CNL-25-016, Comments on Preliminary Accident Sequence Precursor Program Analysis
| ML25017A071 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah  |
| Issue date: | 01/16/2025 |
| From: | Marshall T Tennessee Valley Authority |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| CNL-25-016 | |
| Download: ML25017A071 (1) | |
Text
Sequoyah Nuclear Plant, Post Office Box 2000, Soddy Daisy, Tennessee 37384 CNL-25-016 January 16, 2025 10 CFR 50.4 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, D.C. 20555-0001 Sequoyah Nuclear Plant, Units 1 and 2 Renewed Facility Operating License Nos. DPR-77 and DPR-79 NRC Docket Nos. 50-327 and 50-328
Subject:
Tennessee Valley Authority Comments on Preliminary Accident Sequence Precursor Program Analysis
References:
- 1. NRC electronic mail to TVA, ³Transmittal of Preliminary Sequoyah Nuclear Plant, Unit 1, 1B Emergency Diesel Generator Accident Sequence Precursor Report,' dated November 22, 2024
- 2. NRC electronic mail to TVA, ³RE: RE: Transmittal of Preliminary Sequoyah Nuclear Plant, Unit 1, 1B Emergency Diesel Generator Accident Sequence Precursor Report,' dated December 18, 2024 In Reference 1, the U.S. Nuclear Regulatory Commission (NRC) requested comments on the subject preliminary Accident Sequence Precursor (ASP) Program Analysis report and requested that Tennessee Valley Authority (TVA) respond by December 22, 2024. In Reference 2, the NRC granted an extension of this comment period to January 20, 2025.
The Enclosure to this submittal provides TVA¶s comments on the subject preliminary ASP Program Analysis report.
There are no new regulatory commitments in this letter. Please address any questions regarding this submittal to Rick Medina, Manager, Sequoyah Site Licensing, at rmedina4@tva.gov.
Respectfully, Thomas B. Marshall Site Vice President Sequoyah Nuclear Plant
U.S. Nuclear Regulatory Commission CNL-25-016 Page 2 January 16, 2025
Enclosure:
Comments on Preliminary Accident Sequence Precursor Program Analysis Report cc (Enclosure):
NRC Regional Administrator +/- Region II NRC Senior Resident Inspector +/- Sequoyah Nuclear Plant NRC Project Manager +/- Sequoyah Nuclear Plant
Enclosure CNL-25-016 Comments on Preliminary Accident Sequence Precursor Program Analysis Report
Enclosure CNL-25-016 E1 of 2 INTRODUCTION TVA herewith provides comments on the preliminary Accident Sequence Precursor Program Analysis report (Reference 1) associated with the failure of Emergency Diesel Generator (EDG) 1B during testing at Sequoyah Nuclear Plant, Units 1 and 2, on September 19, 2023. TVA appreciates this opportunity to comment on the subject report.
Comment 1 The Executive Summary in Reference 1 refers to an outdated Inspection Report detailing the background of the failure. Specifically, the second paragraph of the Executive Summary does not use the most current NRC information to accurately describe the event.
Therefore, the Executive Summary in Reference 1 should be updated with the details below from Reference 2:
TVA representatives, including diesel engine experts, provided relevant information resulting from a root cause investigation and detailed analyses. These attributed the DG failure to a material flaw developed at a fatigued location in the rear outboard valve bridge lash adjuster spring that was not reasonably within TVA¶s ability to foresee and prevent. The information TVA provided also included a technical explanation for the loosening of the valve bridge adjuster screw and locknut caused by the lash adjuster spring failure.
Additionally, TVA used a detailed material evaluation performed by TVA Central Lab
& Services to compare Sequoyah¶s failure against other well understood operating experience. TVA¶s diesel experts shared that in cases of maintenance-induced failure such as due to improper tightening of the rocker arm valve spring adjuster screw and locknut, the DG failed within minutes of restoration. This does not align with Sequoyah¶s DG failure. Additionally, the experts provided examples of operating experience that showed that while the lash adjuster spring may have failed prior to surveillance testing on September 19, 2023, the engine remained functional until the complete failure of the valve spring seat. TVA concluded that the combination of the material characteristics of the exhaust valve spring seat with the unusual loads developed by the fatigue failed lash adjuster spring resulted in the DG failure.
The NRC performed an independent review of the information that you presented at the May 2, 2024, regulatory conference meeting including equipment history, operating experience, and completed equipment and causal analyses. Based upon the newly provided data, the NRC determined that there was no performance deficiency directly associated with the failure of the 1B-B DG. However, the NRC has concluded that a minor violation of TS 5.4.1, ³Procedures,' occurred associated with the licensee's failure to adequately establish and implement maintenance instructions and practices. This minor violation could not be directly attributed to the 1B-B DG failure observed on September 19, 2023, and is documented in the enclosed inspection report.
Moreover, the agency found through its independent review of the best available information that a loss of design basis functionality prior to the failure of the 1B-B DG did not occur. Therefore, the agency has determined that the AV associated with TS 3.8.1, ³AC Sources +/- Operating,' communicated in NRC inspection report
Enclosure CNL-25-016 E2 of 2 05000327, -328/2024090, dated March 19, 2024 (ADAMS Accession Number ML24066A197) no longer applies.
Comment 2 The last paragraph of the Executive Summary in Reference 1 states: ³Since there was no performance deficiency associated with this degraded condition, no finalized detailed risk evaluation was performed.'
However, in an effort to provide the NRC with additional information, TVA performed a detailed risk analysis of the event. This evaluation was completed and provided to NRC Region II on February 15, 2024, as described in Reference 3.
Therefore, the last paragraph of the Executive Summary in Reference 1 should be updated to reflect that the evaluation was completed and submitted to the NRC for review.
Comment 3 TVA presented the detailed risk analysis of this event as White (<1E-05) for Unit 1 and Green (<1E-06) for Unit 2. The analysis in Reference 1 presents the results calculated by the NRC as Yellow in terms of SDP classification (1E-05) for Unit 1 and White (5E-06) for Unit 2, with no mention of TVA¶s detailed risk analysis. Since there was no performance deficiency associated with this failure, the results of the detailed risk analysis that the NRC and TVA calculated were not finalized for significance. Therefore, the increase in the core damage probability was not determined.
Comment 4 TVA has two permanently installed 480V FLEX diesel generators located on the roof of the Sequoyah Auxiliary Building. Minimal effort is required to align and operate these FLEX diesel generators in order to supply station battery chargers. Emergency Raw Cooling Water or High Pressure Fire Protection is also credited to provide water to the steam generators upon loss of auxiliary feedwater. Restoration of Reactor Coolant System makeup after onset of reflux cooling but before core damage could be considered a success, thus allowing additional time for recovery. With consideration of these factors, the top cutsets in Table 6 and Table 7 of Reference 1 may be conservative.
References
- 1. ³Preliminary Accident Sequence Precursor (ASP) Program Analysis Report: Failure of Emergency Diesel Generator µ1B¶ During Testing at Sequoyah Nuclear Plant, Units 1 and 2 +/- Event Date September 19, 2023,' dated November 22, 2024
- 2. NRC letter to TVA, ³Sequoyah, Units 1 and 2 +/- Final Significance Determination +/- NRC Inspection Report 05000327/2024091 and 05000328/2024091 (EA-24-008),' dated May 30, 2024 (ML24145A085)
- 3. NRC letter to TVA, ³Sequoyah, Units 1 and 2 +/- NRC Inspection Report 05000327/2024090 and 05000328/2024090 and Preliminary Greater-than-Green Finding and Apparent Violation (EA-24-008),' dated March 19, 2024 (ML24066A197)