ML24199A199

MD 8.3 Evaluation for D.C. Cook Unit 2 2AB Emergency Diesel Generator Frequency Issue on 5/21/24

ML24199A199
Person / Time
Site:	Cook
Issue date:	05/30/2024
From:	Billy Dickson NRC/RGN-III/DORS/ERPB
To:
Shared Package
ML24165A108	List: ML24199A199
References
Download: ML24199A199 (1)

Text

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

PLANT:

D.C. Cook EVENT DATE:

5/21/2024 DETERMINISTIC CRITERIA EVALUATION DATE:

5/30/2024 During a planned surveillance run on May 21, 2024, Emergency Diesel Generator (EDG) 2AB failed to maintain its frequency. When the frequency indication was erratic the licensee was able to take manual control locally at the EDG and used the governor control to raise the frequency to 60 Hz, where it remained. This is a step in the procedures for operating the EDGs.

The licensee determined that the cause of the failure to be corrosion on a relay that interfered with the setpoint signal for frequency. The relay was subsequently cleaned and placed back in service.

The 2AB EDG was successfully run twice during post-maintenance testing.

The inspectors verified that the 2AB EDG was within its planned preventative maintenance (PM) frequency of 12 refueling cycles (18 years). That PM was last performed 13 years ago.

Over the past 2 years the licensee experienced issues with failures of the 2CD EDG caused by malfunctioning digital reference units (DRU). These DRU issues were ultimately attributed to a manufacturing issue (10 CFR 21 ADAMS# ML23312A231). The DRU issues were inspected and determined to be a manufacturing issue outside of the licensees ability to foresee and correct.

Details of the previous issues are below:

During the refueling outage of October 2022, the 2CD EDG failed a test due to unacceptable frequency drop. The EDG was not required to be operable at that point. The licensee identified the cause as a malfunctioning digital reference unit (DRU) and proceeded to replace it.

Subsequently, the licensee sent the defective DRU to the vendor; however, the vendor was unable to reproduce the issue. Consequently, the vendor returned the DRU without refurbishing it. As a precautionary measure, the licensee redirected the DRU to its training department rather than returning it to service.

While at power, on August 10, 2023, the 2CD EDG failed a slow-speed start monthly surveillance due to another unacceptable frequency drop. Again, the licensee determined the cause of the issue was a degraded DRU. The licensee promptly replaced the DRU and declared the EDG operable. This time, the vendor discovered a cracked solder connection within the DRU. Based on this information, they re-examined the DRU that failed in October 2022 and a refurbished DRU in on-site storage. The outcome was the discovery of similar solder connection cracks in both DRUs.

To provide context, each reactor unit is equipped with two redundant EDGs, resulting in a total of four EDGs. All of the in-service DRUs had been inspected and there is no immediate concern related to the previously discussed DRU condition (cracked solder connections). With the discontinuation of DRU manufacturing, the licensee turns to refurbished units to address this issue. While the licensee has not confirmed the use of refurbished DRUs by other licensees, they remain unaware of alternative sources for acquiring DRUs.

2 Y/N DETERMINISTIC CRITERIA N

1. Involved operations that exceeded, or were not included in, the design bases of the facility Remarks: No operations that exceeded or were not included in the design basis.

N

2. Involved a major deficiency in design, construction, or operation having potential generic safety implications Remarks: No major deficiencies having generic safety implications.

N

3. Led to a significant loss of integrity of the fuel, primary coolant pressure boundary, or primary containment boundary of a nuclear reactor Remarks: No known loss of integrity to any fuel barrier.

N

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

N

5. Involved possible adverse generic implications Remarks: No generic implications.

N

6. Involved significant unexpected system interactions Remarks: No significant unexpected system interactions.

Y

7. Involved repetitive failures or events involving safety-related equipment or deficiencies in operations Remarks: As discussed in the description on page 1 the recent failure of 2AB EDG is the fourth failure of an EDG related to a frequency issue at DC Cook. The previous failures were on the 2CD EDG and were attributed to a DRU manufacturing issue. Initial review of the current issue does not appear to be directly related to DRU issues and instead appears to be attributed to maintenance periodicity. However, this is a repetitive failure involving safety-related equipment (EDGs) at DC Cook.

