B12955, Provides NRC W/Plans for Mods Re Degraded Grid Protection for Class 1E Power Sys.Delays Occurred as Result of Identification of Significant Concerns During Review. Requests NRC Initiation of Review of Design

Provides NRC W/Plans for Mods Re Degraded Grid Protection for Class 1E Power Sys.Delays Occurred as Result of Identification of Significant Concerns During Review. Requests NRC Initiation of Review of Design

ML20207F912
Person / Time
Site:	Millstone
Issue date:	08/15/1988
From:	Mroczka E, Sears C NORTHEAST NUCLEAR ENERGY CO., NORTHEAST UTILITIES
To:	NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM)
References
B12955, NUDOCS 8808230283
Download: ML20207F912 (8)

Text

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NORTHEAST UTILITIES o.n.r.i Ome.. . s.m.n su..i. 8.mn. conn.cticut 1 EvYwYIEEU*S P.O. box 270

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' M m .c w i i % . w ., (203) 665-5000 August 15, 1988 Docket No. 50-245 B12955 Re: Degraded Grid Protection ISAP Topic 1.23 ISAP Topic 1.25 U.S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555 Gentlemen:

Millstone Nuclear Power Station, Unit No.1 Dearaded Grid Protection for Class 1E Power Systems The purpose of this letter is to provide the NRC Staff with No,theast Nuclear Energy Company's (NNECO) plans for modifications related to degraded grid protection for Class IE power systems at Hillstone Unit No. 1. NNECO believes that the modifications illustrated in Attachment 1 to this letter, which are currently scheduled in the Hillstone Unit No.1 Integrated Implementation Schedule (IIS) to be implemented during the 1989 refueling outage, meet the intent of NRC Branch Technical Position (BTP) PSB-1, "Adequacy of Station Electric Distribution System Voltages." NNECO has evaluated Hillstone Unit No l's power supply system against 10CFR50 Appendix A, General Design Criteria (GDC) 17 with reoard to the ability of the plant to cope with a degraded grid voltage condition under Topics 1.23 and 1.25 of the Integrated Safety Assessment Program (ISAP).

In a letter dated May 22,1987,III NNECO provided a summary of our efforts to incorporate a loss of voltage and degraded voltage detection and actuation scheme to meet the intent of BTP PSB-1. A description of the existing under-voltage protection scheme and a comparison of it with the criteria of BTP PSB-1 was included.

Our May 22, 1987 letter stated that the deferral of the degraded grid protec-tion modifications until the 1989 refueling outage was necessary. Testing of the proposed scheme, which was to be implemented during the 1987 refueling outage, on Hillstone Unit No. l's plant-specific simulator showed that the (1) E. J. Hroczka letter to U.S. Nuclear Regulatory Commission. "Degraded Grid Protection for Class IE Power Systems," dated May 22, 1987.

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U.S. Nuclear Regulatory Commission B12955/Page 2 August 15, 1988 i operation of the auxiliary electrical system would be severely degraded due to l t1e asymmetrical bus arrangement. The then-)roposed design, which was based 1

! on the requirement to split the output loac shed and loss of normal power (LNP) logic into independent S1 and S2 circuits, would have provided for independent emergency power systems and the automatic reinstatement of the  !

load shed feature. However, this design also introduced the possibility of a .

l partial LNP (i.e., one safety division able to experience an LNP without the other and vice-versa) which resulted in operator confusion while assessing the i l

daveloping scenario. (Note that the existing loss of voltage scheme is '

designed to create a single response--that being to put the unit into a full loss of off site power condition.) NNECO was concerned that the proposed scheme could have hindered the operators' ability to assess a loss of power  ;

event in time to prevent such events as serious level transients since the .

reactor would not be directly scrammed.  !

The revised design does not split the output load shed and LNP logic, thus l

alleviating the concern with the asymmetrical bus configuration. Instead, I redundant LNP channels will be provided to ensure the adequacy of the power l supply to safety related equipment. Automatic reinstatement of the load shed feature is provided for with individual bus voltage relaying that will load i shed the respective bus should voltage be lost. In addition, a reactor scram [

signal will be generated should 4-kV Bus 14A, 148,14C, or 14D become de-  ;

energized, in order to minimize the transient expected upon the loss of a i l primary bus. ,

In a lettsr dated June 9, 1987,(2) the NRC Staff accepted our proposal to delay the final plant undervoltage detection modifications until the 1989 refueling outage. The acceptability of the existing undervoltage protection system and the degraded grid voltage procedures for providing an adequate level of safety until the final modifications are completed was reaffirmed.

Delays in the final implementation of modifications related to degraded grid t protection have occurred as a result of the identification of significant ,

concerns during comprehensive review of this issue. While we did not antici-  :

pate the discovery of potential adverse i.npacts of the previous designs, which caused implementation delays, our use of probabilistic safety techniques and the plant-specific simulator will result in improved overall plant safety.

NNECO is hereby submitting this information to enable the NRC Staff to initi-ate their review of the above mentioned design. We are prepared to meet with the Staff to further discuss this issue. A license amendment request relating to the planned modifications will be submitted by December 1,1988.  ;

(2) C. O. Thomas letter to E. J. Mrotzka, "Implementation of Degraded Grid Protection for Class IE Power Systens " dated June 9, 1987. j

U.S. Nuclear Regulatory Commission B12955/Page3 August 15, 1988 NNECO is confident that Millstone Unit No. I will continue to operate safely under the >roposed undervoltage detection system. If you have any questions, please fee'l free to contact my staff.

Very truly yours, NORTHEAST NUCLEAR ENERGY COMPANY E .d . N M.

E. J. Mroczka "

Senior Vice President By: C. F. Sears Vice Presid2nt cc: W. T. Russell, Region 1 Administrator M. L. Boyle, NRC Project Manager, Millstone Unit No. I W. J. Raymond, Senior Resident inspector, Millstone Unit Nos. 1, 2, and 3 L. Kolownauski, Resident inspector, Millstone Unit No. 1

Docket No. 50-245 812955 Attachment 1 Millstone Unit No. 1 Undervoltage Detection Modifications August 1988

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