B17429, Forwards Supplemental Info Related to Request to Withdraw ATWS Commitment for Millstone Unit 2.Info Provided Is Clarification of Info Submitted in Util 980506 Response to RAI

Forwards Supplemental Info Related to Request to Withdraw ATWS Commitment for Millstone Unit 2.Info Provided Is Clarification of Info Submitted in Util 980506 Response to RAI

ML20153B378
Person / Time
Site:	Millstone
Issue date:	09/14/1998
From:	Bowling M NORTHEAST NUCLEAR ENERGY CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
B17429, TAC-M99614, NUDOCS 9809230091
Download: ML20153B378 (9)

Text

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. Northeast i R pe Fmy RddRoute 156), Watufwd, Cr 06385

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Nucl:ar Energy _ htone Nociear power stason Northeast Nuclear Energy Company P.O. Box 128 Wr.terford, Cr 06385-0128 (860) 447-1791 Fax (860) 444-4277 The Northeast Utilities System SEP I g 1998 Docket No. 50-336 B17429 Re: 10CFR50.62 U.S. Nuclear Regulatory Commission ,

Attention: Document Control Desk )

Washington, DC 20555 Millstone Nuclear Power Station, Unit No. 2 Request to Withdraw an ATWS Test Commitment (TAC No. M99614) -

Clarifications to Response to Reauest for Additional Information The purpose of this letter is to provide supplemental information related to a request to i withdraw an Anticipated Transient Without Scram (ATWS) commitment for Millstone ~

Nuclear Power Station, Unit No. 2. In a letter dated September 5,1997,W Northeast

, Nuclear Energy Company (NNECO) requested approval to withdraw a commitment to revise procedures to test certain alarms at altemate refueling outages. In a letter dated January 23,1998,* the Staff requested additional information needed to complete its l review of the proposed commitment withdrawal. NNECO responded to this request for additional information in a letter dated May 6,1998."

As a result of discussions with the Staff, NNECO has elected to clarify the information provided in the May 6,1998, letter by providing additional details for Question 1 and a restated response for Question 2b. These supplemental discussions are contained in ,

Attachment 1 to this letter.

M M. L. Bowling, Jr. letter to the U.S. Nuclear Regulatory Commission, " Millstone Nuclear l /

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Power Station, Unit No. 2, ATWS Modifications (TAC No. 59114), Withdrawal of I ,

Commitment," dated September 5,1997, i A D. G. Mcdonald, Jr. (USNRC) letter to M. L Bowling, " Request for Additional Information A Relating to the Request to Withdraw an Anticipated Transient Without Scram (ATWS) Test 0 ,

Commitment - Millstone Nuclear Power Station, Unit No. 2 (TAC No. M99614)," dated January 23,1998.

m M. L. Bowling, Jr. letter to the U.S. Nuclear Regulatory Commission, " Millstone Nuclear Power Station, Unit No. 2, Request for Additional Information Relating to the Request to Withdraw an ATWS Test Commitment (TAC No. M99614)," dated May 6,1998.

9809230091 990914 $

PDR ADOCK 05000336 i p PDR 1

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. U.E. Nuciatr R3gulatory Commission B17 429\Page 2 COMMITMENTS

' Attachment 2 identifies those actions committed to by NNECO in this letter There are .

no commitments contained within this letter.

Should you have any questions on the information provided herein, please contact Mr.

R. G. Joshi at (860) 440-2080.

Very truly yours, r

NORTHEAST NUCLEAR ENERGY COMPANY For: Martin L. Bowling, Jr.

Recovery Officer - Technical Services By:

David B. Amerine Vice President - Human Services -

i Attachments (2) cc: H. J. Miller, Region i Administrator D. G. Mcdonald, Jr., NRC Senior Project Manager, Millstone Unit No. 2 D. P. Beaulieu, Senior Resident inspector, Millstone Unit No. 2

- E. V. Imbro, Director, Millstone ICAVP inspections 4

s Docket No. 50-336 B17429 l

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Attachment 1 Millstone Nuclear Power Station, Unit No. 2 Request to Withdraw an ATWS Test Commitment (TAC No. M99614)-

Clarifications to Response to Request for Additional Information I

l September 1998

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U.S. Nucletr Rngulatory Commission B17429\ Attachment 1\Page 1 Millstone Nuclear Power Station, Unit No. 2 j l

l Request to Withdraw an ATWS Test Commitment (TAC No. M99614) - '

Clarifications to Response to Request for Additional Information Question 1.

