B17217, Provides Addl Info Relating to Request to Withdraw Anticipated Transient W/O Scram Commitment for Mnps,Unit 2

Provides Addl Info Relating to Request to Withdraw Anticipated Transient W/O Scram Commitment for Mnps,Unit 2

ML20247J178
Person / Time
Site:	Millstone
Issue date:	05/06/1998
From:	Bowling M NORTHEAST NUCLEAR ENERGY CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
B17217, TAC-M99614, NUDOCS 9805210353
Download: ML20247J178 (8)

Text

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Northeast ""I" F*"r "dd""" 2*""'"'d' "385 Nuclear Energy wai, ion, Nociear re-er statum Northeast Nuclear Energy Company P.O. Box 128

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• Waterford, CT 06385-0128 (860) 447-1791 Fax (860) 444 4277 The Northeast Utilitics System MAY 6 1998 1 Docket No. 50-336 B17217

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Re: 10CFR50.62 U.S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, DC 20555 Millstone Nuclear Power Station, Unit No. 2 I Request for Additional Information Relating to the Reauest to Withdraw an ATWS Test Commitment (TAC No. M99614)

The purpose of this letter is to provide additional information relating to a request to withdraw an Anticipated Transient Without Scram (ATWS) commitment for Millstone Nuclear Power Station, Unit No. 2. In a letter dated September 5,1997,' 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 review of this request.

BACKGROUND in a letter dated December 13,1988,8 the NRC issued a Safety Evaluation and stated that the Millstone Unit No. 2 ATWS design met the requirements of 10CFR50.62. This determination was based, in part, on information provided in a NNECO letter dated

' M. L. Bowling, Jr. letter to the U.S. Nuclear Regulatory Commission, " Millstone Nuclear Power Station, Unit No. 2, ATWS Modifications (TAC No. 59114),  !

Withdrawal of Commitment," dated September 5,1997. ]

D. G. Mcdonald, Jr. (USNRC) letter to M. L. Bowling, " Request for Additional Information Relating to the Request to Withdraw an Anticipated Transient Without Scram (ATWS) Test Commitment - Millstone Nuclear Power Station, Unit No. 2 (TAC No. M99614), dated January 23,1998.

D. H. Jaffee (USNRC) letter to E. J. Mroczka, "ATWS Rule,10 CFR 50.62 (TAC No. 59114)," dated December 13,1988.

9805210353 980506 PDR ADOCK 05000336

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U.S. Nuclear Regulitory Commission B17217/Page 2

' July 25,1988,' which responded to a Staff request for additional information dated June 8l 1988,' regarding proposed ATWS modifications. In response to Question 2.a.2, NNECO committed to revise procedures to test at alternate refueling outages one set of AC voltage output low alarms, charger trouble alarms, and DC bus undervoltage alarms.

DISCUSSION As discussed in the withdrawal request, NNECO has determined that testing one set of AC voltage output low alarms, charger trouble alarms, and DC bus undervoltage alarms at alternate refueling outages is not.necessary to demonstrate operability of the Millstone Unit No. 2 ATWS design. In reviewing this determination, the Staff has-identified an issue regarding the common power supply between the Reactor Protective System (RPS) and the Diverse Scram System (DSS). NNECO's response to the specific questions posed by the Staff are as follows.

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 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).

Response

During normal plant operation, the power distribution system is subject to constant monitoring in the control room on the main control board. Routine rounds are conducted by the operators to verify main control board indications and to confirm local indications and satisfactory operation of equipment in the field. The periodicity of these routine rounds is commensurate with the surveillance requirements of the Technical Specification (TS) for the electrical distribution system. The combination of observation of control board indications and routine rounds provides detection of abnormal or d

E. J. Mroczka letter to the U.S. Nuclear Regulatory Commission, " Millstone Nuclear Power Station, Unit No. 2, ATWS Modifications (TAC # 59114)," dated July 25, 1988.

D. H. Jaffee (USNRC) letter to E. J. Mroczka, " Millstone Nuclear Power Station, Unit No. 2 (TAC # 59114)," dated June 8,1988.

U.S. Nuclear Regulatory Commission B17217/Page 3 degraded conditions. Aberrant indications would be identified in the performance of the rbutine' rounds.

I The AC Voltage Output Low alarms, Charger Trouble alarms, and DC Bus I

Undervoltage alarms only facilitate detection of degraded conditions. As with all .

nonsafety-related equipment, NNECO utilizes these alarms, if available, since they i provide timely information to the operating shift personnel. With the exception of the Charger Trouble alarms, each of these alarms is subject to standard periodic surveillance testing.

Question 2.

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 1 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  ;

l mitigating system (which includes the DSS) and the RPS from performing their intended functions.

a. Please verify that the likely combination of degraded conditions on battery Bus 201B, which would impact two channels (associated with Vital Bus VA20 and VA40) and, channel A high pressurizer pressure of the RPS in bypass, will not 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 of the RPS in bypass, will not prevent the ATWS mitigating system (which includes the DSS) and the RPS from performing their intended functions.

Response

There are two safety-related (vital) battery busses, 201 A and 2018, which are normally supplied by safety-related 480 VAC chargers,201 A/DC1 and 201B/DC2, respectively.

Charger 201A/DC1 is powered from the Facility Z1 electrical bus (22E), and charger 2018 is powered from the Facility Z2 electrical bus (22F). A swing safety-related charger,201C/DC3, powered from bus 22E, can supply either of these battery busses.

