power to all of the stack monitors had to be interrupted for approximately five minutes, time enough to disconnect all of the cables to the Stack Gas I monitor and then restore power to the remaining monitors. The reactor was operating at approximately 37% power at this time. Stack Gas II monitor was removed from the same control room instrument panel on July 9, 1974, so that it could be repaired. This time i power had to be interrupted to the remaining monitors twice in order to perform the repair, once to remove the monitor and again to reinstall it, i

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s O O VERMONT YANKEE NUCLEAR POWER CORPORATION l

l Directorate of Regulatory Operations July 15, 1974 Page 3 The plant was operating at approximately 74% power at this time. A means to remove one of the monitors from the instrument panel without interrupting ,

power to the remaining monitors will be implemented in the near future.

The Plant Operations Review Committee met to review this Abnormal '

Occurrence and the events associated with it and concluded the following:

1. The loss of the stack gas monitors was the direct result of the plant stack being struck by lightning. ,

2. The stack gas release rate was at all times within the limits established by the State and the Technical Specifications. This determination was based on the results of grab samples taken at the stack and the fact that the stack monitors were left in service and did provide an indication of relative release rates even though they were not adequately calibrated.

In an effort to prevent the consequences of a direct lightning strike to the plant stack and the problets associated with a plant shutdown without the stack gas nonitors in service, the Plant Operations Review Committee

d the fellering ree =mendations:

1. Install an additional grounding network at the plant stack that I will be connected to the existing plant ground network.

2. Revise the applicable plant procedure to provide a means for operating personnel to electrically disarm and disconnect the cables from one of the two stack gas monitors when an electrical storm appears eminent. This action will ensure that at least one of the monitors is operable following an electrical storm. This is considered to be an interim measure to be implemenced until a '

permanent solution can be found.

3. Install electrical surge suppressors in the circuits of those

. control room annunciators specified by the Nuclear Services Division.

4. In an attempt to remove potential sources of conductance from the stack to the plant buildings, the stack base fan indicating lights will be removed from the control room.

5. Fabricate a calibration chamber that will shield the stack gas detectors from background radiation levels when the detectors are being source calibrated.

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• * '1ERMONT YANKEE NUCLEAR POWER CORPORATION Directorate of Regulatory Operations .

July 12, 1974 I Page 4 l

6. Alter the power source inputs to the stack monitors in such a manner that one of the monitors may be removed for repair i without interrupting the service of the remaining monitors, t

7. Continue the investigation of the c9use of component failures resulting from severe electrical storns. l Very truly yours,

. VERMONT YANKEE NUCLEAR POWER CORPORATION B.W. Riley H Plant Superintendent WPM /kbd 9

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STACKRELEASERATESF0i.LOWINGTHESHUTDOWNOF 5 July 1974 j Date Time Release Rate j 7/5/74 1354 <3,000 pCi/sec

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l 7/5/74 1444 <5,000 pCi/sec 7/5/74 1531 <5,000 pCi/see 1

7/5/74 1654 .

6,866 pCi/sec 7/5/74 1751 .

11,881 pC1/sec

! 7/5/74 2003 15,981 pCi/sec 7/6/74 08'03 3,284 uCi/sec 7/6/74 1308 15,941 pCi/sec 7/6/74 -

1707 111,119 pCi/sec

, 7/7/74 1411 11,823 pCi/sec 7/8/74 0730 16,730 pCi/sec j 7/9/74 1450 <5,000 pCi/sec i

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