ML25324A237
| ML25324A237 | |
| Person / Time | |
|---|---|
| Site: | Callaway  |
| Issue date: | 12/02/2025 |
| From: | Tony Nakanishi Plant Licensing Branch IV |
| To: | Karenina Scott Ameren Missouri, Callaway Energy Center |
| References | |
| EPID L-2025-LLR-0054, EPID L-2025-LLR-0059 | |
| Download: ML25324A237 (0) | |
Text
December 2, 2025 CALLAWAY PLANT, UNIT NO. 1 - AUTHORIZATION AND SAFETY EVALUATION FOR ALTERNATIVE REQUEST NO. I5R-01 AND I5R-02 (EPID L-2025-LLR-0054 AND EPID L-2025-LLR-0059 LICENSEE INFORMATION Recipients Name and Address:
Kent Scott Senior Vice President and Chief Nuclear Officer Ameren Missouri Callaway Energy Center 8315 County Road 459 Steedman, MO 65077 Licensee:
Union Electric Company, doing business as Ameren Missouri Plant Name and Unit:
Callaway Plant, Unit No. 1 (Callaway)
Docket No.:
50-483 APPLICATION INFORMATION Submittal Dates: May 22, 2025, and June 11, 2025 Submittal Agencywide Documents Access and Management System (ADAMS) Accession Nos.: Packages ML25142A385 for I5R-01 and ML25163A037 for I5R-02 Supplement Date(s): May 28, 2025, and June 24, 2025 Supplement ADAMS Accession No(s).: Packages ML25148A289 for I5R-01 and ML25175A342 for I5R-02 Applicable Inservice Inspection (ISI) Interval and Interval Start/End Dates: The Callaway fifth ISI interval began on December 19, 2024, and ends on December 18, 2036.
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: American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code),Section XI, IWA-4412, Defect Removal; IWA-3300, Flaw Characterization; IWB-3142.4, Acceptance by Evaluation; IWB-3420, Characterization; IWB-3660, Evaluation Procedure and Acceptance Criteria for PWR [Pressurized-Water Reactor] Reactor Vessel Head Penetration Nozzles; IWB-2420, Successive Inspections; 10 CFR 50.55a(g)(6)(ii)(E); and ASME Code Case N-638-11, Similar and Dissimilar Metal Welding Using Ambient Temperature Machine GTAW [Gas Tungsten Arc Welding] Temper Bead Technique Section XI, Division I, requirements once this alternative case was implemented by the licensee.
Applicable Code Edition: 2019 Edition of the ASME Code,Section XI Brief Description of the Proposed Alternative: The licensee proposed alternatives to (1) ASME Code Case N-638-11, paragraph 4(a)(2), to remove the 48-hour hold time prior to performing non-destructive examination (NDE), and (2) the flaw characterization requirements of Section XI, IWA-3300 and IWB-3420 and to the flaw removal requirements of IWA-4412 on the basis that performing the half nozzle repair modification, as described in the licensees submittals, provides an acceptable level of quality and safety.
For additional details on the licensees request, please refer to the documents located at the ADAMS Accession Nos. identified above.
STAFF EVALUATION During the refueling outage in spring 2025, the licensee detected boric acid leakage from the reactor vessel bottom head (RVBH) bottom mounted instrumentation (BMI) penetration nozzle No. 48. As part of the extent-of-condition investigation, the licensee performed ultrasonic testing and visual examinations on all 58 BMI nozzles and identified 4 penetration nozzles, which require repair, numbers 30, 35, 48 and 57. The licensee repaired these penetration nozzles with a half nozzle repair modification as described in proposed alternative I5R-01, which addresses a 48 hour5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> hold time as part of the welding process, and proposed alternative I5R-02, which is related to flaw characterization and removal associated with the existing J-groove weld. The U.S. Nuclear Regulatory Commission (NRC) verbally authorized the licensees proposed alternatives I5R-01 and I5R-02 on May 29, 2025 (ML25153A581), and June 25, 2025 (ML25178A223), respectively. The following evaluation documents the NRC staffs technical bases for these authorizations.
