L-25-227, Response to Request for Additional Information (RAI) Regarding License Amendment Request Alternate Completion Time for Technical Specification (TS) 3.8.1 Action A.3

Response to Request for Additional Information (RAI) Regarding License Amendment Request Alternate Completion Time for Technical Specification (TS) 3.8.1 Action A.3

ML25356A382
Person / Time
Site:	Davis Besse
Issue date:	12/22/2025
From:	Craven R Vistra Operations Company
To:	Office of Nuclear Reactor Regulation, Document Control Desk
References
L-25-227
Download: ML25356A382 (0)

Text

Davis-Besse Nuclear Power Station Robert B. Craven Site Vice President 5501 N. State Route 2 Oak Harbor, Ohio 43449 December 22, 2025 L-25-227 10 CFR 50.90 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, DC 20555-0001

Subject:

Davis-Besse Nuclear Power Station, Unit No. 1 Docket No. 50-346, License No. NPF-3 Response to Request for Additional Information (RAI) Regarding License Amendment Request Alternate Completion Time for Technical Specification (TS) 3.8.1 Action A.3 By letter dated October 22, 2025 (ADAMS Number ML2529A487), Vistra Operations Company LLC (Vistra OpCo) submitted a request for an amendment to the renewed facility operating license for the Davis-Besse Nuclear Power Station, Unit No. 1 (DBNPS). The proposed change would revise TS 3.8.1, AC Sources -

Operating, Action A.3 to allow an alternative Completion Time of 15 days.

By electronic mail dated December 18, 2025, the Nuclear Regulatory Commission (NRC) staff requested additional information to complete the review of the license amendment request.

Vistra OpCos response to the RAI is provided in Attachment 1. An updated markup of the TS page is provided in Attachment 2 and supersedes the previously provided markup. The proposed markup of the TS Bases is provided in Attachment 3. Attachment 4 provides the page from approved Emergency Procedures to restore power to the essential buses from the SBODG.

This supplement to the license amendment request does not change the conclusion of the no significant hazards consideration.

There are no regulatory commitments contained in this submittal. If there are any questions, or if additional information is required, please contact Mr. Jack Hicks, Senior Manager, Licensing, at (254) 897-6725 or jack.hicks@vistracorp.com.

6555 SIERRA DRIVE IRVING, TEXAS 75039 o 214-812-4600 VISTRACORP.COM

Davis-Besse Nuclear Power Station, Unit No. 1 L-25-227 Page 2 I declare under penalty of perjury that the foregoing is true and correct. Executed on December 22, 2025.

Sincerely, Robert B. Craven Attachments:

1. Response to Request for Additional Information

2. Proposed Technical Specification Pages Markup 3 Proposed Technical Specification Bases Markup 4 Power restoration procedure from the SBODG cc:

NRC Region III Administrator NRC Resident Inspector NRC Project Manager Utility Radiological Safety Board Executive Director, Ohio Emergency Mgmt. Agency

L-25-227 Response to Request for Additional Information Page 1 of 7 By letter dated October 22, 2025 (ADAMS Number ML2529A487), Vistra Operations Company LLC (Vistra OpCo) submitted a request for an amendment to the renewed facility operating license for the Davis-Besse Nuclear Power Station, Unit No. 1 (DBNPS). The proposed change would revise TS 3.8.1, AC Sources - Operating, Action A.3 to allow an alternative Completion Time of 15 days. By electronic mail dated December 18, 2025, the Nuclear Regulatory Commission (NRC) staff requested additional information to complete the review of the license amendment request. The NRC request for additional information (RAI) questions are provided below in bold font, followed by the Vistra OpCo response.

Question 1:

BTP 8-8 provides guidance from a deterministic perspective, including defense-in-depth aspects, for one-time or permanent AOT extensions for the EDGs and offsite power sources from the current AOT up to 14 days.

