Text

(Vcsns), Unit 1 - License Amendment Request - Inverter Allowed Outage Time (AOT) Extension
	ML23317A224
Person / Time
Site:	Summer
Issue date:	11/09/2023
From:	James Holloway; Dominion Energy Services, Dominion Energy South Carolina
To:	; Office of Nuclear Reactor Regulation, Document Control Desk
References
	23-177
	Download: ML23317A224 (1)
	v • d • e

Dominion Energy South Caro lin a, In c.

5000 Dominion Boulevard, Glen Allen, VA 23060 Dominion Dominion Energy.com Ill"":; Energy November 9, 2023 Attn: Document Control Desk *Serial No.: 23-177 U. S. Nuclear Regulatory Commission NRA/YG: RO Washington, DC 20555-0001 Docket No.: 50-395 License No.: NPF-12 DOMINION ENERGY SOUTH CAROLINA (DESC)

VIRGIL C. SUMMER NUCLEAR STATION (VCSNS) UNIT 1 LICENSE AMENDMENT REQUEST - INVERTER ALLOWED OUTAGE TIME (AOT}

EXTENSION Pursuant to the provisions of Title 10 of the Code of Federal Regulations (10 CFR Part 50.90), Dominion Energy South Carolina (DESC) is hereby submitting a License Amendment Request for Virgil C. Summer Nuclear Station (VCSNS) Unit 1.

The proposed amendment would modify VCSNS Technical Specification (TS) Limiting Condition for Operation (LCO) 3.8.3.1, Action C, concerning inoperable Alternating Current (A.C.) Inverters, of TS Section 3/4.8.3, Onsite Power Distribution. The proposed change would modify the existing Action C wording to simplify the TS Action C statement and increase the inverter, associated with the 120-Volt A.C. Vital Susses, Allowed Outage Time (AOT) from 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months to 7 days. to this letter provides a description and assessment of the proposed change. The assessment consists of a deterministic engineering evaluation supported by a risk insights assessment. Attachment 2 provides the current VCSNS TS marked-up to show the proposed change. Attachment 3 provides the revised (clean) VCSNS TS page. Attachment 4 provides the proposed TS Bases change. Attachment 5 provides a list of references.

DESC requests approval of the proposed license amendment by November 9, 2024, with a 60-day implementation period.

In accordance with 10 CFR 50.91, "Notice for Public Comment; State Consultation," a copy of this application, with attachments, is being provided to the designated South Carolina State Official.

Should you have any questions, please contact Yan Gao at (804)273-2768.

Respectfully, Ja~ ~ ~7 Vice President - Nuclear Engineering and Fleet Support

Serial No.23-177 Docket No. 50-395 Page 2 of 3 Commitments made in this letter: None.

COMMONWEALTH OF VIRGINIA COUNTY OF HENRICO The foregoing document was acknowledged before me, in and for the County and Commonwealth aforesaid, today by James E. Holloway, who is Vice President - Nuclear Engineering and Fleet Support of Dominion Energy South Carolina, Inc. He has affirmed before me that he is duly authorized to execute and file the foregoing document on behalf of that Company, and that the statements in the document are true to the best of his knowledge and belief.

Acknowledged before me this ~ day of '/JtJy/lY,1. be f" 2023.

My Commission Expires: __l_[;....../_'?>_1---4/....2._J.....IL.-_

ATTACHMENTS

1. Description and Assessment

2. Current VCSNS TS Page Mark-up

3. Revised (Clean) VCSNS TS Page

4. Proposed TS Bases Change

5. References

Serial No.23-177 Docket No. 50-395 Page 3 of 3 cc: U.S. Nuclear Regulatory Commission, Region II Marquis One Tower 245 Peachtree Center Avenue, NE Suite 1200 Atlanta, Georgia 30303-1257 Mr. G. Edward Miller NRC Senior Project Manager U. S. Nuclear Regulatory Commission One White Flint North Mail Stop 09 E-3 11555 Rockville Pike Rockville, Maryland 20852-2738 NRC Senior Resident Inspector V.C. Summer Nuclear Station Ms. Robin S. Mark Bureau of Environmental Health Services South Carolina Department of Health and Environmental Control 8500 Farrow Road - Building 17 Columbia, SC 29203 Mr. G. J. Lindamood Santee Cooper - Nuclear Coordinator 1 Riverwood Drive Moncks Corner, SC 29461

Serial No.23-177 Docket No. 50-395 ATTACHMENT1 DESCRIPTION AND ASSESSMENT Virgil C. Summer Nuclear Station (VCSNS) Unit 1 Dominion Energy South Carolina, Inc. (DESC)

Serial No.23-177 Docket No. 50-395 Attachment 1: Page 1 of 29 TABLE OF CONTENTS 1.0

SUMMARY

DESCRIPTION .......................................................................................................................... 2 2.0 DETAILED DESCRIPTION ............................................................................................................................. 3 2.1 SYSTEM DESIGN AND OPERATION ......................................................................................................................... 3 2.2 CURRENT TS REQUIREMENTS ..............................................................................................................................7 2.3 REASON FOR THE PROPOSED CHANGE ...................................................................................................................8

2.4 DESCRIPTION

OF THE PROPOSED CHANGE ............................................................................................................11

3.0 TECHNICAL EVALUATION

......................................................................................................................... 12 3.1 DETERMINISTIC ASSESSMENT .............................................................................................................................12 3.1.1 Defense-In-Depth ...................................................................................................................................12 3.1.2 Safety Margin Evaluation ......................................................................................................................17 3.1.3 VCSNS Maintenance Rule Monitoring ....................................................................................................19 3.2 RISK INSIGHTS ASSESSMENT ..............................................................................................................................19 3.2.1 Tier 1 - Risk Assessment Results .............................................................................................................19 3.2.2 Tier 2 Assessment -Avoidance of Risk Significant Plant Configurations ............................................... 22 3.2.3 Tier 3 Assessment- Configuration Risk Management Program ............................................................ 23 3.2.4 Risk Insights Conclusion .........................................................................................................................24

4.0 REGULATORY EVALUATION

..................................................................................................................... 24 4.1 APPLICABLE REGULATORY REQUIREMENTS AND CRITERIA ........................................................................................ 24 4.2 PRECEDENT ....................................................................................................................................................26 4.3 No SIGNIFICANT HAZARDS CONSIDERATION .........................................................................................................27

4.4 CONCLUSION

..................................................................................................................................................29

5.0 ENVIRONMENTAL CONSIDERATION

........................................................................................................ 29

Serial No.23-177 Docket No. 50-395 Attachment 1: Page 2 of 29 DESCRIPTION AND ASSESSMENT 1.0

SUMMARY

DESCRIPTION This License Amendment Request (LAR) would change Dominion Energy South Carolina (DESC) Virgil C. Summer Nuclear Station (VCSNS), Unit 1, Technical Specification (TS)

[5.1 ], Section 3/4.8.3, Onsite Power Distribution, Limiting Condition for Operation (LCO) 3.8.3.1, Action C, concerning inoperable Alternating Current (A.C.) Inverters. The proposed change would simplify the existing Action C wording and increase the inverter, associated with the 120-Volt A.C. Vital Susses, Allowed Outage Time (AOT) from 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months to 7 days.

The proposed AOT extension will provide operational and maintenance flexibility to allow performance of adequate preventative and corrective maintenance and ensure long term 120-Volt A.C. Vital Susses reliability and availability. The proposed change provides flexibility to resolve unanticipated inverter deficiencies and avoid a potential unplanned plant shutdown should an emergent condition occur online requiring inverter corrective maintenance beyond the current 24-hour TS Action time.

This LAR submittal is based on a deterministic engineering evaluation supported by a risk insights assessment, and on risk management considerations.

Table 1.0-1 below provides a decision summary using the integrated risk evaluation insights and process laid out in the NRC Be riskSMART framework [5.27].

Be ... Clear 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months of AOT under VCSNS TS LCO 3.8.3.1, Action C, for about the 120-Volt A.C. Vital Susses (A.C. Inverters), is insufficient for roblem on-line maintenance and testin activities.

Spot... What can Extending the AOT to 7 days will reduce the potential for go right? unnecessary plant shutdowns due to inverter corrective maintenance activities and avoid unnecessa shutdown risk.

What can Maintaining the current 24-hour AOT will result in more go wrong? challenges (more consequences) from inverter on-line maintenance and testin activities.

How likely It is with high probability and high confidence to predict that is it? on-line digital inverter maintenance activities will exceed the 24-hour AOT.

Manage What you On-line maintenance and testing activities risk assessment can and management actions in accordance with 10 CFR 50.56 (a)(4), and defense-in-depth design features will provide sufficient com ensato measures.

Serial No.23-177 Docket No. 50-395 Attachment 1: Page 3 of 29 on a Based on the high probability and high consequence of the decision challenge, DESC is requesting an AOT extension from 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months to 7 days for the inverters. This LAR only applies to a single inverter AOT and without a SBO condition. If more than a single inverter is out of service or under a SBO condition, the sub*ect TS Action statement would not be a licable.

Realize The result VCSNS programs and processes, such as training, corrective actions, Maintenance Rule, etc., ensure any lessons learned from a 7-da inverter AOT will be ca tured.

2.0 DETAILED DESCRIPTION 2.1 System Design and Operation The nominal 120-Volt A.C. Vital Bus system consists of six (6) panels and four (4) inverters which provide power to four (4) independent channels of Engineered Safety Features (ESF) instrumentations. Channels A and B consist of two (2) panels and one (1) inverter each, while Channels D and E consist of one (1) panel and one (1) inverter each.

Six (6) 120-Volt AC. Vital Susses are provided. Four (4) of the six (6) Susses, APN5901 (120-Volt Vital AC. Distribution Panel 1, NSSS), APN5902 (120-Volt Vital AC.

Distribution Panel 2, NSSS), APN5903 (120-Volt Vital AC. Distribution Panel 3, NSSS),

and APN5904 (120-Volt Vital AC. Distribution Panel 4, NSSS), are supplied by one (1) of four (4) single phase static inverters to each of them. The other two (2) of the six (6)

Busses, APN5907 (120-Volt Vital AC. Distribution Panel 7, NSSS) and APN5908 (120-Volt Vital AC. Distribution Panel 8, NSSS), are powered by two (2) of the four (4) UPS backed busses. The normal feed to Panel APN5907 is from APN5901. The normal feed to Panel APN5908 is from APN5903.

One (1) Channel A and one (1) Channel D inverters are connected to ESF battery 1A, and one (1) Channel Band one (1) Channel E inverters are connected to ESF battery 1B.

The 120-Volt A.C. Vital Susses constitute a reliable electrical system which provides a stable power supply to vital equipment and guarantees proper action when power is required while eliminating spurious shutdowns. Controls for the backup groups of heaters, pressurizer level transmitters, and pressurizer relief block valve operators receive their power from this Vital AC. Bus source through the emergency power busses.

Control power for the block valves is supplied from an emergency power bus different from that which supplies the associated PORV. Safety grade circuit breakers and fuses are used for circuit protection.

The normal source of power for the 120-Volt AC. Vital Bus inverters is through the inverter static rectifier. These inverter rectifiers are fed from 480-Volt Susses 1 DA2 and

Serial No.23-177 Docket No. 50-395 Attachment 1: Page 4 of 29 10B2. The station batteries and battery chargers constitute the standby power source.

The battery chargers are fed from 480-Volt Susses 1DA2 and 10B2. In the event of loss of 480-Volt power, the power source for the Vital Bus inverters is the station batteries.

