ML25036A116
| ML25036A116 | |
| Person / Time | |
|---|---|
| Site: | Millstone  |
| Issue date: | 03/20/2025 |
| From: | Gonzales H NRC/NRR/DORL/LPL1 |
| To: | Carr E Dominion Energy Nuclear Connecticut |
| References | |
| EPID L-2024-LLR-0025 | |
| Download: ML25036A116 (1) | |
Text
March 20, 2025 Eric S. Carr Chief Nuclear Officer Dominion Energy Nuclear Connecticut, Inc.
Innsbrook Technical Center 5000 Dominion Boulevard Glen Allen, VA 23060-6711
SUBJECT:
MILLSTONE POWER STATION, UNIT NO. 3 - ALTERNATIVE REQUEST IR-4-13 TO SUPPORT STEAM GENERATOR CHANNEL HEAD DRAIN MODIFICATION (EPID L-2024-LLR-0025)
Dear Eric Carr:
By letter dated April 1, 2024 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML24093A102), as supplemented by letter dated March 17, 2025 (ML25077A198), Dominion Energy Nuclear Connecticut, Inc. (DENC, the licensee) submitted alternative request IR-4-13 for the Millstone Power Station (Millstone), Unit 3, to the U.S.
Nuclear Regulatory Commission (NRC or Commission) to propose an alternative to the requirements of the American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code), Code Case (CC) N-839, Similar and Dissimilar Metal Welding Using Ambient Temperature SMAW [shielded metal arc welding] Temper Bead Technique, at Millstone, Unit 3.
Specifically, pursuant to Title 10 of the Code of Federal Regulations (10 CFR) 50.55a(z)(2), the licensee submitted IR-4-13 for NRCs approval for relief from the CC requirements to use a 48-hour hold time and volumetric inspection method for final examination. The licensee has concluded that complying with the specified CC requirements to perform a temper bead repair to the channel head bowl drain connection on each steam generator (SG) at Millstone, Unit 3, to alleviate future potential primary water stress corrosion cracking concerns would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety.
In its supplemental letter dated March 17, 2025, the licensee clarified that the associated SG channel head drain modifications may take place either during the spring 2025 or fall 2026 refueling outages for Millstone, Unit 3, pending the results of visual inspections.
The NRC staff has reviewed the subject request and concludes, as set forth in the enclosed safety evaluation, that the licensee has adequately addressed all the regulatory requirements set forth in 10 CFR 50.55a(z)(2). The NRC staff has determined that complying with the specified requirements described in the licensees request 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 SG channel head bowl drain modifications. Therefore, the NRC staff authorizes the use of the proposed alternative for Millstone, Unit 3, for the repair and replacement activity for the four SGs.
E. Carr All other ASME Code,Section XI requirements for which alternative was not specifically requested and authorized remain applicable, including third party review by the Authorized Nuclear Inservice Inspector.
If you have any questions, please contact the Project Manager, Richard Guzman, at 301-415-1030 or Richard.Guzman@nrc.gov.
Sincerely, Hipólito González, Chief Plant Licensing Branch I Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket No. 50-423
Enclosure:
Safety Evaluation cc: Listserv HIPOLITO GONZALEZ Digitally signed by HIPOLITO GONZALEZ Date: 2025.03.20 11:48:43 -04'00'
STAFF EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION ALTERNATIVE REQUEST IR-4-13 TO SUPPORT STEAM GENERATOR CHANNEL HEAD DRAIN MODIFICATION DOMINION ENERGY NUCLEAR CONNECTICUT, INC.
MILLSTONE POWER STATION, UNIT 3 DOCKET NO. 50-423
1.0 INTRODUCTION
By letter dated April 1, 2024 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML24093A102), as supplemented by letter dated March 17, 2025 (ML25077A198), Dominion Energy Nuclear Connecticut, Inc. (DENC, the licensee) submitted alternative request IR-4-13 for the Millstone Power Station (Millstone), Unit 3, to the U.S.
Nuclear Regulatory Commission (NRC or Commission) to propose an alternative to the requirements of the American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code), Code Case (CC) N-839, Similar and Dissimilar Metal Welding Using Ambient Temperature SMAW [shielded metal arc welding] Temper Bead Technique, at Millstone, Unit 3.
