ML23256A288
| ML23256A288 | |
| Person / Time | |
|---|---|
| Site: | Wolf Creek  |
| Issue date: | 09/20/2023 |
| From: | Jennifer Dixon-Herrity Plant Licensing Branch IV |
| To: | |
| Lee S, 301-415-3168 | |
| References | |
| EPID L-2023-LLR-0010 I4R-08 | |
| Download: ML23256A288 (1) | |
Text
September 20, 2023 WOLF CREEK GENERATING STATION, UNIT 1 - AUTHORIZATION AND SAFETY EVALUATION FOR ALTERNATIVE REQUEST NO. I4R-08 (EPID L-2023-LLR-0010)
LICENSEE INFORMATION Recipients Name and Address: Mr. Cleveland Reasoner Chief Executive Officer and Chief Nuclear Officer Wolf Creek Nuclear Operating Corporation P.O. Box 411 Burlington, KS 66839 Licensee: Wolf Creek Nuclear Operating Corporation Plant Name and Unit: Wolf Creek Generating Station, Unit 1 (Wolf Creek)
Docket No.: 50-482 APPLICATION INFORMATION Submittal Date: March 16, 2023 Submittal Agencywide Documents Access and Management System (ADAMS) Accession No.: ML23075A048 Applicable Inservice Inspection (ISI) Program Interval and Interval Start/End Dates:
The Wolf Creek fourth 10-year ISI interval, began on September 3, 2015, and is scheduled to end on September 2, 2025.
Alternative Provision: The licensee requested an alternative under Title 10 of the Code of Federal Regulations (10 CFR) 50.55a(z)(2), Hardship without a compensating increase in quality and safety.
ISI Requirement: 10 CFR 50.55a(g)(6)(ii)(D)(5), Peening.
Applicable Code Edition and Addenda: American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code),Section XI, Rules for Inservice Inspection of Nuclear Power Plant Components, 2007 Edition through 2008 Addenda.
Brief Description of the Proposed Alternative: The licensee implemented an Ultra-High-Pressure Cavitation Peening (UHPCP) process at Wolf Creek during the spring 2021 refueling outage 24, RF24. In accordance with Materials Reliability Program (MRP) topical report MRP-335, Revision 3-A, Materials Reliability Program: Topical Report for Primary Water Stress
Corrosion Cracking [PWSCC] Mitigation by Surface Stress Improvement,1 the reactor pressure vessel (RPV) head penetration nozzle (RPVHPN) follow-up volumetric examination is currently scheduled for the spring 2024 refueling outage, RF26, at Wolf Creek. However, the warranty from the licensees vendor that performed the UHPCP application specifies that a follow-up volumetric examination be conducted in the third refueling outage after peening.
To eliminate performance of equivalent examinations in two sequential refueling outages, the licensee is requesting deferral of the Wolf Creek RPVHPN post-peening follow-up volumetric examination scheduled for the spring 2024 refueling outage, RF26, until the fall 2025 refueling outage, RF27. The licensee confirms there will be no deviation in visual examination requirements, as bare metal visual examinations will be performed each refueling outage in accordance with topical report MRP-335, Revision 3-A, section 4.3.4.
STAFF EVALUATION The U.S. Nuclear Regulatory Commission (NRC) staff reviewed and evaluated the licensees request on the basis of 10 CFR 50.55a(z)(2), such that compliance with the specified requirements would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety. The specified requirement under 10 CFR 50.55a(g)(6)(ii)(D),
Augmented ISI requirements: Reactor vessel head inspections, is performance of a follow-up volumetric examination after two operating cycles following the application of peening on the RPVHPNs. The licensee noted that the vendor for the peening process requires a volumetric examination after three operating cycles to support the warranty for the peening process. The purpose of the licensees proposed alternative is to defer the regulatory required volumetric examination to be in conjunction with the vendors warranty required volumetric examination.
The licensee noted the hardship in performing the examination in two sequential refueling outages would add unnecessary radiological dose. The nozzles to be examined are in a locked high radiation area and estimated personnel radiological exposure to perform a volumetric examination of all nozzles would be approximately 732 mrem (millirem) (7.32 mSv (millisievert)).