These batteries are floating, on standby service. The change in power source, from normal to standby, occurs without exceeding the stated inverter output voltage and frequency regulation. The station batteries are sized to carry this additional inverter load without being charged for no less than 4 hours4.62963e-5 days 0.00111 hours 6.613757e-6 weeks 1.522e-6 months . The battery chargers are sized to recharge from a design minimum charge to full charge in 12 hours1.388889e-4 days 0.00333 hours 1.984127e-5 weeks 4.566e-6 months while carrying the largest combined demand from the various steady-state loads. An alternate power supply for the 120-Volt A.C. Vital Busses is provided through 480-Volt to 120-Volt transformers from 480-Volt Susses 1DA2 and 1D82 for use when the inverters are out of service.

The Vital A.C. inverter in each system provides continuous power to the 120-Volt Vital A.C. Busses. The output of the inverter is a regulated supply. Transfer from one (1) input to the other is accomplished without interruption of the output. Each inverter is protected by circuit breakers on the 480-Volt A.C. input side, 120-Volt Vital A.C. output side and 125-Volt D.C. input side. Each dual input inverter is provided with an A.C. and a D.C.

ammeter. Abnormal conditions in the dual input inverter, including loss of A.C. input, loss of D.C. input, and loss of A.C. output voltage cause alarms to occur in the control room.

The output of each inverter is connected to a distribution cabinet through an automatic static transfer switch and a normally closed-circuit breaker. An alternate backup 480-Volt to120-Volt Transformer Class 1E power source is provided through the automatic static transfer switch. The feed to the transformers is from a 480-Volt Motor Control Center with emergency diesel generator backup. The distribution cabinets have appropriately sized branch circuit breakers to feed reactor protection and other vital instrument channels.

Most reactor protective schemes have three (3) or four (4) channels. Redundant instrument channels are fed from redundant vital busses. Because of the preferred failure mode defined for the reactor protective instrumentation, failure of an instrument channel

_power source results in a reactor trip signal from the affected channel. Multiple power supplies are provided to prevent a single power supply failure from initiating a false reactor trip.

Operable Vital AC. Inverters require the associated Vital AC. Instrumentation Bus to be powered by the inverter and connected to the 125-Volt D.C. battery.

With a Vital A.C. inverter inoperable and the associated Vital AC. Instrumentation Bus supplied from the alternate (interruptible) regulated A.C. electrical power source, a loss of offsite power will result in loss of power to the associated Vital A.C. Instrumentation Bus. Power would be restored once the associated Diesel Generator (DG) re-energized the Bus. Following restoration of Vital A.C. Instrumentation Bus power, Bus loads would be restored.

Only one of the four Vital A.C. Instrument Busses APN5901, 2, 3 or 4-EV can be energized via the non-regulated alternate source BOP instrument power supply at any time in accordance with TS Section 3/4.8.3. Therefore, a minimum of three (3) out of the four (4) redundant channels of NSSS and BOP instrumentation are provided with a regulated uninterruptible source of power for monitoring and actuating the Reactor

Serial No.23-177 Docket No. 50-395 Attachment 1: Page 5 of 29 Protection System. This limitation ensures vital A.C. system availability and NSSS instrument operability.

Following the loss of function of inverter XIT5901, 2, 3, or 4, the inverter will automatically switch to the backup source without loss of A.C. output. Additionally, inverters XIT5901, 2, 3, 4 are equipped with an Independent Static Bypass Switch (ISBS); in the event of a failure of the UPS controller, the unit will default to the backup source. Annunciator Response Procedures (ARP) guide the operator to verify that following a loss of inverter function, the applicable vital instrument panel is still energized by dispatching an operator to check the output ammeter on the applicable UPS.

Loss of Power Effects for 120-Volt A.C. Distribution Panels The principal impacts due to loss of power for each distribution panel are listed in the following tables. Unused or spare breakers are not listed.

APN5901

- - J:,:; {h,. ,:..*..... /,,.*. :*,,.' . *" ** * *E'r*>'::";:.,:;:_, .****** ;,:; , *** **':> ,,.. ** <T*** ..

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02 Feeds lncore Thermocouple Analog Isolator Cabinet A 04 Feeds Nuclear Instrument System Channel I Control Power 05 Feeds Nuclear Instrument System Channel I Instrument Power 06 Feeds Main Control Board Bus 1 Instrument Power 07 Feeds Solid State Protection System Cabinet Train A (Input and Output) 08 Feeds Solid State Protection System Cabinet Train B 09 Feeds Transmitter Power Supply Cabinet Channel A 10 Feeds Process l&C Rack Protection Set I 11 Feeds IE Auxiliary Relay Rack 12 Feeds Reactor Protection Time Delay Panel XPN06011 13 Feeds Reactor Vessel Level Cabinet-Right 14 Feeds HVAC Control Board XCP6210 15 Feeds Nuclear Steam Supply Aux Relay Rack 1 XPN7031 16 Feeds ESF Loading Sequence Control Panel Train A 17 Feeds Auxiliary Safeguards Cabinet Train A 18 Feeds Reactor Core Cooling Monitor Unit A 20 Feeds NI Channel I (A) Amplifier Assembly XPN7303-NI 21 Feeder for APN5907 (APN5907) 27 Main Incoming Feed from XIT5901 APN5902

Serial No.23-177 Docket No. 50-395 Attachment 1: Page 6 of 29 01 Feeds NI Channel II D Amplifier Source and Interim Range 02 Feeds Control Board Instrument Bus 11 Channel D 04 Feeds NIS Channel II Control Power XCP7072 05 Feeds NIS Channel II Instrument Power XCP7072 07 Feeds Transmitter Power Supply Cabinet Channel D 08 Feeds Solid State Protection System Channel II Train A XPN7010 09 Feeds Solid State Protection System Test Panel Train 10 Feeds Solid State Protection System Cabinet Channel 11 Train B 12 Feeds Process Rack Protection Set II XPN7002 13 Feeds Process Rack Control Group 2 XPN7006 15 Feeds Reactor Protection Time Delay Relay Panel 25 Main Incoming Feed from Inverter 5902 (XIT5902)

APN5903

';';'<?">} *** , .** *****.... >. **\;":: .. ;,*. , ** ;i:;.*ii\'*******,,1 .'./:>7'::*';it]'.'.'f{'?t, **.?* ***J:>:;Y,,: .

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04 Feeds NIS Channel Ill Control Power XCP7073 05 Feeds NIS Channel Ill Instrument Power XCP7073 06 Feeds Process l&C Rack Protection Set 111 Power Supply #1 07 Feeds 1E Auxiliary Relay Rack 08 Feeds Solid State Protection System Cabinet Channel II Train A 09 Feeds Solid State Protection System Cabinet Channel II Train B 10 Feeds Transmitter Power Supply Cabinet Channel B 12 Feeds Main Control Board Instrument Bus 3 13 Feeds Reactor Protection Under Frequency Time Delay Relay Panel 14 Feeds HVAC Control Board XPN7176 15 Feeds Earthquake Instrumentation Control Panel 16 Feeds ESF Loading Sequencer Control Panel Train B 17 Feeds NSS Auxiliary Relay Rack 2 18 Feeds Auxiliary Safeguard Cabinet Train B 19 Feeds lncore Thermocouple Analog Isolator Cabinet B 21 Feeder for APN5908 25 Normal Incoming Feeder from Inverter 5903 (XIT5903)

Serial No.23-177 Docket No. 50-395 Attachment 1: Page 7 of 29 APN5904 04 Feeds NIS Channel IV Instrument Power 05 Feeds NIS Channel IV Control Power 07 Feeds Solid State Protection System Cabinet 08 Feeds Solid State Protection System Cabinet Train B (Input and Output) 09 Feeds Solid State Protection System Test Panel Train B 10 Feeds Process l&C Rack Protection Set IV 11 Feeds Process l&C Rack Control Group 4 13 Feeds Reactor Vessel Level Cabinet-Left 16 Feeds Control Board Instrument Bus 4 20 Feeds Reactor Core Cooling Unit 23 Normal Incoming Feeder from Inverter No. 4 (XIT5904) 2.2 Current TS Requirements TS 3/4.8.3 [5.1 ], "ONSITE POWER DISTRIBUTION - Operating," LIMITING CONDITION FOR OPERATION (LCO) 3.8.3.1 provides that the following electrical busses shall be energized in the specified manner with the breakers open between redundant busses during Modes 1, 2, 3, & 4:

a. Train A A.C. Emergency Susses consisting of:

1. 7200 volt Emergency Susses # 1DA and 1EA.

2. 480 volt Emergency Susses # 1DA 1, 1DA2 and 1EA 1.

b. Train B A. C. Emergency Susses consisting of:

1. 7200 volt Emergency Susses# 1DB and 1EB.

2. 480 volt Emergency Susses# 1DB1, 1DB2, and 1EB1.

c. 120 volt A.C. Vital Susses# 5902 and 5901 energized from an associated inverter connected to D.C. Bus# 1HA*.

d. 120 volt A.C. Vital Susses# 5904 and 5903 energized from an associated inverter connected to D.C. Bus# 1HB*.

e. 120 volt A.C. Vital Bus #5907 energized.

f. 120 volt A.C. Vital Bus #5908 energized.

g. 125 volt D.C. Bus 1HA energized from Battery Bank #1A.

h. 125 volt D.C. Bus 1HS energized from Battery Bank #1 B.

Action statement C of TS LCO 3.8.3.1 states, "With one of A. C. Vital Busses #5901, 5902, 5903, or 5904 either not energized from its associated inverter or with the inverter not 1

connected to its associated D. C. Bus re-energize the A. C. Vital Bus from its associated inverter connected to its associated D. C. Bus within 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months or be in at least HOT

Serial No.23-177 Docket No. 50-395 Attachment 1: Page 8 of 29 STANDBY within the next 6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months and in COLD SHUTDOWN within the following 30 hours3.472222e-4 days 0.00833 hours 4.960317e-5 weeks 1.1415e-5 months ."

TS LCO 3.8.3.1 page further has an asterisk that relates to conditions C & D that states the following: "*The inverters may be disconnected from their D. C. Bus for up to 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months as necessary for the purpose of performing an equalizing charge on their associated battery bank provided (1) their vital busses are energized, and (2) the vital busses associated with the other battery bank are energized from their associated inverters and connected to their associated D. C. Bus."

2.3 Reason for the Proposed Change VCSNS TS LCO 3.8.3.1, Action C currently allows for 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months to troubleshoot, repair, perform post-maintenance testing, and return to service any Vital A.C. Bus that is not energized by its associated inverter which is in turn connected to its associated D.C. Bus.

The nomenclature of this TS reflects the original plant design where each Vital A.C. Buss utilized a manually interlocked bypass breaker as part of the distribution panel that allowed either the Alternate A.C. power source OR the associated two (2) input inverters to energize the panel. Current plant design, which was changed in 1991 via VCSNS design modification (MRF21432), has a single input to each Vital A.C. Bus via the three source Inverter (UPS) output. As such, updating the TS wording to match the plant design more closely, and the treatment of Vital A.C. Bus energization issues as related to the OPERABILTY of the inverter will allow more clarity and flexibility for plant personnel when utilizing TS LCO 3.8.3.1 Action statements.