BTP 8-8, in section B, states, in part, that:

A supplemental power source should be available as a backup to the inoperable EDG or offsite power source, to maintain the defense-in-depth design philosophy of the electrical system to meet its intended safety function. The supplemental source must have capacity to bring a unit to safe shutdown (cold shutdown) in case of a loss of offsite power (LOOP) concurrent with a single failure during plant operation (Mode 1)

The permanent or temporary power source can be either a diesel generator, gas or combustion turbine, or power from nearby hydro units.

Confirm that the SBODG will be employed as a supplemental power source and discuss conformance to BTP 8-8. If the SBODG does not conform to BTP 8-8, please provide justification as to why conformance to BTP 8-8 is not needed.

Response

Davis-Besse confirms that the Station Blackout Diesel Generator (SBODG) will be credited as a supplemental power source in accordance with BTP 8-8. The SBODG is capable of bringing the unit to cold shutdown after a loss of offsite power and a failure of both EDGs.

The Davis-Besse Updated Final Safety Analysis (UFSAR) Section 8.3 discusses the Station Blackout Diesel Generator as an Alternate AC Source (AAC).

L-25-227 Page 2 of 7 The purpose of the non-class 1E 4.16kV, 2865KW Station Blackout Diesel Generator (SBODG) is to provide AC power to all systems required for coping with a Station Blackout as defined in 10 CFR 50.2. This alternate AC power source meets the criteria specified in Appendix B to NUMARC 87-00, Guidelines and Technical Bases for NUMARC Initiative Addressing Station Blackout at Light Water Reactors and in Regulatory Guide 1.155, Station Blackout.

The SBODG is capable of supplying either of the Stations essential 4.16kV buses through nonessential Bus D2 and is available within ten minutes of the onset of Station Blackout.

During an emergency, the SBODG can be manually started and loaded onto Bus D2 from the Control Room.

Question 2:

BTP 8-8 further states that the availability of the alternate AC (AAC) source should be verified within the last 30 days before entering extended AOT by operating or bringing the power source to its rated voltage and frequency for 5 minutes and ensuring all its auxiliary support systems are available or operational. LAR section 3.2 states that the SBODG will be demonstrated functional by verifying its periodic test is current prior to the start of the extended 15-day CT. Will verification of the periodic testing of the SBODG meet the guidance in BTP for testing the availability of the AAC? If not, describe how reliance on the periodic testing will adequately demonstrate the availability of the SBODG as the AAC. If additional testing is needed to ensure availability of the SBODG as the AAC will this additional testing be included as compensatory measures?

Response

The SBODG Monthly Test is performed every 30 days. This test verifies proper frequency and voltage and all support systems are operating properly. The SBODG is paralleled to the grid and maintained at peak load for at least 5 minutes and runs for at least 60 minutes. This test will be performed prior to start of the extended CT. Performance of the SBODG Monthly Test does meet the guidance outlined in BTP 8-8; no additional testing is necessary.

Question 3:

BTP 8-8 states that the plant should have formal engineering calculations for equipment sizing and protection and have approved procedures for connecting the AAC or supplemental power sources to safety buses.

1) Discuss or provide a summary of the calculations for equipment sizing and protection and 2) describe the approved procedures for connecting the SBODG.

Response

1) The NRCs Safety Evaluation of the DBNPS Station Blackout Rule 10 CFR 50.63, Section 2.2.2 dated March 7, 1991 included a recommendation that the proposed AAC power source L-25-227 Page 3 of 7 have approximately the same capacity as one of the existing Emergency Diesel Generators (EDGs), and if the proposed AAC source has less capacity, then a coping analysis should be performed to show the plant can cope with and recover from an SBO for the required duration.

As installed, the SBODG capacity (2865 kW) satisfies the NUMARC 87-00 capacity attribute discussed in the NRCs Safety Evaluation Section 2.2.1 and exceeds the recommendation discussed in the NRCs Safety Evaluation Section 2.2.2 to respond to a station blackout.

