ML15292A214

Response to Request for Additional Information on Proposed License Amendment Request (LAR) TS 3.8.1 - AC Sources - Operating Revised Surveillance Requirement

ML15292A214
Person / Time
Site:	North Anna
Issue date:	10/13/2015
From:	Clark G Virginia Electric & Power Co (VEPCO)
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
15-476
Download: ML15292A214 (13)

Text

VIRGINIA ELECTRIC AND POWER COMPANY RICHMOND, VIRGINIA 23261 October 13, 2015 U. S. Nuclear Regulatory Commission Serial No.:

15-476 Attention: Document Control Desk NLOS/DEA:

R0 Washington, DC 20555-0001 Docket No.:

50-339 License No.:

NPF-7 VIRGINIA ELECTRIC AND POWER COMPANY (DOMINION)

NORTH ANNA POWER STATION UNIT 2 RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION PROPOSED LICENSE AMENDMENT REQUEST (LAR)

TS 3.8.1 - AC SOURCES - OPERATING REVISED SURVEILLANCE REQUIREMENT In a May 19, 2015 letter (Serial No.15-065) Dominion requested an amendment, in the form of a change to the Technical Specifications (TS) for Facility Operating License Number NPF-7 for North Anna Power Station Unit 2, to modify Surveillance Requirement (SR) 3.8.1.8.

In an email dated September 9, 2015, the Nuclear Regulatory Commission (NRC) transmitted a request for additional information (RAI) related to the proposed [AR. The Dominion response to the RAI is provided in Attachment 1. In response to RAI 1, a copy of drawing 1300016-11715-FE-IBB, Sheet I of I is provided in Attachment 2 and a copy of drawing 11715-FE-I1BG, Sheet I of I is provided in Attachment 3.

Should you have any questions or require additional information, please contact Ms. Diane E. Aitken at (540) 894-2574.

Sincerely, Gianna C. Clark Vice President-Nuclear Support Services l,

"OTciL AilR" I

Commonwealth of. Virginia "Reg,

1. t 1410542 COMMNWEATH O VIRINIA

)

fy,Crnmossion.Expires*May 31, 2018 COUNTY OF HENRICO

)

The foregoing document was acknowledged before me, in and for the County and Commonwealth aforesaid, today by Ms. Gianna C. Clark, who is Vice President -

Nuclear Support Services, of Virginia Electric and Power Company. She has affirmed before me that she is duly authorized to execute and file the foregoing document in behalf of that company, and that the statements in the document are true to the best of her knowledge and belief.

7" 05 Acknowledged before me this 3.=_,day of 0*

2015.'/*

MY Com mission Expires:~

3/,/.

?

.:O*'*_,

Serial No.15-476 Docket No. 50-339 Page 2 of 2 Commitments contained in this letter: None Attachments:

1. Response to Request for Additional Information

2. Drawing 1300016-11715-FE-1BB, Sheet I of I

3. Drawing 11715-FE-I1BG, Sheet 1 of 1 cc:

U.S. Nuclear Regulatory Commission - Region II Marquis One Tower 245 Peachtree Center Avenue, NE Suite 1200 Atlanta, GA 30303-1257 Mr. J. E. Reasor, Jr.

Old Dominion Electric Cooperative Innsbrook Corporate Center 4201 Dominion Blvd.

Suite 300 Glen Allen, Virginia 23060 Dr. V. Sreenivas NRC Project Manager-NAPS U.S. Nuclear Regulatory Commission One White Flint North Mail Stop 08 G-9A 11555 Rockville Pike Rockville, MD 20852-2738 Ms. Karen Cotton-Gross NRC Project Manager U.S. Nuclear Regulatory Commission One White Flint North Mail Stop 08 G-9A 11555 Rockville Pike Rockville, MD 20852-2738 NRC Senior Resident Inspector North Anna Power Station

Serial No.15-476 Docket No. 50-339 Attachment I RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION Virginia Electric and Power Company (Dominion)

North Anna Power Station Unit 2

Serial No.15-476 Docket No. 50-339 RAI - LAR TS 3.8.1 AttachmentI Page 1 of6 RAIl1 Provide a brief description, including drawings, of the proposed changes in order to remove Note 1 from surveillance SR 3.8.1.8. The description and simplified drawings should identify the proposed configuration of the plant during all modes of operation.

Dominion Response For Unit 1, preferred alternate supplies (bus ties) from the Station Service Bus to Emergency Bus are provided that function as an alternate offsite AC circuit. These bus ties exist between Station Service Bus 1 B and Emergency Bus 1 H and between Station Service Bus 2B and Emergency Bus 1J. These bus ties have a normally open breaker at the Station Service Bus and Emergency Bus.