The 24-hour AOT was based on engineering judgment, taking into consideration the time required to repair an inverter or reconnect the D.C. Bus to a de-energized or disconnected inverter and the additional risk to which the unit is exposed because of a loss of an uninterruptible power source. Mitigating strategies have been implemented to address emergent issues within the current 24-hour AOT. These include prepared safety tag outs for inverter troubleshooting, and repair and maintaining limited stocks of capacitors and burned-in replacement circuit cards. However, as discussed below, the 24-hour AOT can be insufficient in certain instances to support on-line troubleshooting, corrective maintenance, and post-maintenance testing in response to emergent issues; especially instances involving long lead time parts or required oversight / on-site assistance from the vendor. VCSNS performs preventative maintenance on the safety related UPS units during each refueling outage. There are no current plans to perform routine preventive maintenance on a scheduled basis with the unit on line. Should the need for such maintenance be identified because of component performance, the necessary preventive maintenance would be planned and scheduled in accordance with DESC procedures for on-line work management.

If an inverter becomes inoperable due to an emergent issue, the inverter troubleshooting and repair process requires proper electrical safety tagging to be established. The troubleshooting process begins with physical inspection of the UPS panels, fuses, and alarm checks. Upon completion of repairs and depending on which circuit cards have

Serial No.23-177 Docket No. 50-395 Attachment 1: Page 9 of 29 been removed and replaced, adjustments are performed. Post maintenance testing is required before returning the UPS to service. Depending on the corrective maintenance, an inverter functional test may also be required.

Experiences both at VCSNS and at other nuclear power plants have shown that the current 24-hour AOT for restoration of an inoperable inverter is insufficient in certain instances to support complex on-line troubleshooting, corrective maintenance, and post-maintenance testing while the unit is at power. This is especially true of the newer style inverter/UPS systems that utilize more digital components to perform their function as these components are generally not simple to troubleshoot. Further, as is common with digital systems vs. analog systems, the post-maintenance testing window needs to be long enough to account for the potential for intermittent or non-repeatable (i.e., ability to trigger the condition vice it repeating at a specific interval) issues.

Plant-specific operating experience does not include instances in which more than 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months was required to return an inoperable inverter to operable status while online.

However, during post maintenance testing for the new installation of XIT5904 (120-Volt Vital A.C. 10 KVA UPS), during refueling outage RF27 (Spring 2023), VCSNS experienced multiple swaps to hard bypass events three (3) times with a 3-4-hour window of normal operation. The swap to bypass was initiated by the Independent Static Bypass Switch (ISBS) as designed based on the reset of the main controller. Discussions with the vendor led to the conclusion that the control cards for these units needed to be replaced with a spare. During the interim time while a vendor Field Service Engineer (FSE) was being dispatched, the main controller card was replaced, and appropriate microcontrollers swapped by plant maintenance. All PMT was completed SAT and the inverter was put on plant loads and put in normal configuration.

Approximately 36 hours4.166667e-4 days 0.01 hours 5.952381e-5 weeks 1.3698e-5 months later the system automatically transferred to and from bypass, with less than two (2) seconds in between states. With the vendor onsite, troubleshooting was performed, and the inverter left in Test position with the loads powered by the bypass source through the UPS. Over the next three (3) days of testing this issue reoccurred with 23- and 32-hour windows between each. It was determined via troubleshooting with the vendor that this was likely an issue with the control board stack, and a different, known good set was installed.

The complex troubleshooting of these devices, down to the microcontroller level, and need to have the vendor onsite to perform calibration, testing, and card swap spanned multiple days. These activities would not have been able to be completed within the 24-hour AOT window as currently stipulated in the TS if these issues had presented themselves during plant operation. During a more recent online event the inverter was reset, as per the design of the system, and the backup ISBS appropriately took control and put the inverter into bypass. After reset, the INOPERABLE inverter was run until the end of the 24-hour AOT window before being declared Operable.

In the event of an inverter failure with the plant online (not fixed by resetting the main controller) that requires complex troubleshooting as described above, the required time to resolve the issue would likely be well in excess of the current 24-hour allowable outage time. The inverter would first need to be tagged out by operations personnel to ensure

Serial No.23-177 Docket No. 50-395 Attachment 1: Page 10 of 29 the safety of both workers and the inverter. A multi-discipline troubleshooting team would have to be assembled consisting of maintenance, engineering, and operations personnel to diagnose and resolve the equipment malfunction. This team would then create a troubleshooting plan which requires a proper level of risk review and would be subject to independent challenge or possibly independent third-party review before implementation.

Once approved, the troubleshooting plan would then have to be executed through an emergent work order. This entire process of assembling a team, developing a plan, and planning the work order could realistically take over the current allowable 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months depending on the nature of the inverter failure. Following the work order planning, maintenance would need to implement the troubleshooting plan. Further, due to the nature of the currently installed inverters, a vendor representative would typically be requested/required to travel to the site. The mobilization of a vendor representative typically would take a minimum of 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months , dependent on day, time, and schedule.

Vendor time may be parallel to other activities but can also be in line with timing depending on the exact issue.

A postulated timeline for failure of a Class 1E A.C. Vital Bus Inverter is provided below.

Additional time could be required if corrective maintenance required replacement of cards for which burned-in spares were not immediately available:

Activity Activity duration (hours)

Assemble troubleshooting team 4 Staff outage control center (OCC) and review situation 4 Tag-out equipment 4 Develop complex troubleshooting plan** 12 Plan troubleshooting work order** 2 Implement troubleshooting plan** 12 Plan corrective maintenance work order 4 Perform corrective maintenance** 36 Calibrate any replaced cards** 12 Perform de-energized test** 12 Release tags 4 Energize inverter** 2 Post maintenance test** 8 Total duration 116

- Request Vendor representative support onsite. Minimum 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months , dependent on day, time, and schedule. Vendor mobilization time may be a parallel action but can also be a series action with timing depending on the exact issue.

In addition to the above-mentioned maintenance needs for the proposed change, the impact of the proposed change would allow the station to enhance safety decision-making by the use of risk insights, be more efficient in implementation of station resources, and to reduce unnecessary burden on the station staff, which are consistent with the

Serial No.23-177 Docket No. 50-395 Attachment 1: Page 11 of 29 objectives of NRG Final Policy Statement [5.17]. The proposed inverter AOT extension would provide these benefits, as shown in the following table, by supporting the ability to complete on-line corrective or planned maintenance of an inoperable inverter.

NRG Final Policy Statement [5.17]

Anticipated Benefits of Proposed Inverter Enhanced Efficient Reduction in AOT Extension Decision- Use of Unnecessary making Resources Burden Provide additional time to complete repairs X X following and inverter malfunction; Avert unplanned unit shutdowns and X X X minimize the potential need for NOED; Increase the time to perform troubleshooting, repair, and testing following inverter equipment problems, which will enhance the X X safety and reliability of equipment and personnel; Allow time to perform routine maintenance activities on the inverters in MODES 1 through 4, enhancing the ability to focus X X X quality resources on the activity and the availability of the inverters during refueling outage periods.

2.4 Description of the Proposed Change The following revision is proposed to TS LGO 3.8.3.1, Action G and the associated TS Bases.

Current TS LCO 3.8.3.1, Action C states:

c. With one of A. C. Vital Busses #5901. 5902. 5903, or 5904 either not energized from its associated inverter. or with the inverter not connected to its associated D. C. Bus re-energize the A. C. Vital Bus from its associated inverter connected to its associated D.C. Bus within 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months or be in at least HOT STANDBY within the next 6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months and in COLD SHUTDOWN within the following 30 hours3.472222e-4 days 0.00833 hours 4.960317e-5 weeks 1.1415e-5 months .

Proposed TS LCO 3.8.3.1, Action C will state:

c. With one of A.G. Vital Bus lnverter(s) #5901. 5902. 5903, or 5904 INOPERABLE, restore the Inverter to OPERABLE status within 7 days or be in at least HOT STANDBY within the next 6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months and in COLD SHUTDOWN within the following 30 hours3.472222e-4 days 0.00833 hours 4.960317e-5 weeks 1.1415e-5 months .

Further, the following text will be added to the TS Bases for LCO 3.8.3.1:

Serial No.23-177 Docket No. 50-395 Attachment 1: Page 12 of 29 "Energized" A. C. Vital Busses #5901. 5902. 5903. or 5904 require the panels to be energized to their proper voltage from the associated inverter via inverted D. C. voltage.

inverter using the normal A. C. source. or Class 1E backup A. C. source via voltage regulator. OPERABLE inverters require the associated 120 Volt A. C. Vital Busses #5901.

5902. 5903. or 5904 to be powered by the inverter with output voltage within tolerances.

and power input to the inverter from the associated D. C. Bus. Alternatively, the power supply may be from an internal A. C. source via rectifier as Jong as the OPERABLE D. C.

Bus is available as the uninterruptible power supply."

3.0 TECHNICAL EVALUATION

3.1 Deterministic Assessment DESC has determined that the proposed Vital A.C. Inverter Completion Time Extension from 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months to 7 days is necessary to support online troubleshooting, corrective maintenance, and post-maintenance testing of an inoperable inverter while the unit is at power (Modes 1-4). Traditional engineering considerations need to be addressed, including defense-in-depth and safety margins, in addition to risk insights assessments.

The fundamental safety principles which the plant design was based on cannot be compromised. Design Basis Accidents are used to develop the plant design. These are a combination of postulated challenges and failure events that are used in the plant design to demonstrate safe plant response. Defense-in-depth, the single failure criterion, and adequate safety margins in relation to the proposed change need to be considered in compliance with NRC plant-specific and risk insights informed TS change guidance [5.4].

3.1.1 Defense-In-Depth VCSNS has two (2) Vital A.C. Inverters per independent train (Train A and Train B). This results in total of four (4) Vital A.C. Inverters (XIT5901, XIT5902, XIT5903, XIT5904) with Channels A and Don A Train and Channels Band Eon B Train. The system configuration ensures that each of the four (4) channels is electrically and physically isolated. If an inverter is inoperable or is to be removed from service for maintenance or testing, the associated Vital A.C. instrumentation bus is powered from a separate Class 1E regulated power supply via a regulating transformer.

As described in the VCSNS UFSAR Section 8.3.1 [5.2], the four (4) Vital A.C. Channels provide independent power to the four (4) RPS schemes. Most reactor protective schemes have 3 or 4 channels. Redundant instrument channels are fed from redundant vital busses. Because of the preferred failure mode defined for the reactor protective instrumentation, failure of an instrument channel power source results in a reactor trip signal from the affected channel. Multiple power supplies are provided to prevent a single power supply failure from initiating a false reactor trip.

Refer to the below Defense-in-Depth Review from VCSNS UFSAR [5.2] Section 7.6.1.2:

Serial No.23-177 Docket No. 50-395 Attachment 1: Page 13 of 29 VCSNS UFSAR [5.2], Section 7.6.1.2, states:

There are 2 independent 480 volt a-c power sources. each serving 2 inverters. Therefore, loss of either of the two 480 volt a-c power sources affects only 2 of the 4 inverters.

There are 2 independent Class 1E batteries and battery chargers. Each battery is attached to a bus serving 2 inverters.

There is a third battery charger provided, which serves as a standby charger. This charger is provided for use during maintenance of. and backup to the normal battery chargers. The standby charger has mechanically interlocked circuit breakers on the a-c input and d-c output such that only the 2 circuit breakers associated with Channel A or the 2 circuit breakers associated with Channel B can close at one time.

Since not more than 2 inverters are connected to the same bus, a loss of a single bus can only affect 2 of the 4 inverters.