2) Davis-Besse has both abnormal and emergency procedures which direct restoring power to essential power buses via the SBODG. Attachment 4 provides the page from the emergency procedure to restore power to essential buses C1 or D1.

Question 4:

BTP 8-8 states that the AOT should be limited to 14 days and the licensee must provide justification for the duration of the requested AOT (actual hours plus margin based on plant-specific past operating experience). LAR section 2.3 states that the 13 days provides the duration necessary to restore the X02 transformer tap changer, including two days for replacement of the transformer bushings, for a total of 15 days.

Discuss the specific tasks and duration of the tasks for an extended CT duration of 15 days. Provide the justification of the duration of the extended CT (i.e., based on operating experience, etc.).

Response

The following items illustrate the scope of the startup transformer X02 maintenance Tap Changer Repair and Bushing Scope:

Establish Conditions /Hang Clearance Hang Grounds & Install Ground Truck De-terminate High and Low Voltage and Neutral Bushings/Lightning Arrester Pre-Testing Leak Check/Inspection Contingent Leak Repair Drain Down Remove Covers Internal Inspection Tap Changer Replacement/Repairs Gasket Replacement Bushing Replacement L-25-227 Page 4 of 7 Install Covers Leak Check Prepare Cold Weather Fill Oil Fill Under Vacuum Add Oil Circulation Passes Post Fill Testing Restore from Cold Weather Fill Re-terminate High and Low Voltage Bushings Modify Bushing Drops Hang Revised Clearance Remove Grounds & Ground Trucks Post Maintenance Testing The anticipated tap changer repair and bushing replacement duration of 315 hours0.00365 days <br />0.0875 hours <br />5.208333e-4 weeks <br />1.198575e-4 months <br /> is approximately 88 percent of the requested time (360 hours0.00417 days <br />0.1 hours <br />5.952381e-4 weeks <br />1.3698e-4 months <br />). The remaining 45 hours5.208333e-4 days <br />0.0125 hours <br />7.440476e-5 weeks <br />1.71225e-5 months <br /> of the requested time will accommodate unexpected maintenance or weather delays.

Question 5:

BTP 8-8 provides guidance that 1) the availability of the AAC shall be checked every 8-12 hours (once per shift), 2) if the AAC becomes unavailable any time during the extended AOT, the unit shall enter the LCO and start shutting down within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />, 3) preplanned maintenance will not be scheduled if severe weather conditions are anticipated, and 4) no discretionary switchyard maintenance will be performed.

Provide justification for why the above items are not needed or add them to the compensatory measures. Confirm that the remaining operable offsite circuit through transformer X01 is protected.

Response

1) Shiftly checks of the SBODG (AAC) will be performed as compensatory measures The SBODG will be demonstrated functional by verifying its periodic test is current prior to start of the extended 15 day CT. In addition, plant operators will perform routine shiftly walkdowns of the SBODG and document in the electronic logging system (eSOMS).

(2) Based on a review of the BTP 8-8, the 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> shutdown is appliable when the site is in an extended EDG allowed out of service time (AOT) as opposed to an offsite source AOT. In the L-25-227 Page 5 of 7 event the SBODG becomes unavailable, both EDGs will be available to provide power to their respective essential buses.

3) Actions in response to severe weather have been added as compensatory measures Prior to removing startup transformer X02 from service, review the weather forecast for severe weather in the immediate future. In the event of severe weather, all work associated with startup transformer X02 will be suspended and the station procedures for severe weather will be used to control work processes.

4) Actions to restrict discretionary switchyard maintenance have been added as compensatory measures.

All scheduled work reviewed to ensure no work is performed that could threaten offsite and onsite power sources. Switchyard work will be limited to activities related to startup transformer X02 only.