The NRC staff recognized the licensees concerns and found the licensee has identified a hardship meeting the requirement of 10 CFR 50.55a(z)(2).
The NRC staff reviewed the level of quality and safety of the licensees proposed alternative that the volumetric examinations of the subject RPVHPNs be delayed for one cycle of operation. The licensee provided supporting basis through a flaw analysis, prior volumetric and bare metal visual examination results, and defense-in-depth actions. The NRC staff reviewed each of these factors in evaluating the level of quality and safety in the licensees proposed alternative.
The licensee utilized a flaw analysis performed by Dominion Engineering, Inc., Technical Note TN-4069-00-01, Revision 0, MRP-335 R3-A Matrix of Deterministic Crack Calculations for Tcold Reactor Vessel Top Head Nozzles Evaluated for Alternative Peening Follow-up Volumetric Examination Timing, which was previously submitted to support a proposed alternative for Byron Station, Unit 2 (ML18248A060), to defer some volumetric examinations. The licensee reviewed the flaw analysis and concluded that it was also applicable to Wolf Creek, which uses a nominal 18-month fuel cycle, and operates its RPV upper head at similar cold loop temperature conditions.
1 Electric Power Research Institute (EPRI) submitted MRP-335, Revision 3, for NRC review on February 19, 2016 (ML16055A216). The NRC staff approved MRP-335, Revision 3, on August 24, 2016 (ML16208A485), and requested an approved version of the topical report be published with an -A designation. MRP-335, Revision 3-A, dated November 7, 2016, is publicly available at EPRI.com under Product ID 3002009241.
The NRC staff notes that the degradation mechanism of concern is leakage of primary coolant containing boric acid from the RPVHPNs and/or associated J-groove weld. This leakage can cause two issues to challenge the structural integrity of the reactor coolant pressure boundary of the RPV head or nozzles. The first challenge is circumferential cracking, and thereby, ejection of a penetration nozzle from the RPV head. This could cause a small break loss-of-coolant accident (LOCA) or control rod misalignment. The second challenge is that the leakage could cause boric acid corrosion of the low alloy steel material that comprises the bulk thickness of the RPV head. Boric acid corrosion rates of low alloy steel could be up to 6 inches/year under very severe conditions as discussed in NUREG/CR-6875, Boric Acid Corrosion of Light Water Reactor Pressure Vessel Materials, July 2005 (ML052360563). After sufficient corrosion, a small or medium break LOCA could occur. To prevent significant degradation in RPV heads and penetration nozzles, 10 CFR 50.55a(g)(6)(ii)(D) requires an examination program for these components, including volumetric examinations and bare metal visual examinations. The NRC staff further notes that the licensee applied peening on the subject nozzles and associated J-groove weld surfaces, in accordance with MRP-335, Revision 3-A, to mitigate against PWSCC initiation in the components.
The NRC staff reviewed TN-4069-00-01, Revision 0, and determined that the crack growth analyses were based on conservative assumptions and industry-wide crack size measurement data applicable for Wolf Creek. The analysis included a matrix of deterministic PWSCC crack growth calculations. The matrix considered various crack growth cases that involve different initial crack sizes, crack aspect ratios, operating temperatures, and severity levels of stress profiles. The crack growth analysis discussed the effectiveness of follow-up volumetric examination to monitor pressure boundary leakage of the nozzles. The analysis further estimates the growth of hypothetical, shallow PWSCC cracks that may have been missed in previous examinations. The report evaluation indicated that extending the currently approved examination schedule by one cycle of operation would result in a very low fraction of cases that could cause nozzle leakage. Further, the report concluded the possibility of leaking nozzles after the follow-up volumetric examination would be the same for examinations performed after two or three cycles of operation.
The NRC staff reviewed the licensees assessment of TN-4069-00-01, Revision 0, and determined that it is representative for Wolf Creek. The NRC staff notes that leakage is required to establish the necessary environmental conditions for circumferential cracking of the nozzle above the J-groove weld or boric acid corrosion of the low alloy steel RPV head. Therefore, additional time would be required to initiate and grow a circumferential crack in the nozzle material above the J-groove weld or produce sufficient boric acid corrosion of the upper head material to challenge the structural integrity of the RPV head. The NRC staff notes that while the possibility of leakage from a nozzle or J-groove weld cannot be completely discounted, the time necessary for any such hypothetical leakage was evaluated to determine the potential to challenge structural integrity of the RPV head or nozzle.
