ML20030A907

Trip Rept of 810709 Site Visit to Observe Effects of Main Phase B Transformer Fire Which Occurred on 810703

ML20030A907
Person / Time
Site:	North Anna
Issue date:	07/22/1981
From:	Engle L Office of Nuclear Reactor Regulation
To:	Novak T Office of Nuclear Reactor Regulation
References
NUDOCS 8107290174
Download: ML20030A907 (8)

Text

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-L.s-JUL 2 21981 mi~

d Docket Nos. 50-338 and 50-339 i

76 I

D C.

MEMORANDUM TO: Thomas M. Novak, Assistant Director

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'I for Operating Reactors

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THRU:

Robert A. Clark, Chief w

Operating Reactors Branch #3 W / IM \\

FROM:

Leon B.. Engle, Project Manager Operating Reactors Branch #3 TRIP REPORT:

NORTH ANNA POWER STATION, UNITS 1 AND 2 / OBSERVATIONS ON JULY 3, 1981 UNIT 2 MAIN TRANSFORMER FIRE On July 9,1981, NRC staff members made a site visit to the North Anna Power Station to observe the effects of the Unit 2 main phase B trans-former fire which occurred on July 3,1981. A list of attendees is pro-vided in the attachment to this memorandum.

Background:

At 7:22 A.M., July 3,1981, an electrical fault accurred in the Unit 2 main phase B transform r.

This fault caused a major fire.

The loss-of-load on the B transformer initiated a generator / turbine / reactor trip on Unit 2.

At the tira of the trip, Unit 2 and Unit I were at 17 and 100 percent power, respectively.

v Because of the then uncontrolled fire, precautionary measures were initi-ated which manually tripped two Reactor Coolant Pumps (RCPs) for Unit 2 2 0:

to shed load and Unit 1 began a load reduction from 100 to 30 percent

N power.

It is noted that the Unit 2 reactor trip and manual trip of 2 E8 RCPs locked in a real low-low T signal (2/3) on the Unit 2 Solid State AV gg Protection System (SSPS).

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The fire then shorted out the overhead aluminum bus bars from the Reserve Station Service Transformer C (RSST-C). The RSST-C tripped out at 7:45 i

30o8 AM. This trip caused a loss-of-power to the IC, 2C and F station service 2

f

. buses. The F service bus interrupt caused a loss-of-power to the 1H(Unit 1)

OJ and 2J(Unit 2) emergancy buses (See Figure 1). The 1H and 2J diesels m a.u. -

came on automatically to restore power to the emergency buses. The in-terrupt in normal power to the 2J bus caused an undervoltage transient in 1

the Unit 2 vital but inverters and the SSPS.

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kg g6, c This undervoltage transient caused a spurious high-steam flow signal to register in the Unit 2 SSPS bistaples which when combined with the already signal activated Unit 2 Safety Injection (SI). SI was present low-low-TAV terminated and reset within two minutes.

The Station Emergency Director dec1ared a Site Emergency at 7:50 AM.

State and Federal Agencies were notified in accordance ith the licensee's Emergency Response Plan.

The fire was extinguished at 8:45 A.M. by the station fire brigade and other plant personnel with assistance from off-site fire companies from Louisa County, Spotsylvania County and the nearby town of Mineral. The Station Alert was terminated at 9:20 AM.

Initiating Event:

The above scenario of events is believed to have been caused by a fault in the main phase B transformer 500 K11ovolt bushing inside the transformer.

This short circuit ruptured and distorted the transformer casing and allowed about 9,000 gallons of oil to spill into the B transformer area.

The B transformer was,everely damaged and will be shipped to Westinghouse to determine the cause f the fault.

Observations:

Three single phase main transformers A, B, and C (Unit 2) are located in a coninon bay and separated from each other by concrete walls (See Figute 2 as attached). The transformers are located near the north turbine butid-ing wall. A spare transformer area adjacent to and northwest of the B-transformer is also part of the conwon bay area. The main phase transfor-mers A, B, nd C have automatic fire suppression systems with a capacity for spraying 2,000 gallons per minute. The water from these systems is carried off from the common bay area by two six-inch gravity feed flow drains.

Also, it should be neted that the Reserve Station Service Transformer C bus bars passsdirectly overhead of the spare transformer bay area.

The B transformer fault caused oil leakage and a fire in the B-transformer bay area. Immediately, the fire suppression (deluge) systems for the A and B tran formers came on automatically. The transformer C suppression system was manually activa'ed by contrcl room operators.

