CNL-24-064, Response to Request for Additional Information Regarding the Watts Bar Nuclear Plant, Unit 2 Steam Generator Tube Inspection Report for U2R5
| ML24309A265 | |
| Person / Time | |
|---|---|
| Site: | Watts Bar  |
| Issue date: | 11/04/2024 |
| From: | Hulvey K Tennessee Valley Authority |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| CNL-24-064, EPID L-2024-LRO-0022 | |
| Download: ML24309A265 (1) | |
Text
10 CFR 50.4 1101 Market Street, Chattanooga, Tennessee 37402 CNL-24-064 November 4, 2024 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, D.C. 20555-0001 Watts Bar Nuclear Plant, Unit 2 Facility Operating License No. NPF-96 NRC Docket No. 50-391
Subject:
Response to Request for Additional Information Regarding the Watts Bar Nuclear Plant, Unit 2 Steam Generator Tube Inspection Report for U2R5 (EPID L-2024-LRO-0022)
References:
Cycle 5 Steam Generator Tube Inspection Report, dated May 16, 2024 (ML24137A267) 2.
NRC electronic mail to TVA, Request for Additional Information regarding the Watts Bar Unit 2 Steam Generator Tube Inspection Report for U2R5 (EPID L-2024-LRO-0022), dated September 10, 2024 (ML24260A032)
In Reference 1, Tennessee Valley Authority (TVA) submitted information summarizing the results of the fall 2023 steam generator inspections performed at the Watts Bar Nuclear Plant, Unit 2, during the Unit 2 Cycle 5 refueling outage (U2R5).
In Reference 2, the Nuclear Regulatory Commission issued a request for additional information (RAI) and requested that TVA respond by October 10, 2024. On October 8, 2024, Shawna Hughes contacted Kimberly Green of your staff to request an extension to TVAs response due date. The due date was extended to November 7, 2024. The enclosure to this letter provides the response to the RAI.
There are no new regulatory commitments in this letter. Please address any questions regarding this request to Amber V. Aboulfaida, Senior Manager, Fleet Licensing, at avaboulfaida@tva.gov.
Respectfully, Kimberly D. Hulvey General Manager, Nuclear Regulatory Affairs & Emergency Preparedness Enclosure cc: See Page 2 Digitally signed by Edmondson, Carla Date: 2024.11.04 12:37:51 -05'00'
U.S. Nuclear Regulatory Commission CNL-24-064 Page 2 November 4, 2024
Enclosure:
Response to NRC Request for Additional Information cc (Enclosure):
NRC Regional Administrator - Region II NRC Senior Resident Inspector - Watts Bar Nuclear Plant NRC Project Manager - Watts Bar Nuclear Plant
Enclosure CNL-24-064 E1 of 2 Response to NRC Request for Additional Information Introduction By letter dated May 16, 2024 (ADAMS Accession No. ML24137A267), TVA (the licensee),
submitted information summarizing the results of the fall 2023 steam generator (SG) inspections performed at Watts Bar Nuclear Plant, Unit 2 (WBN 2), during the Unit 2 Cycle 5 refueling outage (U2R5). To complete its review of the inspection report, the U.S. Nuclear Regulatory Commission (NRC) staff requests the following additional information.
Regulatory Basis All pressurized-water reactors have Technical Specifications (TS) according to Section 50.36 of Title 10 of the Code of Federal Regulations that include a SG Program with specific criteria for the structural and leakage integrity, repair, and inspection of SG tubes. WBN 2 TS Section 5.9.9 requires that a report be submitted within 180 days after the initial entry into hot shutdown (Mode 4) following completion of an inspection of the SGs performed in accordance with TS 5.7.2.12, which requires that a SG Program be established and implemented to ensure SG tube integrity is maintained. The TS 5.7.2.12 (d) states, in part, In addition to meeting the requirements of d.1, d.2, and d.3 below, the inspection scope, inspection methods, and inspection intervals shall be such as to ensure that SG tube integrity is maintained until the next SG inspection.
RAI 1
The WBN 2 inspection during the fall 2023 outage (U2R5) was the first inspection of the SGs following replacement in 2022. The Westinghouse Model 68AXP replacement SGs had operated for about 1.33 effective full power years (EFPY) at the time of the U2R5 outage. Based on the inspection results, 56 tubes were plugged due to wear at horizontal advanced tube support grids (ATSG). Condition monitoring was met for tube wear at the ATSG with a maximum wear depth of 28 percent through-wall. The steam generator inspection report also identifies a sub-population of perimeter tubes as having a different wear rate from the interior tubes. Provide the following:
a.
If known, describe the root cause(s) for the greater than anticipated wear at the ATSG observed during U2R5, including if the hot leg and cold leg tube wear have different causes. If the root cause has not yet been determined, provide any insights that have been gained about the potential reason(s) for the tube wear.
b.
For the tubes that were preventively plugged, describe the shape of the wear scars (e.g., tapered, flat).
Tennessee Valley Authority (TVA) and Westinghouse have conducted investigations and analysis into the causes for the wear found in the WBN Unit 2 replacement SGs. In accordance with the TVA corrective action program, no root cause evaluation was required. An apparent cause evaluation was performed and it was determined that the higher local cross-flow velocity in the preheater region from the recirculation window and leakage from the feedwater box, likely increased above calculated values due to local gaps, coupled with a potentially ineffective ATSG support (due to fabrication anomaly not detected) are the apparent causes.
Enclosure CNL-24-064 E2 of 2 b.
The tubes plugged in U2R5 were classified as having flat wear scars.
RAI 2
Section 9 of the WBN 2 tube inspection report discusses two operational assessment methods to predict tube integrity conditions at the next scheduled inspection. The deterministic model evaluated all SGs while the fully probabilistic model determined the probability of burst and probability of leakage only for the limiting steam generator (SG2). Results for both the deterministic and the probabilistic analyses (ATSG Wear) are close to the acceptance criteria for a projected 2.7 EFPY interval. When considering tube wear, Revision 5 of the EPRI Steam Generator Integrity Assessment Guidelines states:
((
))
Given the uncertainty associated with projecting the greater than expected tube wear rates experienced at some of the ATSGs during the first operating cycle for the replacement SGs, please discuss the date and provide justification for the next planned SG tube inspection.
TVA Response to RAI 2 TVA will perform SG inspections in the next WBN Unit 2 refueling outage (scheduled for spring 2025). Although the WBN U2R5 operational assessment justified a two-cycle operating interval, TVA made the decision to reinspect WBN U2 SGs after one-cycle given the uncertainty associated with projecting the greater than expected tube wear rates.