ML23342A080
| ML23342A080 | |
| Person / Time | |
|---|---|
| Site: | Vogtle  |
| Issue date: | 12/08/2023 |
| From: | Andrea Johnson NRC/NRR/DORL/LPL2-1 |
| To: | Pournaras D Southern Nuclear Operating Co |
| References | |
| L-2023-LRO-0067 | |
| Download: ML23342A080 (4) | |
Text
From:
Andrea Johnson Sent:
Friday, December 8, 2023 9:52 AM To:
Pournaras, DeLisa S.
Cc:
Chamberlain, Amy Christine; Lowery, Ken G.; Paul Klein; John Lamb; RMJOYCE@southernco.com
Subject:
Request for Additional Information (RAI) - Vogtle, Unit 1 - Review of SG Tube Inspection Report for Refueling Outage 24 (L-2023-LRO-0067)
- DeLisa, By letter dated September 22, 2023 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML23265A249), Southern Nuclear Operating Company (SNC, the licensee) submitted the Steam Generator (SG) Tube Inspection Report for the SG tube inspection performed during the twenty-fourth refueling outage on Vogtle Electric Generating Plant (Vogtle), Unit 1 (1R24) in accordance with the requirements of Vogtle Technical Specification 5.6.10 as updated by TS Amendment 211 (ML21316A055).
The U.S. Nuclear Regulatory Commission (NRC) staff reviewed the letter dated September 22, 2023, and determined that additional information is needed to complete its review.
On November 29, 2023, the NRC staff provided draft RAI questions to SNC to make sure that the RAI is understandable, the regulatory basis is clear, to ensure there is no proprietary information, and to determine if the information was previously docketed. On December 8, 2023, SNC stated that it would provide the RAI response within 45 days from the date of this email.
If you have any questions, please contact me at (301) 415-2890.
Sincerely, Andrea M. Johnson Project Manager US Nuclear Regulatory Commission NRR/DORL/LPL2-1 REQUEST FOR ADDITIONAL INFORMATION (RAI):
By letter dated September 22, 2023 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML23265A249), Southern Nuclear Operating Company (SNC, the licensee), submitted information summarizing the results of the spring 2023 steam generator (SG) inspections performed at Vogtle Electric Generating Plant (Vogtle), Unit 1, during the twenty-fourth refueling outage (1R24).
All pressurized water reactors have Technical Specifications (TS) according to § 50.36 of Title 10 of the Code of Federal Regulations (10 CFR) that include a SG Program with specific criteria for the structural and leakage integrity, repair, and inspection of SG tubes. Vogtle, Unit 1, TS Section 5.6.10 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 Section 5.5.9, which requires that a SG Program be established and implemented to ensure SG tube integrity is maintained.
To complete its review of the inspection report, the U.S. Nuclear Regulatory Commission (NRC) staff requests the following additional information:
Regulatory Evaluation The SG tubes function as an integral part of the reactor coolant pressure boundary (RCPB) and, in addition, serve to isolate radiological fission products in the primary coolant from the secondary coolant and the environment. For the purposes of this SG Tube Inspection Report
review, tube integrity means that the tubes are capable of performing these functions in accordance with the plant design and licensing basis.
The regulation 10 CFR 50.36, the Commission established its regulatory requirements related to the content of the TSs. Pursuant to 10 CFR 50.36, Technical Specifications (TSs) are required to include items in the following five specific categories related to station operation: (1) safety limits, limiting safety system settings, and limiting control settings; (2) limiting conditions for operation (LCOs); (3) surveillance requirements; (4) design features; and (5) administrative controls. The rule does not specify the particular requirements to be included in a plants TSs. In 10 CFR 50.36(d)(5), administrative controls are stated to be the provisions relating to organization and management, procedures, recordkeeping, review and audit, and reporting necessary to assure operation of the facility in a safe manner. This also includes the programs established by the licensee and listed in the administrative controls section of the TSs for the licensee to operate the facility in a safe manner. The requirements for SG tube integrity and SG tube reporting on inspections and repair for Vogtle, Unit 1, are in TS 3.4.17, Steam Generator (SG) Tube Integrity, and TS 5.6.10, Steam Generator Tube Inspection Report, respectively. In addition, TS 5.5.9 contains the requirements for the SG Program The regulation 10 CFR establishes the fundamental regulatory requirements with respect to the integrity of the SG tubing. Specifically, the General Design Criteria (GDC) in Appendix A to 10 CFR Part 50 state that the RCPB shall have an extremely low probability of abnormal leakage
... and gross rupture (GDC 14), shall be designed with sufficient margin (GDC 15 and 31),
shall be of the highest quality standards possible (GDC 30), and shall be designed to permit periodic inspection and testing... to assess... structural and leak tight integrity (GDC 32). To this end, 10 CFR 50.55a specifies that components which are part of the RCPB must meet the requirements for Class 1 components in Section III of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (Code). Section 50.55a further requires, in part, that throughout the service life of a pressurized-water reactor (PWR) facility, ASME Code, Class 1 components meet the requirements, except design and access provisions and pre-service examination requirements, in Section XI, Rules for Inservice Inspection [ISI] of Nuclear Power Plant Components, of the ASME Code, to the extent practical. This requirement includes the inspection and repair criteria of Section XI of the ASME Code.Section XI requirements pertaining to ISI of SG tubing are augmented by additional SG tube surveillance requirements in the TS.