Specifically, pursuant to Title 10 of the Code of Federal Regulations (10 CFR) 50.55a(z)(2), the licensee submitted IR-4-13 for NRCs approval for relief from the Code Case requirements to use a 48-hour hold time and volumetric inspection method for final examination. The licensee has concluded that complying with the specified CC requirements to perform a temper bead repair to the channel head bowl drain connection on each steam generator (SG) at Millstone, Unit 3 to alleviate future potential primary water stress corrosion cracking (PWSCC) concerns would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety.
2.0 REGULATORY EVALUATION
Adherence to Section XI of the ASME Code is mandated by 10 CFR 50.55a(g)(4), which states, in part, that ASME Code Class 1, 2, and 3 components will meet the requirements, except the design and access provisions and the pre-service examination requirements, set forth in the ASME Code,Section XI.
Paragraph 10 CFR 50.55a(z) states, in part, that alternatives to the requirements of paragraphs (b) through (h) of 10 CFR 50.55a or portions thereof may be used when authorized by the NRC if the licensee demonstrates that: (1) the proposed alternative would provide an acceptable level of quality and safety; or (2) compliance with the specified requirements of this Section would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety.
Based on the above, and subject to the following technical evaluation, the NRC staff has determined that regulatory authority exists for the licensee to request the use of an alternative and the NRC to authorize it.
3.0 LICENSEES PROPOSED ALTERNATIVE 3.1 Proposed Alternative IR-4-13 3.1.1 ASME Code Components Affected ASME Code Class 1 SGs 3RCS*SG1A 3RCS*SG1B 3RCS*SG1C 3RCS*SG1D 3.1.2 Applicable Code Edition and Addenda Construction: The American Society of Mechanical Engineers (ASME),Section III, 1971 Edition, Summer 1973 Addenda.
In-Service Inspection: ASME Boiler and Pressure Vessel Code,Section XI, Rules for Inservice Inspection of Nuclear Power Plant Components, 2013 Edition.
3.1.3 Applicable Code Requirement ASME Code,Section XI, Subsection IWA, provides requirements for repair/replacement activities including the following: IWA-4400 details the requirements for welding, metal removal, fabrication, and installation. IWA-4411 specifies that welding preheat and post-weld heat treatment shall be in accordance with the Construction Code for the Item. CC N-839 states that in lieu of the preheat and post-weld heat treatment requirements of IWA-4411, the materials and welds specified in 1(a) of the CC may be repaired using the manual SMAW temper bead technique without the specified preheat or post-weld heat treatment of the Construction Code.
The specific CC N-839 requirements that are the subject of this request are contained in Section 4 Examination, paragraph (a)(2), which states: (2) When ferritic filler materials are used, the weld shall be nondestructively examined after the completed weld has been at ambient temperature for at least 48 hr. When austenitic filler materials are used, the completed weld shall be nondestructively examined after the initial three temper bead layers (i.e., layers 1, 2, and 3) have been in place for at least 48 hr. Examination of the welded region shall include both volumetric and surface examination methods. Demonstration for ultrasonic examination of the repaired volume is required using representative samples which contain construction type flaws.
3.1.4 Reason for Request The licensee requests an alternative from the ASME Code requirements based on implementing CC N-839 except for the requirements to 1) hold for 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> before examination and
- 2) perform volumetric examination of the welded region, as these requirements would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety.in accordance with 10 CFR 50.55a(z)(2).
3.1.5 Proposed Alternative As an alternative to the final volumetric inspection per CC N-839, the licensee is proposing to perform progressive surface exams with the Liquid Dye Penetrant (PT) Examination process.
Intentional 48-hour holds will not be employed prior to any of the exams. Three temper bead layers will be applied. PT examination will be performed on 100% of each layer's surface. The weld area will then be prepared for installation of the coupling. PT examination will be performed at 3/16 increments when filling the groove weld, and on the final groove and fillet weld surfaces.
3.2
NRC Staff Evaluation
The design and manufacture of the four SGs at Millstone, Unit 3, included a drain line coupling at the center of the channel head bowl. The drain line was fabricated by filling a machined cavity in the channel head with Alloy 182 weld metal and subsequent post-weld heat treatment of the weld deposit. A through-the-shell hole was then drilled in the channel head and an Alloy 600 tube was hard rolled into the hole to provide a primary water flow path and seal welded at the inside diameter and outside diameter of the SG. The weld buildup was then machined for installation of a stainless steel coupling, which was welded with a partial penetration weld joint and a reinforcing fillet weld.
