Text

Integrated Inspection Report 05000275/2025003 and 05000323/2025003
	ML25350C095
Person / Time
Site:	Diablo Canyon ; (DPR-080, DPR-082)
Issue date:	12/18/2025
From:	Ami Agrawal; NRC/RGN-IV/DORS/PBB
To:	Gerfen P; Pacific Gas & Electric Co
References
	IR 2025003
	Download: ML25350C095 (0)
	v • d • e

December 18, 2025

SUBJECT:

DIABLO CANYON POWER PLANT, UNITS 1 AND 2 - INTEGRATED INSPECTION REPORT 05000275/2025003 AND 05000323/2025003

Dear Paula A. Gerfen:

On September 30, 2025, the U.S. Nuclear Regulatory Commission (NRC) completed an inspection at Diablo Canyon Power Plant, Units 1 and 2 and discussed the results of this inspection with Jordan Tyman, Director, Nuclear Risk and Compliance, and other members of your staff. The results of this inspection are documented in the enclosed report.

Due to the temporary cessation of government operations, which commenced on October 1, 2025, the NRC began operating under its Office of Management and Budget-approved plan for operations during a lapse in appropriations. Consistent with that plan, the NRC operated at reduced staffing levels throughout the duration of the shutdown. However, the NRC continued to perform critical health and safety functions and make progress on other high-priority activities associated with the ADVANCE Act and Executive Order 14300. On November 13, 2025, following the passage of a continuing resolution, the NRC resumed normal operations. However, due to the 43-day lapse in normal operations, the Office of Nuclear Reactor Regulation granted the Regional Offices an extension on the issuance of the calendar year 2025 inspection reports that should have been issued by November 13, 2025, to December 31, 2025. The NRC resumed the routine cycle of issuing inspection reports on November 13, 2025.

Two findings of very low safety significance (Green) are documented in this report. Two of these findings involved violations of NRC requirements. We are treating these violations as non-cited violations (NCVs) consistent with Section 2.3.2 of the Enforcement Policy.

If you contest the violations or the significance or severity of the violations documented in this inspection report, you should provide a response within 30 days of the date of this inspection report, with the basis for your denial, to the U.S. Nuclear Regulatory Commission, ATTN:

Document Control Desk, Washington, DC 20555-0001; with copies to the Regional Administrator, Region IV; the Director, Office of Enforcement; and the NRC Resident Inspector at Diablo Canyon Power Plant, Units 1 and 2. If you disagree with a cross-cutting aspect assignment in this report, you should provide a response within 30 days of the date of this inspection report, with the basis for your disagreement, to the U.S. Nuclear Regulatory Commission, ATTN: Document Control Desk, Washington, DC 20555-0001; with copies to the Regional Administrator, Region IV; and the NRC Resident Inspector at Diablo Canyon Power Plant, Units 1 and 2.

This letter, its enclosure, and your response (if any) will be made available for public inspection and copying at http://www.nrc.gov/reading-rm/adams.html and at the NRC Public Document Room in accordance with Title 10 of the Code of Federal Regulations 2.390, Public Inspections, Exemptions, Requests for Withholding.

Sincerely, Ami N. Agrawal, Chief Reactor Projects Branch B Division of Operating Reactor Safety Docket Nos. 05000275 and 05000323 License Nos. DPR-80 and DPR-82

Enclosure:

As stated

Inspection Report

Docket Numbers:

05000275 and 05000323

License Numbers:

DPR-80 and DPR-82

Report Numbers:

05000275/2025003 and 05000323/2025003

Enterprise Identifier:

I-2025-003-0010

Licensee:

Pacific Gas and Electric Company

Facility:

Diablo Canyon Power Plant, Units 1 and 2

Location:

Avila Beach, CA

Inspection Dates:

July 1, 2025, to September 30, 2025

Inspectors:

E. Garcia, Resident Inspector

M. Hayes, Senior Resident Inspector

S. Hedger, Senior Emergency Preparedness Inspector

Approved By:

Ami N. Agrawal, Chief

Reactor Projects Branch B

Division of Operating Reactor Safety

SUMMARY

The U.S. Nuclear Regulatory Commission (NRC) continued monitoring the licensees performance by conducting an integrated inspection at Diablo Canyon Power Plant, Units 1 and 2, in accordance with the Reactor Oversight Process. The Reactor Oversight Process is the NRCs program for overseeing the safe operation of commercial nuclear power reactors. Refer to https://www.nrc.gov/reactors/operating/oversight.html for more information.