48 Hour Hold Time Alternative I5R-01 In the performance of the half nozzle repair, the licensee utilized the NRC approved ASME Code Case N-638-11 for the ambient temperature temper bead welding technique to install a weld pad, thereby precluding post weld heat treatment of the affected area of the RVBH.
Subparagraph 4(a)(2) of ASME Code Case N-638-11 requires a 48-hour hold time after welding the temper bead layers to perform NDE. The purpose of this hold time is to allow sufficient time for any potential hydrogen induced cracking (HIC) to develop prior to NDE. The licensee provided a technical basis for the elimination of the 48-hour hold time requirement in attachment 1 of the May 22, 2025, submittal. The NRC staff reviewed the licensees basis and considered the operating experience of using this technique for the past 20 years at over 50 nuclear plants with no indications of HIC. The NRC staff found the licensees basis for the proposed alternative acceptable and consistent with previous NRC approvals for the elimination of the 48-hour hold time at other nuclear plants, as listed in section 7 of the licensees submittal dated May 22, 2025.
Therefore, the NRC staff finds that the licensees proposed alternative, to eliminate the 48-hour hold requirement of subparagraph 4(a)(2) of ASME Code Case N-638-11 for the installation of weld pads for RVBH BMI nozzles 30, 35, 48 and 57, provides an acceptable level of quality and safety through the end of plant life for Callaway.
Flaw Characterization and Removal Alternative I5R-02 The licensee also provided a technical basis to ensure the structural integrity of the half nozzle repairs and lower reactor pressure vessel head through an ASME Code,Section III design analysis, a corrosion evaluation, a flaw evaluation, a loose parts assessment and a leakage detection program for one operating cycle.
The NRC staff reviewed the half nozzle repair modification as explained in section 5 of the June 11, 2025, submittal, utilizing ASME Code,Section XI, Code Case N-638-11 and ASME Code,Section III requirements. The NRC staff found the licensees method was consistent with previous NRC approvals of the half nozzle repair methodology used to repair bottom mounted nozzle penetrations at two plants (ML14093A407 and ML032130436). The NRC staff utilized these previous reviews to supplement the evaluation of the licensees proposed alternative as an efficiency, due to the limited review time available for the verbal authorization.
The NRC staff reviewed the licensees ASME Code,Section III design and stress analysis provided in attachment 1 of enclosure 2 of the June 11, 2025, submittal. The NRC staff notes that this analysis was for only one cycle of operation but also evaluated licensees design specifications for the life of the repair. The NRC staff found the assumptions were justified and design inputs were comprehensive and sufficient to verify adequacy of the licensees bounding analyses. The NRC staff found the calculation of allowable stresses utilized the limits of ASME Code,Section III. The NRC staff noted the loading and loading conditions were conservative.
Further, the NRC staff found the loading conditions and pressure stress identified were consistent with generally acceptable flaw evaluations. The NRC review of weld size requirements per design requirements of ASME Code,Section III were met and ranges were clarified by the licensee during the review process. The NRC staff determined that the reinforcement area met design limits and qualitative analysis was acceptable for one cycle of operation. Therefore, the NRC staff review found the licensees design analysis was acceptable for BMI nozzles numbers 30, 35, 48 and 57 for one cycle of operation.
The NRC staff reviewed the licensees corrosion evaluation to ensure no structural impact for one cycle of operation. The licensee provided information to support the design life of the repairs, defined as the remainder of the 60-year licensed operational life of the plant, through 2044. The NRC staff found the analysis was comprehensive for degradation mechanisms and materials analyzed. Further, the results were consistent with other corrosion evaluations of similar repairs exposing low alloy steels. Therefore, the NRC staff found the licensees corrosion evaluation was acceptable for one cycle of operation.
