ML24131A007
| ML24131A007 | |
| Person / Time | |
|---|---|
| Site: | Byron  |
| Issue date: | 06/20/2024 |
| From: | Joel Wiebe Plant Licensing Branch III |
| To: | Rhoades D Constellation Energy Generation |
| Wiebe J | |
| References | |
| EPID L-2023-LLR-0050 | |
| Download: ML24131A007 (1) | |
Text
June 20, 2024
BYRON STATION, UNIT NO. 2 - AUTHORIZATION AND SAFETY EVALUATION FOR ALTERNATIVE REQUEST NO. I4R-24 (EPID L-2023-LLR-0050)
LICENSEE INFORMATION
Recipients Name and Address : David P. Rhoades Senior Vice President Constellation Energy Generation, LLC President and Chief Nuclear Officer (CNO)
Constellation Nuclear 4300 Winfield Road Warrenville, IL 60555
Licensee: Constellation Energy Generation, LLC
Plant Name and Unit: Byron Station, Unit No. 2
Docket No.: 50-455
APPLICATION INFORMATION
Submittal Date: September 29, 2023
Submittal Agencywide Documents Access and Management System (ADAMS) Accession No.: ML23272A242
Applicable Inservice Inspection (ISI) Program Interval and Interval Start/End Dates: Byron Station (Byron), Unit No. 2, is currently in the fourth ISI interval which began on July 16, 2016, and is scheduled to end on July 15, 2025. This alternative request will carry into the fifth ISI interval which would be expected to end in July 2035.
Alternative Provision: Constellation Energy Generation, LLC, the licensee, requested an alternative under Title 10 of the Code of Federal Regulations (10 CFR), paragraph 50.55a(z)(2).
ISI Requirement: 10 CFR 50.55a(g)(6)(ii)(D)(5 ), requires the use of MRP-335, Revision 3-A, Topical Report for Primary Water Stress Corrosion Cracking Mitigation by Surface Stress Improvement (ML16319A282). Specifically, Table 4-3 of MRP-335, Revision 3-A, provides the inspection requirement for Alloy 600 reactor pressure vessel head (RPVH) penetration nozzles which have been mitigated by peening.
Applicable Code Edition and Addenda: The current applicable American Society of Mechanical Engineers Boiler and Pressure Vessel (ASME) Code is the 2007 Edition with the 2008 Addenda.
Brief Description of the Proposed Alternative: During the spring 2022 (B2R23) refueling outage at Byron, Unit No. 2, the licensee identified a flaw in the RPVH Core Exit Thermocouple
(CETC) Penetration, No. P-75. The flaw was found outside of the peening coverage area but within the required ISI examination area for the penetration nozzle. The licensee found the flaw to have an axial orientation with a length of 0.197 inches and a depth of 0.141 inches from the outside diameter surface of the nozzle. Since MRP-335 does not address flaws found outside the peening area, it is assumed that the entire P-75 is flawed. As a result, Table 4-3 of MRP-335, Revision 3-A, requires the entire penetration nozzle P-75 to be reexamined every other refueling outage until repaired.
The licensee is requesting a proposed alternative to perform the post-peening volumetric examinations of penetration nozzle P-75 in accordance with Item No. B4.60, of ASME Code Case N-729-6 once per inspection interval like the rest of the RPVH penetration nozzles at Byron, Unit No. 2. The licensee explained that the next examination for the rest of the RPVH penetration nozzles (Item No. B4.60) is scheduled to be performed during the fifth ISI interval as required by 10 CFR 50.55a(g)(6)(ii)(D). The proposed alternative is based on fatigue and primary water stress corrosion cracking crack growth analyses.
The licensee noted that performing this examination every other refueling outage as required by Table 4-3 of MRP-335, Revision 3-A until the flaw is repaired is a hardship due to person-rem exposure and increased outage complexity. Based on these factors, the licensee has identified the performance of this volumetric reexamination as a hardship without a compensating increase in the level of quality and safety in accordance with 10 CFR 50.55a(z)(2).
For additional details on the licensees request, please refer to the documents located at the ADAMS Accession No. identified above.
STAFF EVALUATION
The NRC staff reviewed the licensee proposed alternative under 10 CFR 50.55a(z)(2). The licensee-identified hardship is due to person-rem exposure and increased outage complexity of being required to perform a volumetric or wetted surface examination of penetration nozzle P-75 every other refueling outage versus once each ISI interval as allowed by 10 CFR 50.55a(g)(6)(ii)(D)(5) for effectively peened penetration nozzles. The NRC staff finds the additional work in and around a high radiation area for multiple inspections is a hardship due to radiological dose and additional risk to personnel and equipment safety. As such, the NRC staff finds that these factors are a hardship to the licensee consistent with 10 CFR 50.55a(z)(2).