List of Findings and Violations

Failure to Perform Preventive Maintenance Replacement Strategy Cornerstone Significance Cross-Cutting Aspect Report Section Initiating Events Green NCV 05000323/2025003-01 Open/Closed

[P.3] -

Resolution 71111.12 The inspectors identified a Green finding and associated non-cited violation of Technical Specification (TS) 5.4.1.a, Procedures, when PG&E failed to establish and implement a preventive maintenance strategy for safety-related fuse FU-2213 after removing the fuse replacement frequency from an already established preventive maintenance strategy.

Specifically, the licensee removed the previously in place 4-year fuse replacement interval from maintenance plan 19782 and failed to establish a new maintenance plan for the fuse.

Subsequently, the fuse failed and tripped all bistables associated with protection set II of the Eagle 21 system.

Failure to Correct a Condition Adverse to Quality for Diesel Emergency Generator 2-3 Cornerstone Significance Cross-Cutting Aspect Report Section Mitigating Systems Green NCV 05000323/2025003-02 Open/Closed

[P.2] -

Evaluation 71152A The inspectors identified a Green finding and associated non-cited violation of Title 10 of the Code of Federal Regulations (10 CFR) Part 50, Appendix B, Criterion XVI, "Corrective Action," for failure to correct a condition adverse to quality associated with diesel emergency generator (DEG) 2-3. Specifically, the licensee failed to correct a condition where a fuel linkage cotter pin was contacting a mechanical travel limiter causing interference with the diesels response to fuel demands.

Additional Tracking Items

None.

PLANT STATUS

Unit 1 operated at or near rated thermal power for the entire inspection period.

Unit 2 operated at or near rated thermal power for the entire inspection period with the exceptions of reducing power to approximately 55 percent for a wildfire on July 4 through 6, 2025, and to approximately 46 percent for a wildfire on August 4 through 15, 2025.

INSPECTION SCOPES

Inspections were conducted using the appropriate portions of the inspection procedures (IPs) in effect at the beginning of the inspection unless otherwise noted. Currently approved IPs with their attached revision histories are located on the public website at http://www.nrc.gov/reading-rm/doc-collections/insp-manual/inspection-procedure/index.html. Samples were declared complete when the IP requirements most appropriate to the inspection activity were met consistent with Inspection Manual Chapter (IMC) 2515, Light-Water Reactor Inspection Program - Operations Phase. The inspectors performed activities described in IMC 2515, Appendix D, Plant Status, observed risk significant activities, and completed on-site portions of IPs. The inspectors reviewed selected procedures and records, observed activities, and interviewed personnel to assess licensee performance and compliance with Commission rules and regulations, license conditions, site procedures, and standards.

REACTOR SAFETY

71111.01 - Adverse Weather Protection

Seasonal Extreme Weather Sample (IP Section 03.01) (1 Sample)

(1) The inspectors evaluated readiness for seasonal extreme weather conditions prior to the onset of seasonal hot temperatures for the following systems:

diesel generators, component cooling water, and 480 V switchgear ventilation on July 18, 2025

71111.04 - Equipment Alignment

Partial Walkdown Sample (IP Section 03.01) (3 Samples)

The inspectors evaluated system configurations during partial walkdowns of the following systems/trains:

(1) Unit 2 diesel generators 2-1 and 2-2 while diesel generator 2-3 was out of service for planned maintenance on July 16, 2025

(2) Units 1 and 2 offsite power alignment while in a single output breaker configuration on August 8, 2025

(3) Unit 2 auxiliary building ventilation system supply fan S-34 while supply fan S-33 was out of service for planned maintenance on September 16, 2025

71111.05 - Fire Protection

Fire Area Walkdown and Inspection Sample (IP Section 03.01) (3 Samples)

The inspectors evaluated the implementation of the fire protection program by conducting a walkdown and performing a review to verify program compliance, equipment functionality, material condition, and operational readiness of the following fire areas:

(1) Units 1 and 2 turbine building 85-foot elevation 12 kV bus rooms on August 4, 2025

(2) Unit 2 storage of combustible material near the hot shutdown panel on August 25, 2025

(3) Unit 2 turbine building 140-foot elevation on September 26, 2025

Fire Brigade Drill Performance Sample (IP Section 03.02) (1 Sample)

(1) The inspectors evaluated the onsite fire brigade training and performance during an announced fire drill on July 31, 2025.

71111.06 - Flood Protection Measures

Flooding Sample (IP Section 03.01) (1 Sample)

(1) The inspectors evaluated internal flooding mitigation protections in the:

Unit 1 auxiliary building 73-foot elevation on July 9, 2025

71111.11Q - Licensed Operator Requalification Program and Licensed Operator Performance

Licensed Operator Performance in the Actual Plant/Main Control Room (IP Section 03.01) (1 Sample)

(1) The inspectors observed and evaluated licensed operator performance in the control room during Unit 2 downpower to 55 percent due to the Madre fire approaching offsite 500 kV transmission lines on July 4, 2025.