The NRC staff reviewed the licensees analysis for the remaining flaws in the original Alloy 600 nozzle or J-groove weld materials. The licensee noted there was insufficient time to complete a detailed life of repair finite element analysis, therefore a previous life of repair analysis was utilized from a plant with a similar half nozzle repair for cracking identified in a bottom mounted nozzle penetration and weld. The licensee noted the consistency between Callaways BMI nozzles and the evaluated plants BMI nozzles in dimensions, materials, flaw sizes, and weld residual stress. The NRC staff noted these values were consistent or conservative for Callaway.
The NRC staff also found the Callaway transient cycles comparison for only one cycle as compared to the 50 years of the original analysis was conservative. The licensee concludes that the comparative evaluation of fatigue crack growth and primary stress limits are met considering one cycle of operation. The NRC staff review found this assessment conservative and acceptable for one cycle of operation.
The NRC staff reviewed the licensees loose parts consideration due to potential debris from the cracked remnants of the J-groove welds. The licensee noted that there were minimal driving forces for cracks to be formed to produce a loose part. Further, the licensee explained that operating history of numerous repairs leaving similar cracked J-groove welds in place had no known cases of debris generation during their full operating lifetimes. The NRC staff found the licensees qualitative assessment was sufficient for one cycle of operation and noted the licensee would provide a detailed loose parts analysis for operation beyond one cycle.
Finally, the NRC staff reviewed the licensees reactor coolant system leakage detection requirements and capabilities to address any potential leakage from the subject penetrations.
The licensee provided details on Technical Specification 3.4.13, RCS [Reactor Coolant System] Operational LEAKAGE. Further, the licensee noted actions would be taken in the event that unidentified leakage increased greater than 0.1 gallons per minute above the normal, steady-state value for a given plant condition. The NRC staff reviewed the requirements and action steps and found they would provide defense-in-depth to identify leakage prior to loss of structural integrity for the repaired penetrations and the bottom reactor vessel head at Callaway.
In summary, the NRC staff reviewed the licensees basis and found, for the proposed duration of one operating cycle, that: (1) the ASME Code,Section III design and stress requirements are met, (2) all reasonable corrosion mechanisms were assessed and will not challenge the structural integrity of the RVBH, (3) remaining flaws in the original penetration or J-groove weld will not challenge the structural integrity of the RVBH, (4) the stress assessment and past history of these repairs adequately support that generation of loose parts is unlikely, and (5) that the licensees leak detection system and associated operator actions will provide defense-in-depth measures and reasonable assurance that any potential repair issues will be identified prior to challenging structural integrity of the RVBH.
Therefore, the NRC staff finds that the licensees proposed alternative for the half nozzle repairs of RVBH BMI nozzles 30, 35, 48 and 57 provides an acceptable level of quality and safety for the duration of Callaway, Cycle 28, which is currently scheduled to conclude in the fall of 2026.
CONCLUSION The NRC staff has determined that the proposed alternatives I5R-01 and I5R-02 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 verbally authorized the use of proposed alternative I5R-01 on May 29, 2025 (ML25153A581) at Callaway Unit 1 through the end of the current license, which is October 18, 2044.
The NRC staff verbally authorized the use of proposed alternative I5R-02 on June 25, 2025 (ML25178A223) at Callaway Unit 1 for the duration of Cycle 28, which is currently scheduled to conclude in the fall of 2026.
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 Contributors: Jay Collins, NRR John Tsao, NRR Date: December 2, 2025 Tony Nakanishi, Chief Plant Licensing Branch IV Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation cc: Listserv TONY NAKANISHI Digitally signed by TONY NAKANISHI Date: 2025.12.02 10:07:47 -05'00'
- via eConcurrence NRR-028 OFFICE NRR/DORL/LPL4/PM NRR/DORL/LPL4/LA NRR/DNRL/NPHP/BC NAME TByrd PBlechman MMitchell DATE 11/24/2025 11/24/2025 11/24/2025 OFFICE NRR/DNRL/NVIB/BC NRR/DORL/LPL4/BC NRR/DORL/LPL4/PM NAME ABuford TNakanishi TByrd DATE 12/1/2025 12/2/2025 12/2/2025