ML24270A166
| ML24270A166 | |
| Person / Time | |
|---|---|
| Site: | Diablo Canyon  |
| Issue date: | 10/09/2024 |
| From: | Jennivine Rankin Plant Licensing Branch IV |
| To: | Peck A Pacific Gas & Electric Co |
| Lee S, 301-415-3158 | |
| References | |
| EPID L-2024-LLR-0043 | |
| Download: ML24270A166 (6) | |
Text
October 9, 2024 DIABLO CANYON NUCLEAR POWER PLANT, UNITS 1 AND 2 - AUTHORIZATION AND SAFETY EVALUATION FOR ALTERNATIVE REQUEST NO. REP-RHR-SWOL (EPID L-2024-LLR-0043)
LICENSEE INFORMATION Recipients Name and Address:
Adam Peck Acting Chief Nuclear Officer and Site Vice President Pacific Gas and Electric Company Diablo Canyon Power Plant P.O. Box 56, Mail Code 104/6/605 Avila Beach, CA 93424 Licensee:
Pacific Gas and Electric Company (PG&E)
Plant Name(s) and Unit(s):
Diablo Canyon Nuclear Power Plant (Diablo Canyon, Units 1 and 2)
Docket No(s).:
50-275 and 50-323 APPLICATION INFORMATION Submittal Date: June 27, 2024 Submittal Agencywide Documents Access and Management System (ADAMS) Accession No.: ML24180A205 Applicable Inservice Inspection (ISI) Program Interval: The remainder of the fourth and through the fifth ISI interval, which ends May 6, 2035, for Unit 1, and March 12, 2036, for Unit 2.
Alternative Provision: The licensee requested an alternative under Title 10 of the Code of Federal Regulations (10 CFR) 50.55a(z)(1), Acceptable level of quality and safety.
ISI Requirement: The provision of article IWA4000, Repair/Replacement Activities and mandatory appendix VIII, Performance Demonstration for Ultrasonic Examination Systems, Supplement 11, Qualification Requirements for Full Structural Overlaid Wrought Austenitic Piping Welds.
Applicable Code Edition and Addenda: American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code),Section XI, 2007 Edition with the 2008 Addenda.
Brief Description of the Proposed Alternative: PG&E submitted a request for approval of alternative for application of a full structural weld overlay (SWOL) for the Diablo Canyon, Units 1 and 2 by letter dated September 26, 2017 (ML17269A220). By letter dated January 2, 2018, the NRC staff approved the relief request, through the operating license dates of November 2, 2024, for Unit 1, and August 26, 2025, for Unit 2 (ML17338A131). PG&E installed the SWOL at weld joint WIB-245 during the Unit 2, 20th refueling outage (2R20) in 2018, and at weld joint WIB-228 during the Unit 1, 21st refueling outage (1R21) in 2019.
In November 2023, PG&E submitted a license renewal application for Diablo Canyon, Units 1 and 2 to the U.S. Nuclear Regulatory Commission (NRC) (ML23311A154). As a result, PG&E is requesting NRC to extend the approval of the subject SWOLs to the end of the fifth 10-year ISI interval by PG&E Letter DCL-24-066 as an extension of alternative request REP-RHR-SWOL.
For additional details on the licensees request, please refer to the documents located at the ADAMS accession numbers identified above.
STAFF EVALUATION The licensee submitted the request on the basis that the proposed alternative would provide an acceptable level of quality and safety in accordance with 10 CFR 50.55a(z)(1). Based on this regulation, the NRC staff finds that regulatory authority exists to review and, if found acceptable, authorize an alternative to ASME Code,Section XI as requested by the licensee.
By letter dated January 2, 2018, the NRC staff authorized the use of the proposed alternative REP-RHR-SWOL for the fourth 10-year ISI interval at Diablo Canyon, Unit 1, which ends on November 2, 2024, and at Diablo Canyon, Unit 2, which ends on August 26, 2025. The licensee is requesting to revise the previous proposed alternative REP-RHR-SWOL through the fifth 10-year ISI interval at Diablo Canyon, which ends on May 6, 2035, for Unit 1, and March 12, 2036, for Unit 2. The licensee has already installed the SWOLs at weld joint WIB-245 during the Unit 2, 20th refueling outage (2R20) in 2018 and at weld joint WIB-228 during the Unit 1, 21st refueling outage (1R21) in 2019. The NRC staff authorization of the original proposed alternative approved the SWOLs general requirements, material specifications, installation, examinations (pre-overlay, acceptance, preservice and inservice), pressure testing and crack growth analysis and design. The NRC staff finds the impacts of the licensees revised proposed alternative, in extending the service life for the SWOLs, requires verification of the crack growth analysis for design life and confirming that the proposed inservice examination remains consistent with the previous authorization and current requirements to ensure an acceptable level of quality and safety for the revised proposed alternative.