Licensed Operator Requalification Training/Examinations (IP Section 03.02) (1 Sample)

(1) The inspectors observed and evaluated a crew of licensed operators during simulator training on August 6, 2025.

71111.12 - Maintenance Effectiveness

Maintenance Effectiveness (IP Section 03.01) (3 Samples)

The inspectors evaluated the effectiveness of maintenance to ensure the following structures, systems, and components (SSCs) remain capable of performing their intended function:

(1)power distribution panel fuses for 120 Vac on July 8, 2025 (2)35 percent atmospheric steam dumps on July 22, 2025

(3) Unit 2 condensate booster pump sets mechanical seal leakage on August 21, 2025

Quality Control (IP Section 03.02) (1 Sample)

The inspectors evaluated the effectiveness of maintenance and quality control activities to ensure the following SSC remains capable of performing its intended function:

(1) Unit 2 diesel generator 2-2 replacement parts on August 18, 2025

71111.13 - Maintenance Risk Assessments and Emergent Work Control

Risk Assessment and Management Sample (IP Section 03.01) (2 Samples)

The inspectors evaluated the accuracy and completeness of risk assessments for the following planned and emergent work activities to ensure configuration changes and appropriate work controls were addressed:

(1) Green risk configuration requiring three out of six auxiliary feedwater pumps protected due to aggregate risk calculation on July 1, 2025

(2) Green risk configuration while diesel generator 2-2 is out of service for planned maintenance and unplanned isolation of condensate booster pump 2-2 due to a water seal leak on August 19, 2025

71111.15 - Operability Determinations and Functionality Assessments

Operability Determination or Functionality Assessment (IP Section 03.01) (4 Samples)

The inspectors evaluated the licensee's justifications and actions associated with the following operability determinations and functionality assessments:

(1) Unit 1 missed intermediate range detector surveillance after completing maintenance during refueling outage 1R25 on August 12, 2025

(2) Unit 2 diesel generator 2-2 return to service following unexpected shutdowns during testing on September 4, 2025

(3) Unit 2 diesel generator 2-2 jacket water leak following return to service on September 9, 2025

(4) Unit 2 steam dumps in steam pressure mode due to fluctuations in steam dump input signal on September 10, 2025

71111.18 - Plant Modifications

Temporary Modifications and/or Permanent Modifications (IP Section 03.01 and/or 03.02) (1 Sample)

The inspectors evaluated the following temporary or permanent modifications:

(1)10 percent atmospheric steam dump controller logic change on July 21, 2025

71111.24 - Testing and Maintenance of Equipment Important to Risk

The inspectors evaluated the following testing and maintenance activities to verify system operability and/or functionality:

Post-Maintenance Testing (PMT) (IP Section 03.01) (6 Samples)

(1) Unit 2 diesel generator 2-3 following major maintenance outage window on July 22, 2025

(2) Unit 1 safety injection pump 1-2 post-maintenance testing on July 30, 2025

(3) Unit 1 auxiliary saltwater pump 1-2 following pump repacking on August 5, 2025

(4) Unit 2 resetting compensated auctioneered Tavg signal on August 13, 2025

(5) Unit 1 charging pump 1-1 following motor maintenance on September 8, 2025

(6) Unit 2 diesel generator 2-2 post-maintenance testing after mechanical governor following a failure to start during solid state protection system (SSPS) logic testing on September 29, 2025

Surveillance Testing (IP Section 03.01) (2 Samples)

(1) Unit 1 diesel generator 1-1 routine surveillance test on July 2, 2025

(2) Unit 2 main steam safety valve testing on September 30, 2025

Diverse and Flexible Coping Strategies (FLEX) Testing (IP Section 03.02) (1 Sample)

(1) FLEX equipment set 2: annual inspection and test run of FLEX emergency auxiliary feed water pump EAFWP2 on August 13, 2025

71114.02 - Alert and Notification System Testing

Inspection Review (IP Section 02.01-02.04) (1 Sample)

(1) The inspectors evaluated the maintenance and testing of the alert and notification system between September 2, 2023, and August 16, 2025.

71114.03 - Emergency Response Organization Staffing and Augmentation System

Inspection Review (IP Section 02.01-02.02) (1 Sample)

(1) The inspectors evaluated the readiness of the Emergency Response Organization between September 2, 2023, and August 16, 2025. Inspectors also evaluated the licensee's ability to staff the emergency response facilities in accordance with emergency plan commitments.

