ML20086D686

AO 73-16:on 730612,during Severe Electrical Storm in Which Direct Lightning Strike to Top of Plant Ventilation Stack Observed,Detonation in Offgas Sys Caused Sufficient Back Pressure to Fracture Air Ejector Rupture Disc

ML20086D686
Person / Time
Site:	Vermont Yankee
Issue date:	06/22/1973
From:	George Thomas VERMONT YANKEE NUCLEAR POWER CORP.
To:	US ATOMIC ENERGY COMMISSION (AEC)
Shared Package
ML20086D688	List: ML20086D686 ML20086D692
References
AO-73-16, VYV-2749, NUDOCS 8312020443
Download: ML20086D686 (4)

Text

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1 0 0 VERMONT Y. ANKEE NUCLEAR POWER CORPORATION GCVCNTY SCVEN OROVC STHCCT RuTI AND, VEltMONT 03701 REPt.Y TO:

VYV-2749 p, g, gox 337 VERNON, VERMONT 05354 June 22, 1973 l

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Director Directorate of Licensing y pi M_~D go] 3 7_.'

U.S. Atomic Energy Commission  ! zi y 7 Washington, D.C. 20545 t 34

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REFERENCE:

Operating License DPR-28 Docket No. 50-271 YN.))2 g , t'WI /

Abnormal Occurrence Na A0-73-16

Dear Sir:

As defined by Technical Specifications for the Vermont Yankee Nuclear rowet Station. $cclica 6.7.D.d av ace reporting the .followin5. Abnormal occurrence as A0-73-16.

At 1750 on June 12, 1973, during a severe electrical storm in'which a direct lightning strike to the top of the plant ventilation stack was observed, a detonation in the Off-gas System caused suf ficient back pressure to fracture the air ejector rupture disc, thereby degrading a boundary designed to contain radioactive materials as defined in Technical Specifications-Section 1.A.5.

Concurrently, the area gamma radiation monitor on the perimeter fence,' -

became inoperab1n. This instrumentation,is required at all times as a Limiting Condition for Operation by Technical Specifications Section 3.8. A.1.

Prior to, this incident the plant was operating normally at approximately 78% power with the Interim of f-gas System in service.

The cause of the detonation and instrument malfunct' ion is believed to-have been th'c lightning ettike. . This act of nature resulted-in the following events which necurred essentially simultaneously:

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1. Interim off-gas system tripped off due to loss of power

2. Stack gas monitoring system I tripped off

3. Aren gamma monitor at perineter fence inoperabic. ,

4. Off-gas flow indicator on CRP 9-6 inoperable

5. Panalarm system on ~ CRP 9-6 and 9-7. inoperabic (135 circuit boards i involyed) ~l Air. ejector rupture disc fractured

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KEE NUCLEAR POWER CORPORATIOM i l

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Director 1 June 22, 1973 Page 2 Following the incident, temporary repairs were made to the rupture disc to minimize gaseous radioactivity in the Turbine Building. Air samples taken in the air ejector room of the Turbine Building indicated a maximum noble gas activity of 8.45x10-4 p Ci/cc. The continuous air monitor in the Turbine llall indicated a maximum particulate activity of 2x10-8 p C1/cc. The Turbine Building roof louvers were closed at the time of the incident. The Stack Gas Monitoring System I was returned to service at 1815, and the perimeter fence monitor was restored to operation at 2010, and the reactor was completely shutdown at 0345 on June 13, 1973.

During the shutdown period, the air ejector rupture disc was replaced, the "A" stack particulate filter was replaced, the of f-gas piping was inspected for damage, all instrumentation connected to the SJAE off-gas system was calibrated, and defective panalarm cards were repaired. The plant returned to operation on June 14, 1973, without the IOG system in service.

It was further determined that;

1. ine poureu concrete stacx nas tnree z.o- 11gntning arrestors at the top with external metal platforms and a grounded ladder spanning its length. The stack is unlined; however, two 1" stainless steel monitoring lines run up the inside to within about 100 feet from the top.

2. There was no unmonitored release of radioactivity. The maximum indicated release rate was 6000 pCi/sec. Meteorological conditions at the time of the incident were:

Wind speed 18 to 23 mph Lower AT -0.2*F Wind direction 300 to 360 Upper AT +1.0*F Temperature 66*F Precipitation - heavy rain Dew point 63.5*F

3. Investigation and evaluation of Interim Off-Gas System (IOC) damage indicated the associated electronics suf fered the most extensive damage and will require major replacement and retesting of components before it can be returned to service.

The Plant Operations Review Committee (PORC) met to review the incident and made the following recommendations:

1. Determine the extent of damage to the IOG system, repair, and retest prior to returning the system to service.

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- U VERMONT Y;w(KEE NUCLEAR POWER CORPOHATION Director June 22, 1973 Page 3

2. Determine if the existing stack and IOG Building lightning protection systems are operating properly by checking for excessive resistance in the grounding cables.

3. Perform an engineering review of the stack and IOG Building liditning arrestor systems to evaluate their design adequacy.

4. Attempt to determine more precisely the cause of the detonation.

In response to the above recommendations, the Nuclear Services Division -

performed an engineering evaluation which resulted in the following comments:

1. It is believed that the IOG electronic failures were caused because the grounding of the IOG rack and panel was inadequate.

As a result, the subject equipment did not remain at the same potential as the grounding grid during the lightning stroke discharge, and thus, sensitive components failed due to this abnormal potential difference.

z. It is rurtner postulated that the stack received a direct ligntning strike which was intercepted by the gas column issuing f rom the stack, The Jightning discharge then continued through the stack gas to the piping at the base of the stack, and thus ignited the gas.

Their corrective recommendations are as follows:

1. A ground connection l a made between the IOG panel and the adjacent control board ground bus in the Control Room.

2. The grounding connections at the repeater rack in the IOG Building be inspected and remade if necessary to insure a proper connection to station ground.

3. Check that all other IOG electrical equipment in the IOG shack and elseuhere is properly grounded to station ground.

4. To increase the probability of intercepting lightning strokes above the top of the stack by the lightning rods instead of the gas l column, extend the rods to approximately fif teen feet above the top of the stack. i l

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' . e e VCRMONT Y,uYKEE NUCLEAR POWER CORPORATION Director

June 22, 1973

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The indicated corrective actions are currently underway and vill be cornpleted as soo.1 as possible.

Very truly yours, ,

VERMONT YANKEE NUCLEAR POWER CORPORATION j .b<<c.tO G.S. Thomas Assistant Plant Superintendent BMB/kbd 1

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