Unit 2 does not have Station Service Bus to Emergency Bus preferred alternate supplies (bus ties) that provide an alternate offsite AC circuit to each emergency bus similar to Unit 1. On Unit 2, the Emergency Buses (2H and 2J) may be interconnected by a breaker that is normally removed from its cubicle located on Bus 2H. The breaker on Bus 2H is under strict operational supervision and is provided for maintenance purposes.

Dominion is currently developing a plant modification to install an alternate offsite AC circuit to each Unit 2 Emergency Bus, 2H and 2J. In addition, the existing manual cross-tie between Buses 2H and 2J will be permanently removed. The additional offsite circuits will require testing in accordance with TS SR 3.8.1.8. As a result, the limitation in SR 3.8,1.8, Note 1, will no longer be necessary and Dominion proposes that it be removed.

A markup of drawing NA-DWG-000-1300016-11715-FE-1BB, SH-001 and 11715-FE-1 BG, SH-001 are provided in Attachments 2 and 3, respectively, which illustrate the proposed Unit 2 bus preferred alternate supplies (bus ties) and backup alternate supply.

The proposed configurations of the plant during all modes of operation with the new Unit 2 bus ties installed are tabulated below:

Serial No.15-476 Docket No. 50-339 RAI - LAR TS 3.8.1 _

Page 2 of 6 Emergency [

Unit Status 1 Transmission

[

Offsite I

Station Bus

__________________J Network

[

Transformer Transformer Unit 2 Generator /

tGU i

SST 2C Unt1Oln r

500kV Switchyard ni(Note 1)

Offline, Unit 2 Online [500kV Switchyard SRT-l to Bus 3 RSST C 230kV Switchyard ][SRT-3 to Bus 5

]

(Note 1) 1 if Unit 2 GSU Bus 2H UnitOnlne rwitchyard___

j (Links Removed)

SST 2C Offline, Unit 20Offline [500kV Switchyard ]

SRT-1 to Bus 3 ]

RSST C 230kV Switchyard SRT-3 to Bus 5 ]

(Note 1)

[Backup Alternate - Unit 500kV Switchyard SRT-2 to Bus 4

]

RSST A I Online or Offline, 1i (Through 0L Unit 2 Online or Offline [230kV SwitchyardI SRT-3 to Bus 5 Bus)

Unit I Generator/

UntI/U 1 STI Uni 2 nlne r fflne 500V Sithyad SR-2to Bu 4S ] SST 1A Bus 2J

[ 23~~~~50kV Switchyard SR3toBs5]

(te)

Unit I Offline,

[50vSicyrUnt1GUSTA Unit 2 Online or Offlin e [500kv Switchyard SRT-2 to Bus 4 ]

RSST A Bu J230kV Switchyard IISRT-3 to Bus 5 1 (oe1 Note 1: When the specified alternate alignment to the emergency bus is placed in service, the other emergency bus must also be placed in an alternate alignment for both buses (2H and 2J) to be considered operable. This satisfies the requirement for two qualified circuits between the offsite transmission network and the onsite Class 1 E AC electrical power distribution system per TS 3.8.1.a.

RAI 2

The intent of the proposed plant modification is to provide flexibility in plant operation to ensure that two qualified offsite power sources are available per TS 3.8.1.a. After installation of the modification, a combination of offsite power sources will be available to supply power to safety busses of both units. As an example, the staff notes that in some cases, offsite power from Unit 1 (or Unit 2) busses may be used to supply Unit 2 (Unit 1) safety busses. To verify compliance with Criterion 17, please provide:

a. A summary of load flow analyses (limiting cases) performed for the proposed electrical paths.

Serial No.15-476 Docket No. 50-339 RAI - LAR TS 3.8.1

__Attachment 1__

Page 3 of 6

b. The reason for mega volts-amps reactive (MVAR) limitations imposed on the Main Generator when Bus 2H or 2J is connected to the corresponding bus 2C or lA respectively. For a plant configuration using offsite power through busses 2C and 1A, please provide a summary of safety bus voltages for a postulated accident in Unit 2, a controlled safe shutdown of Unit I with offsite power system (grid) at the minimum allowable voltage and the Main Generator at the maximum allowable MVAR limit. Please include a profile of bus voltages at the onset of the event, during load sequencing and steady state conditions.

c. Provide clarification on the paths that will be tested when the limitation in Ts SR 3.8.1.8 is removed.