71114.04 - Emergency Action Level and Emergency Plan Changes

Inspection Review (IP Section 02.01-02.03) (1 Sample)

(1) The inspectors evaluated the following submitted Emergency Plan changes (with associated implementing procedure changes, notified on April 10, 2025):

Diablo Canyon Power Plant Emergency Plan, Section 6, Revision 5.01

Diablo Canyon Power Plant Emergency Plan, Section 7, Revision 5.04

Diablo Canyon Power Plant Emergency Plan, Section 8, Revision 5.00 In addition, the inspectors evaluated the 10 CFR 50.54(q) emergency plan change process and practices between September 2, 2023, and August 16, 2025. The evaluation reviewed screenings and evaluations documenting implementation of the process. The reviews of the change process documentation or the emergency plan changes do not constitute NRC approval.

71114.05 - Maintenance of Emergency Preparedness

Inspection Review (IP Section 02.01 - 02.11) (1 Sample)

(1) The inspectors evaluated the maintenance of the emergency preparedness program between September 2, 2023, and August 16, 2025. The evaluation reviewed evidence of completing various emergency plan commitments, the conduct of drills and exercises, licensee audits and assessments, and the maintenance of equipment important to emergency preparedness.

OTHER ACTIVITIES - BASELINE

71151 - Performance Indicator Verification The inspectors verified licensee performance indicators submittals listed below:

MS08: Heat Removal Systems (IP Section 02.07) (2 Samples)

(1) Unit 1 (July 1, 2024, through June 30, 2025)

(2) Unit 2 (July 1, 2024, through June 30, 2025)

MS09: Residual Heat Removal Systems (IP Section 02.08) (2 Samples)

(1) Unit 1 (July 1, 2024, through June 30, 2025)

(2) Unit 2 (July 1, 2024, through June 30, 2025)

MS10: Cooling Water Support Systems (IP Section 02.09) (2 Samples)

(1) Unit 1 (July 1, 2024, through June 30, 2025)

(2) Unit 2 (July 1, 2024, through June 30, 2025)

EP01: Drill/Exercise Performance (DEP) Sample (IP Section 02.12) (1 Sample)

(1) July 1, 2024, through June 30, 2025 EP02: Emergency Response Organization (ERO) Drill Participation (IP Section 02.13) (1 Sample)

(1) July 1, 2024, through June 30, 2025 EP04: Emergency Response Facility and Equipment Readiness (ERFER) (IP Section 02.14) (1 Sample)

(1) July 1, 2024, through June 30, 2025 71152A - Annual Follow-up Problem Identification and Resolution

Annual Follow-up of Selected Issues (Section 03.03) (2 Samples)

The inspectors reviewed the licensees implementation of its corrective action program related to the following issues:

(1) Unit 2 downpower due to Gifford fire potentially affecting 500 kV transmission lines on August 4, 2025

(2) Unit 2 diesel generator 2-3 stop screw design change review on August 12, 2025

INSPECTION RESULTS

Failure to Perform Preventive Maintenance Replacement Strategy Cornerstone Significance Cross-Cutting Aspect Report Section Initiating Events Green NCV 05000323/2025003-01 Open/Closed

[P.3] -

Resolution 71111.12 The inspectors identified a Green finding and associated non-cited violation of Technical Specification (TS) 5.4.1.a, Procedures, when PG&E failed to establish and implement a preventive maintenance strategy for safety-related fuse FU-2213 after removing the fuse replacement frequency from an already established preventive maintenance strategy.

Specifically, the licensee removed the previously in place 4-year fuse replacement interval from maintenance plan 19782 and failed to establish a new maintenance plan for the fuse.

Subsequently, the fuse failed and tripped all bistables associated with protection set II of the Eagle 21 system.

Description:

The purposes of the Eagle 21 system and SSPS are to implement the reactor protection system (RPS) and engineered safety features actuation system (ESFAS) functions to initiate a reactor trip and safety injection when certain plant parameters exceed their setpoints. There are four protection sets that receive input for actuation. Both RPS and ESFAS will initiate their protection schemes when two of the four protection sets receive the same demand input for actuation.

On July 7, 2025, Unit 2 experienced a complete de-energization of all four racks of protection set II for the Eagle 21 system. This caused a trip input into the Eagle 21 system bistables which called for a reactor trip and safety injection into the reactor vessel. Protection set II was the only set of the four to experience a loss of power and therefore, was the only set to have the bistables trip and a reactor shutdown and safety injection was not initiated.

Plant personnel went to the protection set racks to find the cause of the de-energization. A blown fuse was discovered inside the protection set II rack. This fuse was associated with the power supply for all input from protection set II into Eagle 21. The licensee began an investigation to determine the cause of failure. At the time, routine maintenance work was scheduled to be performed on the protection set II racks. Instrumentation and controls technicians were inside one of the protection set II cabinets, according to their work order, but were not actively performing work when the de-energization occurred. A blower fan, used to keep the cabinet cool during normal operation, was found tripped inside the cabinet. The licensee removed the fan and tested it in a new power slot inside the cabinet, and it functioned properly. The licensee ruled out the technicians performing work and the tripped blower fans as credible causes for the blown fuse in the cabinet. The rest of the investigation did not reveal any new information or probable causes of failure. The blown fuse was attributed to age-related degradation as the most likely failure mechanism.

