ML25192A063

MD 8.3 Evaluation for DC Cook, 2025 Emergency Diesel Generator Thermal Overload Failures

ML25192A063
Person / Time
Site:	Cook
Issue date:	07/07/2025
From:	Nestor Feliz-Adorno NRC/RGN-III/DORS/ERPB
To:
Shared Package
ML25192A048	List: IR 05000315/2025003 ML25192A063 ML25363A015
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Download: ML25192A063 (0)

Text

MD 8.3 Evaluation Decision Documentation for Reactive Inspection (Deterministic and Risk Criteria Analyzed)

PLANT:

D.C. Cook EVENT DATE:

7/2/2025 DETERMINISTIC CRITERIA EVALUATION DATE:

7/7/2025 On April 22, 2025, at 2142, Unit 2 AB Emergency Diesel Generator (EDG) failed its slow speed start surveillance (Technical Specification Surveillance Requirement 3.8.1.2). The EDG was tripped due to no voltage or frequency indication being observed, even after an attempt was made to flash the field locally. The 2AB EDG was declared inoperable and unavailable, placing Unit 2 in a 14-day Limiting Condition for Operation (LCO).

The licensees investigation determined that the thermal overload (TOL) for the automatic voltage regulator (AVR) had tripped. Forensic testing of the TOL identified physical degradation of the trip mechanism. The Unit 2 AB EDG AVR (2-DGAB-VRCKT) is on a 5-refueling outage preventative maintenance (PM) interval, revised in 2022 from an 8-year online interval. The AVR was last replaced in U2C22 (April 2015), with PM originally scheduled for U2C27 (October 2022). A 25 percent grace period extended this due date to U2C28 (April 2024). However, an issue with the replacement AVR during U2C28 led the station to reinstall the original AVR, and the licensee incorrectly credited this work order as completing the PM. A Preventive Maintenance Change Request (PMCR) subsequently reset the due date to U2C29 (October 2025), extending beyond the grace period without a required risk assessment for exceeding the drop-dead date per procedure.

An IFRB determined on June 12, 2025, that the PM oversight represented a performance deficiency but was not the proximate cause of the degraded condition because the defect was unforeseeable. Specifically:

(Mainly) Had the licensee followed its PM deferral process, the defective AVR would still have remained in service, since the defect was not foreseeable.

Had the AVR been replaced per the PM schedule, it would have undergone vendor refurbishment; however, the defective subcomponent (the TOL) was not within the scope of the refurbishment, so the defect would have remained undetected and uncorrected.

The licensees vendor identified two manufacturing defects in the TOL: (1) a heater coil directly contacting the bimetal strip that senses overcurrent (determined to be the primary failure mechanism in the 2AB case), and (2) misrouting of the braided wire connecting the control contacts. Importantly, the vendor determined that the braided wire defect does not cause EDG failures.

Corrective actions included replacing the degraded TOL, bench testing the replacement, and restoring the EDG to operable status on April 24, 2025. The licensee also conducted an extent of condition of the other EDGs.

On June 25, 2025, as part of the extent of condition, x-ray imaging of the 1CD TOL revealed the braided wire defect but not the heater coil contact issue. As a precaution, the licensee replaced this TOL with a unit that passed x-ray inspection and subsequent post-maintenance testing.

2 On July 2, 2025, at 2208, Unit 1 CD EDG failed a slow speed start surveillance with a similar symptomno voltage or frequency indication, even after an attempt was made to flash the field locally. The 1CD EDG was declared inoperable and unavailable, entering a 72-hour LCO (with potential extension to 14 days).

The licensees FIP team identified a tripped TOL on the AVR. Unlike the 2AB failure, mechanical agitation of the TOL during troubleshooting did not replicate the trip. The initial evaluation determined the 1CD TOL failed due to a cardboard cover resting on the TOLs reset button. After becoming loose, the cover exerted a pulling load on the button (rather than pushing it), causing the button to open electrical contacts within the TOL. An extent-of-condition review revealed similar covers installed on two other EDGs (2AB and 2CD). These covers were subsequently removed from all EDGs that had them (1CD, 2AB, and 2CD).

Y/N DETERMINISTIC CRITERIA

1. Involved operations that exceeded, or were not included in, the design bases of the facility N

Remarks: No operations that exceeded or were not included in the design basis.