Dominion Response

a. The proposed Unit 2 alternate offsite power supply alignments have been evaluated for conformance with General Design Criterion (GDC) 17. When the preferred alternate offsite power supplies are in service and an accident or a trip occurs on Unit 2, Station Service Bus 20 will auto-transfer to Reserve Station Service Transformer (RSST) C, which is supplied by a qualified offsite source, while Bus 2J will continue to be supplied by the Unit I Main Generator (or the 500 kV switchyard) through Station Service Bus 1A. If an accident or a trip occurs on Unit l, the Unit 1 Generator Output Breaker (GOB) will open allowing Bus 2J to continue to be fed from the Unit 1 Station Service Transformer (SST) 1A via the 500 kV switchyard, while Bus 2H will continue to be supplied by the Unit 2 Main Generator through Station Service Bus 2C.

To assess the effects on auxiliary power system voltages, bounding analyses were performed using the ETAP computer program with the proposed modifications implemented.

For evaluation of the new bus preferred alternate offsite power supplies, the first analysis modeled a Unit 2 Containment Depressurization Actuation (CDA) followed by a Unit 2 Station Service Bus auto-transfer at 30 seconds. In this analysis, emergency Bus 2H was loaded on SST 20 (which is auto-transferred to RSST C at 30 seconds), while emergency Bus 2J was loaded on Station Service Bus IA. The second analysis modeled a Unit 2 trip event with the preferred alternate offsite power supplies in place followed by a Unit 2 Station Service Bus 20 auto-transfer to RSST C at 30 seconds. Based on the analyses performed, sufficient margins exist to allow the preferred alternate offsite power supplies to be used during these bounding scenarios and the proposed new paths are sufficient to satisfy GDC 17 safety function requirements.

Serial No.15-476 Docket No. 50-339 RAI - LAR TS 3.8.1 A ttachrnent~l

-Page 4of 6

b. The MVAR output limitation (+300 MVAR out -

lagging) for Unit I or 2 with the preferred alternate offsite power supplies in place is added in order to ensure adequate Bus 2J and Bus 2H voltage in accordance with GDC17. This limitation is modeled in the load flow analysis performed for Unit 2 as well as in the load flow analysis previously performed for the Unit I preferred alternate offsite power supplies.

To assess the impact of the preferred alternate offsite power supplies on the auxiliary system voltages, bounding ETAP runs were performed with the proposed preferred alternate offsite power supplies installed. One set of runs modeled a Unit 2 CDA followed by a Unit 2 Station Service Bus transfer at 30 seconds. In this analysis the Emergency Bus 2H was loaded on Station Service Transformer 2C (transferred to RSST C at 30 seconds) while the Emergency Bus 2J was loaded on Station Service Bus IA. Based on the performed analysis, sufficient margins exist to allow the preferred alternate offsite power supplies to be used during these bounding scenarios. A summary of safety bus voltages is provided below:

Voltages Prior to Transfer:

Load Sequence Begins at 0+

Generator and Switchyard Generation Categories Change to Post Event Lows.

ETAP Time Bus Votg 0-BUS 1 H - 4KV 4280 0

BUSI1H -4KV 4092 6

BUS 1H-4KV 4120 15 BUSIH -4KV 4196 20 BUS 1H -4KV 4247 30 BUS 1H -4KV 4274 0-BUS 1J -4KV 4283 0

BUS 1J-4KV 4108 6

BUSI1J-4KV 4138 15 BUS 1J -4KV 4218 20 BUS 1J -4KV 4271 30 BUSI1J -4KV 4271 0-BUS 2H -4KV 4313 0

BUS 2H -4KV 3640 6

BUS 2H -4KV 3961 15 BUS 2H -4KV 3961 20 BUS 2H - 4KV 3875 30 BUS 2H - 4KV 3953 Voltages after Station Service Load Transfers onto the Reserve Station Transformers:

ETAP Time BusVotg 0-BUS IH -4KV 4074 0

BUSI1H -4KV 4074 6

BUSI1H-4KV 4126 60 BUSI1H -4KV 4276 0-BUS1J -4KV 4073 0

BUS1J - 4KV 4073 6

BUS 1J -4KV 4099 60 BUSI1J -4KV 4276 0-BUS 2H -4KV 4030 0

BUS 2H - 4KV 4030 6

BUS 2H -4KV 4083 60 BUS 2H -4KV 4234 0-BUS 2J - 4KV 4087 0

BUS 2J - 4KV 4087 6

BUS 2J -4KV 4113 60 BUS 2J -4KV 4314

Serial No.15-476 Docket No. 50-339 RAI - LAR TS 3.8.1

&ttacbrnentIl Page 5 of 6 Voltages Prior to Transfer:

Load Sequence Begins at 0+

Generator and Switchyard Generation Categories Change to Post Event Lows.