The TS 5.5.9(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. Therefore, NRC staff questions related to a specific SG inspection technique used to detect tube cracking and the techniques probability of detection, which is an input used in projecting future tube integrity, is appropriate.
Request For Additional Information Vogtle, Unit 1, has recurring circumferential outside diameter stress corrosion cracking (ODSCC) at the hot leg top of tubesheet in the expansion transition. The 1R24 inspection report (Table 8) provides information about five tubes with ODSCC in the hot leg top of tubesheet expansion transition region. This topic has been the subject of a previous NRC staff Request for Additional Information (RAI) dated June 6, 2022 (ML22157A090). During the review of the previous 1R23 inspection report dated April 1, 2022 (ML22091A261), the NRC staff issued an RAI related to the depth of circumferential ODSCC associated with reliable eddy current detection. The SNC response dated July 19, 2023 (ML22200A301), indicated that the ODSCC detection and sizing was based on examination technique specification sheet (ETSS) 21410.1.
In addition, the response stated a site-specific probability of detection (POD) function was developed using the Electric Power Research Institute (EPRI) MAPOD (model assisted
probability of detection) Code. The RAI response also provided proprietary 50th percentile and 95th percentile maximum depth values of the POD curve from the MAPOD simulation.
During the review of the 1R24 SG inspection report, the NRC staff compiled the hot leg expansion transition circumferential ODSCC maximum depths reported from 1R14 to 1R24.
These maximum depths did not include data from look-back sizing, which would bias the detected depths in a nonconservative manner. The NRC staff determined the approximate reported maximum depths where 50 percent of the cracks had been detected for three different cases: (1) all data from 1R14 through 1R24; (2) data from the more recent 1R20 to 1R24 outages; and (3) data from all inspections that had an inspection in the prior (N-1) refueling outage. The maximum depth where 50 percent of the cracks had been reported for each of these three cases was significantly greater than the proprietary 50th percentile depth provided in the 1R23 RAI response. While the NRC staff understands that an empirically based analysis of a small dataset will result in a different outcome from a statistical model such as MAPOD, additional information is needed for the NRC staff to complete the review.
- a. For the ETSS of interest, the site-specific POD function developed with MAPOD seems to be optimistic relative to the units historical probe performance for ODSCC at the hot leg expansion transition region. Please discuss what steps were taken during and after development of the POD function to ensure the results are reasonably consistent with probe performance.
- b. Did the site-specific POD function developed for ETSS 21410.1 at the top of tubesheet expansion region receive a peer review? If so, please discuss the organization that performed the peer review and the scope of the peer review.
- c. The 1R23 and 1R24 SG tube inspection reports indicate that enhanced probe inspections were performed of tubes with potentially increased susceptibility to degradation. Please specify if any of the tubes with stress corrosion cracking indications in 1R23 and 1R24 occurred in potentially high stress tubes (e.g., -2 sigma tubes).
Hearing Identifier:
NRR_DRMA Email Number:
2333 Mail Envelope Properties (BY5PR09MB486715CDBD95F1B49CC5CC3A978AA)
Subject:
Request for Additional Information (RAI) - Vogtle, Unit 1 - Review of SG Tube Inspection Report for Refueling Outage 24 (L-2023-LRO-0067)
Sent Date:
12/8/2023 9:51:55 AM Received Date:
12/8/2023 9:51:00 AM From:
Andrea Johnson Created By:
Andrea.Johnson@nrc.gov Recipients:
"Chamberlain, Amy Christine" <ACCHAMBE@southernco.com>
Tracking Status: None "Lowery, Ken G." <KGLOWERY@southernco.com>
Tracking Status: None "Paul Klein" <Paul.Klein@nrc.gov>
Tracking Status: None "John Lamb" <John.Lamb@nrc.gov>
Tracking Status: None "RMJOYCE@southernco.com" <RMJOYCE@southernco.com>
Tracking Status: None "Pournaras, DeLisa S." <DSPOURNA@SOUTHERNCO.COM>
Tracking Status: None Post Office:
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