The NRC staff performed a series of independent evaluations to verify the licensees assessment. Based on MRP-335, Revision 3-A, the NRC staff determined that there is reasonable assurance that peening of the Wolf Creek nozzles will mitigate new crack initiation.
The NRC staff also determined that the bare metal visual examination of the RPV head, to be performed during each refueling outage, ensures there is currently no active indication of nozzle leakage, and any potential leakage will be identified. The NRC staffs independent evaluations found some cases of crack growth and specific weld residual stress profiles where leakage could result if the examination frequency was increased by one cycle of operation. However, the NRC staff evaluations showed insufficient time for these cases, either in the nozzle or J-groove weld, to allow leakage to challenge the structural integrity of the RPV head. The NRC staff based this conclusion on the need for additional circumferential crack growth for nozzle ejection
or the leaking flaw to grow to allow leakage rates to cause boric acid corrosion rates identified in NUREG/CR-6875. Therefore, the NRC staff determined that the conclusions of the licensees assessment are acceptable.
The NRC staff reviewed the operating experience available for Wolf Creek and other peened RPVHPNs. The NRC staff noted that no previous indications of PWSCC have been identified in the Wolf Creeks RPVHPNs or associated J-groove welds. Additionally, during follow-up volumetric examinations of RPVHPNs of similar operating temperature conditions, no new indications of PWSCC were identified in the peened areas of RPVHPNs at four other plants.
Further, no indications of leakage were found at these plants through the J-groove weld by the volumetric leak path examination. The NRC staff determined that these examination results provide additional assurance that indicates the margin of the postulated flaw analyses utilized by the licensee and NRC are conservative.
The NRC staff further assessed the adequacy of the defense-in-depth of the licensees examination and monitoring requirements to evaluate the structural integrity of the upper head and nozzles. The NRC staff notes that the licensee confirmed that a bare metal visual examination was and will be performed on each nozzle for evidence of pressure boundary leakage every refueling outage in accordance with MRP-335, Revision 3-A. The NRC staff finds that the visual examination is an effective defense-in-depth inspection. The NRC staff also notes that technical specifications of Wolf Creek require operational leakage monitoring. The NRC staff finds the ongoing leakage monitoring program at Wolf Creek, during the additional cycle of operation, provides an effective defense-in-depth basis to ensure the structural integrity of the RPV head and nozzles at Wolf Creek for the period of the licensees proposed alternative. The NRC staff also notes that if any leakage is identified, it would be required to be repaired and the examination requirements of 10 CFR 50.55a(g)(6)(ii)(D) would be implemented.
Given the licensees identified hardship, the NRC staff finds that the licensee has provided an adequate technical basis to extend the follow-up volumetric examination of the subject RPVHPNs for one operating cycle. The NRC staff also finds that the defense-in-depth bare metal visual examination, along with operational leakage monitoring, provides reasonable assurance that the structural integrity of the RPV upper head and nozzles are maintained.
Therefore, the NRC staff finds that complying with the current volumetric examination requirement in the spring 2024 refueling outage, RF26, along with the licensees warranty obligation in the fall 2025 refueling outage, RF27, would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety.
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 structural integrity of the RPV head.
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-08 at Wolf Creek until the fall 2025 refueling outage, RF27.
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: September 20, 2023 Jennifer L. Digitally signed by Jennifer L. Dixon-Herrity Dixon-Herrity Date: 2023.09.20 11:45:54 -04'00' Jennifer L. Dixon-Herrity, Chief Plant Licensing Branch IV Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation cc: Listserv
ML23256A288 *by email OFFICE NRR/DORL/LPL4/PM NRR/DORL/LPL4/LA* NRR/DNRL/NPHP/(A)BC*
NAME SLee PBlechman SCumblidge DATE 9/14/2023 9/14/2023 9/8/2023 OFFICE NRR/DORL/LPL4/BC*
NAME JDixon-Herrity DATE 9/20/2023