ETAP Time Bus Voae 0-BUS 2J -4KV 4319 0

BUS 2J - 4KV 3717 6

BUS 2J -4KV 4098 15 BUS 2J -4KV 4098 20 BUS 2J - 4KV 3996 30 BUS 2J - 4KV 4088 Note: Some Voltages drop below DVR drop-out (3783.3V). All voltages recover prior to the minimum DVR drop-out time delay of 6 seconds.

C. The paths that will be periodically tested when the limitation in TS SR 3.8.1.8 is removed are as follows:

The preferred alternate path from Unit I Station Service Bus 1A to Unit 2 Emergency Bus 2J,

• The preferred alternate path from Unit 2 Station Service Bus 2C to Unit 2 Emergency Bus 2H, and

• The backup alternate path from RSST A to "D" Transfer bus through 0L bus to "E" Transfer bus to Emergency Bus 2H.

The described alternate 4.16kV connections are tested; however, it is not intended that the upstream paths be tested. All tests are performed per procedures which prevent alignments that are not in accordance with GDC-17.

This is similar to the existing Unit 1 Periodic Test procedure and procedures for transferring of 4160 Volt busses.

RAI 3

With reference to GDC Criterion 5, please provide a summary of the analysis performed to demonstrate that degraded or fault conditions in one electrical path will not adversely impact safe shutdown of dual units.

Dominion Response GDC 5 states that the important to safety systems, structures or components that are shared between units should not significantly impair the safety functions of either unit.

Serial No.15-476 Docket No. 50-339 RAI - LAR TS 3.8.1

__ Attachment 1 Page 6 of 6 Each unit at North Anna Power Station has redundant emergency buses which are able to safely shutdown each unit. An electrical fault or degraded condition at the points common to both units in the system are detected by plant specific undervoltage relays at the Emergency buses. The Emergency bus relaying then separates the impacted bus(es) from the offsite supply and starts the associated diesel generators to provide emergency bus power. All redundant emergency busses would have power, either offsite power or diesel power. The conditions at both plants would then be evaluated to determine the need to shutdown and cooldown the unit, which could be done in an orderly manner, as required. With the new Unit 2 preferred alternate offsite power supplies utilized, in the event of an accident on either unit, the two emergency busses on each unit continue to be supplied by a separate RSST or SST. Hence, the addition of the new Unit 2 preferred alternate offsite power supplies does not add an additional failure method nor does it impair the safety functions or the ability to safely shutdown or cooldown either unit.

RAI 4

Since a Generator Output Breaker is not planned for the Unit 2 main generator, the RSSTs or alternate offsite power sources may be used to power the auxiliaries. During this period, the SSTs and the RSSTs or alternate paths are paralleled prior to transfer to maintain uninterrupted power to the plant busses. Confirm that fault conditions or other electrical transients during this period will not adversely impact operation of dual unit or dual unit safe shutdown capability.

Dominion Response The transfer of the auxiliary busses from the SSTs to the RSSTs will occur in the same manner that it presently does after the Unit 1 preferred alternate offsite power supplies are utilized. During the manual transfer, for a short duration, the SSTs and RSSTs are paralleled. During this short period of time it is plausible that the circuit breaker interrupting ratings at the 2J or 2H buses are exceeded. As stated in IEEE Std 666-2007 it is an acceptable practice to design for the single-source condition if the duration of parallel operation is short. The proposed manual transfer to the preferred alternative feed is controlled by procedures and occurs in a short amount of time, thus meeting this practice. There are no other electrical transients expected during the manual transfer.

The system conditions following the transfer have been previously evaluated and no short circuit or load flow concerns have been determined to exist with the alternate offsite power supplies being utilized.

