ML20155C844

Safety Evaluation Accepting Licensee Proposal to Withdraw ATWS Test Commitment

ML20155C844
Person / Time
Site:	Millstone
Issue date:	10/29/1998
From:	NRC (Affiliation Not Assigned)
To:
Shared Package
ML20155C827	List: ML20155C823 ML20155C844
References
NUDOCS 9811030024
Download: ML20155C844 (6)

Text

-._. . _ ._ _ . . _ _ _ _ . __ _ . . _ _. _ ._ _._ _ _ _.__.,

i am p UNITED STATES 3

• NUCLEAR REGULATORY COMMISSION -

WASHINGTON, D.C. 30006 4001 SAFE TY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION REQUEST TO WITHDRAW AN ANTICIPATED TRANSIENT WITHOUT SCRAM TEST COMMITMENT NORTHEAST N1 CLEAR ENERGY COMPANY MILLSTONE NUCLEAR POWER STATION. UNIT NO. 2 DOCKET NO. 50-336 l

1.0 INTRODUCTION

I By letter dated September 5,1997, Northeast Nuclear Energy Company (NNECO, licensee) requested NRC staff approval to withdraw a commitment made on July 25,1988, as part of its anticipated transient without scram (ATWS) modifications for the Millstone Nuclear Power Plant, Unit 2 (Millstone Unit 2). A request for additional Information (RAI) related to this topic was sent to the licensee on January 23,1998, and a response to the RAI was provided on May 6,1998. In addition, conference calls took place on June 22 and July 10,1998, between the staff and NNECO to discuss the failure modes of the shared power sources between the Reactor Protection System (RPS) and the Diverse Scram System (DSS), Based on these conversations, NNECO submitted additional information on September 14,1998.

2.0 BACKGROUND

The ATWS Rule (10 CFR 50.62) requires that the equipment / systems installed to prevent and/or mitigate the consequences of ATWS events be electrically independent of the existing RPS to

. minimize the potential for common mode failures (CMFs) that could affect both RPS and ATWS circuits. The Millstone Unit 2 DSS is part of the ATWS mitigation system. Electrical independence of the DSS from the existing RPS should be provided from the sensor outputs up to and including the final actuation device. The ATWS design at Millstone Unit 2 shares a common power supply between the RPS and DSS that deviates from the staff guidance provided in the supplemental information concerning electrical independence that was published with the ATWS Rule.

l The staff has allowed exceptions to the electricalindependence guidance for the DSS and RPS power supply circuits where these systems are designed, procured, installed, and maintained as fully redundant safety-related (Class 1E) circuits. NNECO was also required to demonstrate that CMFs of the shared power sources would not result in loss of both RPS and ATWS prevention / mitigation functions as discussed in item 2.a.2 of the staff's RAI dated June 8,1988.

The staff asked NNECO to explain why a CMF affecting the RPS power distribution system, including degraded voltage and frequency conditions, that would compromise both the RPS and the ATWS prevention / mitigation functions, cannot go undetected. The staff further indicated that, if alarms were relied on to provide early detection of degraded voltage and/or frequency conditions, NNECO was requested to identify the specific alarms provided along with a discussion of the periodic surveillance testing performed to verify alarm operability.

9811030024 981029 PDR ADOCK 05000336 P PDR Ehclosure

2-NNECO responded by stating that vital aternating current (ac) power to the four-channel RPS and the four-channel DSS is supplied from four vital (Class 1E safety related) inverters. These inverters are equipped with the following local and control room alarms and indications;

a. Input Breaker Trip l b. AC Voltage Output Low (Setpoint of 108 VAC)

! c. AC Ground Fault

d. High Temperature (Setpoint of 175 deg. F)

The inverters are powered by a station 125 VDC system, which consists of two batteries, three l battery chargers, and two battery busses. Each bus powers two inverters which, in tum, provide the four separate channels of vital 120-volt ac for the four-channel RPS and DSS. The battery bus and battery charger systems are monitored and alarmed for various conditions including:

a. Charger Trouble:

1. High Voltage at 150 VDC

2. Low Voltage at 120 VDC

b. DC Bus Undervoltage at 126 VDC

c. Ground Alarms

d. Indications are provided for battery current and voltage and direct current (dc) bus current and voltage.