The current failure also appears to originate from a different part of the speed circuit than the prior failures associated with the DRUs.

N

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

The EDG operating procedures contained steps for using the governor control to raise the frequency to 60 Hz. The operators took appropriate actions to adjust and maintain the frequency.

3 CONDITIONAL RISK ASSESSMENT RISK ANALYSIS BY: David Werkheiser DATE: 5/30/2024 Brief Description of the Basis for the Assessment (may include assumptions, calculations, references, peer review, or comparison with licensees results):

A regional senior reactor analyst (SRA), using SAPHIRE 8 Version 8.2.10, and the Donald C. Cook Unit 1 & 2 SPAR Model Version 8.82 ran an Event Condition Assessment for the degraded 2AB EDG frequency control condition. The SRA used the 1AB EDG failure to start (FTS) basic event as a surrogate since the SPAR model is combined, multi-unit, model. FTS is used since the EDG was neither electrically loaded nor reached a steady state operating condition. The exposure period is estimated to be 30 days, which corresponds to the last successful test of the EDG. The supplemental diesels were available for use. Nominal test and maintenance model was used.

A bounding CCDP estimate is represented by setting the EPS-DGN-FS-1AB equal to TRUE.

This fails the 1AB EDG and invokes a potential common cause failure (CCF) to start of the other train EDG. The estimated conditional core damage probability (CCDP) and incremental CCDP are: 2.5E-6/year and 5E-7/year, respectively. The dominant accident sequence is Loss of Offsite Power (weather), CCF of the EDGs, failure to restore Offsite Power or Emergency on-site power.

Based on discussions with inspectors, the SRA assessed that the degraded condition (frequency oscillations) was potentially recoverable for this event. This was successfully demonstrated by the licensee staff by taking local manual control and stabilizing EDG speed/frequency before securing the EDG using readily observable cues and procedural guidance. There is uncertainty if this operator action would be successful if the EDG was loaded. Using SPAR-H, a human error probability (HEP) of 0.2 was estimated (1 in 5 chance of failure) for this action.

A best estimate was run by substituting the 1AB EDG failure probability to the recovery HEP (0.2). In addition, to account for potential latent issues / common cause aspects, all EDG start, run, and CCF event probabilities were doubled. The estimated conditional core damage probability (CCDP) and incremental CCDP are: 2E-6/year and 1E-7/year, respectively. The dominant accident sequence is Loss of Offsite Power (weather), CCF of the EDGs, failure to restore Offsite Power or Emergency on-site power.

Sensitivity cases showed the best estimate CCDP is insensitive to the HEP, but ICCDP was sensitive. Both cases were insensitive to FLEX credit.

This assessment did not account for contribution by external events, which would only increase the risk. Fire risk would be considered a non-trivial contributor.

The estimated conditional core damage probability (CCDP) is 2 to 2.5E-6/yr and places the risk in the range of no additional follow-up and special inspection.

4 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 recommendation is not to perform a reactive inspection at this time. When the erratic frequency indication occurred, the licensee was able to take manual control locally at the EDG and used the governor control to raise the frequency to 60 Hz, where it remained. The frequency issue appears to be different from the DRU issues that occurred during the previous 2 years. The DRU issue was inspected and determined to be a manufacturing issue outside of the licensees ability to foresee and correct. The current issue points to a maintenance practice.

The recommendation is to follow up the issue with a focused baseline inspection and gain a better understanding of the licensees maintenance practices.

BRANCH CHIEF: Dariusz Szwarc /RA/

DATE: 06/13/2024 SRA: David Werkheiser /RA/

DATE: 06/14/2024 DIVISION DIRECTOR: Billy Dickson /RA/

DATE: 07/17/2024 DIVISION DIRECTOR:

DATE:

RA (if reactive inspection is initiated)

DATE:

ADAMS ACCESSION NUMBER: ML24199A199 ADAMS PACKAGE ACCESSION NUMBER: ML24165A108 EVENT NOTIFICATION REPORT NUMBER (as applicable): N/A Internal Distribution List is at the end of this document.