Serial No.15-476 Docket No. 50-339 Page 1 of 2 DRAWING 130001 6-11715-FE-I1BB, Sheet I of I ONE LINE DIAGRAM ELECTRICAL DISTRIBUTION SYSTEM NORTH ANNA POWER STATION UNITS 1 & 2 Virginia Electric and Power Company (Dominion)

North Anna Power Station Unit 2

Serial No.15-476 Docket No. 50-339 Page 1 of 2 DRAWING 1171 5-FE-I BG, Sheet I of I ONE LINE DIAGRAM ELECTRICAL DISTRIBUTION SYSTEM STATION BLACKOUT SYSTEM NORTH ANNA POWER STATION Virginia Electric and Power Company (Dominion)

VIRGINIA ELECTRIC AND POWER COMPANY RICHMOND, VIRGINIA 23261 October 13, 2015 U. S. Nuclear Regulatory Commission Serial No.:

15-476 Attention: Document Control Desk NLOS/DEA:

R0 Washington, DC 20555-0001 Docket No.:

50-339 License No.:

NPF-7 VIRGINIA ELECTRIC AND POWER COMPANY (DOMINION)

NORTH ANNA POWER STATION UNIT 2 RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION PROPOSED LICENSE AMENDMENT REQUEST (LAR)

TS 3.8.1 - AC SOURCES - OPERATING REVISED SURVEILLANCE REQUIREMENT In a May 19, 2015 letter (Serial No.15-065) Dominion requested an amendment, in the form of a change to the Technical Specifications (TS) for Facility Operating License Number NPF-7 for North Anna Power Station Unit 2, to modify Surveillance Requirement (SR) 3.8.1.8.

In an email dated September 9, 2015, the Nuclear Regulatory Commission (NRC) transmitted a request for additional information (RAI) related to the proposed [AR. The Dominion response to the RAI is provided in Attachment 1. In response to RAI 1, a copy of drawing 1300016-11715-FE-IBB, Sheet I of I is provided in Attachment 2 and a copy of drawing 11715-FE-I1BG, Sheet I of I is provided in Attachment 3.

Should you have any questions or require additional information, please contact Ms. Diane E. Aitken at (540) 894-2574.

Sincerely, Gianna C. Clark Vice President-Nuclear Support Services l,

"OTciL AilR" I

Commonwealth of. Virginia "Reg,

1. t 1410542 COMMNWEATH O VIRINIA

)

fy,Crnmossion.Expires*May 31, 2018 COUNTY OF HENRICO

)

The foregoing document was acknowledged before me, in and for the County and Commonwealth aforesaid, today by Ms. Gianna C. Clark, who is Vice President -

Nuclear Support Services, of Virginia Electric and Power Company. She has affirmed before me that she is duly authorized to execute and file the foregoing document in behalf of that company, and that the statements in the document are true to the best of her knowledge and belief.

7" 05 Acknowledged before me this 3.=_,day of 0*

2015.'/*

MY Com mission Expires:~

3/,/.

?

.:O*'*_,

Serial No.15-476 Docket No. 50-339 Page 2 of 2 Commitments contained in this letter: None Attachments:

1. Response to Request for Additional Information

2. Drawing 1300016-11715-FE-1BB, Sheet I of I

3. Drawing 11715-FE-I1BG, Sheet 1 of 1 cc:

U.S. Nuclear Regulatory Commission - Region II Marquis One Tower 245 Peachtree Center Avenue, NE Suite 1200 Atlanta, GA 30303-1257 Mr. J. E. Reasor, Jr.

Old Dominion Electric Cooperative Innsbrook Corporate Center 4201 Dominion Blvd.

Suite 300 Glen Allen, Virginia 23060 Dr. V. Sreenivas NRC Project Manager-NAPS U.S. Nuclear Regulatory Commission One White Flint North Mail Stop 08 G-9A 11555 Rockville Pike Rockville, MD 20852-2738 Ms. Karen Cotton-Gross NRC Project Manager U.S. Nuclear Regulatory Commission One White Flint North Mail Stop 08 G-9A 11555 Rockville Pike Rockville, MD 20852-2738 NRC Senior Resident Inspector North Anna Power Station

Serial No.15-476 Docket No. 50-339 Attachment I RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION Virginia Electric and Power Company (Dominion)

North Anna Power Station Unit 2

Serial No.15-476 Docket No. 50-339 RAI - LAR TS 3.8.1 AttachmentI Page 1 of6 RAIl1 Provide a brief description, including drawings, of the proposed changes in order to remove Note 1 from surveillance SR 3.8.1.8. The description and simplified drawings should identify the proposed configuration of the plant during all modes of operation.

Dominion Response For Unit 1, preferred alternate supplies (bus ties) from the Station Service Bus to Emergency Bus are provided that function as an alternate offsite AC circuit. These bus ties exist between Station Service Bus 1 B and Emergency Bus 1 H and between Station Service Bus 2B and Emergency Bus 1J. These bus ties have a normally open breaker at the Station Service Bus and Emergency Bus.

