ML20137S751

Ro:On 851121,during Refueling Outage,Breakers 5T-2 & 6T-2 Opened.Caused by Manual Tripping by Test Personnel in Switchyard.Switchyard Breakers Reclosed & 345 Kv Line Reconnected

ML20137S751
Person / Time
Site:	Millstone
Issue date:	11/25/1985
From:	Opeka J NORTHEAST NUCLEAR ENERGY CO.
To:	Crutchfield D NRC
References
NUDOCS 8512060303
Download: ML20137S751 (3)

Text

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cene,.i O,r.ces . seiaen street. seriin, connecticut 9 n mumnuac c "* P O. BOX 270 H ARTFORD. CONNECTICUT 06141-0270 k k J N N $ N Y. . . (203) 665-5000 November 25,1935 Docket No. 50-245 Bf1902 Mr. Dennis M. Crutchfield Assistant Director for Safety Assessment Division of Licensing U.S. Nuclear Regulatory Commission Washington, D.C. 20555 Gentlemen:

Millstone Nuclear Power Station, Unit No.1 Momentary Loss of AC Power On November 21, 1935 Northeast Nuclear Energy Company (NNECO) activated the Millstone Nuclear Power Station 52.mergency Plan for a brief period by declaring an ALERT following a loql of normal power for Millstone Unit No.1.

Although normal reporting procedures and communication channels are being followed, this informational letter is being provided to inform the Staff of the circumstances surrounding the events which occurred during the af ternoon of November 21,1935.

On November 21, Millstone Unit No. I was in the.26th day of a normal refueling outage. With the Reserve Station Service transformer out of service for normal outage related preventative maintenance, off-site power was being received by backfeeding through the main transformer and the 23 kV line (the " Flanders line") into the station was available. As a part of routine outage maintenance -

the Emergency Diesel Generator was not inservice.

During switchyard transfer-trip preventative maintenance testing involving 345 kV breakers ST-2 and 6T-2, both breakers were inadvertently opened, resulting in a momentary loss of AC power for Millstone Unit No.1. Because of the plant configuration for the refueling outage, there was not automatic initiation of the loss of normal power (LNP) logic as all logic requirements were not satisfied. Technical Specifications for this particular plant configuration would have allowed up to a four hour, period without on-site power before being reportable. The LNP logic was manuany initiated to strip the loads fram the emergency buses in preparation for the start and sequencing oi' the gas turbine generator so that loads may be restored to the buses in a controlled manner.

During the gas turbine start sequence, the gas turbine tripped as a result of a high exhaust temperature indication. Since the 23 kV line feeder breaker opens with the initiation of a LNP signal for emergency bus load sequencing purposes, the failure of the gas turbine resulted in a loss of on-site AC power.

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v e ,,e Following the loss of the ST-2 and 6T-2 breakers, the reason for their loss was determined to be a manual tripping by test personnel in the switchyard. Thus, following the loss of on-site power the Connecticut Valley Exchange (CONVEX) was requested to reclose the switchyard breakers and the 345 kV line was reconnected within IM minutes of its loss. During this period the incoming Flanders line was still available to the station at the bus breaker but the breaker was open as a result of the load sequencing logic.

While this event was not reportable under 10CFR50.72 and 50.73 and power from the Flanders line was still available to the station Revision 0 of the Millstone Nuclear Power Station Emergency Plan (l) specifies in Table 4-I A that NNECO designate an ALERT condition for loss of off-site power and loss of all on-site AC power for less than 15 minutes. The actual time that power was not on the emergency buses was between three (3) and four (4) minutes. While perhaps not technically required because off-site power to the station was never truly unavailable, the decision was made to be conservative and declare the ALERT.

With the unit in its 26th day of a refueling outage and no core alterations in progress, the decay heat load was minimal and the implications of the event correspondingly low.

Throughout the events described above, Millstone Unit No. 2 remained on line in normal operation at 100% power. The circumstances of this event similarly had no impact on Millstone Unit No. 3, which is undergoing final preparations for initial fuel load.

Informal discussions with the Staff have revealed that considerable confusion may exist within the NRC regarding what happened at Millstone Unit No. I on November 21, and the significance of the events. The following considerations are relevant:

o The event occurred twenty-six (26) days-into a refueling outage, at which time the decay heat load is an extremely small fraction (approximately 0.7%) of the decay heat load following a trip from 100%

power.

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o The event is not reportable pursuant to 10CFR50.72 and 50.73 criteria.

o The plant was in a unique configuration with respect to on-site and off-site power availability for the purposes of conducting tests and l maintenance on. AC power-related components. This configuration and i

consequently the sequence of events which unfolded could not have occurred during power operation.

o Although conservatively classified as a loss of off-site and on-site power for purposes of activating the emergency response organization, off-site power was available to the unit at al times through both the i main transformer and the Flanders line. Operator actions were f

necessary to bring power to the plant buses from either of the two i sources available.

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(1) J. F. Opeka letter to E. J. Butcher, " Millstone Nuclear Power Station Emergency Plan," dated October 31,1985.

o The emergency plan _ incident classification criteria are clearly conservative, perhaps excessively so. The criterien which requires declaration of an ALERT following a loss of all on-site and off-site (a debatable point in this instance) power does not distinguish between 100% power operation and refueling outages or other periods where the decay heat load is very small. We will be evaluating the appropriateness of any changes to the incident classification scheme.

We ~ are hopeful that the factual information provided above will put the November 21, 1985 event in its proper perspective. In particular, we believe it would be misleading to characterize this event as a " station blackout l' unless considerable amplification accompanied this term. - As always, we would be happy to answer any questions you may have on this matter.

Very truly yours, NORTHEAST NUCLEAR ENERGY COMPANY N , F. [Su.Sw

3. F. OpeRs ') ()

Senior Vice President

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