2. Involved a major deficiency in design, construction, or operation having potential generic safety implications N

Remarks: No major deficiencies having generic safety implications.

3. Led to a significant loss of integrity of the fuel, primary coolant pressure boundary, or primary containment boundary of a nuclear reactor N

Remarks: No known loss of integrity to any fuel barrier.

4. Led to the loss of a safety function or multiple failures in systems used to mitigate an actual event N

Remarks: No loss of safety function or multiple failures in systems used to mitigate an actual event occurred.

5. Involved possible adverse generic implications N

Remarks: No generic implications.

6. Involved significant unexpected system interactions N

Remarks: No significant unexpected system interactions.

7. Involved repetitive failures or events involving safety-related equipment or deficiencies in operations Y

Remarks: As discussed in the description on page 1, the recent failure of the 1CD EDG is the second failure of an EDG attributed to AVR TOLs in the past few months. The previous failure occurred on the 2AB EDG. The licensee attributed the first failure to a degraded coil within the TOL and the second failure to a loose cardboard cover pulling

3 on the TOLs reset button. Both failures displayed similar symptoms: no voltage or frequency indication, even after an attempt was made to flash the field locally.

Cooks EDG reliability has been a recent concern and was the subject of a Special Inspection in 2024 following four repeated failures of a different component, involving refurbishment of that obsolete component (see ML24214A330 and Inspection Report 05000315/2024050 and 05000316/2024050).

The similar symptoms associated with the same component type, the lack of definitive evidence that the degraded coil caused the first failure, the presence of the cardboard cover on both EDGs, the unexpected potential opening of electrical contacts from a slight pulling pressure on the TOLs reset button, and a recent history of EDG failures involving refurbishment of obsolete components raise questions about the licensees maintenance practices and problem identification and resolution efforts. Additionally, the repeated EDG failures raise concerns about the overall reliability of the EDGs.

8. Involved questions or concerns pertaining to licensee operational performance N

Remarks: There were no questions or concerns pertaining to operational performance.

4 CONDITIONAL RISK ASSESSMENT RISK ANALYSIS BY: Lionel Rodriguez / Josh Havertape DATE: 7/10/25 Brief Description of the Basis for the Assessment (may include assumptions, calculations, references, peer review, or comparison with licensees results):

A regional SRA using Saphire 8, Version 8.2.12, and the D.C. Cook SPAR model, Version 8.83, completed a condition assessment for the initial failure of the 2AB EDG on April 22, 2025. The analyst determined the EDG was not capable of performing its probabilistic risk assessment (PRA) function because it failed its surveillance test and was not capable of supplying voltage. Since the EDG was not electrically loaded and did not reach a steady-state operating condition, it was modeled as a Failure to Start. This was done by setting the EPS-DGN-FS-1AB basic event to TRUE in the model, which fails the EDG and adjusts the common cause failure probability of the other EDGs to start as a result the failure. The 1AB EDG was used as a surrogate for the 2AB EDG since the SPAR model is a combined, multi-unit, model. This was required to correctly reflect the risk impact on Unit 2, which was the most affected unit. The analyst assumed the exposure time was 31 days, from the last successful EDG start on March 25, 2025, until it was restored to operable on April 24, 2025.

FLEX was credited in the assessment. The nominal test and maintenance model was used since the plant-specific configuration during the exposure time was not readily available to the analyst. No additional recovery credit was provided because when the failure occurred, operators attempted to locally flash the EDG field and were not successful. This resulted in an estimated conditional core damage probability (CCDP) of 2.60E-6 and an incremental CCDP (ICCDP) of 6.17E-7 for internal events.

In addition, the licensee had previously provided fire risk results for the 2AB EDG failure. The ICCDP due to fire risk was 1.20E-6 for the exposure period. Therefore, the combined ICCDP due to internal events and fire was 1.82E-6.

The analyst also performed a similar condition assessment for the failure of the 1CD EDG on July 2, 2025. The risk of that failure was lower given the exposure time was less (9 days versus 31 days).

The analyst then performed sensitivity analyses by failing the opposite units EDG for the given exposure times, and then adding the risk results, to account for the accumulated risk on a per unit basis. The CCDP and ICCDP results were in the same order of magnitude as those reported above for just the 2AB EDG failure and did not cause the decision threshold to be crossed.