Unit 2 does not have Station Service Bus to Emergency Bus preferred alternate supplies (bus ties) that provide an alternate offsite AC circuit to each emergency bus similar to Unit 1. On Unit 2, the Emergency Buses (2H and 2J) may be interconnected by a breaker that is normally removed from its cubicle located on Bus 2H. The breaker on Bus 2H is under strict operational supervision and is provided for maintenance purposes.

Dominion is currently developing a plant modification to install an alternate offsite AC circuit to each Unit 2 Emergency Bus, 2H and 2J. In addition, the existing manual cross-tie between Buses 2H and 2J will be permanently removed. The additional offsite circuits will require testing in accordance with TS SR 3.8.1.8. As a result, the limitation in SR 3.8,1.8, Note 1, will no longer be necessary and Dominion proposes that it be removed.

A markup of drawing NA-DWG-000-1300016-11715-FE-1BB, SH-001 and 11715-FE-1 BG, SH-001 are provided in Attachments 2 and 3, respectively, which illustrate the proposed Unit 2 bus preferred alternate supplies (bus ties) and backup alternate supply.

The proposed configurations of the plant during all modes of operation with the new Unit 2 bus ties installed are tabulated below:

Serial No.15-476 Docket No. 50-339 RAI - LAR TS 3.8.1 _

Page 2 of 6 Emergency [

Unit Status 1 Transmission

[

Offsite I

Station Bus

__________________J Network

[

Transformer Transformer Unit 2 Generator /

tGU i

SST 2C Unt1Oln r

500kV Switchyard ni(Note 1)

Offline, Unit 2 Online [500kV Switchyard SRT-l to Bus 3 RSST C 230kV Switchyard ][SRT-3 to Bus 5

]

(Note 1) 1 if Unit 2 GSU Bus 2H UnitOnlne rwitchyard___

j (Links Removed)

SST 2C Offline, Unit 20Offline [500kV Switchyard ]

SRT-1 to Bus 3 ]

RSST C 230kV Switchyard SRT-3 to Bus 5 ]

(Note 1)

[Backup Alternate - Unit 500kV Switchyard SRT-2 to Bus 4

]

RSST A I Online or Offline, 1i (Through 0L Unit 2 Online or Offline [230kV SwitchyardI SRT-3 to Bus 5 Bus)

Unit I Generator/

UntI/U 1 STI Uni 2 nlne r fflne 500V Sithyad SR-2to Bu 4S ] SST 1A Bus 2J

[ 23~~~~50kV Switchyard SR3toBs5]

(te)

Unit I Offline,

[50vSicyrUnt1GUSTA Unit 2 Online or Offlin e [500kv Switchyard SRT-2 to Bus 4 ]

RSST A Bu J230kV Switchyard IISRT-3 to Bus 5 1 (oe1 Note 1: When the specified alternate alignment to the emergency bus is placed in service, the other emergency bus must also be placed in an alternate alignment for both buses (2H and 2J) to be considered operable. This satisfies the requirement for two qualified circuits between the offsite transmission network and the onsite Class 1 E AC electrical power distribution system per TS 3.8.1.a.

RAI 2

The intent of the proposed plant modification is to provide flexibility in plant operation to ensure that two qualified offsite power sources are available per TS 3.8.1.a. After installation of the modification, a combination of offsite power sources will be available to supply power to safety busses of both units. As an example, the staff notes that in some cases, offsite power from Unit 1 (or Unit 2) busses may be used to supply Unit 2 (Unit 1) safety busses. To verify compliance with Criterion 17, please provide:

a. A summary of load flow analyses (limiting cases) performed for the proposed electrical paths.

Serial No.15-476 Docket No. 50-339 RAI - LAR TS 3.8.1

__Attachment 1__

Page 3 of 6

b. The reason for mega volts-amps reactive (MVAR) limitations imposed on the Main Generator when Bus 2H or 2J is connected to the corresponding bus 2C or lA respectively. For a plant configuration using offsite power through busses 2C and 1A, please provide a summary of safety bus voltages for a postulated accident in Unit 2, a controlled safe shutdown of Unit I with offsite power system (grid) at the minimum allowable voltage and the Main Generator at the maximum allowable MVAR limit. Please include a profile of bus voltages at the onset of the event, during load sequencing and steady state conditions.

c. Provide clarification on the paths that will be tested when the limitation in Ts SR 3.8.1.8 is removed.