NNECO also stated that the Millstone Unit 2 procedures call for testing of de bus undervoltage alarms at every other outage. However, NNECO indicated that revised procedures, which would be implemented at the next planned refueling outage, include testing one set of ac voltage output low alarms, charger trouble alarms, and de bus undervoltage alarms at alternate refueling outages. The revised procedures would ensure that one set of these alarms are tested during each outage. By letter dated September 5,1997, the staff was informed that provisions to test the above alarms were not implemented and that NNECO was requesting withdrawal of the testing commitment it made on July 25,1988, as part of its ATWS modifications.

3.0 EVALUATION As previously noted, an RAI was issued on January 23,1998, requesting clarification on the testing of the power supply circuit alarms. NNECO was asked if it had the capability to detect degraded conditions on the power distribution system without reliance on or credit for the alarms in question (ac voltage output low alarms, charger trouble alarms, and de bus undervoltage alarms). By response dated May 5,1998, as clarified on September 14,1998, NNECO indicated that, during normal plant operation, the power distribution system is subject to constant l

monitoring of instruments located on the main control board in the control room. Routine rounds are also 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 technics! specifications for the electrical distribution system. The combination of the observation of control board indications

, and routine rounds provides detection of abnormal or degraded conditions. Any indications of a l deviation from normal conditions would be identified in the observation of control board I

indications and the performance of the routine rounds. With the exception of the charger trouble l alarms, one train of the de bus undervoltage alarms is tested each refueling cycle on an attemating basis. All the ac vcitage output alarms are tested once per refueling cycle.

3-Failure of Common Power Suoolv The staff also requested confirmation that a failure of a power supply (degraded conditions on a 125 VDC or 120 vital ac inverter No. 2 -Vital Bus VA20) 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, it should be noted that NNECO has submitted a proposed license amendment to the Millstone Unit 2 Technical Specifications to limit the RPS bypass, including high pressurizer pressure, to 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> from the current requirement that allows indefinite bypass. This change will decrease the time that Chanael 'A' High Pressurizer Pressure of the RPS can be in bypass.

For this postulated situation, 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 availability of the ATWS trip logic to one-out-of-three.

l

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

3. A degraded Vital Bus VA20 will affect the Channel'B' sensors, which are used for both the ATWS mitigating system and the RPS. The combinations affected are 'AB,BC,' and 'BD' l

logic matrices.

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 concem would be the possibility of a single failure within the common bus system propagating through the logic matrix power supplies into the matrix circuitry.

ATWS Evaluation For the ATWS mitigating system, based on the preceeding common power supplies, [..SS log c matrices 'AB,AC,AD,BC,' and 'BD' could be lost by a single failure. However, DGE MW matrix 'CD' would not be affected and would remain available to process the ATWS trip. Tr:is 1:

because the ATWS circuitry employs an auctioneered power supply design that supplies both the six combinational trip logic matrices and the associated actuation relays. One power apply is fed from Vital Bus VA10 and the other is fed from Vital Bus VA20. Therefore, a!thou;;h the matrix power supply fed from Vital Bus VA20 may be affected, the auctioneered power supply fed

! from Vital Bus VA10 would be available and the ATWS mit; gating system (which includes the DSS) will not be prevented from performing its intended function.

RPS Evaluation l

The RPS logic matrices could be vulnerable to a degraded Vital Bus VA20. This vulnerability results from a modification to the original RPS design to prevent spurious trips. In analyzing a potential logic matrix circuitry failure, due to a vital bus single failure, NNECO determined that only a failure that resulted in the welding closed of the logic matrix relay contacts would prevent j performance of the required protective function. Since the contacts are normally closed (Iow resistance), insignificant self heating of the contacts could occur. In event heat levels in the relay l

e 4

. 4 coils would rise, however, one or both relay coils could fail. Such relay coil failures are not of concem since failure of one or both of these coils will deenergize the respective trip circuit breaker control relay resulting in a half trip condition. Thus, no RPS power supply single failure resulting in loss of RPS function is credible.

NNECO also found no failures that would cause a high-voltage condition at the output terminals of a power supply. NNECO's evaluation documented the following conditions with no effect on the RPS logic matrices.

s. single phase to ground faults and su'rges applied to a vital ac source,

b. continuous phase-to-phase short-circuit of the vital ac input to one matrix power supply, and

c. transverse mode surges or a continuous high voltage were applied to a matrix power supply.

Based on the proceeding discussion, for the postulated scenario, the RPS will not be prevented from performing its intended function.