NNECO stated that "[Given the] routine inspections performed on the various power ,

distribution components, and the manifestation of degraded voltage and/or frequency i conditions by off-normal responses from a variety of systems and indications, these occurrences are considered detectable."

Please verify that, based on the above statement, NNECO has the capability to detect degraded conditions on the power distribution system without reliance on or taking credit for the alarms in question (AC Voltage Output Low alarms, Charger Trouble alarms, and DC Bus Undervoltage alarms). I i

Issue:  ;

NNECO previously responded that the AC Voltage Output Low alarms and the DC Bus p Undervoltage alarms are subject to standard periodic surveillance testing.

Supplemental Response:  !

One train of the DC Bus Undervoltage alarms is tested each refueling cycle on an alterna'.ing basis. All of the AC Voltage Output Low alarms are tested once per refueling cycle.

Question 2.b.

NNECO stated that: (a) Degraded conditions on a battery bus or charger would, in the worst case, impact two (out of the four) channels of both the ATWS mitigating system and the RPS and, (b) Degraded conditions on a 125 VDC or 120 Vital AC inverter or panel would, in the worst case, impact one (out of four) channels of both the ATWS mitigating system and the RPS. Neither of these occurrences would prevent the ATWS mitigating system (which includes the DSS) and the RPS from performing their intended functions.

b. Please verify that the likely combination of degraded conditions on a 125 VDC or 120 Vital AC inverter #2 (Vital Bus VA20), which would impact logic matrices BC, BD, and CD and, channel A high pressurizer pressure cf the RPS in bypass, will not l prevent the ATWS mitigating system (which includes the DSS) and the RPS from l performing their intended functions.

' 1 U.S. Nucirr Regulatory Commission '

B17429\ Attachment 1\Page 2 Issue: . .

In discussing why neither the ATWS Mitigating System nor the Reactor Protective System (RPS) would be prevented from performing their intended functions, NNECO l did not fully address a potential concern with respect to common power sources. This l RPS power supply concern was introduced by the implementation of a Combustion Engineering (CE) modification to the original plant RPS design.

I Restated Response:

The ATW3 Mitigating System and the RPS share common power sources. The four l protection channels yield six combinational trip logic matrices 'AB', 'AC', 'AD', 'BC',

! 'BD' and 'CD'. For the postulated scenario, the following conditions apply:

1. With Channel 'A' high pressurizer pressure of the RPS in bypass, ATWS Channel 'A' is also ' bypassed as a precautionary measure to preclude a

! reduction in the ATWS trip logic.

l l 2. When both RPS and ATWS Channel'A' are bypassed, Channel'A'is precluded ,

l from providing a trip input. The combinations affected are 'AB', 'AC', end 'AD'. 1 f 1

3. A degraded Vital Bus VA20 will affect the Channel 'B' sensors which are used f for both the ATWS Mitigating System and the RPS. The combinations afiected i are 'AB', 'BC', and 'BD'.

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4. A degraded Vital Bus VA20 may also affect RPS logic matrix 'CD', because the '

power supply design for this RPS logic matrix includes both Vital Bus VA20 and Vital Bus VA40. The concern would be the possibility of a single failure within the common bus system propagating through the logic matrix power supplies

, into the matrix circuitry. ,

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ATWS Evaluation For the ATWS Mitigating System, Diverse Scram System (DSS) logic matrices 'AB',

'AC', 'AD', 'BC', and 'BD' could be lost. However, DSS logic matrix 'CD' would not be affected and would remain available to process the ATWS trip. This is because the ATWS circuitry employs an auctioneered power supply design which supplies both the six combinational trip logic matrices and the associated actuation relays. One power supply is fed from Vital Bus VA10 and the other is fed from Vital Bus VA20. Therefore, although the matrix power supply fed from VA20 may be affected, the power supply fed from VA10.would be available and the ATWS Mitigating System (which includes the

DSS) will not be prevented from performing its intended function.