Question 6:

10 CFR 50.36(c)(2) requires that TSs include Limiting Condition for Operation (LCOs).

Per 10 CFR 50.36(c)(2)(i), LCOs are the lowest functional capability or performance levels of equipment required for safe operation of the facility. The regulation also requires that when an LCO of a nuclear reactor is not met, the licensee shall shut down the reactor or follow any remedial action permitted by the TS until the condition can be met.

The NRC staff noted that the Davis-Besse markup TS page (Attachment 2 to the LAR) is missing some information and requirements. The current proposed changes appear to have potential for misinterpretation by licensee operators.

Justify how the proposed TS provide assurance that the specification would not be misinterpreted or revise the proposed Davis-Besse TS 3.8.1 Required Action A.3 marked-up pages, as needed, to have an appropriate amount of detail and include the following elements:

1. An expiration date for using the extended CT;

2. Limit one-time use of the 15-day CT to specific equipment, regardless of whether maintenance is completed during the attempt;

3. Specify applicable equipment by designation (e.g., transformer X02) - one option is to add the requirement into the compensatory measures list;

4. Conditions for when the offsite circuit CT extension is applicable, (e.g., startup transformer repairs);

L-25-227 Page 6 of 7

5. Remedial actions for compensatory measure equipment if discovered unavailable during the maintenance evolution (e.g., restore to available status within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />); and

6. In TS 3.8.1 Required Action A.3, reference the document that contains the list of all risk management actions and compensatory measures; (i.e., Vistra OpCo letter).

An effective option is to place an asterisk at the existing CT and create a footnote with the details at the bottom of the TS page.

Response

The proposed TS 3.8.1 Required Action A.3 has been modified to use the footnote below.

This footnote addresses items (1), (2), (3), (4) and (6). The updated TS page is provided in to this letter and supersedes the previously provided TS page markup.

A one-time Completion Time (CT) extension for an inoperable offsite circuit allows 15 days to restore the inoperable Startup Transformer X02 to OPERABLE status.

Compensatory measures within Vistra Operations Company LLC letter L-25-227 dated 12/22/2025 shall be implemented and shall remain in effect during the extended CT period. The one-time extension shall expire upon completion of the maintenance to restore Startup Transformer X02 to OPERABLE status or by 0600 on March 6, 2026, whichever occurs earliest The list of compensatory measures has been revised to include the measures indicated in the RAI responses. The compensatory measures listed below supersede the previously provided list.

The following compensatory measures will be implemented to limit the risk impact during the proposed startup transformer X02 maintenance outage.

Any emergent work will be assessed and managed in accordance with 10 CFR 50.65(a)(4) and the applicable TS.

Daily communication will be held with the grid operator to monitor grid conditions and ensure no significant grid perturbations are expected during the extended completion time period.

Plant administrative procedures governing protected equipment postings will be utilized to protect risk significant plant equipment. This includes the EDGs, SBODG, EFW System and startup transformer X01.

There will be no scheduled work that would result in more than GREEN (low)

Probabilistic Risk Assessment (PRA) risk impact.

All scheduled work reviewed to ensure no work is performed that could threaten offsite and onsite power sources. Switchyard work will be limited to activities involving startup transformer X02 only.

L-25-227 Page 7 of 7 Emergent issues resulting in unplanned inoperability of risk significant systems will be addressed in a timely manner.

The SBODG will be demonstrated functional by verifying its periodic test is current prior to start of the extended 15 day CT. In addition, plant operators will perform routine shiftly walkdowns of the SBODG and document in the electronic logging system (eSOMS).

Both EDGs will be demonstrated OPERABLE by verifying their respective surveillance are current prior to the start of the extended 15 day CT.

Reserve Source Selector Switches (RSSS) will both be aligned to the startup transformer X01 Prior to removing startup transformer X02 from service, review the weather forecast for severe weather in the immediate future. In the event of severe weather, all work associated with startup transformer X02 will be suspended and the station procedures for severe weather will be used to control work processes.

Around the clock staffing will occur in the Outage Control Center to address issues affecting compensatory measure equipment without undue delay. Any subsequent limiting condition required actions will be entered, and followed as appropriate, for an off-site circuit inoperable.