Since each of the 4 instrument channels is supplied power by independently connected inverters. the loss of an inverter cannot affect more than 1 of the 4 instrument channels.

Each distribution panel can receive power from the 120 volt Class 1E a-c backup regulated source through an automatic static transfer switch or through a manual bypass switch. The inverter power source and the backup source are aligned such that the distribution panels cannot be connected to both sources at the same time.

Therefore, no single failure in the Instrumentation and Control Power Supply System or its associated power supplies can cause a loss of power to more than one of the redundant loads.

The inverters are designed to maintain their outputs within acceptable limits. The loss of the a-c or d-c inputs is alarmed in the Control Room, as is the loss of an inverter's output.

There are no inverter breaker controls on the control board, as no manual transfers are necessary in the event of Joss of the 480 volt a-c preferred power source. The a-c and d-e inputs are diode isolated in the UPS.

The defense-in-depth philosophy requires multiple means or barriers to be in place to accomplish safety functions and prevent the release of radioactive material. During operation with an out-of-service vital instrument bus inverter, the associated vital A.C.

instrument bus is energized from its alternate source, a dedicated voltage regulating transformer. The source of power for the back-up supply is a safety-related Motor Control Center (MCC), which relies on the EOG as the back-up power supply. In the event of a failure of a Vital A.C. Inverter, the static transfer switch will shift the instrument bus to its alternate source with no interruption of power to the instrument bus.

Should a loss of off-site power (LOOP) occur while inverter XIT5902 or XIT5904 is aligned to its back-up source, the instrument bus will remain de-energized until the DG starts and energizes the back-up power supply. For the instrument bus associated with either XIT5902 or XIT5904 to remain de-energized, the EOG would have to fail or the MCC that provides the back-up power source would have to fail.

Serial No.23-177 Docket No. 50-395 Attachment 1: Page 14 of 29 However, should a LOOP occur while XIT5901 or XIT5903 is aligned to its back-up source, the instrument bus will remain de-energized due to loss of power to the ESF Load Sequencer. In this situation, there are a series of mitigating actions that will be taken to restore power to either diesel backed 7 .2 KV switchgear XSW1 DA or XSW1 DB. These actions include going from normal power feed to emergency feed for the effected bus (115kV XTF4/5 to 230kV/XTF31 for the 7.2 kV Emergency Bus Train A, and 230kV/XTF31 to 11 SkV XTF4/5 for the 7.2 kV Emergency Bus Train B), aligning the effected bus to the Alternate A.C. via XSW1 DX, or manual DG loading. These actions are performed based on station procedures SOP-304 and AOP-304.1A/B, respectively.

For this LAR, only one ESF Load Sequencer and up to one EOG may be affected TS 3.8.3.1, Action C to be applicable, and if a redundant channel should fail or be taken out of service during the extended AOT, VCSNS would be in TS 3.0.3, requiring a plant shutdown.

A failure to energize a vital A.C. instrument bus following a LOOP does not impact the ability of the RPS or ESFAS to actuate because of the installed redundancy. The remaining three 120-volt vital A.C. instrument busses will be unaffected by the loss of offsite power because they will be supplied from their associated inverter batteries.

The impact of a loss of power to the single affected 120-Volt A.C. vital instrument bus on plant operations will be minimal. The plant will be shutdown due to a loss of offsite power, and the remaining three instrument busses will still provide the required two-out-of-three channel actuation logic to the RPS and ESFAS. TS permits no more than one vital instrument bus inverter to be inoperable, so that when one vital instrument bus is aligned to its alternate source, the redundant instrument bus inverters will be operable and aligned to a D.C. power supply.

The unavailability of a single vital A.C. inverter due to entry into the TS Action Statement for inverter maintenance does not reduce the amount of available equipment to a level below that necessary to mitigate a design basis accident. The two protective trains, with adequate independence and backup power supplies ensure proper mitigation of postulated accidents. The proposed change will continue to provide multiple means to accomplish safety functions and prevent the release of radioactive material, consistent with the defense-in-depth philosophy.

Further, consistent with the defense-in-depth philosophy as stated in [5.4], the following items are addressed:

1) Preservation of a reasonable balance among the layers of defense The proposed TS change does not significantly reduce the effectiveness of a layer of defense that exists in the plant design before the implementation of the proposed TS change.

The defense-in-depth approach to designing and operating VCSNS was and continues to be used to prevent and mitigate accidents that could release radiation or hazardous materials. This approach provides for multiple independent and redundant layers of defense to compensate for potential human and mechanical failures which ensures that

Serial No.23-177 Docket No. 50-395 Attachment 1: Page 15 of 29 no single layer, no matter how robust, is exclusively relied upon. Defense-in-depth includes the use of access controls, physical barriers, redundancy, diverse key safety functions, and emergency response measures.

The robust plant design to survive hazards and minimize challenges that could result in the occurrence of an event is not affected by the proposed change. The proposed extension to the AOT does not increase the likelihood of initiating events or create new significant initiating events. There are several actions that are completed by the operations staff as required for the automatic response of the plant when there is a loss of vital instrumentation. This LAR has the effect of balancing some of the demands on the operations staff. Once approved, the LAR will permit more focused operator and maintenance technician attention upon risk significant actions, as compared to actions that are based upon qualitative completion times.

The availability and reliability of Systems, Structures, or Components (SSC) providing the safety functions that prevent plant challenges from progressing to core damage are not significantly impacted. The remaining OPERABLE 120-Volt A.C. inverters can supply the required loads to safely shutdown the plant.

The proposed extension of the TS 3.8.3.1, Action C AOT from 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months to 7 days does not significantly reduce the effectiveness of the emergency preparedness program, including the ability to detect and measure releases of radioactivity, notify offsite agencies and the public, and shelter or evacuate the public as necessary.

2) Preservation of adequate capability of design features without an overreliance on programmatic activities as compensatory measures The proposed TS change does not substitute programmatic activities for design features to an extent that significantly reduces the reliability and availability of design features to perform their safety functions without overreliance on programmatic activities.

There are no programmatic activities that will be required as compensatory measures to preserve adequate capability of design features during use of the extended AOT. Only three (3) inverter channels, as described in Section 2.1, are necessary to supply the safety related equipment required for (1) the safe shutdown of the facility, and (2) the mitigation and control of accident conditions within the facility. Therefore, one inverter channel being inoperable does not impact the ability of the system to perform its required function.

VCSNS uses ARPs as described in Section 2.1 to guide the operator to verify that following a loss of inverter function, the applicable vital instrument panel is still energized.

While timely actions are required when there is a loss of vital instrumentation, this LAR will reduce the immediate demands on the operations staff to prepare for a potential plant shutdown.

Administrative controls consistent with other licensees that have received similar extensions of the inverter allowed out-of-service time, as described in Section 4.2 of this Attachment will be implemented. The administrative controls described below are qualitative, prudent actions. Entry into the extended inverter AOT will not be planned concurrent with EOG maintenance, and entry into the extended inverter AOT will not be

Serial No.23-177 Docket No. 50-395 Attachment 1: Page 16 of 29 planned concurrent with planned maintenance on another ECCS or isolation actuation instrumentation channel that could result in that channel being in a tripped condition.

These administrative controls recognize that with an inverter inoperable and the distribution panel being powered by the backup A.C. distribution system, continued instrumentation power for that train is dependent on power from the associated EOG following a loss of power event.

3) Preservation of system redundancy, independence, and diversity commensurate with the expected frequency and consequences of challenges to the system, including consideration of uncertainty The proposed TS change does not significantly reduce the redundancy, independence, or diversity of systems.

The proposed extended AOT makes no changes to the system operation or design and therefore has no effect on the extended frequency of challenges or result in a decrease in redundancy, independence, or diversity of the 120-Volt A.C. power system. Also, redundant power supplies and operator actions are not impacted by these changes. If a redundant channel should fail or be taken out of service during the extended AOT, VCSNS would be in TS 3.0.3, requiring a plant shutdown. This requirement is unchanged by this LAR.

The proposed extended AOT is consistent with the assumptions in the plant's safety analysis and does not result in a significant increase in risk.

4) Preservation of adequate defense against potential Common Cause Failures (CCFs)

The proposed TS change does not significantly reduce defenses against CCFs that could defeat the redundancy, independence, or diversity of the layers of defense; fission product barriers; and the design, operational, or maintenance aspects of the plant.

The extension requested does not reduce defenses against CCF. In fact, these extensions allow more deliberate and thorough troubleshooting following an emerging failure, which can improve the causal evaluations performed for equipment issues. Better understanding of any emergent failure causes could lead to investigations or actions that improve the reliability of the unaffected inverters.

In addition, the operating environment for these components remains unchanged and there are no changes to the design or operation of the inverters associated with the proposed change, so new common cause failure modes are not introduced. There are no changes to the common environment, inverter, or support system design, therefore there are no changes to existing coupling factors. The extent of condition assessment performed as a part of any failure of a safety-related piece of equipment will address these issues as required by plant administrative procedures.

Therefore, the defense against potential CCFs remains adequate.

5) Maintain multiple fission product barriers

Serial No.23-177 Docket No. 50-395 Attachment 1: Page 17 of 29 The proposed TS change does not significantly reduce the effectiveness of the multiple fission product barriers.

The fission product barriers (fuel cladding, reactor coolant system, and containment) and their effectiveness are maintained. The proposed changes do not affect the integrity of fission product barriers to limit leakage to the environment. Extending the AOT of the 120-Volt AC inverters does not result in a significant increase in the frequency of existing challenges to the integrity of the barriers, significant increase the failure probability of any individual barrier, or introduce new or additional failure dependencies among barriers.

6) Preserve sufficient defense against human errors The proposed TS change does not significantly increase the potential for or create new human errors that might adversely impact one or more layers of defense.

The proposed extended AOT does not require any new operator actions for the existing plant equipment or introduce the potential for new human errors. Existing plant on-line maintenance activities risk assessment program will incorporate compensatory measures which control what equipment or systems will not be allowed to be taken out of service concurrent with an inverter out of service for planned maintenance. This will not significantly increase the potential for, or create new, human errors that might adversely impact one or more layers of defense. No new operating, maintenance, or test procedures are required due to the change, and no new at-power tests or maintenance activities are expected to occur because of the change. The plant will continue to be operated and maintained as before.

7) Continue to meet the intent of the plant's design criteria The proposed TS change does not affect the plant's ability to meet the intent of the design criteria referenced in the licensing basis.

The intent of the VCSNS design criteria is maintained. The plant will continue to be operated and maintained as before. The proposed changes do not involve any physical changes to the design or operation of the 120-Volt A.C. Distribution system. The ability of the remaining TS required inverters to perform their required functions is maintained.

8) Integrated Evaluation of the Defense-in-Depth Considerations There are no changes to the current plant design. The intent of each defense-in-depth consideration addressed above would still be met following implementation of the proposed extended AOT. Therefore, the proposed TS change maintains consistency with the defense-in-depth philosophy.

3.1.2 Safety Margin Evaluation The proposed extension of the vital A. C. inverter TS completion time remains consistent with the codes and standards applicable to VCSNS onsite A.C. sources and electrical distribution system. Safety analysis acceptance criteria in the VCSNS UFSAR are met, and the safety analysis acceptance criteria, as stated in the VCSNS UFSAR, are not

Serial No.23-177 Docket No. 50-395 Attachment 1: Page 18 of 29 impacted by these proposed changes. Redundant channels will be maintained. Diversity, with regards to ensuring that sufficient power will be available to supply the safety equipment required for 1) the safe shutdown of the facility and 2) the mitigation and control of accident conditions within the facility will be maintained. In combination with other methods, such as bus voltage monitoring for offsite power, Open Phase Isolation System (OPIS) is used to determine the availability of preferred off-site sources. The minimum specified independent and redundant A.C. and D.C. power sources and distribution systems will continue to satisfy the requirements of General Design Criterion 17 of Appendix A to 10 CFR 50. The proposed changes will not allow plant operation in a configuration outside the design basis.

