ML17252B626
| ML17252B626 | |
| Person / Time | |
|---|---|
| Site: | Dresden  |
| Issue date: | 03/23/1977 |
| From: | Bolger R Commonwealth Edison Co |
| To: | Ziemann D Office of Nuclear Reactor Regulation |
| References | |
| Download: ML17252B626 (2) | |
Text
Common..&h Edison One First Nain'"Plaza, Chicago, Illinois Address Reply to: Post Office Box 767 Chicago, Illinois 60690 March 23, 1977 REBllLAfllll\\' llr.m nu:,.
Mr*. Dennis L. Ziemann, Chief Operating Reactors -
Bra~h 2 Division of Operating Reactors U.S. Nuclear Regulatory Conunission Washington, D.C.
20555
Subject:
Dresden Station Unit 2 ECCS Appendix K Single Failure Analysis NRC Docket No.- 50-237 References (a):
G. A. Abrell'letter to D. L. Ziemann dated May 17, 1976.
.~.
(b):
R. L. Bolger* letter to B. C. Rusche dated May 21, 1976.
Dear Mr. Ziemann:
Reference (a) transmitted information concerning the automated transfer scheme on MCC 28-7/29-7.
Reference (b) stated that (1) the impact on plant safety with removal of the automated. transfer.scheme would be evaluated and (2) either additional justificat~on for the present design or a proposed design change would be transmitted.
if the auto transfer is removed,. the single failure of any part* of the safety system that causes a loss of Bus 28 or Bus 29 will
- cause the loss of the LPCI system in addition to the loss of one core spray pump.
It is our opinion that,this is an unacceptable result.
Since removal is unacceptable, a review of the consequences of leaving the transfer in is required. This review r.eveals that the main area of concern would be a fault on either MCC28-7 or MCC29-7 which could possibly cause the loss of Bus 29 and the auto transfer of the fault to Bus 28 causing the loss of that bus. Also, if the fault.
only caused the loss of Bus 29 and then after the transfer it cleared before causing the loss.of Bus 28, the result would be the loss of LPCI and one core spray pump.
It is our opinion that the loss of either Bus 29 or both Busses 28.and 29 is not a credible.event under the conditions previously*
mentioned.
The loss of Bus 29 would require three failures, while the loss of both Bus 28 and Bus 29 would require five failures*.* *The three failures would be (1) the fault on either MCC28-7 or MCC29-7, (2) the
- failure of breaker.2971 to trip before Bus 29 feed breaker MF 29 trips, and (3) the failure of breaker 2972 to trip before Bus 29 feed breaker MF 29 trips.
The five failures would be the same three failures above and (4-) the failures of 2871 to trip be.fore Bus 28 feed breaker MF 28 trips, and (5) the failure of 2872 to trip before Bus 28 feed breaker MF 28 trips. _
(~
Mr* Dennis L. Ziemann 2 -
March.23, 1977 A double failure of the breakers to trip as,require4 is not considered a credible event.in light o.f the fact that the:trip coordina-
. tion between MCC 'feed breakers* 2~71, 2.87~, 2971. and 29?i and.bus feed breakers* MF 28
- a_nd Mf: 29. i.s-such that.the MCC :feed breakers have a 500%
faster trip time than the bus feed breakers.
Based on the above evaluation, we believe the auto transfer should not be removed and that the system is adequate as is to meet single failure requirements.
Future proposals for the system require that Conunonwealth Edison be furnished an official statement of your specific concerns for the present arrangement.
?ti~
Assistant Vice President