4.0 .QQNCLUSION Based on the proceeding evaluation, the staff has concluded that NNECO's attemate proposal

] (one train of the de bus undervoltage alarms is tested each refueling cycle on an attemating basis, and all the ac voltage output alarms tested once per refueling cycle) is acceptable for ensuring power availability to the RPS and ATWS systems. The staff has further concluded that NNECO has provided adequate justification that the ATWS mitigating system and the RPS will continue to perform their intended functions when subject to a failure of a common power supply and the bypass of Channel'A' high pressurizer pressure.

Principal Contributor. J. Joyce Date:- October 29, 1998 i

9

l C

Millst:na Nucirr Power Station Unit 2 cc:

Lillian M. Cuoco, Esquire Mr. F. C. Rothen Senior Nuclear Counsel Vice President - Work Services Northeast Utilities Service Company Northeast Utilities Service Company P. O. Box 270 P. O. Box 128 Hartford, CT 06141-0270 Waterford, CT 06385 Mr. John Buckingham Emest C. Hadley, Esquire Department of Public Utility Control 1040 B Main Street Electric Unit P.O. Box 549 10 Liberty Square West Wareham, MA 02576 l New Britain, CT 06051 Mr. John F. Streeter Edward L. Wilds, Jr., Ph.D. Recovery Officer- Nuclear Oversight Director, Division of Radiation Northeast Utilities Service Company Department of Environmental Protection P. O. Box 128 79 Elm Street Waterford, CT 06385 l Hartford, CT 06106-5127 Mr. John Carlin Regional Administrator, Region i Vice President- Human Services U.S. Nuclear Regulatory Commission Northeast Utilities Service Company l 475 Allendale Road P. O. Box 128 King of Prussia, PA 19406 Waterford, CT 06385 First Selectmen Mr. Allan Johanson, Assistant Director Town of Waterford Office of Policy and Management l 15 Rope Ferry Road Policy Development and Planning Waterford, CT 06385 Division 450 Capitol Avenue - MS# 52ERN Mr. Wayne D. Lanning, Director P. O. Box 341441 Millstone inspections Hartford, CT 06134-1441 Office of the Regional Administrator 475 Allendale Road Mr. M. H. Brothers i

King of Prussia, PA 19406-1415 Vice President - Operations Northeast Nuclear Energy Company Charles Brinkman, Manager P.O. Box 128 l Washington Nuclear Operations Waterford, CT 06385 i ABB Combustion Engineering l 12300 Twinbrook Pkwy, Suite 330 Mr. J. A. Price Rockville, MD 20852 Director- Unit 2 Northeast Nuclear Energy Company Senior Resident inspector P.O. Box 128 l Millstone Nuclear Power Station Waterford, CT 06385 l

clo U.S. Nuclear Regulatory Commission P.O. Box 513 ,

i Niantic, CT 06357 l

l 1

- - - - . . - . . . . - - . _ . _ - . . . - .- - . - . _.. -.- - . . _ . . - ._ -. - . -_~. -

l Millstona Nucle r Power Station Unit 2 I

cc:

Mr. Leon J. Olivier Attomey Nicholas J. Scobbo, Jr.

Chief Nucler Officer- Millstone Ferriter, Scobbo, Caruso, Rodophele, PC Northeast Nuclear Energy Company 1 Beacon Street,11th Floor P.O. Box 128 Boston, MA 02108 Waterford, CT 06385 Mr. J. P. McElwain Citizens Regulatory Commission Recovery Officer- Millstone Unit 2 ATTN: Ms. Susan Pony Luxton Northeast Nuclear Energy Company i 180 Great Neck Road P. O. Box 128 Waterford,CT 06385 Waterford, Connecticut 06385 Deborah Katz, President Citizens Awareness Network P. O. Box 83 Shelbume Falls, MA 03170 The Honorable Terry Concannon Co-Chair i Nuclear Energy Advisory Council Room 4035 Legislative Office Building Capitol Avenue Hartford, CT 06106 Mr. Evan W. Woollacott Co-Chair Nuclear Energy Advisory Council 128 Terry's Plain Road Simsbury, CT 06070 Little Harbor Consultants, Inc.

Millstone -ITPOP Project Office P. O. Box 0630 Niantic, CT 06357-0630 l

Mr. Daniel L. Curry Project Director Parsons Power Group Inc.

2675 Morgantown Road Reading, PA 19607

-. - . . _ _ _ .