The licensee stated that industry operating experience has shown that Alloy 600 base material and Alloy 82/182 weld filler material is susceptible to PWSCC as several plants with similar designs have experienced borated water leakage through this connection. The licensee is planning an ASME Code,Section XI repair and replacement activity that will fully remove the Alloy 600 leak path by removing the existing Alloy 182 weld metal and replacing it with weld material as identified in a Westinghouse Technical Summary Proprietary report documented in of the request.
Repair and replacement activity will include all the steps as outlined in Attachment C of the request, starting with removal and verification of the existing coupling and Alloy 600 material.
After VT-1 visual examination, a weld seal bead will be placed to seal the gap between the drain pipe and the channel head prior to performing a PT examination. Three layers of temper bead welding will be applied for the buttering of the newly machined channel head cavity surfaces with the SMAW process per CC N-839 and in accordance with all other requirements specified in ASME Code,Section XI IWA-4400. The licensee stated that each layer of the buttering applied will be PT examined in accordance with the requirements of ASME Code Section III, Subsection NB-5350 of the 1973 Addenda.
After applying each layer, the surface will be prepared mechanically for PT examination prior to the next layer. After the third layer, the cavity will be prepared for the coupling fit-up. A first layer of the partial joint penetration weld will then be applied, mechanically prepped and PT examined. Welding will continue with progressive PT examinations until a minimum thickness is achieved and the partial joint penetration (PJP) weld is even with the end of the temper bead weld before a final PT examination of the PJP weld and installation of the coupling fillet weld. At the conclusion of the welding, a final visual and PT examination will be performed. The licensee stated that all PT examinations will be in accordance with the requirements of ASME Code,Section III, Subsection NB-5350 of the 1973 Addenda.
The licensee stated that the drain modification has been designed and qualified in accordance with the design requirements of the original code of construction, which is ASME Code,Section III, Division 1, 1971 Edition through Summer 1973 Addenda. Later editions of this code allow for progressive surface examination for temper bead repairs to partial penetration welds in lieu of volumetric examinations if meaningful results cannot be obtained. The licensee provided discussion on a project that intended to develop volumetric examination of SG channel head drains modifications in Attachment F of the request. The licensee stated that attempts to develop an ultrasonic examination method have been unsuccessful, largely due to the geometry of the bowl drain, and the partial penetration weld joint design. These attempts resulted in an inability to accurately detect and characterize identified indications. In addition, the licensee also stated that the configuration of the channel head also results in an inability to obtain meaningful results with the radiographic examination method as stated in Attachment D.
The NRC staff reviewed the licensees technical justification to eliminate the 48-hour hold time when using austenitic filler materials in the temper bead welding process for P-1 and P-3 ferritic materials. The licensee also referenced a white paper titled, "Elimination of the 48-Hour Hold for Ambient Temperature Temper Bead Welding With Austenitic Weld Metal, from Request 2TYP-4-RV-06 (ML23249A184) as a precedent that detailed technical discussion of industrys operational experience and a list of similar repairs in support for justification of their technical bases for the request to eliminate the 48-hour hold for final nondestructive inspection.
The NRC staff found the licensees basis for the proposed repair to be consistent with previous NRC approvals for the elimination of the 48-hour hold time (ML23156A682). The NRC staff is also unaware of any instances of hydrogen-induced cracking, the primary concern for the 48-hour hold time, in the hundreds of welds that have been completed by the nuclear industry over two decades. Given this basis, the staff finds the elimination of the 48-hour hold time requirements of ASME Code Case N-839 to be acceptable.
4.0 CONCLUSION
As set forth above, 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 SG channel head bowl drain modifications. Accordingly, the NRC staff concludes that the licensee has adequately addressed the regulatory requirements set forth in 10 CFR 50.55a(z)(2).
Therefore, the NRC staff authorizes the use of proposed alternative IR-4-13 for Millstone, Unit 3, for the repair and replacement activity for the four SGs.
All other ASME Code,Section XI requirements for which alternative was not specifically requested and authorized remain applicable, including third party review by the Authorized Nuclear Inservice Inspector.
Principal Contributor: O. Kahn, NRR Date: March 20, 2025
ML25036A116 OFFICE NRR/DORL/LPL1/PM NRR/DORL/LPL1/LA NRR/DNRL/NPHP/BC NAME RGuzman KEntz MMitchell DATE 2/5/2025 2/5/2025 1/27/2025 OFFICE NRR/DORL/LPL1/BC NRR/DORL/LPL1/PM NAME HGonzález RGuzman DATE 3/20/2025 3/20/2025