The NRC staff reviewed the licensees proposed alternative by evaluating the licensees flaw evaluation to assess if it provides reasonable assurance of structural integrity of the reactor coolant pressure boundary associated with penetration nozzle P-75 and the RPVH for the duration of the alternative. The flaw is located on the penetration nozzle well below the interface with the nozzle to the partial penetration weld. This location is outside of the reactor coolant pressure boundary. This is reflected in the acceptance criteria of the ASME Code,Section XI, subparagraph IWB-3663, for this flaw which allows up to a through-wall flaw to be acceptable for flaws in the penetration nozzle below the partial penetration weld. This is because any flaw in that area would not allow primary coolant leakage as both the outside and inside diameter of the penetration nozzle are reactor coolant wetted surfaces. The safety concerns would be for a circumferential flaw to grow to an extent to allow a loose part of the nozzle or an axial flaw to grow to such an extent that it interacted with the nozzle at the partial penetration weld interface.
The licensee provided the evaluation of an axial flaw in Section 6.1 of Attachment 4 to its September 29, 2023, submittal and the evaluation of a circumferential flaw in Section 6.2.
The NRC staff evaluated the licensees flaw analysis by reviewing the methodology, assumptions, and inputs of the calculations. The NRC finds the licensees methodology to
evaluate both axial and circumferential flaws for both fatigue and primary water stress corrosion cracking mechanisms are appropriate. The NRC staff additionally finds the licensees use of both the ASME Code and latest Electric Power Res earch Institute crack growth rates for primary water stress corrosion cracking for the penetrati on nozzle material is conservative. The NRC staff reviewed the licensees assumptions for the calculations and finds them consistent with other head nozzle penetration flaw calculations with the head temperature analysis being reasonably thorough. RPVH temperature is a signifi cant factor in flaw growth rates and the NRC staff finds the licensees assumptions to be reasonably conservative. Finally in reviewing the licensees inputs, the NRC staff concluded that they were consistent with design and the calculated residual stresses are reasonable given the weld fillet size and location near the flaw.
Additionally, the NRC staff finds the multiple residual stress path flaw analysis performed by the licensee also was conservative. Therefore, t he NRC staff finds the licensees analysis is acceptable.
The licensees final results concluded that the axial flaw would be acceptable per the ASME Code requirements for 15.3 effective full power years (EFPY). If turned circumferential, the licensee concluded that it would take 25 EFPY for the flaw to become unacceptable per the ASME Code. Additionally, the minimum distance between the top of the final axial flaw length and the bottom of the partial penetration weld is 3.7 inches. The implication is that any axial flaw would have to continue to grow an additional 3.7 inches along the outside diameter surface of the nozzle to interact with the partial penetration weld or affect the reactor coolant pressure boundary. The duration of the licensees proposed alternative is until the next volumetric inspection of the penetration nozzle P-75, which is not to exceed one inspection interval (nominally 10 calendar years) from the refueling outage in which the flaw was found. As such, the licensee finds the flaw will not lead to a safety concern or an unacceptable probability of leakage in the that time interval until the next examination per the proposed alternative. The NRC staff performed a series of flaw analyses to evaluate safety margin for the hypothetical flaw growth, including an axial through-wall flaw gr owing from the location of the current flaw to the partial penetration weld. The NRC staff analys is supports the licensees conclusion and finds the licensees proposed alternative will provide reasonable assurance of the structural integrity of penetration nozzle P-75 and the RPVH at Byron, Unit No. 2.
CONCLUSION
The NRC staff has determined that complying with the specified requirements described in the licensees request referenced above would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety.
The proposed alternative provides reasonable assurance of the structural integrity of penetration nozzle P-75 and the RPVH at Byron, Unit No. 2.
The NRC staff concludes that the licensee has adequately addressed the regulatory requirements set forth in 10 CFR 50.55a(z)(2).
The NRC staff authorizes the use of proposed alternative I4R-24 at Byron, Unit No. 2, until the next scheduled inspection for Item No. B4.60 at Byron, Unit No. 2, as required by 10 CFR 50.55a(g)(6)(ii)(D).
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
Date: June 20, 2024
Jeff A. Whited, Chief Plant Licensing Branch III Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation
cc: Listserv
ML24131A007 OFFICE NRR/DORL/LPL3/PM NRR/DORL/LPL3/LA NRR/DNRL/NPHP/BC NAME JWiebe SRohrer MMitchell DATE 5/9/2024 5/14/2024 5/6/2024 OFFICE NRR/DORL/LPL3/BC NAME JWhited DATE 6/20/2024