Dominion Response

a. The proposed Unit 2 alternate offsite power supply alignments have been evaluated for conformance with General Design Criterion (GDC) 17. When the preferred alternate offsite power supplies are in service and an accident or a trip occurs on Unit 2, Station Service Bus 20 will auto-transfer to Reserve Station Service Transformer (RSST) C, which is supplied by a qualified offsite source, while Bus 2J will continue to be supplied by the Unit I Main Generator (or the 500 kV switchyard) through Station Service Bus 1A. If an accident or a trip occurs on Unit l, the Unit 1 Generator Output Breaker (GOB) will open allowing Bus 2J to continue to be fed from the Unit 1 Station Service Transformer (SST) 1A via the 500 kV switchyard, while Bus 2H will continue to be supplied by the Unit 2 Main Generator through Station Service Bus 2C.

To assess the effects on auxiliary power system voltages, bounding analyses were performed using the ETAP computer program with the proposed modifications implemented.

For evaluation of the new bus preferred alternate offsite power supplies, the first analysis modeled a Unit 2 Containment Depressurization Actuation (CDA) followed by a Unit 2 Station Service Bus auto-transfer at 30 seconds. In this analysis, emergency Bus 2H was loaded on SST 20 (which is auto-transferred to RSST C at 30 seconds), while emergency Bus 2J was loaded on Station Service Bus IA. The second analysis modeled a Unit 2 trip event with the preferred alternate offsite power supplies in place followed by a Unit 2 Station Service Bus 20 auto-transfer to RSST C at 30 seconds. Based on the analyses performed, sufficient margins exist to allow the preferred alternate offsite power supplies to be used during these bounding scenarios and the proposed new paths are sufficient to satisfy GDC 17 safety function requirements.

Serial No.15-476 Docket No. 50-339 RAI - LAR TS 3.8.1 A ttachrnent~l

-Page 4of 6

b. The MVAR output limitation (+300 MVAR out -

lagging) for Unit I or 2 with the preferred alternate offsite power supplies in place is added in order to ensure adequate Bus 2J and Bus 2H voltage in accordance with GDC17. This limitation is modeled in the load flow analysis performed for Unit 2 as well as in the load flow analysis previously performed for the Unit I preferred alternate offsite power supplies.

To assess the impact of the preferred alternate offsite power supplies on the auxiliary system voltages, bounding ETAP runs were performed with the proposed preferred alternate offsite power supplies installed. One set of runs modeled a Unit 2 CDA followed by a Unit 2 Station Service Bus transfer at 30 seconds. In this analysis the Emergency Bus 2H was loaded on Station Service Transformer 2C (transferred to RSST C at 30 seconds) while the Emergency Bus 2J was loaded on Station Service Bus IA. Based on the performed analysis, sufficient margins exist to allow the preferred alternate offsite power supplies to be used during these bounding scenarios. A summary of safety bus voltages is provided below:

Voltages Prior to Transfer:

Load Sequence Begins at 0+

Generator and Switchyard Generation Categories Change to Post Event Lows.

ETAP Time Bus Votg 0-BUS 1 H - 4KV 4280 0

BUSI1H -4KV 4092 6

BUS 1H-4KV 4120 15 BUSIH -4KV 4196 20 BUS 1H -4KV 4247 30 BUS 1H -4KV 4274 0-BUS 1J -4KV 4283 0

BUS 1J-4KV 4108 6

BUSI1J-4KV 4138 15 BUS 1J -4KV 4218 20 BUS 1J -4KV 4271 30 BUSI1J -4KV 4271 0-BUS 2H -4KV 4313 0

BUS 2H -4KV 3640 6

BUS 2H -4KV 3961 15 BUS 2H -4KV 3961 20 BUS 2H - 4KV 3875 30 BUS 2H - 4KV 3953 Voltages after Station Service Load Transfers onto the Reserve Station Transformers:

ETAP Time BusVotg 0-BUS IH -4KV 4074 0

BUSI1H -4KV 4074 6

BUSI1H-4KV 4126 60 BUSI1H -4KV 4276 0-BUS1J -4KV 4073 0

BUS1J - 4KV 4073 6

BUS 1J -4KV 4099 60 BUSI1J -4KV 4276 0-BUS 2H -4KV 4030 0

BUS 2H - 4KV 4030 6

BUS 2H -4KV 4083 60 BUS 2H -4KV 4234 0-BUS 2J - 4KV 4087 0

BUS 2J - 4KV 4087 6

BUS 2J -4KV 4113 60 BUS 2J -4KV 4314

Serial No.15-476 Docket No. 50-339 RAI - LAR TS 3.8.1

&ttacbrnentIl Page 5 of 6 Voltages Prior to Transfer:

Load Sequence Begins at 0+

Generator and Switchyard Generation Categories Change to Post Event Lows.