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U.S. Nuclear Regulatory Commission B17429\ Attachment 1\Page 3 RPS Evaluation l

The RPS logic matrices could be vulnerable to a degraded Vital Bus VA20. This vulnerability results from a CE modification to the original RPS design as discussed in a 1984 submittal." In this submittal, NNECO presented an evaluation of the RPS

, circuitry, including the 28 VDC logic matrix power supplies, to demonstrate the independence of the vital buses. NNECO determined that only a failure which resulted in the welding of the logic matrix relay contacts would prevent performance of the l required protective function. However, as pad of the evaluation, NNECO also concluded that no such single failure is credible. i In analyzing a potential logic matrix circuitry failure due to a vital bus single failure, NNECO concluded that only transverse mode surges or a continuous high voltage

,~ applied to a matrix regulated power supply would have any potential for causing logic i matrix circuitry failures. If it is assumed that a higher than normal voltage were to result at the output terminals of the regulated power supply, a higher than normal current

would flow through matrix logic contacts and the two matrix relays associated with the power supply. Since the contacts are normally c; ad (Iow resistance), insignificant  ;

self heating of the contacts would occur. Heat in the relay coils would rise, however, possibly causing one or both relay coils to fail. Such relay coil failures are not of i concan =ince failure of one or both of these coils will de-energize the respective trip

i. circuit breaker control relay resulting in a half-trip condition. Contact welding would not occur due to either self heating or contact opening, since such welding requires both molten contact material and contact pressure.

NNECO also found no failure which would cause a high-voltage < sndition at the output terminals of a power supply. The vital buses are normally pov ad from inveders, and the maximum output voltage of an inverter is limited by the input battery voltage. The distribution circuits from the vital buses are provided with fuse; and circuit breaker protection to assure individual circuit faults are isolated close to the fault source.

This evaluation fudher documented the following conclusions.

. Single phase to ground faults and surges applied to a vital AC source will have no effect whatsoever. Since the circuits are ungrounded, no current will flow.

. A continuous phase-to-phase short-circuit of the vital AC input to one matrix power supply will have no effect upon the output of the other matrix power supply or its

- vital AC input. A half-trip condition will result from the loss of output of the matrix power supply whose input is shod circuited.

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• W. G. Counsil letter to J. R. Miller (USNRC), " Millstone Nuclear Power Station, Unit No. 2 -

Reactor Protection and Engineered Safeguards System Actuation Logic," dated June 25,

1984.

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. U.S. Nuclarr R:gulatory Commission B17429%ttachment 1\Page 4

. Even if transverse mode surges or a continuous high voltage were applied to a matrix power supply and effects were assumed to propagate through the regulated power supply to the matrix circuits and relays, the redundant vital AC supply would be effectively isolated from these effects by 1) the inherent DC to AC blocking of the associated power supply,2) a reverse biased diode, and 3) the impedance of the j matrix relays plus the shunting effect of the normally closed matrix contacts.

In addition, NNECO has submitted a proposed license amendment" to limit the RPS bypass to 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br />. This limiting condition is consistent with the standard bypass requirement for similar CE RPS designs.

Based on the preceding discussion, NNECO concludes that for the postulated scenario, the RPS will not be prevented from performing its intended function.

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• M. L. Bowling, Jr. letter to USNRC, " Millstone Nuclear Power Station, Unit No. 2 - Proposed Revision to Technical Specifications, Reactor Protective and Engineered Safety Feature Actuation System Instrumentation," dated May 14,1998.

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Docket No. 50-336 B17429 l

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Attachment 2 Millstone Nuclear Power Station, Unit No. 2 Request to Withdraw an ATWS Test Commitment (TAC No. M99614)-

Clarifications to Response to Request for Additional Information

. List of Regulatory Commitments l

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! September 1998 1

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U.S. Nucl=r R*gulttory Commission B17429\ Attachment 2\Page 1 List of Regulatory Commitments The following table identifies those actions committed to by NNECO in this document.

Any other actions discussed in the submittal represent intended or planned actions by NNECO.' They are described to the NRC for the NRC's information and are not i

regulatory commitments. The Manager - Regulatory Compliance, Millstone Unit No. 2, should be notified of any questions regarding this document or any associated regulatory commitments.

REGULATORY COMMITMENT COMMITTED DATE OR OUTAGE None N/A 3

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