The fuse was located in safety-related power distribution panel PY22. This panel distributes safety-related power from the 480 V power source to the 120 Vac instrumentation system.

The power distribution panels for Unit 2 were upgraded in 2008, and the fuse was last replaced in 2014. Maintenance plan 19782 governed fuse replacement frequency and included cleaning, inspecting, and cycling the panel breaker on a 4-year periodicity. The fuses were replaced during refueling outages when the associated safety-related bus was due for maintenance. Prior to the panel upgrade in 2008, the fuse replacement was accomplished by using a test jumper to prevent the fuse from producing inputs into Eagle 21 during maintenance. When fuse replacement was scheduled during the refueling outage in 2012, it was discovered that the new upgraded panels made this impossible to accomplish and the panels would need to have power completely removed from them for fuse replacement. This was an undesirable situation for the unit since RPS and ESFAS bistables would be tripped and provide their input to their respective protection schemes. The maintenance was deferred until a procedure was developed that allowed use of a test jumper similar to what was done on the older panels. Maintenance personnel re-evaluated the issue and determined there was no requirement to have a 4-year replacement interval for these fuses. An engineering evaluation was performed to justify extending this interval to 10 years to match the fuse replacement schedule in place for safety-related vital inverters. A task was created to develop the necessary test jumper procedure and to remove the fuse replacement from the previous maintenance plan and create a new maintenance plan for the 10-year fuse replacement. The fuse replacement was removed from maintenance plan 19782 and development began on a new procedure. However, it was determined that use of a test jumper would not be feasible with the new panels and the procedure development ceased.

The fuse replacement was never added back to the old maintenance plan and the new 10-year replacement interval maintenance plan was never developed. During the 2014 refueling outage, the PY22 panel had power removed for maintenance and the fuse was replaced.

There are six safety-related power distribution panels per unit for transferring 480 V power to the 120 Vac instrumentation system. The fuses in each of the Unit 2 panels had all been replaced as of 2016. The fuses in three Unit 1 panels were last replaced during the Unit 1 panel upgrade in 2009. The new 10-year interval maintenance plan would then be due for 2019. In 2018, the engineering evaluation for fuse replacement was reviewed again and a new 15-year interval was concluded to be an acceptable frequency. This changed the next fuse replacement for all panels to be due beyond the dates for permanent shutdown of both units in 2024 and 2025. That same engineering review determined that despite permanent shutdown, the fuse replacement maintenance plans should still be created and remain active until it was known which panel loads would be required after shutdown. This maintenance plan was never created. Both Diablo Canyon units are pursuing license renewal and are currently operating in the period of extended operation with no immediate plans to shut down.

Corrective Actions: The licensee replaced the blown fuse in the rack and completed return to service testing for protection set II on July 8, 2025.

Corrective Action References: SAPNs 51292654, 51292817

Performance Assessment:

Performance Deficiency: The inspectors determined the failure to establish, implement, and maintain written procedures for performing maintenance on safety-related equipment was within the licensees ability to foresee and correct.

Screening: The inspectors determined the performance deficiency was more than minor because it was associated with the Procedure Quality attribute of the Initiating Events cornerstone and adversely affected the cornerstone objective to limit the likelihood of events that upset plant stability and challenge critical safety functions during shutdown as well as power operations. The inspectors determined that the failure to replace the fuse in accordance with an established preventive maintenance strategy caused the trip of Eagle 21 bistables making this issue more than minor and had the potential to result in RPS and ESFAS actuation.

Significance: The inspectors assessed the significance of the finding using IMC 0609 Appendix A, The Significance Determination Process (SDP) for Findings At-Power. The inspectors determined this finding to be of very low safety significance (Green) because the finding did not cause a reactor trip and the loss of mitigation equipment relied upon to transition the plant from the onset of the trip to a stable shutdown condition.

Cross-Cutting Aspect: P.3 - Resolution: The organization takes effective corrective actions to address issues in a timely manner commensurate with their safety significance. Specifically, there were multiple opportunities to establish and implement a maintenance plan between the time it was discovered that a test jumper could not be used for fuse replacement in 2012 and when the fuse failed in 2025.

Enforcement:

Violation: Diablo Canyon TS 5.4.1.a, Procedures, requires in part, written procedures shall be established, implemented, and maintained covering the activities referenced in Regulatory Guide (RG) 1.33, Appendix A, Revision 2, February 1978. RG 1.33, Appendix A, Section 9.a, Procedures for Performing Maintenance, states, in part, maintenance that can affect the performance of safety-related equipment should be properly pre-planned and performed in accordance with written procedures, documented instructions, or drawings appropriate to the circumstances. The licensee established maintenance plan 19782, in part, to meet these regulatory requirements.