In enclosure 1 in a letter dated September 11, 2018 (ML18254A426), the licensee submitted Design Report for the Qualification of the Structural Weld Overlay Repair of Residual Heat Removal Welds WIB-245 and WIB-228 for Diablo Canyon Power Plant, Units 1 and 2. The NRC staff reviewed the documents in accessing the proposed extension of service life for the SWOLs. The NRC staff found the flaw analysis in the design report addressed all potential degradation methods and ensured the design of the overlay would put the original flaw, as well as a hypothetical flaw up to 75 percent through-wall, in a compressive stress state sufficient enough to reduce crack growth intensity factors to allow zero growth for stress corrosion cracking and only minimal (0.0004 inch) growth for a postulated circumferential flaw under fatigue crack growth for 60 years of inservice temperature and environmental conditions.
Additionally, the NRC staff confirmed the SWOLs met the ASME Code,Section III primary stress criteria, primary plus secondary stress criteria, thermal ratcheting criteria, allowable limits for stress intensity ranges and fatigue usage for 60 years of service life. Therefore, the NRC staff confirmed the licensees original design for the SWOLs is acceptable for the licensees proposed period of extended service life.
In enclosure 2 of the same letter dated September 11, 2018, the licensee submitted a change to the original design in the report, Impact of Added Second Buffer Layer of Stainless Steel to RHR [Residual Heat Removal] Weld Overlay. The reason for the design change was to add a second buffer layer of stainless steel material to be the first layer of the overlay to reduce the potential for fabrication cracking during the installation process. Additionally, the welding vendor requested an extension of the weld overlay over the inside of the pipe bend to allow for constant orbital welding. The NRC staff reviewed the licensees assessment of the modifications through a weld residual stress comparison, thermal mixing cycling, weight and weld shrinkage comparison. The axial and hoop through-wall residual stress changes due to the additional material were very similar with a continued magnitude of compression at 75 percent through-wall of the original weld; therefore the impact will have no adverse effect on the original weld residual stress reported in the design report. A comparison of the stress intensity analysis for the ASME Code,Section III qualification showed an improvement for the previous design maximum stress intensity location. The NRC staff verified the licensees analysis of the change in weight did not impact the piping system deadweight and seismic behavior, and the conclusions of the original design were unchanged. The NRC staff verified the effects of weld shrinkage were considered and found the original design calculation had sufficient margin to not be adversely altered by the modified design. Therefore, the NRC staff finds the modified design of the SWOLs continue to be acceptable for the licensees proposed period of extended service life.
A third report in enclosure 3 in the same letter dated September 11, 2018, provided notification of the final acceptance report of the weld overlay as installed at Diablo Canyon, Unit 2. A letter dated July 1, 2019 (ML19182A361), provided the final acceptance report of the weld overlay as installed at Diablo Canyon, Unit 1. However, the Unit 1 SWOL, while meeting the thickness requirements, did exceed the length allowance listed on the design drawing. The licensee submitted an engineering evaluation to show acceptability of the Diablo Canyon, Unit 1 SWOL with the additional length of 0.419 inch to 0.32 inch onto the elbow of the original pipe. The licensee assessed the structural capacity and mechanical stress impacts, the residual and thermal stress impacts on ASME Code,Section III allowables, the crack growth analysis, licensing requirements, non-destructive examination impact as well as shrinkage and weight concerns. The NRC staff reviewed the licensees analysis and finds that with the thickness dimensions meeting the design, the overall length impact, as discussed by the licensee, is similar to the analysis performed in the impact of the extension of the weld overlay over the inside of the pipe bend to allow for constant orbital welding discussed above. The NRC staff found the assessment consistent with the previous analysis and acceptable for the small increase in length of the overlay. The NRC staff notes that the licensee verified that the final ultrasonic examination was successfully performed with no restrictions or limitations on coverage, which is one of the primary design concerns with the overall length of a weld overlay, to allow the probes a sufficient scanning surface for the material of concern. The NRC staff finds that the measured axial shrinkage was within the qualification analysis and therefore acceptable. Therefore, the NRC staff finds the licensees final acceptance reports for the SWOLs at Diablo Canyon, Units 1 and 2 to be acceptable and continue to show that the modified design of the SWOLs are acceptable for the licensees proposed period of extended service life.