Its important to note that for this analysis, the ICCDP value is a better representation of the risk significance since its a condition assessment for a degraded plant condition. It would be appropriate to compare the ICCDP value against the recommended event response thresholds of IMC 0309 for decision-making (as stated in previous versions of IMC 0309).

The estimated CCDP is 2.60E-6 for internal events. The estimated ICCDP is 1.82E-6 including internal events and fire. This places the risk in the range of no additional follow-up and special inspection.

5 RESPONSE DECISION USING THE ABOVE INFORMATION AND OTHER KEY ELEMENTS OF CONSIDERATION AS APPROPRIATE, DOCUMENT THE RESPONSE DECISION TO THE EVENT OR CONDITION, AND THE BASIS FOR THAT DECISION DECISION AND DETAILS OF THE BASIS FOR THE DECISION:

The risk assessment for these events places the estimated risk in the overlap region between no additional follow-up and a special inspection. A focused baseline inspection will be conducted to assess the adequacy of the licensees causal evaluation, extent-of-condition, and corrective actions for the TOL failures. However, the inspection team will continuously evaluate whether escalation to an SIT is warranted based on the identification of broader programmatic issues or significant gaps in the licensees problem identification and resolution efforts.

BRANCH CHIEF: Néstor Féliz Adorno DATE: 07/11/2025 SRA: Lionel Rodriguez DATE: 07/11/2025 DIVISION DIRECTOR: Jason Kozal DATE: 07/14/2025 DIVISION DIRECTOR:

DATE:

RA: MShuaibi for JGiessner DATE: 07/14/2025 ADAMS ACCESSION NUMBER: ML25192A063 ADAMS PACKAGE ACCESSION NUMBER: ML25192A048 EVENT NOTIFICATION REPORT NUMBER (as applicable): N/A Internal Distribution List is at the end of this document.

Gilliam, Jasmine signing on behalf of Feliz-Adorno, Nestor on 07/11/25 Signed by Rodriguez, Lionel on 07/11/25 Signed by Kozal, Jason on 07/14/25 Shuaibi, Mohammed signing on behalf of Giessner, Jack on 07/14/25

6 Decision Documentation for Reactive Inspection (Deterministic-only Criteria Analyzed)

PLANT:

D.C. Cook EVENT DATE:

7/2/2025 EVALUATION DATE:

7/7/2025 Brief Description of the Significant Event or Degraded Condition: Refer to Page 1.

REACTOR SAFETY Y/N IIT Deterministic Criteria 1.

Led to a Site Area Emergency N

Remarks: No emergency declaration was made.

2.

Exceeded a safety limit of the licensee's technical specifications N

Remarks: No safety limits were exceeded.

3.

Involved circumstances sufficiently complex, unique, or not well enough understood, or involved safeguards concerns, or involved characteristics the investigation of which would best serve the needs and interests of the Commission N

Remarks: The issue occurred during a planned surveillance and was neither complex nor unique.

Y/N SI Deterministic Criteria N

4.

Significant failure to implement the emergency preparedness program during an actual event, including the failure to classify, notify, or augment onsite personnel Remarks: Licensee did not meet the criteria to declare an event.

N 5.

Involved significant deficiencies in operational performance which resulted in degrading, challenging, or disabling a safety system function or resulted in placing the plant in an unanalyzed condition for which available risk assessment methods do not provide an adequate or reasonable estimate of risk.

Remarks: Operators responded as expected.

7 RADIATION SAFETY Y/N IIT Deterministic Criteria 1.

Led to a significant radiological release (levels of radiation or concentrations of radioactive material in excess of 10 times any applicable limit in the license or 10 times the concentrations specified in 10 CFR Part 20, Appendix B, Table 2, when averaged over a year) of byproduct, source, or special nuclear material to unrestricted areas N

Remarks: No radiological release was involved.

2.

Led to a significant occupational exposure or significant exposure to a member of the public. In both cases, significant is defined as five times the applicable regulatory limit (except for shallow-dose equivalent to the skin or extremities from discrete radioactive particles)

N Remarks: No occupational or public exposure was involved.

3.

Involved the deliberate misuse of byproduct, source, or special nuclear material from its intended or authorized use, which resulted in the exposure of a significant number of individuals N

Remarks: No deliberate misuse and no exposure.

4.

Involved byproduct, source, or special nuclear material, which may have resulted in a fatality N

Remarks: Did not involve nuclear material which may have resulted in a fatality.

5.