Contrary to the above, from January 2018, to July 2025, the licensee failed to establish and implement maintenance instructions for the replacement of safety-related fuses for vital 120 Vac power distribution panels. This resulted in fuse FU-2213 failing due to age-related degradation and tripping all Unit 2 Eagle 21 protection set II bistables unexpectedly.

Enforcement Action: This violation is being treated as a non-cited violation, consistent with Section 2.3.2 of the Enforcement Policy.

Failure to Correct a Condition Adverse to Quality for Diesel Emergency Generator 2-3 Cornerstone Significance Cross-Cutting Aspect Report Section Mitigating Systems Green NCV 05000323/2025003-02 Open/Closed

[P.2] -

Evaluation 71152A The inspectors identified a Green finding and associated non-cited violation of Title 10 of the Code of Federal Regulations (10 CFR) Part 50, Appendix B, Criterion XVI, "Corrective Action," for failure to correct a condition adverse to quality associated with diesel emergency generator (DEG) 2-3. Specifically, the licensee failed to correct a condition where a fuel linkage cotter pin was contacting a mechanical travel limiter causing interference with the diesels response to fuel demands.

Description:

On September 14, 2022, initial attempts to load DEG 2-3 to a full load of 2790 kW were not successful. While taking speed control in the raise direction, load would not rise above 2700 kW. Speed control was taken in the lower direction and a second attempt to raise load was made and DEG 2-3 was able to achieve and maintain the 2790 kW load.

The mechanical governor terminal shaft controls fuel supply to each engine cylinder through a series of mechanical linkages. Several of these linkages are pinned together with cotter pin joints. The last linkage in the series prior to the connection to the control shaft has a mechanical travel limiter in the form of a stop screw. The stop screw is an asset protection device that limits the amount of fuel that the governor may demand. This design feature helps prevent an engine overspeed event as well as an exhaust fire during the engine start sequence when fuel demand is highest. DEG 2-3 is the only diesel at Diablo Canyon that has a stop screw installed. Operations had observed the cotter pin closest to the stop screw was sticking straight out of its hole rather than having the pins ears folded back around the joint as is normal practice. Operations also noted the close proximity of the stop screw to this cotter pin and surmised the abnormal cotter pin configuration may have initially interfered with the stop screw while trying to raise load. An engineering evaluation was performed to evaluate if the mechanical stop screw was correctly positioned and that the cotter pin was not creating an interference. The recommended corrective actions from the evaluation were to bend the cotter pin ears back dog-ear, and to evaluate a design change to remove the stop screw considering the benefits and risks of such a change. The decision was made not to pursue this design change.

In February of 2024, the licensee performed preventative maintenance on DEG 2-3. During the maintenance, personnel routinely manipulate cotter pins on the mechanical governor. This involves straightening the cotter pins for removal and inserting the cotter pins back into the governor linkage. During this maintenance period the inspectors could not find any evidence that maintenance instructions were updated to ensure cotter pins were bent back before final restoration.

On July 21, 2025, following a maintenance outage window, a full load rejection test was being performed on DEG 2-3 when the diesel experienced an overspeed trip. Two cotter pins on the fuel linkage were identified as not properly installed with their dog-ears not folded back over the pin. The dog-ears of the lower pin were able to wedge themselves against the fuel rack stop screw under the right combination of load and vibration. This would cause the fuel control linkage to lock in a fixed position, regardless of fuel demands from the governor actuator. The governor would have been demanding less fuel on a load rejection but with the fuel linkage stuck, the diesel would not have been able to respond to that demand and would trip on overspeed. The possibility of this cotter pin and stop screw interaction influencing diesel operation was a condition adverse to quality and was known about from the previous issue in 2022. However, the station only took action to fix that specific issue at the time and did not adequately address the overall condition adverse to quality. The station had evidence from previous corrective actions taken that bending the cotter pin ears back would eliminate the interaction but did not ensure this action was complete through their procedures following diesel maintenance. Similarly, if the recommendation for a design change to remove the stop screw had been pursued, then the cotter pin would not have anything to interact with, and the fuel linkage would not be able to be locked in position. Ultimately, the 2022 issue was treated as broke-fix for that scenario and more robust corrective actions for the condition adverse to quality were not implemented.

Corrective Actions: The licensee folded back the dog-ears for the affected cotter pins to ensure they were in their proper configuration. A procedure change to ensure the cotter pins are folded back will be issued. The design change recommendation to remove the stop screw will be performed during the next maintenance outage for DEG 2-3.