It could be clearly seen that the A and C tran formers and bay areas re-s ceived no damage and that the fire suppression (deluge) systems were effective.

Of greater significance is the fact that the B-transformer bay aren (point source of fire) did not show the extensive damage which occurred in the adjacent spare bay area. One can conclude that the fire suppression systems effective in reducing the effects of an oil-water fire.

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- The deluge of water rapidly filled the bay area and the was an egress of oil-water to the spare transformer bay area. At the time of these events there was neither a soare transformer nor a fire suppression system in this area. The build up of-oil / water / fire in the spare bay area eventually caused a deflagration of the mixture and a small fire-ball rapidly moved up along the north est face of the turbine building wall. The pressure differential caused by this deflagration was evident in the turbine building corrugated iron siding being blown outward from the building wall. Also, as stated previously, the overhead aluminum bus bars from RSST-C were damaged by the fire in this area, and cables associated with these buses which were mounted in vertical cable trays on the turbine building wall were badly burned.

Offspecial interest was the condition of the concrete wall which separated the B and spare tr nsformer areas. The surface of this wall in the B area showed little or no damage (light smoke stain), whereas, the surface of this wall in the spare bay area showed the effects of intense heat-fire and spalling of the concrete wall.

Eventually, the total volume of water from the fire deluge systems became greater than the capacity of the two six-inch gravity feed mains in the common bay area and the oil-water-fire overflowed the bay area. The fire then moved to areas in the Unit 3 base matt construction area and down a northwesterly slope toward Lake Anna.

Before the fire was extinguished ($1 /2 hours) about 50 station personnel were assisting in fire fighting activities. The fire was fought from inside/

outside the turbine butiding and from the turbine building roof. Assistance was also received from off-site fire companies as mentioned previously.

==

Conclusions:==

1.

An electrical fault can fracture the casings of large transformers with resultant oil spillage and fire.

2.

Fire protection suppression (deluge) systems greatly reduce the damage from oil fires.

't. The spreading of oil-watar fires in comon bay areas could be more effect-ively contained with sub-divided bays and adequately sized gravity drains.

4 The effects of transformer fires should be considered when evaluating the implementation of GDC-17 requirements for physical and electrical sep-aration of preferred power source connections to the on-site distribution system.

Odginst signed by:

Leon B. Engle, Project Manager Operating Reactors Branch #3, DL Attachments: List of Attendees, rigur : 1-and 2.

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P. D. Box 4579 Socorro, New Mexico 87891 7

Boulder, Colorado 80306 Mrs. Margaret Dietrich Route 2, Box 568 Michael W. Haupin, Esquire Gordensville, Virginia 22042-I Hunton, Williams, Cay and Gibson P. O. Box 1535 Richmond, Virginia 23212 Mr. James C. Dunstance State Corporation Conraission

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Alderma'n Library Cor.raonucalth of Virginia Blandon Building Manusbripts Department Richmond, Virginia 23209 University of Virginia Charlottesville, Virginia 22901 Mrs. June Allen

!! orth Anna Environnental Coalition Mr. Edward Kube

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',{F Board of Supervisors Wichita, Kansas 67207 Louisa County Courthouse P. O. Box 27 U.S. Environmental Protection Agency 1

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Ellyn R. Weiss Esquire Curtis Building Sheldon, Harman, Roisman and Weiss 6th and Walnut Streets 1725 I Street, N.W. Suite 506 Philadelphia, Pennsyivania 19106

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20006 Mr. Pau1 ~W. Purdem Mr.11. R. Cartwright, Station Mcnager Environmental Studies Institute P. O. Box 402 Drexel University lii.neral, Virginia 23117 32nd and Chestnut Streets Philadelphia, Pennsylvania 19104 Mr. Ar+hony Gar.bardella Office of the Attorney General Atcaic Safety and Licensing 11 South 12th Street - Rcoa 208 Appeal Coard Panel l*

Richmond, Virg#nia 23219 U.S. Nuclear F.egulatory Ccr=ission Washington, D. C.

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Route 2 Box 78A Mir.cral, Virginia 23117 Mr. 3.11. Ferguscn

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SITE VISIT TO NORTH ANNA POWER STATION 4

ei JULY 9, 1981

r LIST OF ATTENDEES NRC VEPC0 V.*Benaroya W. Cartwright M. Chiramal E. Harrell L. Engig J. Davis R. Ferguson C. Kefauner Jr.

W. Miller G. Ketz D. Speidell Jr.

H. Stewart H. Sutton R. Terrier W. Thc,mpson Gage-Babcock and Associates, Inc.

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