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

PLANT:

EVENT DATE:

EVALUATION DATE:

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

REACTOR SAFETY Y/N IIT Deterministic Criteria N

1. Led to a Site Area Emergency Remarks: No emergency declaration was made.

N

2. Exceeded a safety limit of the licensee's technical specifications Remarks: No safety limits were exceeded.

N

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 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.

6 RADIATION SAFETY Y/N IIT Deterministic Criteria N

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 Remarks: No radiological release was involved.

N

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)

Remarks: No occupational or public exposure was involved.

N

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 Remarks: No deliberate misuse and no exposure.

N

4. Involved byproduct, source, or special nuclear material, which may have resulted in a fatality Remarks: Did not involve nuclear material which may have resulted in a fatality.

N

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 Remarks: No sufficiently complex circumstances warranting Commission interest.

Y/N AIT Deterministic Criteria N

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)

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

7 N

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 Remarks: No deliberate misuse of materials was involved.

N

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 Remarks: No failure of radioactive material packaging was involved.

N

9. Involved the failure of the dam for mill tailings with substantial release of tailings material and solution off site Remarks: No failure of a dam for mill tailings was involved.

Y/N SI Deterministic Criteria N

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

N

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)

Remarks: No unplanned occupation exposure was involved.

N

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

N

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

8 N

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

N

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

N

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 Remarks: No failure of radioactive materials packaging was involved.

N

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

SAFEGUARDS/SECURITY Y/N IIT Deterministic Criteria N

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 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).

9 Remarks: No safeguards initiated event was involved.

N

3. Actual intrusion into the protected area Remarks: No intrusion into the protected area.

Y/N AIT Deterministic Criteria N

4. Involved a significant infraction or repeated instances of safeguards infractions that demonstrate the ineffectiveness of facility security provisions Remarks: No infractions demonstrating ineffectiveness of facility security provisions were involved N

5. Involved repeated instances of inadequate nuclear material control and accounting provisions to protect against theft or diversions of nuclear material Remarks: No instances of inadequate nuclear material control and accounting provisions were involved.

N

6. Confirmed tampering event involving significant safety or security equipment Remarks: No tampering was involved.

N

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

Y/N SI Deterministic Criteria N

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)

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

10 Remarks: No safeguards infractions were involved.

N

10. Confirmation of lost or stolen weapon Remarks: No weapons were lost or stolen.

N

11. Unauthorized, actual non-accidental discharge of a weapon within the protected area Remarks: No discharge of a weapon occurred.

N

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

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

N

13. Failure to the licensees package/personnel search procedures which results in contraband or an unauthorized individual being introduced into the protected area Remarks: No failures of package/personnel search procedures were involved.

N

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 Remarks: No tampering or vandalism was involved.

11 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: Dariusz Szwarc /RA/

DATE: 06/13/2024 SRA: David Werkheiser /RA/

DATE: 06/14/2024 DIVISION DIRECTOR: Billy Dickson /RA/

DATE: 07/17/2024 DIVISION DIRECTOR:

DATE:

ADAMS ACCESSION NUMBER: ML24199A199 ADAMS PACKAGE ACCESSION NUMBER: ML24165A108 EVENT NOTIFICATION REPORT NUMBER (as applicable): N/A Distribution: Michelle.Simmons@nrc.gov; Scott.Morris@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; LaDonna.Suggs@nrc.gov; Laura.Pearson@nrc.gov; Jason.Kozal@nrc.gov; Billy.Dickson@nrc.gov; David.Curtis@nrc.gov; Jonathan.Feibus@nrc.gov; Geoffrey.Miller@nrc.gov; Michael.Hay@nrc.gov; Thomas.Briley@nrc.gov; Doris.Chyu@nrc.gov; Joshua.Havertape@nrc.gov; Dariusz.Szwarc@nrc.gov; Thomas.Hartman@nrc.gov; NRR_Reactive_Inspection.Resource@nrc.gov