The NRC staff also reviewed the licensees proposed inservice examination program through the proposed period of extended service life for each unit. By NRC Inspection Reports 05000275/2018001, 05000323/2018001, and 07200026/2018001 dated April 24, 2018 (ML18114A835), the NRC staff, as part of the NRC Inspection Procedure 71111.08 for ISI, reviewed the licensees procedure 266373-TR-002, RHR Structural Weld Overlay Repair, and ultrasonic examination procedures and found no violations of regulatory requirements. The NRC staff noted that the licensee has performed satisfactory acceptance and preservice examinations on both of the SWOLs. The licensee proposes to continue to inspect both welds in accordance with ASME Code Case N-740-2, Dissimilar Metal Weld Overlay for Repair of Class 1, 2, and 3 Items,Section XI, Division 1, for the fifth ISI interval. While ASME Code Case N-740-2 is currently unacceptable for use by the NRC in table 2 of NRC Regulatory Guide 1.193, Revision 7, the conditions for unacceptability do not include the inspection program. Further, the inspection program of N-740-2 was approved for the SWOLs in the authorization of the original proposed alternative REP-RHR-SWOL approved by the NRC in letter dated January 2, 2018. In accordance with ASME Code Case N-740-2, the last inservice examinations were performed during the first period of the fourth ISI interval for Diablo Canyon, Unit 2 and the second period of the fourth ISI interval for Diablo Canyon, Unit 1 and found both welds to be acceptable. The licensee describes how both SWOLs were placed into a population to be examined on a 25 percent sample basis once during each inspection interval. This sampling program is consistent with the current requirements for examination of cracked ASME Code Class 1 dissimilar metal butt welds repaired by SWOL in 10 CFR 50.55a(g)(6)(ii)(F),
Augmented ISI requirements: Examination requirements for Class 1 piping and nozzle dissimilar-metal butt welds. The licensee explains that with currently only having one such SWOL in this population at each plant, this requirement will result in each weld being examined once per ISI interval. The NRC staff finds that the inservice examination program remains consistent with the inservice examination requirements of ASME Code Case N-504-4, Alternative Rules for Repair of Class 1, 2, and 3 Austenitic Stainless Steel Piping,Section XI, Division 1, and the ASME Code,Section XI, Nonmandatory Appendix Q. Therefore, the NRC staff continues to find the licensees inservice examinations are acceptable for the licensees proposed period of extended service life for the subject SWOLs.
Given the above review, the NRC staff finds the subject SWOLs were designed, installed and will continue to be inspected during the period of extended service to provide reasonable assurance of structural integrity of the SWOLs at Diablo Canyon, Units 1 and 2 through the proposed duration of the licensees revised proposed alternative REP-RHR-SWOL per PG&E Letter DCL-24-066.
CONCLUSION The NRC staff has determined that the proposed alternative in the licensees request referenced above would provide an acceptable level of quality and safety.
The NRC staff concludes that the licensee has adequately addressed the regulatory requirements set forth in 10 CFR 50.55a(z)(1).
The NRC staff authorizes the use of proposed revised alternative REP-RHR-SWOL per PG&E Letter DCL 24-066 at Diablo Canyon, Units 1 and 2 to the end of the fifth 10-year ISI interval, which is currently scheduled to end on May 6, 2035, for Unit 1, and March 12, 2036, for Unit 2.
All other ASME Code,Section XI requirements for which an alternative was not specifically requested and authorized remain applicable, including third-party review by the Authorized Nuclear Inservice Inspector.
Principal Contributor: Jay Collins, NRR Date: October 9, 2024 Jennivine K. Rankin, Chief Plant Licensing Branch IV Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation cc: Listserv Jennivine K.
Rankin Digitally signed by Jennivine K. Rankin Date: 2024.10.09 10:40:02 -04'00'
- concurrence via email OFFICE NRR/DORL/LPL4/PM NRR/DORL/LPL4/PM*
NRR/DORL/LPL4/LA*
NAME HMcLatchie SLee PBlechman DATE 9/25/2024 10/7/2024 9/30/2024 OFFICE NRR/DNRL/NPHP/BC*
NRR/DORL/LPL4/BC*
NAME MMitchell JRankin DATE 9/13/2024 10/9/2024