L-25-227 Proposed Technical Specification Changes (Mark-Up)

(1 pages follows)

AC Sources - Operating 3.8.1 Davis-Besse 3.8.1-2 Amendment TBD ACTIONS (continued)

CONDITION REQUIRED ACTION COMPLETION TIME A. (continued)

A.3 Restore offsite circuit to OPERABLE status.

72 hour8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />s*

B. One EDG inoperable.

B.1 Perform SR 3.8.1.1 for OPERABLE offsite circuit(s).

AND B.2 Declare required feature(s) supported by the inoperable EDG inoperable when its redundant required feature(s) is inoperable.

AND B.3.1 Determine OPERABLE EDG is not inoperable due to common cause failure.

OR B.3.2 Perform SR 3.8.1.2 for OPERABLE EDG.

AND B.4 Restore EDG to OPERABLE status.

1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> AND Once per 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> thereafter 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> from discovery of Condition B concurrent with inoperability of redundant required feature(s) 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> 24 hours 7 days A one-time Completion Time (CT) extension for an inoperable offsite circuit allows 15 days to restore the inoperable Startup Transformer X02 to OPERABLE status. Compensatory measures within Vistra Operations Company LLC letter L-25-227 dated 12/22/2025 shall be implemented and shall remain in effect during the extended CT period. The one-time extension shall expire upon completion of the maintenance to restore Startup Transformer X02 to OPERABLE status or by 0600 on March 6, 2026, whichever occurs earliest.

L-25-227 Proposed Technical Specification Bases (Mark-Up)

(2 pages follows)

AC Sources - Operating B 3.8.1 Davis-Besse B 3.8.1-6 Revision 32 BASES ACTIONS (continued)

A.2 Required Action A.2, which only applies if the train cannot be powered from an offsite source, is intended to provide assurance that an event coincident with a single failure of the associated EDG will not result in a complete loss of safety function of critical redundant required features.

These features are powered from the redundant AC electrical power train.

The Completion Time for Required Action A.2 is intended to allow the operator time to evaluate and repair any discovered inoperabilities. This Completion Time also allows for an exception to the normal "time zero" for beginning the allowed outage time "clock." In this Required Action, the Completion Time only begins on discovery that both:

a.

The train has no offsite power supplying its loads; and

b.

A redundant required feature on the other train is inoperable.

If at any time during the existence of Condition A (one offsite circuit inoperable) a redundant required feature subsequently becomes inoperable, this Completion Time begins to be tracked.

Discovering no offsite power to one train of the onsite Class 1E Electrical Power Distribution System coincident with one or more inoperable redundant required support or supported features, or both, that are associated with the other train that has offsite power, results in starting the Completion Time for the Required Action. Twenty-four hours is acceptable because it minimizes risk while allowing time for restoration before subjecting the unit to transients associated with shutdown.

The remaining OPERABLE offsite circuit and EDGs are adequate to supply electrical power to essential bus C1 and essential bus D1 of the onsite Class 1E Distribution System. The 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> Completion Time takes into account the component OPERABILITY of the redundant counterpart to the inoperable required feature. Additionally, the 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> Completion Time takes into account the capacity and capability of the remaining AC sources, a reasonable time for repairs, and the low probability of a DBA occurring during this period.

A.3 Consistent with Regulatory Guide 1.93 (Ref. 6), operation may continue in Condition A for a period that should not exceed 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />. With one offsite circuit inoperable, the reliability of the offsite system is degraded, and the potential for a loss of offsite power is increased, with attendant potential for a challenge to the unit safety systems. In this Condition, No Changes - Included for context

AC Sources - Operating B 3.8.1 Davis-Besse B 3.8.1-7 Revision 32 BASES ACTIONS (continued) however, the remaining OPERABLE offsite circuit and EDGs are adequate to supply electrical power to the onsite Class 1E Distribution System.

