SNRC-1140, Discusses NRC Concerns Re Postulated Failure in Electrical Circuit Breaker 460 Between Reserve Station Svc Transformer & Bus 11 That Can Disable Both Alternate Ac Power Supplies. Util to Rack Down 4 Kv Breaker 460

Discusses NRC Concerns Re Postulated Failure in Electrical Circuit Breaker 460 Between Reserve Station Svc Transformer & Bus 11 That Can Disable Both Alternate Ac Power Supplies. Util to Rack Down 4 Kv Breaker 460

ML20114D296
Person / Time
Site:	Shoreham File:Long Island Lighting Company icon.png
Issue date:	01/29/1985
From:	Leonard J LONG ISLAND LIGHTING CO.
To:	Harold Denton Office of Nuclear Reactor Regulation
References
CON-#185-363 OL, SNRC-1140, NUDOCS 8501310185
Download: ML20114D296 (2)

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LONG ISLAND LIGHTING COMPANY gjm__md SHOREHAM NUCLEAR POWER STATION P.O. BOX 618, NORTH COUNTRY RO AD . WADING RNER. N.Y.11792 JOHN D. LEONARD.JR.

VICE PMESIDENT . NUCLE AR 0PE R ATIONS January 29, 1985 SNRC-ll40 l Mr. Harold R. Denton, Director Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Washington, DC 20555 '

Single Failpre Concerns Alternate AC Power Supplies Shoreham Nuclear Power Station - Unit 1 Docket No. 50-322

Dear Mr. Denton:

It has come to our attention, as a result of recent phone conver-sations between members of our respective staffs, that your Staff has raised questions concerning a postulated single failure that they believe vill disable both of LILCO's alternate AC power supplies (the 20 MW gas turbine and the four 2.5 MW mobile diesel generators). Your Staff post 11ates the following conditions simultaneously exist: (a) loss of all offsite AC power, (b) a design basis loss of coolant accident, (c) inability to start any of the multiple offsite gas turbine units to restore power to the site and, (d) an electrical fault occurs in electrical circuit breaker number 460 between the reserve station sc rvice trans-former-(RSST) and bus 11 as shown in FSAR Figure 8.2.1-1. Given this set of conditions they are concerned that this single fault can render both sources of AC alternate power incapable of restoring power to an emergency bus. If their concern is valid the Staff believes that this would not be consistent with previous Staff. findings on this subject as described in SSERs 5 and 6 and in Staff testimony before the Low Power Licensing Board.

The purpose of this letter is to demonstrate that the concern raised by the Staff does not affect its prior testimony concerning the susceptibility of LILCO's alternate power supplies to single failures.

Breaker number 460, the reserve station service transformer supply breaker to bus 11, is normally open. This reserve station service transformer supply breaker and all other interconnecting tie supply breakers on buses 1A, 1B, 11 and 12 are normally open and the verification of this is required per Shoreham Nuclear 8501310185 850129 D PDR ADOCK 05000322 \

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. Power Station Technical Specification 4.13.1.1.2.a.6. First, the

= fault postulated.by the Staff on-normally open breaker 460 is not

-a-credible failure mode. It has been our experience that electrical: faults,.such as the one being postulated by the Staff do not occur when the breaker is in the open position. As noted

'above, the open status of-the breaker is verified periodically per the: technical ~ specifications. Thus, it's not credible to i assume that the breaker fails in this mode while it is open.

'Second, even if a fault occurred as postulated in tie breaker Enumber 460 it is still possible for the 20 MW gas turbine to supply; power to Shoreham. In such an event, .the EMD diesels could.not be connected to the failed bus 11 and the failure of breaker number 460 would also cause a trip of the RSST protective

relays which would trip all 7 RSST 4 KV breakers, the 69 KV l switchyard supply breaker 640 and the 20 MW gas turbine breaker.

The Plant Operator can open ABS 623 and the 4KV switch gear breaker 450 thereby isolating bus number 11 and the faulted 4 KV breaker 460. The Plant Operator can then reset the RSST lock out relay 86T4B, thereby: allowing the 20 MW gas turbine to start in a dead line mode. The System Operator, by rearranging system breakers, can reroute power from the gas turbine via the 69 KV system and.the 138/69 KV step up transformer at the Wildwood-substation on the Shoreham property to the Shoreham 138 KV switchyard. Power is then connected to the emergency 4 KV bus from the-normal station service transformer..

Despite our systems ability to overcome this highly improbable multiple contingency event we.hereby commit to rack down 4 KV breaker-number 460 to eliminate.the Staff concern. We antici-

-pate that this commitment should fully resolve this concern.

Given that the postulated failure is highly-unlikely, that there-

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are: ways.to route power from the 20 MW gas turbine, even if it

' occurred, and that LILCO has made the above commitment with.

respect to breaker nunter 460, we trust that there is no need to change. previous Staff positions:on this. subject.

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-Should you or. members of your Staff have any additional

. questions,.please do not hesitate to call my office.

Very truly yours, 0 [

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Joh .D. Leon r , Jr. /'

Vic President - N lea Operations G.TG:ck cc:'P. Eselgroth, Sr. Resident Inspector n ,

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