Corrective Action References: SAPNs 51294542, 51294745

Performance Assessment:

Performance Deficiency: The inspectors determined that PG&E failed to correct a condition adverse to quality associated with interference of DEG 2-3 fuel control linkage based on a similar issue in 2022, in accordance with 10 CFR Part 50, Appendix B, Criterion XVI, Corrective Action.

Screening: The inspectors determined the performance deficiency was more than minor because it was associated with the Equipment Performance attribute of the Mitigating Systems cornerstone and adversely affected the cornerstone objective to ensure the availability, reliability, and capability of systems that respond to initiating events to prevent undesirable consequences. Specifically, the station failed to correct a condition that caused interference with the fuel control linkage for DEG 2-3, and as a result, fuel control linkage became locked in place and the diesel experienced an overspeed trip during return to service testing.

Significance: The inspectors assessed the significance of the finding using IMC 0609 Appendix A, The Significance Determination Process (SDP) for Findings At-Power. The inspectors determined this finding to be of very low safety significance (Green) in accordance with Exhibit 2, because the degraded condition does not represent a loss of the probabilistic risk assessment (PRA) function of one or more non-technical specification trains of equipment designated as risk significant in accordance with the licensees maintenance rule program for greater than 3 days.

Cross-Cutting Aspect: P.2 - Evaluation: The organization thoroughly evaluates issues to ensure that resolutions address causes and extent of conditions commensurate with their safety significance. Specifically, PG&E did not perform an evaluation to understand how the cotter pin and stop screw interaction could impact diesel performance and therefore, did not adequately resolve the cause of the issue.

Enforcement:

Violation: Title 10 CFR Part 50, Appendix B, Criterion XVI, "Corrective Action," states measures shall be established to assure that conditions adverse to quality, such as failures, malfunctions, deficiencies, deviations, defective material and equipment, and nonconformances are promptly identified and corrected.

Contrary to the above, between February 2024 and July 2025, the licensee failed to correct a condition adverse to quality to ensure that DEG 2-3 remained reliable. Specifically, interaction between a cotter pin and stop screw caused interference of the DEG 2-3 fuel control linkage and as a result, the fuel control linkage locked in place and DEG 2-3 tripped on overspeed during post-maintenance testing.

Enforcement Action: This violation is being treated as a non-cited violation, consistent with Section 2.3.2 of the Enforcement Policy.

EXIT MEETINGS AND DEBRIEFS

The inspectors verified no proprietary information was retained or documented in this report.

On September 30, 2025, the inspectors presented the integrated inspection results to Jordan Tyman, Director, Nuclear Risk and Compliance, and other members of the licensee staff.

On December 8, 2025, the inspectors presented the emergency preparedness program inspection results to Paula A. Gerfen, Senior Vice President, Generation and Chief Nuclear Officer, and other members of the licensee staff.

DOCUMENTS REVIEWED

Inspection

Procedure

Type

Designation

Description or Title

Revision or

Date

Corrective Action

Documents

Notifications

220067, 51229341

71111.01

Procedures

AD7.ID20

Summer Peak Period Readiness

107723-9

Auxiliary Building HVAC

Drawings

2110-1

Single Line Diagram 500/230/25/12/4.16KV Systems

OP J-6B:I-A

Diesel Generator 2-1 Alignment Checklist

1A

71111.04

Procedures

OP J-6B:II-A

Diesel Generator 2-2 Alignment Checklist

1A

111805-49

Unit 1, Turbine Building Elev. 85'

111805-60

Unit 2, Turbine Building Elev. 85'

111805-67

Turbine Building Elev. 140'

111906-11

Unit 2, Turbine Building Elev. 85'

Drawings

111906-6

Unit 1, Turbine Building Elev. 85'