With one of the required 120-Volt A.C. uninterruptible power distribution systems being powered from the 1E constant voltage regulator and backed up by a DG, there is no significant reduction in the margin of safety.

The Vital A.C. inverters are the preferred source of power for the A.C. instrument busses because of the stability and reliability that they provide. The inverters can be powered from an A.C. source or from an associated 125-Volt D.C. battery. The battery provides an uninterruptible power source. Each Vital A.C. inverter is equipped with a safety-related static transfer switch that connects the vital inverter output to its associated instrument bus. The static transfer switch will transfer the instrument bus power source to its alternate source in the absence of an output from the inverter, in the event of an overload condition, or with a degraded A.C. power output at the inverter. The static transfer switch is an electronic, solid-state device that will automatically or manually transfer instrument bus power from the inverter to the alternate source without interruption of power.

In the event of a LOOP while inverter XIT5902 or XIT5904 is aligned to its back-up source, the instrument bus will remain de-energized until the DG starts and energizes the back-up power supply. For the instrument bus associated with either XIT5902 or XIT5904 to remain de-energized, the EOG would have to fail or the MCC that provides the back-up power source would have to fail. The simultaneous failure of an inverter and its back-up supply coincided with a LOOP is unlikely.

However, in the event of a LOOP while inverter XIT5901 or XIT5903 is aligned to its back-up source, the instrument bus will remain de-energized due to loss of power to the ESF Load Sequencer. In this situation, there are a series of mitigating actions that will be taken to restore power to either XSW1 DA or XSW1 DB. These actions include going from normal power feed to emergency feed for the effected bus (115kV XTF4/5 to 230kV/XTF31 for the 7.2 kV Emergency Bus Train A, and 230kV/XTF31 to 115kV XTF4/5 for the 7.2 kV Emergency Bus Train B), aligning the effected bus to the Alternate A.C. via XSW1 DX, or manual DG loading. These actions are performed based on station procedures SOP-304 and AOP-304.1A/B respectively. In this scenario safety margin is maintained due to availability of multiple mitigating strategies to return power to the effected vital AC instrument bus, and the fact that a failure to energize the affected bus following a LOOP does not impact the ability of the RPS or ESFAS to actuate.

Serial No.23-177 Docket No. 50-395 Attachment 1: Page 19 of 29 3.1.3 VCSNS Maintenance Rule Monitoring 10 CFR 50.65, "Requirements for monitoring the effectiveness of maintenance at nuclear power plants," (i.e., the Maintenance Rule) requires an evaluation be performed when equipment covered by the Maintenance Rule does not meet its performance criteria. If the pre-established performance criteria are not achieved for the inverters, they are considered for 10 CFR 50.65(a)(1) actions. These (a)(1) corrective actions require increased management attention and goal setting to restore their performance to acceptable level. The actual out-of-service time for the inverters is minimized to ensure that the performance criteria are met. All VCSNS 120-Volt Vital Bus inverters are currently in Maintenance Rule (a)(2) status.

The vital instrument bus power supply system has four channels, with redundant/parallel paths (inverter/static switch, 2 A.C. inputs, 1 D.C. input) available to provide power to the busses (APN5901, 5902, 5903 & 5904), any failure that would prevent one of the redundant paths from successfully fulfilling the function of providing power to supply the required loads will be included in this criterion. The maintenance preventable functional failure is defined as greater than 1 occurrence per 18 months.

3.2 Risk Insights Assessment This risk insights assessment evaluates the proposed extension of the inverter AOT from the current 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months to 7 days using VCSNS PRA model(s).

The justification for the use of extended AOTs for 120-Volt A.C. inverters is based upon deterministic evaluations plus risk insights gained from VCSNS PRA model results assessments, consisting of three main elements as cited in RG 1.177 [5.4]:

1) Tier 1: Assessment of the impact of the proposed TS change using a valid and appropriate PRA model as compared with appropriate acceptance guidelines.

2) Tier 2: Evaluation of equipment relevant to plant risk while the inverter(s) are in the extended AOT. Combinations of out-of-service equipment can be evaluated for their risk significance to determine if additional compensatory measures may be required.

3) Tier 3: Implementation of the Configuration Risk Management Program (CRMP) while the inverter(s) is/are in the extended AOT. The CRMP is used for all work and helps ensure that there is no avoidable increase in plant risk while any inverter maintenance is performed. These programmatic measures provide additional assurance that critical plant safety functions are preserved during the extended inverter AOT.

3.2.1 Tier 1 - Risk Assessment Results DESC performed a quantitative and qualitative risk assessment to support the conclusion that the change in risk associated with the proposed 24-hour to 7-day AOT extension of the 120-Volt vital A.C/tfus inverter is acceptable. The risk assessment is intended to augment the determirlistic assessment of extending the AOT from 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months to 7 days.

Serial No.23-177 Docket No. 50-395 Attachment 1: Page 20 of 29 The risk assessment was performed to demonstrate with reasonable assurance that the proposed TS change is within the current risk acceptance guidelines in Regulatory Guide (RG) 1.174 [5.3] and RG 1.177 [5.4]. The risk assessment addressed Key Principles 4 and 5 of RG 1.174 [5.3] and RG 1.177 [5.4], and the risk was calculated consistent with NRC guidance provided in these RGs. The guidelines given in RG 1.174 [5.3] for determining an acceptable permanent change are that the ~CDF and ~LERF associated with the change should be less than the specified criteria, which are dependent on the baseline CDF and LERF.

Key Principle 4: Change in Risk is Consistent with the Safety Goal Policy Statement The risk assessment performed for this change addresses the philosophy of risk-informed decision-making. RG 1.177 [5.4] specifies the acceptable incremental conditional core damage probability (ICCDP) and incremental conditional large early release probability (ICLERP) for permanent changes, typically associated with changing an allowed outage time. The guidelines given in RG 1.177 [5.4] for determining an acceptable TS change are that the ICCDP and ICLERP associated with the change should be less than 1E-06 and less than 1 E-07, respectively.

The risk analysis was based on the ~CDF and ICCDP, the ~LERF and ICLERP for the unavailability of each inverter. This analysis demonstrates the acceptability, from a risk perspective, of increasing the VCSNS TS AOT for one inoperable 120-Volt A.C. Vital Bus inverter from the currently specified 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months to 7 days. As such, the change in risk is small and consistent with the intent of the Commission's Safety Goal Policy Statement.

Key Principle 5: Monitor the Impact of the Proposed Change The impact of the proposed change will be monitored for effectiveness in accordance with the existing plant Maintenance Rule program pursuant to 10 CFR50.65(a)(4) and the associated implementation guidance, Regulatory Guide 1.160 [5.28]. The program requires, in part, that performing maintenance activities shall not reduce the overall availability of secs which are important to safety.

Tables 3.2.1-1 and 3.2.1-2 below show the 7-day ~CDF and ICCDP, and ~LERF, and ICLERP, respectively, for each of the internal events, including internal flooding, fire, and seismic, contributions. The b.CDFAvE and b.LERFAvE calculations in this assessment are based on an assumed one LCO entry per inverter per year where the LCO is not exited until the seven-day proposed extended completion time. Therefore, the calculated b.CDFAvE and b.LERFAvE values are equal to the incremental increase in core damage and large early release probabilities, respectively.

Bus/Inverter IE with IF Fire Seismic Total ACDFAvE and ICCDP XIT5901 7.43E-08 3.34E-07 1.05E-07 5.13E-07 XIT5902 1.92E-12 7.67E-09 0.00E+00 7.67E-09 XIT5903 7.0?E-08 2.38E-07 3.85E-08 3.47E-07 XIT5904 1.76e-10 1.19E-07 3.28E-09 1.22E-07

Serial No.23-177 Docket No. 50-395 Attachment 1: Page 21 of 29 Bus/Inverter IE with IF Fire Seismic Total ALERFAvE and ICLERP XIT5901 2.11 E-08 2.92E-08 1.25E-09 5.15E-08 XIT5902 0.00E+00 3.84E-10 0.00E+00 3.84E-10 XIT5903 1.99E-08 1.96E-08 8.49E-11 3.95E-08 XIT5904 6.71 E-12 5.75E-09 7.65E-02 1.34E-08 The ICCDP and ICLERP values for the proposed change are within the acceptance guidelines from RG 1.177 [5.4] and represent a small quantitative impact on plant risk. In addition, the LlCDFAvE and LlLERFAvE results in being in Region Ill for Figures 4 and 5 of RG 1.174 [5.3], based on a total plant risk of 8.BE-05/yr for CDF and 6.4E-06/yr for LERF.

Table 3.2.1 - 3 below lists the total plant baseline risk (all hazards) after the proposed change, assuming a 7-day unavailability on inverter XIT5901, the risk limiting component.

Contribution CDF LERF IE including IF 2.70E-06 4.44E-07 Fire 5.02E-05 2.72E-06 Seismic 3.52E-05 3.26E-06 Total 8.81 E-05 6.42E-06 As shown in Tables 3.2.1 - 1 and 3.2.1 - 2 above, the largest risk increases are associated with inverter XIT5901, which is expected because power from 120-Volt Vital A.C. Panel APN5901 is required for both the Train A Solid State Protection System (SSPS) cabinet and the Train A Engineering Safety Feature (ESF) Load Sequencer Cabinet. Train B power distribution differs in that the output relays for the Train B SSPS cabinet are powered from APN5904, while the Train B ESF Load Sequencer Cabinet is powered from APN5903. The asymmetry in the results among the four inverters is caused by this design difference.

The cutsets for this quantification show that the risk increase of this configuration is primarily caused by the potential for loss of Diesel Generator function in fire scenarios where offsite power can be impacted. The two inverters associated with the Diesel Generator sequencer cabinets are XIT5901 and XIT5903. A lesser, but still significant, contributor to risk increase is associated with the loss of power to the SSPS output relays (XIT5901 and XIT5904), which would prevent automatic operation of mitigating equipment. The results for XIT5902 show that loss of power to one channel of instrument transmitters is relatively insignificant compared to loss of automatic start and loading of a Diesel Generator and loss of ESF actuation signals.

Cutsets associated with the risk increase for the quantifications associated with the ESF load sequencer typically involved failure to mitigate a Station Blackout. The dominant sequences involve a fire-induced loss of offsite power and a loss of one Diesel Generator

Serial No.23-177 Docket No. 50-395 Attachment 1: Page 22 of 29 due to fire damage or random failures and challenge to operation of the redundant EOG due to unavailability of the inverter associated with the LCO. Cutsets associated with the risk increase for quantifications associated with the SSPS cabinets typically involved failures of operators to manually align mitigating equipment when the ESFAS relays became non-functional due to loss of power.

3.2.2 Tier 2 Assessment - Avoidance of Risk Significant Plant Configurations The dominant cutsets for the calculated risk increase when a vital A.C. bus remains energized but not connected to its battery-backed power supply are mostly associated with the loss of automatic ESF Diesel Generator load shed and sequencing (ESFLS) if the offsite power sources become unavailable to the associated ESF switchgear. The two ESFLS cabinets are powered by XIT5901 via APN5901 and XIT5903 via APN5903 respectively. This insight is applicable to all hazard groups including Internal Events, Fire and Seismic.