The two safety-related battery busses are the normal supplies for the Vital 120 VAC busses (panels VA10, VA20, VA30, and VA40) via four inverters. Facility Z1 inverters INV-1 (VA10 - Channel 'A') and INV-3 (VA30 - Channel 'C') are supplied by bus 201 A, and Facility Z2 inverters INV-2 (VA20 - Channel 'B') and INV-4 (VA40 - Channel 'D')

are supplied by bus 201B.

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U.S. Nucl:ar Reguintory Commission B17217/Page 4

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The Vital 120 VAC busses each have alternate sources as follows:

. Vital AC bus VA10 can be powered from either inverter INV-1 (normal source) or the nonsafety-related turbine battery 201D (alternate source) via inverter INV-5.

. Vital AC bus VA30 can be powered from either inverter INV-3 (normal source) or the regulated non-vital instrument panel VR11 (alternate source) which is supplied from vital motor control centers (MCCs) B51 or B52.

. Vital AC bus VA20 can be powered from either inverter INV-2 (normal source) or the nonsafety-related turbine battery 201 D (alternate source) via inverter INV 6.

. Vital AC bus VA40 can be powered from either inverter INV-4 (normal source) or the regulated non-vital instrument panel VR21 (alternate source) which is supplied from vital MCCs B61 or B62,

a. For the postulated scenario of a degiaded condition on the battery bus 201B and Channel 'A' high pressurizer pressure of the RPS in bypass, the following conditions apply:

1. With a degraded condition on battery bus 201B, the worst case impact is on panels VA20 and VA40. VA20 and VA40 will shift to their respective alternate power sources. With diverse alternate power supplies, it is assumed that the loss of both VA20 and VA40 does not occur. Therefore, at least one panel will be available to provide power to the associated ATWS mitigating system (which includes the DSS) or RPS loads.

2. Vital bus VA30, which is normally powered from the non-degraded battery bus 201 A via inverter INV-3, remains available to process a trip signal. Therefore, Facility Z1 trip signals from either the ATWS mitigating system (energize to trip) or the RPS (de-energize to trip) are unaffected.

, 3. The bypass design of the RPS is configured such that the ATWS signal, although generated by the same sensor, is not directly bypassed. ATWS logic and loop wiring employs a separate bypass scheme which is unaffected by placing an RPS channel in bypass. However, if Channel 'A' high pressurizer pressure of the RPS is being bypassed and tested in accordance with the allowable outage time of the TS, the ATWS Channel 'A' bistable is also i I

bypassed as a precautionary measure to preclude a reduction in the ATWS trip l logic to one-out-of-three.

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b. For the postulated scenario of a degraded condition on a 125 VDC supply or Vital AC inverter INV-2 (Vital Bus VA20) and, Channel 'A' high pressurizer pressure of  !

the RPS in bypass, the following conditions apply:

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1. The four protection channels yield six combinational trip logic matrices 'AB',

'AC', 'AD', 'BC', 'BD' and 'CD'.

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U.S. Nucl:ar Regul: tory Commission B17217/Page 5 2 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 to one-out-of-three.

3. When both RPS and ATWS Channel 'A' are bypassed, Channel 'A' is precluded from providing a trip input. The combinations affected are 'AB', 'AC', and 'AD'.

4. The loss of Vital Bus VA20 will affect Channel 'B' associated logic matrices. The combinations affected are 'AB', 'BC', and 'BD'. Therefore, the case of concern is if Channel 'B' receives the RPS trip signal and the non-bypassed combinations

'BC' and 'BD' fail to trip.

5. The loss of Vital Bus VA20 will not affect logic matrix 'CD'. Assuming the failure of Channel 'B' with Channel 'A' bypassed, the 'CD' combination will provide the RPS trip or ATWS signal.

SUMMARY

NNECO has submitted a request to withdraw a commitment to perform tests of certain nonsafety-related alarms as part of Millstone Unit No. 2 ATWS modifications. These alarms will be retained, but they are not necessary to verify operability of either the RPS or the ATWS prevention / mitigation function. The additional information provided herein verifies that common mode failure of shared power sources will not result in loss of both RPS and DSS functions.

COMMITMENTS Attachment 1 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, NORTHEAST NUCLEAR ENERGY COMPANY Martin L. Bowling, Jr. /

Recovery Officer - Technical Services Attachments (1) cc: See Page 6 1

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U.S. Nuclier R:gulatory Commission B17217/Page 6 l '

cc: W. D. Travers, Ph.D, Director, Special Projects Office H. J. Miller, Region i Administrator D. P. Beaulieu, Senior Resident inspector, Millstone Unit No. 2 D. G. Mcdonald, Jr., NRC Senior Project Manager, Millstone Unit No. 2 P. F. McKee, Deputy Director of Licensing, Special Projects Office W. D. Lanning, Deputy Director of Inspections, Special Projects Office I

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Docket No. 50-336

- . B17217 Attachment 1 Millstone Nuclear Power Station, Unit No. 2 Request for Additional Information Relating to the l Request to Withdraw an ATWS Test Commitment (TAC No. M99614)

List of Regulatory Commitments May 1998

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U.S. Nucl=r Rrgul: tory Commission  !

B17217/ Attachment 1/Page 1 l List of Regulatory Commitments The following table identifies those actions committed to by NNECO in this document. I 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 regulatory commitments. The Manager - Regulatory Compliance, Millstone Unit No. 2, should be notified of any questions regarding this document or any associated i regulatory commitments. I REGULATORY COMMITMENT COMMITTED DATE OR OUTAGE l None N/A i l

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