ETAP Time Bus Voae 0-BUS 2J -4KV 4319 0

BUS 2J - 4KV 3717 6

BUS 2J -4KV 4098 15 BUS 2J -4KV 4098 20 BUS 2J - 4KV 3996 30 BUS 2J - 4KV 4088 Note: Some Voltages drop below DVR drop-out (3783.3V). All voltages recover prior to the minimum DVR drop-out time delay of 6 seconds.

C. The paths that will be periodically tested when the limitation in TS SR 3.8.1.8 is removed are as follows:

The preferred alternate path from Unit I Station Service Bus 1A to Unit 2 Emergency Bus 2J,

• The preferred alternate path from Unit 2 Station Service Bus 2C to Unit 2 Emergency Bus 2H, and

• The backup alternate path from RSST A to "D" Transfer bus through 0L bus to "E" Transfer bus to Emergency Bus 2H.

The described alternate 4.16kV connections are tested; however, it is not intended that the upstream paths be tested. All tests are performed per procedures which prevent alignments that are not in accordance with GDC-17.

This is similar to the existing Unit 1 Periodic Test procedure and procedures for transferring of 4160 Volt busses.

RAI 3

With reference to GDC Criterion 5, please provide a summary of the analysis performed to demonstrate that degraded or fault conditions in one electrical path will not adversely impact safe shutdown of dual units.

Dominion Response GDC 5 states that the important to safety systems, structures or components that are shared between units should not significantly impair the safety functions of either unit.

Serial No.15-476 Docket No. 50-339 RAI - LAR TS 3.8.1

__ Attachment 1 Page 6 of 6 Each unit at North Anna Power Station has redundant emergency buses which are able to safely shutdown each unit. An electrical fault or degraded condition at the points common to both units in the system are detected by plant specific undervoltage relays at the Emergency buses. The Emergency bus relaying then separates the impacted bus(es) from the offsite supply and starts the associated diesel generators to provide emergency bus power. All redundant emergency busses would have power, either offsite power or diesel power. The conditions at both plants would then be evaluated to determine the need to shutdown and cooldown the unit, which could be done in an orderly manner, as required. With the new Unit 2 preferred alternate offsite power supplies utilized, in the event of an accident on either unit, the two emergency busses on each unit continue to be supplied by a separate RSST or SST. Hence, the addition of the new Unit 2 preferred alternate offsite power supplies does not add an additional failure method nor does it impair the safety functions or the ability to safely shutdown or cooldown either unit.

RAI 4

Since a Generator Output Breaker is not planned for the Unit 2 main generator, the RSSTs or alternate offsite power sources may be used to power the auxiliaries. During this period, the SSTs and the RSSTs or alternate paths are paralleled prior to transfer to maintain uninterrupted power to the plant busses. Confirm that fault conditions or other electrical transients during this period will not adversely impact operation of dual unit or dual unit safe shutdown capability.

Dominion Response The transfer of the auxiliary busses from the SSTs to the RSSTs will occur in the same manner that it presently does after the Unit 1 preferred alternate offsite power supplies are utilized. During the manual transfer, for a short duration, the SSTs and RSSTs are paralleled. During this short period of time it is plausible that the circuit breaker interrupting ratings at the 2J or 2H buses are exceeded. As stated in IEEE Std 666-2007 it is an acceptable practice to design for the single-source condition if the duration of parallel operation is short. The proposed manual transfer to the preferred alternative feed is controlled by procedures and occurs in a short amount of time, thus meeting this practice. There are no other electrical transients expected during the manual transfer.

The system conditions following the transfer have been previously evaluated and no short circuit or load flow concerns have been determined to exist with the alternate offsite power supplies being utilized.

Serial No.15-476 Docket No. 50-339 Page 1 of 2 DRAWING 130001 6-11715-FE-I1BB, Sheet I of I ONE LINE DIAGRAM ELECTRICAL DISTRIBUTION SYSTEM NORTH ANNA POWER STATION UNITS 1 & 2 Virginia Electric and Power Company (Dominion)

North Anna Power Station Unit 2

Serial No.15-476 Docket No. 50-339 Page 1 of 2 DRAWING 1171 5-FE-I BG, Sheet I of I ONE LINE DIAGRAM ELECTRICAL DISTRIBUTION SYSTEM STATION BLACKOUT SYSTEM NORTH ANNA POWER STATION Virginia Electric and Power Company (Dominion)