VCSNS Procedure SOP-310, "ESF 120 VAC Instrumentation and Control Power System," cautions the operators to perform procedure steps in STP-506.005, "De-Energization of the ESF Load Sequencer Loss of Voltage and Degraded Voltage Relays A Train, XEG0001A", or STP-506.013, "De-Energization of the ESF Load Sequencer Loss of Voltage and Degraded Voltage Relays B Train, XEG0001 B" to de-energize the ESF Load Sequencer Loss of Voltage and Degraded Voltage Relays prior to removing Inverter XIT5901 or XIT5903 from service. The unavailability of the associated Diesel Generator is tracked with the Removal and Restoration (R&R) form and Technical Specification LCO 3.8.1.1 would apply, which effectively prevents operators from intentionally making both XIT5901 and XIT5903 inoperable simultaneously.

When the associated Diesel Generator becomes inoperable, LCO 3.8.1 .1 Action B would then require VCSNS to demonstrate the operability of the A.C. offsite sources by performing Surveillance Requirement 4.8.1.1.1 within one hour and at least once per eight hours thereafter. If the associated Diesel Generator became inoperable due to any cause other than preplanned preventive maintenance or testing, VCSNS is required to determine the operable Diesel Generator is not inoperable due to a common cause failure within 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months or demonstrate the operability of the diesel by performing Surveillance Requirement 4.8.1.1.2.a.3 within 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months . Additionally, VCSNS is required within four hours to verify that required systems, subsystems, trains, components, and devices that depend on the remaining EOG as a source of emergency power are operable and that the Turbine Driven Emergency Feed Pump is also operable. In addition to the actions required by the Technical Specifications, the condition would also be evaluated using the Risk Monitoring Program in accordance with OAP-102.1, "Conduct of Operations Scheduling Unit", and OAP-100.5, "Guidelines for Configuration Control and Operation of Plant Equipment", and Risk Management Actions would be performed as needed. The existing LCO actions and Risk Management Actions enforced by the Technical Specifications and Risk Monitoring Program are adequate to ensure that the proposed Technical Specification change will result in only small increases in the risk to public

Serial No.23-177 Docket No. 50-395 Attachment 1: Page 23 of 29 safety. No credit is taken for these compensatory measures in the delta risk quantifications.

It should be noted that the affected 120-Volt A.C. Vital Bus remains energized while in the LCO action statement associated with this LAR. If the 120-Volt A.C. Vital Bus loses power, then LCO 3.8.3.1 Action b, with much more restrictive time constraints, would apply. For the condition applying to LCO 3.8.3.1 Action C, the 120-Volt A.C. Vital Bus will remain energized following an initiating event as long as the power supply from the associated train of ESF A.C. power is energized.

3.2.3 Tier 3 Assessment - Configuration Risk Management Program Implementation of the VCSNS Configuration Risk Management Program is consistent with the description contained in Regulatory Guide 1.177, Section 2.3. 7 [5.4]. This program confirms that there is no significant risk increase while maintenance is being performed, including maintenance performed under the proposed change to inverter completion time. This tier is important because not all possible risk-significant configurations can be predicted in the Tier 2 assessment.

VCSNS implements portions of guidance contained within Section 11.0 of NUMARC 93-

01. The VCSNS Risk Monitor (RM) program (Phoenix Program) from the Electric Power Research Institute (EPRI) is used to implement 10 CFR 50.65(a)(4). The RM can evaluate CDF and LERF for internal events. The use of the RM is procedurally controlled by DESC procedures. DESC procedures recognize there are limitations in the RM and specifically direct consideration of external events and site activities that can result in significant plant events. Some conditions are evaluated in RM through multiplication factors; others procedurally lead to risk mitigating actions including plant risk color changes. Fire risk management actions, which are governed by the same set of procedures and implemented by the same staff, are determined based off analyses performed that determined the number of core damage avoidance success paths that are available when certain components are out of service. When maintenance or testing is scheduled, the Operations and Outage & Planning staff perform and review risk analyses using the RM process. For unplanned or emerging equipment failures, Control Room personnel will enter the configuration into RM. In either case, the configuration will be evaluated to assess and manage the risk. The VCSNS operations and work management administrative procedures govern on-line risk assessments. The on-line risk assessment is a blended approach using qualitative or defense-in-depth considerations and quantifiable PRA risk insights when available to complement the qualitative assessment.

VCSNS communicates on-line plant risk using four risk tiers (GREEN, YELLOW, ORANGE and RED). The criteria for these tiers are as follows:

Color Required Action Green No specific actions are required.

Serial No.23-177 Docket No. 50-395 Attachment 1: Page 24 of 29 Color Required Action Requires Superintendent Nuclear Shift Operations approval. The Superintendent Nuclear Shift Operations or Operations Scheduling Unit Yellow Supervisor may require that certain activities be approved at a higher level of management. If a safety function is degraded, work that would lead to degradation of any other safety function should be avoided.

Requires Plant Manager approval to work. Elective entry into a condition that degrades multiple risk factors should be avoided unless a clear operational Orange necessity exists. The Plant Manager may require shop and/or Operations shift pre-job briefings, contingency planning, or temporary equipment staging prior to scheduled implementation.

Red Requires VCSNS Facility Safety Review Committee (FSRC) approval to work.

Additionally, VCSNS maintenance practices involve protecting other inverters coincident with maintenance being performed on a single inverter per OP-VC-601, Protected Equipment. This procedure states that if an inverter train is unavailable as permitted by technical specifications, the remaining operable trains shall be protected. Protecting equipment requires posting of signs and robust barriers to alert personnel not to approach the protected equipment. Work on protected equipment is generally disallowed.

Exceptions for entry into protected spaces exist for activities such as Tech Spec operator rounds, required security patrols, or emergency operations. Other exceptions may be authorized by the station shift manager in writing which would include additional contingency measures. If additional unplanned equipment unavailability occurs, station procedures direct that the risk be re-evaluated, and if found to be unacceptable, compensatory actions are taken until such a time that the risk is reduced to an acceptable level.

3.2.4 Risk Insights Conclusion The ICCDP and ICLERP for the proposed change are within the acceptance guidelines from RG 1.177 [5.4] and represent a small quantitative impact on plant risk. The guidelines of RG 1.177 [5.4] for the increased Allowed Outage Times have been met.

Calculated changes in CDF and LERF due to the inverter AOT extension have been shown to meet the risk significance criteria of Regulatory Guide 1.174 [5.3], Region Ill, which represents "very small risk changes." Therefore, the increase in risk associated with the proposed change is considered acceptable.

4.0 REGULATORY EVALUATION

4.1 Applicable Regulatory Requirements and Criteria The following NRC requirements and guidance documents are applicable to the proposed change.

Serial No.23-177 Docket No. 50-395 Attachment 1: Page 25 of 29 10 CFR 50.36, "Technical Specifications," establishes the requirements related to the content of the TS. For Limiting conditions for operation, 10 CFR 50.36(c)(2) states:

"Limiting conditions for operation are the lowest functional capability or performance levels of equipment required for safe operation of the facility. When a limiting condition for operation of a nuclear reactor is not met. the licensee shall shut down the reactor or follow any remedial action permitted by the technical specifications until the condition can be met."

The proposed change to modify the 120-Volt vital busses associated inverters AOT from 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months to 7 days will not change the intent of the associated limiting conditions for operation (LCO), but rather the time to satisfy the LCO requirement. The design of the inverters is consistent with the initial assumptions of the accident analyses and design basis of the unit.

10 CFR 50 Appendix A General Design Criteria (GDC). The construction permit for VCSNS Unit 1 was issued by the Atomic Energy Commission (AEC) on March 21, 1973.

[5.18] The Operating License was issued on August 6, 1982. [5.19] NUREG 0717, [5.20]

"Safety Evaluation Report related to the operation of Virgil C. Summer Nuclear Station,"

including its supplements, [5.22, 5.23, 5.24, 5.25, 5.26] discussed VCSNS' conformance with the GDC. In a letter dated November 14, 1980, [5.21] VCSNS addressed compliance with 10 CFR Parts 20, 50, and 100 including the GDC. The NRC evaluated the final design and the design criteria and concluded, subject to the applicant's adoption of the additional requirements imposed by the NRC as discussed in the Safety Evaluation Report, that the facility had been designed to meet the requirements of the GDC. In a letter, dated November 14, 1980, [5.21] VCSNS provided a discussion to compare the plant design with the GDC as they appeared in 10 CFR 50 Appendix A. It was this discussion, including the identified exceptions, which formed the original plant licensing basis for compliance with the GDC. This discussion is contained in the UFSAR [5.2]

Section 3.0, "General Design Criteria," with more details provided in other UFSAR sections. Changes have been made to the original UFSAR [5.2] GDC discussions to reflect commitments and changes made to the facility over the life of the plant. Therefore, the GDC discussions in the UFSAR [5.2] constitute the VCSNS Unit 1 licensing basis with respect to compliance with the GDC. A review of the proposed amendment against the VCSNS UFSAR, [5.2] Sections 7 .6.1.2 and 8.3.1, no UFSAR [5.2] changes are required.

10 CFR 50 Appendix A, GDC 17, "Electric Power Systems," requires the onsite electric power supplies, including the batteries, and the onsite electric distribution system, to have sufficient independence, redundancy, and testability to perform their safety functions assuming a single failure. Since no physical changes are being proposed, and current design bases are not being affected, there is no impact on compliance with GDC 17.

10 CFR 50, Appendix A, GDC 18, "Inspection and testing of electric power systems,"

requires that electric power systems that are important to safety must be designed to permit appropriate periodic inspection and testing. The proposed change does not make changes to inverter inspections or testing. Therefore, implementation of the proposed Allowed Outage Time extension will have no significant effect on the continued conformance with GDC 18.

Serial No.23-177 Docket No. 50-395 Attachment 1: Page 26 of 29 10 CFR 50.65, "Requirements for monitoring the effectiveness of maintenance at nuclear power plants," requires that preventive maintenance activities must be sufficient to provide reasonable assurance that SSCs can fulfill their intended functions. As it relates to the proposed inverter Allowed Outage Time extension, 10 CFR 50.65(a)(4) requires the assessment and management of the increase in risk that may result from proposed maintenance activities. The inverter Maintenance Rule program monitors the reliability and availability of the A.C. inverters and ensures that appropriate management attention and goal setting are applied based on pre-established performance criteria. The A.C.

inverters are all currently in the 10 CFR 50.65(a)(2) Maintenance Rule category (i.e., the A.C. inverters are meeting established performance criteria). The VCSNS on-line risk management program is consistent with 10 CFR 50.65(a)(4) and is managed to ensure that risk-significant plant configurations will not be entered for planned maintenance activities, and that appropriate actions will be taken should unforeseen events place the plant in a risk significant configuration during the proposed extended A.C. inverter Allowed Outage Time. Therefore, the proposed extension of the A.C. vital bus inverter Allowed Outage Time from 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months to 7 days are not anticipated to result in exceeding the current established Maintenance Rule criteria for the A.C. inverters.

10 CFR 50.63, "Loss of all alternating current power," requires that nuclear power plants must be able to withstand a loss of all A.C. power for an established period and recover from a station blackout. The proposed extension of the A.C. inverter Completion Time from 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months to 7 days has no significant effect on the ability to withstand a loss of all A.C. power and recover from a station blackout.