Involved circumstances sufficiently complex, unique, or not well enough understood, or involved safeguards concerns, or involved characteristics the investigation of which would best serve the needs and interests of the Commission N

Remarks: No sufficiently complex circumstances warranting Commission interest.

Y/N AIT Deterministic Criteria 6.

Led to a radiological release of byproduct, source, or special nuclear material to unrestricted areas that resulted in occupational exposure or exposure to a member of the public in excess of the applicable regulatory limit (except for shallow-dose equivalent to the skin or extremities from discrete radioactive particles)

N Remarks: No radiological release of nuclear material exceeding regulatory limits.

8 7.

Involved the deliberate misuse of byproduct, source, or special nuclear material from its intended or authorized use and had the potential to cause an exposure of greater than 5 rem to an individual or 500 mrem to an embryo or fetus N

Remarks: No deliberate misuse of materials was involved.

8.

Involved the failure of radioactive material packaging that resulted in external radiation levels exceeding 10 rads/hr or contamination of the packaging exceeding 1000 times the applicable limits specified in 10 CFR 71.87 N

Remarks: No failure of radioactive material packaging was involved.

9.

Involved the failure of the dam for mill tailings with substantial release of tailings material and solution off site N

Remarks: No failure of a dam for mill tailings was involved.

Y/N SI Deterministic Criteria

10. May have led to an exposure in excess of the applicable regulatory limits, other than via the radiological release of byproduct, source, or special nuclear material to the unrestricted area; specifically occupational exposure in excess of the regulatory limits in 10 CFR 20.1201 exposure to an embryo/fetus in excess of the regulatory limits in 10 CFR 20.1208 exposure to a member of the public in excess of the regulatory limits in 10 CFR 20.1301 N

Remarks: No exposure in excess of the applicable regulatory limits was involved.

11. May have led to an unplanned occupational exposure in excess of 40 percent of the applicable regulatory limit (excluding shallow-dose equivalent to the skin or extremities from discrete radioactive particles)

N Remarks: No unplanned occupation exposure was involved.

12. Led to unplanned changes in restricted area dose rates in excess of 20 rem per hour in an area where personnel were present or which is accessible to personnel N

Remarks: No unplanned changes in restricted area dose rates were involved.

13. Led to unplanned changes in restricted area airborne radioactivity levels in excess of 500 DAC in an area where personnel were present or which is accessible to personnel and where the airborne radioactivity level was not promptly recognized and/or appropriate actions were not taken in a timely manner N

Remarks: No unplanned changes in airborne radioactivity levels were involved.

9

14. Led to an uncontrolled, unplanned, or abnormal release of radioactive material to the unrestricted area for which the extent of the offsite contamination is unknown; or, that may have resulted in a dose to a member of the public from loss of radioactive material control in excess of 25 mrem (10 CFR 20.1301(e)); or, that may have resulted in an exposure to a member of the public from effluents in excess of the ALARA guidelines contained in Appendix I to 10 CFR Part 50 N

Remarks: No uncontrolled, unplanned, or abnormal releases of radioactive material to the unrestricted area were involved.

15. Led to a large (typically greater than 100,000 gallons), unplanned release of radioactive liquid inside the restricted area that has the potential for ground-water, or offsite, contamination N

Remarks: No large, unplanned release of radioactive liquid inside the restricted area was involved.

16. Involved the failure of radioactive material packaging that resulted in external radiation levels exceeding 5 times the accessible area dose rate limits specified in 10 CFR Part 71, or 50 times the contamination limits specified in 49 CFR Part 173 N

Remarks: No failure of radioactive materials packaging was involved.

17. Involved an emergency or non-emergency event or situation, related to the health and safety of the public or on-site personnel or protection of the environment, for which a 10 CFR 50.72 report has been submitted that is expected to cause significant, heightened public or government concern N

Remarks: There was no 10 CFR 50.72 report submitted nor expected for this event.

SAFEGUARDS/SECURITY Y/N IIT Deterministic Criteria 1.

Involved circumstances sufficiently complex, unique, or not well enough understood, or involved safeguards concerns, or involved characteristics the investigation of which would best serve the needs and interests of the Commission N

Remarks: The circumstances surrounding the event did not involve safeguards.

N 2.

Failure of licensee significant safety equipment or adverse impact on licensee operations as a result of a safeguards initiated event (e.g., tampering).

10 Remarks: No safeguards initiated event was involved.

3.