Miscellaneous

FP-331A

NFPA 805 Combustible Control Levels

71111.05

Procedures

OM8.ID4

Control of Flammable and Combustible Materials

71111.06

Miscellaneous

DCM T-12

Pipe Break (HELB/MELB), Flooding, and Missiles

71111.11Q

Procedures

OP J-2:VIII

Guidelines for Reliable Transmission Service for DCPP

Corrective Action

Documents

Notifications

50035097, 50059960, 50545465, 50545701, 50672279,

50827110, 50984928, 51045840, 51071157, 51242634,

279641, 51281492, 51285779, 51297581, 51297618,

297743, AR 0428861

Miscellaneous

MP 19782

Maintenance Plan

71111.12

Work Orders

WO 60139124, 60150477, 60162853, 64256604, 64307142

AD7.DC6

On-Line Maintenance Risk Management

AD7.ID14

Assessment of Integrated Risk

B-42

Selecting RMA(s) for Specific RMTS RICT Configurations

71111.13

Procedures

OP1.DC44

Risk Management Actions

71111.15

Corrective Action

Documents

Notifications

50838199, 51297331, 51299444, 51300035

Corrective Action

Documents

Notifications

51134470, 51294145

Miscellaneous

Design Change Package 1000025681

71111.18

Procedures

AD13.ID2

Design Verification Testing

Inspection

Procedure

Type

Designation

Description or Title

Revision or

Date

AD13.ID4

Post-Maintenance Testing

41B

Corrective Action

Documents

Notifications

296963, 51297331

Miscellaneous

Test Procedure

3.10

Long Term Cooling Water Supply from the Raw Water

Reservoir

MP E-21.GOV

Emergency Diesel Generator Electrical Governor

Maintenance

MP M-21.14

Fuel Control Linkage Assembly Routine Minor Maintenance

MP M-21.5

Diesel Engine Water Pump Replacement

MP M-21.8

Diesel Engine Governor Actuator Maintenance

MP M-4.18A

Check of Main Steam Safety Valve Lift Point with the

Tevitest System

OP J-6B:V

Diesel Generators - Manual Operation of DG 2-2

STP M-77

Safety and Relief Valve Testing

43A

STP M-9A1

Diesel Engine Generator 1-1 Routine Surveillance Test

STP M-9A2

Diesel Engine Generator 2-2 Routine Surveillance Test

STP M-9B

Overspeed Trip Test of Diesel Generators

STP M-9D1

Diesel Generator Full Load Rejection Test

STP P-CCP-11

Routine Surveillance Test of Centrifugal Charging Pump 1-1

STP P-SIP-12

Routine Surveillance of Safety Injection Pump 1-2

STP V-3L10B

Exercising Valve SI-8923B, Safety Injection Pump 2 Suction

Valve

7A

Procedures

STP V-3L2B

Exercising Valve SI-1-8821B, Safety Injection Pump

Discharge to RCS Cold Legs

71111.24

Work Orders

WO 60175960, 64277273, 64300110, 64301584, 64308413,

64314201, 64314729

Corrective Action

Documents

Notifications

50592599, 51165000, 51201499, 51269169

Corrective Action

Documents

Resulting from

Inspection

Notifications

298596

Diablo Canyon Power Plant: 2011 EWS Siren Test Report

11/21/2011

71114.02

Miscellaneous

Diablo Canyon Power Plant Performance Indicator

01/02/2025

Inspection

Procedure

Type

Designation

Description or Title

Revision or

Date

Summary, Alert and Notification System (ANS) Reliability -

Early Warning System (EWS) Tests, Report Period:

01/01/2024 to 12/31/2024

Electronic Mail (and Attachment) from

E. Schrader,

NSIR/DPR/POB, to

N. Makris and S. Hedger,

Region IV/DRSS/RCB; Subject: Diablo Canyon ANS Testing

Inspection Concern (ML25322A008)

9/30/2025

Letter from

S. Ziokowski, Acting Chair Regional Assistance

Committee, FEMA Region 9; to

A. Warwick, Nuclear

Emergency Planning, Diablo Canyon Power Plant, RE:

Response to PG&E Letter DCL-24-109, Dated December 4,

24

2/23/2024

Pacific Gas & Electric Company, Diablo Canyon Power Plant

(DCPP), Alert and Notification System (ANS) Design Report

Early Warning System (EWS)

6, 7

DCL-24-099

Docket No. 50-275, OL-DPR-80; Docket No. 50-323, OL-

DPR-82; Diablo Canyon Units 1 and 2; Alert and Notification

System Design Report, Revision 7

10/15/2024

DCL-24-109

Docket No. 50-275, OL-DPR-80; Docket No. 50-323, OL-

DPR-82; Diablo Canyon Units 1 and 2; Alert and Notification

System Siren 0820 Request to Relocate

2/04/2024

EP MT-43,

Annual Full-Scale Audibility Test Results

08/24/2024

Report Number

403.34-18-3

DCPP EWS 2018 Acoustic Test Report

11/02/2018

Report Number

403.34-23.1

DCPP EWS 2023 Acoustic Test Report

11/22/2023

Report Number:

2.34-13.10

2013 DCPP Early Warning System Acoustic Test, August

24, 2013

09/27/2013

Tracking Number:

25-01

50.54(q) Screening Evaluation Form - Movement of EWS

Siren 0820 with FEMA Review

01/14/2025

Tracking Number:

25-22

Diablo Canyon Power Plant Alert and Notification System

Design Report, Revision 7 (FEMA Acceptance)

07/17/2025

Procedures

EP MT-43

Early Warning System Testing and Maintenance

Inspection

Procedure

Type

Designation

Description or Title

Revision or

Date

Corrective Action

Documents

Notifications

226157, 51248230, 51262272, 51262382, 51268360,

270171