The 72 hour8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> Completion Time takes into account the capacity and capability of the remaining AC sources, a reasonable time for repairs, and the low probability of a DBA occurring during this period.

A footnote has been added to provide an alternative completion time to have an offsite source inoperable. The footnote specifies that the alternative 15 day completion time is only applicable during Cycle 24.

This extended completion time is for one-time use to allow maintenance on startup transformer X02 tap changer prior to the 2026 refueling outage.

B.1 To ensure a highly reliable power source remains with an inoperable EDG, it is necessary to verify the availability of the offsite circuits on a more frequent basis. Since the Required Action only specifies "perform,"

a failure of SR 3.8.1.1 acceptance criteria does not result in a Required Action being not met. However, if an offsite circuit fails to pass SR 3.8.1.1, it is inoperable. Upon offsite circuit inoperability, additional Conditions and Required Actions must then be entered.

B.2 Required Action B.2 is intended to provide assurance that a loss of offsite power, during the period that an EDG is inoperable, does not result in a complete loss of safety function of critical systems. These features are designed with redundant safety related trains. Redundant required feature failures consist of inoperable features associated with a train, redundant to the train that has an inoperable EDG.

The Completion Time for Required Action B.2 is intended to allow the operator time to evaluate and repair any discovered inoperabilities. This Completion Time also allows for an exception to the normal "time zero" for beginning the allowed outage time "clock." In this Required Action, the Completion Time only begins on discovery that both:

a.

An inoperable EDG exists; and b.

A redundant required feature on the other train is inoperable.

L-25-227 Power Restoration to Bus C1 or D1 from the SBODG (1 pages follows)

495 DB-OP-02000 Revision 34 ATTACHMENT 28: RESTORE POWER TO C1 OR D1 BUS FROM THE SBODG Page 1 of 1 This attachment is used to restore power to D2 and then either C1 OR D1 Bus from the Station Blackout Diesel Generator. The attachment may be used as directed by the Command SRO to restore power to C1 AND D1, if required, to mitigate multiple equipment failures.

1.0 Reenergization of Bus D2 from SBODG

_____ 1.1 Check AD213 is closed (breaker closed indicates D2 is NOT Locked Out).

_____ 1.2 Verify ABDD2, BUS TIE XFMR BD is open.

_____ 1.3 Verify AD 110 is open.

_____ 1.4 Start the SBODG by pressing START at the SBODG Control Panel C5740.

_____ 1.5 Check SBODG speed approximately 900 RPM.

_____ 1.6 Close AD 301 to energize Bus D2.

_____ 1.7 Verify Bus D2 energized by checking D2 Bus Voltage.

2.0 IF power is being restored to C1 bus, THEN restore Power to C1 from the SBODG via D2 as follows:

_____ 2.1 Verify C1 Sync Switch is OFF OR REMOVED.

_____ 2.2 Verify HBBD is open.

_____ 2.3 Close ABDD2.

_____ 2.4 Close ABDC1 to energize C1 Bus.

_____ 2.5 While monitoring SBODG load (continuous rating 2865 KW), start the required components on C1 Bus.

3.0 IF power is being restored to D1, THEN restore Power to D1 from the SBODG via D2 as follows:

_____ 3.1 Verify D1 Sync Switch is OFF OR REMOVED.

_____ 3.2 Close AD 110 to energize D1 Bus.

_____ 3.3 While monitoring SBODG load (continuous rating 2865 KW), start the required components on D1 Bus.

_____ 4.0 IF C1 AND D1 remain deenergized, THEN notify the Command SRO to declare an Extended Loss of AC Power (ELAP),

AND GO TO DB-OP-02700, Station Blackout. (Time Sensitive Operator Action = 10 minutes)

_____ 5.0 As time permits, initiate action to refuel the SBODG.

REFER TO DB-OP-06273, Diesel Fuel Oil Transfer.