Actual intrusion into the protected area N

Remarks: No intrusion into the protected area.

Y/N AIT Deterministic Criteria 4.

Involved a significant infraction or repeated instances of safeguards infractions that demonstrate the ineffectiveness of facility security provisions N

Remarks: No infractions demonstrating ineffectiveness of facility security provisions were involved.

5.

Involved repeated instances of inadequate nuclear material control and accounting provisions to protect against theft or diversions of nuclear material N

Remarks: No instances of inadequate nuclear material control and accounting provisions were involved.

6.

Confirmed tampering event involving significant safety or security equipment N

Remarks: No tampering was involved.

7.

Substantial failure in the licensees intrusion detection or package/personnel search procedures which results in a significant vulnerability or compromise of plant safety or security N

Remarks: No failures in the licensees intrusion detection or package/personnel search procedures were involved.

Y/N SI Deterministic Criteria 8.

Involved inadequate nuclear material control and accounting provisions to protect against theft or diversion, as evidenced by inability to locate an item containing special nuclear material (such as an irradiated rod, rod piece, pellet, or instrument)

N Remarks: No inadequate nuclear material control and accounting provisions were involved.

N 9.

Involved a significant safeguards infraction that demonstrates the ineffectiveness of facility security provisions

11 Remarks: No safeguards infractions were involved.

10. Confirmation of lost or stolen weapon N

Remarks: No weapons were lost or stolen.

11. Unauthorized, actual non-accidental discharge of a weapon within the protected area N

Remarks: No discharge of a weapon occurred.

12. Substantial failure of the intrusion detection system (not weather related)

N Remarks: There were not failures of the intrusion detection system.

13. Failure to the licensees package/personnel search procedures which results in contraband or an unauthorized individual being introduced into the protected area N

Remarks: No failures of package/personnel search procedures were involved.

14. Potential tampering or vandalism event involving significant safety or security equipment where questions remain regarding licensee performance/response or a need exists to independently assess the licensees conclusion that tampering or vandalism was not a factor in the condition(s) identified N

Remarks: No tampering or vandalism was involved.

12 RESPONSE DECISION USING THE ABOVE INFORMATION AND OTHER KEY ELEMENTS OF CONSIDERATION AS APPROPRIATE, DOCUMENT THE RESPONSE DECISION TO THE EVENT OR CONDITION, AND THE BASIS FOR THAT DECISION.

DECISION AND DETAILS OF THE BASIS FOR THE DECISION:

BRANCH CHIEF: Néstor Féliz Adorno DATE: 07/11/2025 SRA: Lionel Rodriguez DATE: 07/11/2025 DIVISION DIRECTOR: Jason Kozal DATE: 07/14/2025 RA: RA: MShuaibi for JGiessner:

DATE: 07/14/2025 ADAMS ACCESSION NUMBER: ML25192A063 ADAMS PACKAGE ACCESSION NUMBER: ML25192A048 EVENT NOTIFICATION REPORT NUMBER (as applicable): N/A Distribution: Timothy.Steadham@nrc.gov; Sabrina.Atack@nrc.gov; Jason.Carneal@nrc.gov; John.Giessner@nrc.gov; Mohammed.Shuaibi@nrc.gov; Blake.Welling@nrc.gov; Ray.McKinley@nrc.gov; Mark.Franke@nrc.gov; Gregory.Suber@nrc.gov; Laura.Pearson@nrc.gov; LaDonna.Suggs@nrc.gov; Ravi.Penmetsa@nrc.gov; Jason.Kozal@nrc.gov; Billy.Dickson@nrc.gov; Jared.Heck@nrc.gov; Geoffrey.Miller@nrc.gov; Nick.Taylor@nrc.gov; Karla.Stoedter@nrc.gov; Lindsay.Merker@nrc.gov; Doris.Chyu@nrc.gov; Joshua.Havertape@nrc.gov; Lionel.Rodriguez@nrc.gov; Lundy.Pressley@nrc.gov; Nestor.Feliz-Adorno@nrc.gov; NRR_Reactive_Inspection.Resource@nrc.gov Gilliam, Jasmine signing on behalf of Feliz-Adorno, Nestor on 07/11/25 Signed by Rodriguez, Lionel on 07/11/25 Signed by Kozal, Jason on 07/14/25 Shuaibi, Mohammed signing on behalf of Giessner, Jack on 07/14/25