4.2 Precedent Similar LARs have been submitted previously by other nuclear plants and approved by NRC. Examples are provided below:

Hope Creek submitted LAR for inverter AOT extension in 2018 [5.5], NRC approved in 2019 [5.6]

- Salem submitted LAR for vital instrument bus inverter AOT extension in 2018 [5. 7],

NRC approved in 2019 [5.8]

Palo Verde submitted similar LAR in 2009 [5.9], NRC approved in 2010 [5.1 O]

- Clinton submitted similar LAR in 2004 [5.11], NRC approved in 2006 [5.12]

North Anna submitted similar LAR in 2002 [5.13], NRC approved in 2004 [5.14]

Braidwood and Byron submitted LAR in 2002 [5.15], NRC approved in 2003 [5.16]

- Salem deterministic plus risk insights submittal in 2022 [5.29], NRC approval in 2023

[5.30]

Serial No.23-177 Docket No. 50-395 Attachment 1: Page 27 of 29 4.3 No Significant Hazards Consideration Dominion Energy South Carolina (DESC) requests approval of a change to the Virgil C.

Summer Nuclear Station (VCSNS), Unit 1, Technical Specification (TS) Limiting Condition for Operation (LCO) 3.8.3.1, Action C, concerning Alternating Current (A.C.) Vital Bus Inverters.

The proposed change would modify the existing VCSNS TS LCO 3.8.3.1, Action C wording to simply the Action statement and extend the Allowed Outage Time (AOT) for an inoperable inverter from 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months to 7 days. The proposed new allowed outage time (AOT) is based on traditional engineering evaluation and risk insights gained via the application of the VCSNS Probabilistic Risk Assessment (PRA) and on additional risk management considerations. The deterministic engineering analysis plus risk insights supporting the proposed change were developed in accordance with the guidelines established in Regulatory Guide 1.177, "An Approach for Plant Specific Risk-informed Decision-making: Technical Specifications," [5.4] and Regulatory Guide 1.174, "An Approach for Using Probabilistic Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing Basis." [5.3]

DESC has evaluated whether a significant hazards consideration is involved with the proposed amendment by focusing on the three standards set forth in 10 CFR 50.92, "Issuance of amendment," as discussed below:

1. Does the proposed change involve a significant increase in the probability or consequences of an accident previously evaluated?

Response: No.

The proposed TS amendment does not affect the design of the A.C. vital bus inverters, the operational characteristics or functions of the inverters, the interfaces between the inverters and other plant systems, or the reliability of the inverters. An inoperable A.C.

inverter is not considered an initiator of an analyzed event. In addition, TS Actions and the associated Allowed Outage Time are not initiators of previously evaluated accidents.

Extending the Allowed Outage Time for an inoperable A.C. inverter would not have a significant impact on the frequency of occurrence of an accident previously evaluated.

The proposed amendment will not result in modifications to plant activities associated with inverter maintenance, but rather, provides operational flexibility by allowing additional time to perform inverter troubleshooting, corrective maintenance, and post-maintenance testing on-line.

The proposed extension of the Completion Time for an inoperable A.C. vital bus inverter will not significantly affect the capability of the inverter to perform its safety function, which is to ensure an uninterruptible supply of 120-Volt A.C. electrical power to the associated power distribution subsystems. An assessment, using PRA methods, confirmed that the increase in plant risk associated with the implementation of the proposed Allowed Outage Time extension is consistent with the N RC's Safety Goal Policy Statement, as further described in RG 1.174 [5.3] and RG 1.177 [5.4]. In addition, a deterministic evaluation

Serial No.23-177 Docket No. 50-395 Attachment 1: Page 28 of 29 concluded that plant defense-in-depth philosophy will be maintained with the proposed Allowed Outage Time extension.

There will be no impact on the source term or pathways assumed in accidents previously evaluated. No analysis assumptions will be changed and there will be no adverse effects on on site or offsite doses as the resu It of an accident.

Therefore, the proposed change does not involve a significant increase in the probability or consequences of an accident previously evaluated.

2. Does the proposed change create the possibility of a new or different kind of accident from any accident previously evaluated?

Response: No.

The proposed amendment does not involve physical alteration of the 120-Volt A.C. vital bus system. No new equipment is being introduced, and installed equipment is not being operated in a new or different manner. There is no change being made to the parameters within which the 120-Volt A.C. vital bus is operated. There are no setpoints at which protective or mitigating actions are initiated that are affected by this proposed change.

The adoption of a 7-day AOT time for A.C. vital bus inverters do not challenge the VCSNS plant design. The proposed change does not alter assumptions made in the safety analysis. The proposed change will not alter the manner-in-which equipment operation is initiated, nor will the functional demands on credited equipment be changed. No alteration is proposed to the procedures that ensure the 120-Volt vital bus system remains within analyzed limits, and no change is being made to procedures relied upon to respond to an off-normal event. As such, no new failure modes are being introduced.

Therefore, the proposed change does not create the possibility of a new or different kind of accident from any accident previously evaluated.

3. Does the proposed change involve a significant reduction in the margin of safety?

Response: No.

Margin of safety is related to the confidence in the ability of the fission product barriers to perform their design functions during and following an accident. These barriers include the fuel cladding, the reactor coolant system, and the containment system. The proposed change, which would increase the 120-Volt A.C. Vital Bus inverters AOT from 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months to 7 days for an inoperable inverter, does not exceed or alter a setpoint, design basis or safety limit.

Therefore, the proposed change does not involve a significant reduction in a margin of safety.

Based on above, DESC concludes that the proposed change presents no significant hazards consideration under the standards set forth in 10 CFR 50.92(c), and, accordingly, a finding of "no significant hazards consideration" is justified.

Serial No.23-177 Docket No. 50-395 Attachment 1: Page 29 of 29 4.4 Conclusion In conclusion, the proposed AOT increase does not increase the potential for a loss of required instrumentation. While operator actions are required in response to the inoperability of one inverter in a single channel, the proposed change will reduce the immediate demands on the operations staff preparing for a potential plant shutdown.

Once approved, this LAR will better focus the operators on risk significant actions, as compared to actions that are based upon qualitative completion times. It is believed that (1 ), there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner, (2), such activities will be conducted in compliance with the Commission's regulations, and (3), the issuance of the amendment will not be inimical to the common defense and security or to the health and safety of the public.

5.0 ENVIRONMENTAL CONSIDERATION

DESC has determined that the proposed change would change a requirement with respect to installation or use of a facility or component located within the restricted area, as defined in 10 CFR 20, or would change an inspection or surveillance requirement.

However, the proposed amendment does not involve (i) a significant hazards consideration, or (ii) a significant change in the types or a significant increase in the amounts of any effluent that may be released offsite, or (iii) a significant increase in individual or cumulative occupational radiation exposure. Accordingly, the proposed amendment meets the eligibility criterion for a categorical exclusion set forth in 10 CFR 51.22(c)(9). Therefore, pursuant to 10 CFR 51.22(b), no environmental impact statement or environmental assessment need be prepared in connection with the proposed amendment.

Serial No.23-177 Docket No. 50-395 ATTACHMENT 2 CURRENTVCSNSTSPAGEMARK~P Virgil C. Summer Nuclear Station (VCSNS) Unit 1 Dominion Energy South Carolina, Inc. (DESC)

Serial No.23-177 Docket No. 50-395 Attachment 2: Page 1 of 1 ELECTRICAL POWER SYSTEMS 3/4.8~3 OHSll£ POWEROlSJIIBtrrION OPERATING b)NI!lNG ~~l_TtON FQR OPDATION l~S.i:'"1 Th& following electrical bussas sha.11 be energized in the specified mlMtH* with t.ie bNaktr.s. tsPctn b&twe;n redundant bUiHU Ttain A A.. C~ Bergem:y 8u:s.ses consbting of: ~

,1;--* *7200 volt &mrpncy, luSH$ I l°"* and 1~.

2.. 480 volt (N,;-geney Susses II lDAl,. lDA2 *ana lW~

Train_ a A.. c. Eaergency Susses cons11t1ng of:

1~ 7200 volt &lergttrq Busse, I 101 and lElt z.. 480 volt &mrganey ,a.ues 4--lDA*r 11)U., and lEBl.

120 volt A.. C.. Vital luues f 5902 4ftd 5901 e hid from an associated invemr eonn<<ttd to O.C. 8ut I .

lZO vo1t A~C~ Vital Busses I 5904 and 5903 *nt~1Hd f~ an assoc:i1ted inverter connected to o.c. Bus I 1H8 ~

120 volt A.c .. Vital Bus 15907 energized.

120 volt A.. C,. Vital Bus fSHI energiled. .

125 VO, t D.. c. B"s lHA ener,;1 ZN f'l"Oll Battery lank ,u.

l.25 volt D.. t~ Bus 1HB tnsrgized frm lattery Bank Ill ..

APPLICABILITY! MODES 1. a. 3. and 4.

ACTION:

!Bus Inverters

INOPERABLEr restore the inverter to OPERABLE status within 7 days eon~d w its ilHHilMiflt.=CT Bl& re energize the A.G. ~:ital Bus ff'lli its aued&\td irWtPllP EGMOEt.ed ta i"ts asse~iated Ll..C. Bus witftin 24 MUN or be in lt laa,t,HOT STANDBY within the next 6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months and in COLD SKUTDmm ~ithin the fa11aving 30 hour$.

The inverteN NY be ditannect.d fNll tttair O. C. Bus far up to 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months u neceH.ary for the'"'"'" of ptf!forsing an equalidt\9 wrge on tbeir associated bitter, bank pnwided (1) their vital busse1 are energized.I'. lf!d (2) the vital but11s associated with the 1>tner battery bank .are energized fr-om theit associated 'I nvertef'S .aftd connet:Ud to their associated D. c. lus.

3/4 8-13

Serial No.23-177 Docket No. 50-395 ATTACHMENT 3 REVISED (CLEAN) VCSNS TS PAGE Virgil C. Summer Nuclear Station (VCSNS) Unit 1 Dominion Energy South Carolina, Inc. (DESC)

Serial No.23-177 Docket No. 50-395 Attachment 3: Page 1 of 1 ELECTRICAL POWER SYSTEMS 3/4.8.3 ONSITE POWER DISTRIBUTION OPERATING LIMITING CONDITION FOR OPERATION 3.8.3.1 The following electrical busses shall be energized in the specified manner with tie breakers open bet\veen redundant busses:

a. Train A A.C. Emergency Busses consisting ot

1. 7200 volt Emergency Busses # 1DA and 1EA.

2. 480 volt Emergency Bu sses # 1DA 1, 1DA2 and 1EA1.

b. Train B A.C. Emergency Susses consisting of:

1. 7200 volt Emergency Busses # 1DB and 1EB.

2. 480 volt Emergency Susses# 1081, 1DB2, and 1EB1.

c. 120 voit,4..C. Yitai Susses# 5902 and 5901 energized from an associated inverter connected to D.C. Bus# 1HN.

d. 120 volt AC. Yitai Susses# 5904 and 5903 energized from an associated invert.er connected to ELC. Bus# 1HB..,_

e. 120 volt AC. Vital Bus #5907 energized.

t 120 volt AC. Vital Bus #5908 energized.

g_ 125 volt [LC. Bus 1HA energized from Battery Bank #1A.

h. 125 volt D.C. Bus 1HB energized from Battery Bank #18.

APPLICABILITY: MODES 1, 2, 3, and 4.

ACTION:

a. With one of the required trains of A.C. Emergency busses not fully energized, re-energize the division within 8 hours9.259259e-5 days 0.00222 hours 1.322751e-5 weeks 3.044e-6 months or be in at least HOT STANDBY within the next 6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months and in corn SHUTDOWN within the following 30 hours3.472222e-4 days 0.00833 hours 4.960317e-5 weeks 1.1415e-5 months .

b. With one A..C. Vital Bus not energized, re-energize the AC. Vital Bus within 2 hours2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months or be in at least HOT STANDBY within the next 6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months and in COLD SHUTDOWN within the following 30 hours3.472222e-4 days 0.00833 hours 4.960317e-5 weeks 1.1415e-5 months .

c. With one of 120 volt A.C. Vital Bus inverter(s) # 5901, 5902, 5903 and 5904 INOPERABLE; restore the inverter to OPERABLE status within 7 days or be in at least HOT STANDBY within the next 6 hours6.944444e-5 days 0.00167 hours 9.920635e-6 weeks 2.283e-6 months and in COLD SHUTDOWN within the following 30 hours3.472222e-4 days 0.00833 hours 4.960317e-5 weeks 1.1415e-5 months .

The inverters may be disconnected from their D.C. Bus for up to 24 hours2.777778e-4 days 0.00667 hours 3.968254e-5 weeks 9.132e-6 months as necessaryforthe purpose of performing an equalizing charge on their associated battery bank provided ( 1) their vital busses are energized, and (2) the vital lmsses associated with the other battery bank are energized from their associated inverters and connected to their associated D.C. Bus.

SUMMER - UNIT 1 3/4 8-13 Amendment No.

Serial No.23-177 Docket No. 50-395 ATTACHMENT 4 PROPOSED TS BASES CHANGE Virgil C. Summer Nuclear Station (VCSNS) Unit 1 Dominion Energy South Carolina, Inc. (DESC)

Serial No.23-177 Docket No. 50-395 Attachment 4: Page 1 of 1 ELECTRICAL POWER SYSTEMS BASES AC. SOURCES. D.C. SOURCES AND ONSITE POWER DISTRIBUTION SYSTEMS

{Continued)

Systems for Standby Diesel Generators," Revision 1, October 1979, as modified by the NRC's revie11.r and approval of South Carolina Electric & Gas Company's June 10, *1985, December 6, 1985, and November 'ID, 2000 amendment requests.

The Surveillance Requirement that assures the diesel generator is capable of performing its design function follows the guidance of NUREG 1366 and NUREG 143*1. Rev 2. The surveillance tests the capability of the diesel generator to start and close its breaker in the required 10 seconds to support the accident anafysis, and carry the required electrical load while maintaining the voltage and frequency limits necessary to assure OPERABILITY of the loads.

In addition to the Surveillance Requirements, the time for the diesel generator to reach steady state operation, unless the modified start method is utilized, is periodically monitored and the trend evaluated to identify degradation of the governor and voltage regulator performance.

The fuel storage system minimum volume of fuel to demonstrate operability of the diesel generators was based on fuel consumption determined from the development of time dependent loads following a design basis accident and .a loss of off-site power utilizing FSAR Table 8.3-3 for seven days.

All safety-related portions of the VCSNS diesel engine fuel oil storage and transfer system, are Seismic Category I, Safety Class 2b, and designed to ANSI Standard N195-1976 with the provision listed below:

VCSNS will maintain at least 2% margin above the minimum calculated seven day required volume during Modes '1-4. This is an exception to ANSI N*l95-*1976, "Fuel Oil Systems for Standby Diesel Generators," Section 5.4, during Modes 1-4. EOG fuel replenishment is available from multiple sources, including off:.site suppliers. on-site non safety storage in the Auxiliary Boiler Fuel Tank, and the ability to provide fuel from the opposite train EOG Fuel Oil Storage Tank via the fuel oil and transfer system cross-tie.

The 10% fuel margin as recommended in Regulatory Guide 1. *137, Revision *1, "Fuel-Oil Systems for Standby Diesel Generators," position C.1.c.(2) will be met during: Modes 5 and 6.

"Energized" A.G. Vital Busses #5901, 5902, 5903, or 5904 require the panels to be energized to their proper voltage from the associated inverter via inverted D.C. voltage, inverter using the normal AC. source, or Class 1E backup A.C. source via voltage regulator. OPERABLE inverters require the associated 120 VAC A.G. Vital Busses #590*1. 5902, 5903, or 5904 to be powered by the inverter with output voltage within tolerances. and power input to the inverter from the associated D.C. Bus. Alternatively. the power supply may be from an internal A.C. source via rectifier as long as the OPERABLE D.C. Bus is available as the uninterruptible power supply."

SUMMER - UNIT 1 B 3/4 8-3 Amendment No. S4, *139, 160, 164, 178,

Serial No.23-177 Docket No. 50-395 ATTACHMENT 5 REFERENCES Virgil C. Summer Nuclear Station (VCSNS) Unit 1 Dominion Energy South Carolina, Inc. (DESC)

Serial No.23-177 Docket No. 50-395 Attachment 8: Page 1 of 3 5.1 VCSNS Technical Specification 5.2 VCSNS Updated Final Safety Analysis Report 5.3 RG 1.174, "An Approach for Using Probabilistic Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing Basis" 5.4 RG 1.177, "An Approach for Plant-Specific, Risk-Informed Decision-making:

Technical Specifications" 5.5 ADAMS Accession No. ML18103A218, Hope Creek Station LAR, "License Amendment Request: Inverter Allowed Outage Time (AOT) Extension," dated April 13,2018 5.6 ADAMS Accession No. ML19065A156, NRC Safety Evaluation of Hope Creek LAR (5.5), dated March 27, 2019 5.7 ADAMS Accession No. ML18136A866, Salem Station LAR, "License Amendment Request: Vital Instrument Bus Inverter Allowed Outage Time (AOT) Extension,"

dated May 16, 2018 5.8 ADAMS Accession No. 19009A477, NRC Safety Evaluation of Salem LAR (5. 7),

dated January 25, 2019 5.9 ADAMS Accession No. ML092810227, Palo Verde Station LAR, "Request to Amend Technical Specification (TS) 3.8.7, "Inverters-Operating," to Extend Completion Time for Restoration of an Inoperable Inverter, Dated September 28, 2009 5.10 ADAMS Accession No.102670352, NRC Safety Evaluation of Palo Verde LAR (5.9),

dated September 29, 2010 5.11 ADAMS Accession No. ML041210913, Clinton Power Station Unit 1 LAR, "Request for Technical Specification Change to Extend Completion Time for Nuclear System Protection System Inverters", dated April 26, 2004 5.12 ADAMS Accession No. ML061160181, NRC Safety Evaluation of Clinton LAR (5.11 ), dated May 26, 2006 5.13 ADAMS Accession No. ML023600217, North Anna Station LAR, "Proposed Risk-Informed Technical Specifications Change Extended Inverter Allowed Outage Time, dated December 13, 2002 5.14 ADAMS Accession No. ML041380438, NRC Safety Evaluation of North Anna LAR (5.13), dated May 12, 2004 5.15 ADAMS Accession No. ML023020061, Braidwood Station, Units 1 and 2, and Byron Station, Units 1 and 2 LAR, "Request for Technical Specification Change Extension of Completion Time for Instrument Bus Inverters", dated October 16, 2002

Serial No.23-177 Docket No. 50-395 Attachment 8: Page 2 of 3 5.16 ADAMS Accession No. ML032830455, NRC Safety Evaluation of Byron and Braidwood Stations LAR (5.15), dated November 19, 2003 5.17 USNRC, 60 FR 42622, "Use of Probabilistic Risk Assessment Methods in Regulatory Activities; Final Policy Statement," August 16, 1995 5.18 ADAMS Accession No. ML012200324, Virgil C. Summer, Unit 1, Issuance of Construction Permit No. CPPR-94, March 21, 1973 5.19 ADMAS Accession No. ML012200333, Virgil C. Summer, Unit 1, Issuance of Facility Operating License No. NPF-12 to South Carolina Electric & Gas Company, August 06, 1982 5.20 ADMAS Accession No. ML19341C502, NUREG-0717, Safety Evaluation Report related to the operation of Virgil C. Summer Nuclear Station, Unit No. 1, February 28, 1981 5.21 ADMAS Accession No. ML20002A769, Forwards lists of facility compliance w/10CFR20, 10CFR50 & 10CFR100 per RL Tedesco 801027 request, November 14, 1980 5.22 ADMAS Accession No. ML19347E803, NUREG-0717, Safety Evaluation Report related to the operation of Virgil C. Summer Nuclear Station, Unit No. 1, Supplement 1, April 30, 1981 5.23 ADMAS Accession No. ML20004E346, NUREG-0717, Safety Evaluation Report related to the operation of Virgil C. Summer Nuclear Station, Unit No. 1, Supplement 2, May 31, 1981 5.24 ADMAS Accession No. ML20040B267, NUREG-0717, Safety Evaluation Report related to the operation of Virgil C. Summer Nuclear Station, Unit No. 1, Supplement 3, January 31, 1982 5.25 ADMAS Accession No. ML20062N916, NUREG-0717, Safety Evaluation Report related to the operation of Virgil C. Summer Nuclear Station, Unit No. 1, Supplement 4, August 31, 1982 5.26 ADMAS Accession No. ML20070H771, NUREG-0717, Safety Evaluation Report related to the operation of Virgil C. Summer Nuclear Station, Unit No. 1, Supplement 5, November 30, 1982 5.27 ADMAS Accession No. ML21071A238, NUREG/KM-0016, Be riskSMART Guidance for Integrated Risk Insights into NRC Decisions, March 2021 5.28 RG 1.160, "Monitoring the Effectiveness of Maintenance at Nuclear Power Plants" 5.29 ADAMS Accession No. ML22180A268, "License Amendment Request (LAR) to Amend Salem Unit 1 and Unit 2 Technical Specifications (TS) to Extend the Allowed Outage Time for an Inoperable Emergency Diesel Generator from 72 hours8.333333e-4 days 0.02 hours 1.190476e-4 weeks 2.7396e-5 months to 14 Days", June 29, 2022

Serial No.23-177 Docket No. 50-395 Attachment 8: Page 3 of 3 5.30 ADAMS Accession No. ML23081A466, "Salem Nuclear Generating Station, Unit Nos. 1 and 2 - Issuance of Amendment Nos. 346 and 327 Re. Revise Technical Specifications to Extend Allowable Outage Time for Inoperable Emergency Diesel Generator (EPID L-2022-LLA-0095)", May 2, 2023

ML23317A224

Contents

Text

SUMMARY

2.4 DESCRIPTION

3.0 TECHNICAL EVALUATION

4.0 REGULATORY EVALUATION

4.4 CONCLUSION

5.0 ENVIRONMENTAL CONSIDERATION

SUMMARY

3.0 TECHNICAL EVALUATION

4.0 REGULATORY EVALUATION

5.0 ENVIRONMENTAL CONSIDERATION

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ML23317A224

Text

SUMMARY

2.4 DESCRIPTION

3.0 TECHNICAL EVALUATION

4.0 REGULATORY EVALUATION

4.4 CONCLUSION

5.0 ENVIRONMENTAL CONSIDERATION

SUMMARY

3.0 TECHNICAL EVALUATION

4.0 REGULATORY EVALUATION

5.0 ENVIRONMENTAL CONSIDERATION

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