ML23125A061
| ML23125A061 | |
| Person / Time | |
|---|---|
| Site: | Quad Cities  |
| Issue date: | 05/08/2023 |
| From: | V Sreenivas Plant Licensing Branch III |
| To: | Rhoades D Constellation Energy Generation |
| Kuntz R | |
| References | |
| EPID L-2023-LLR-0022 | |
| Download: ML23125A061 (1) | |
Text
May 8, 2023 Mr. David P. Rhoades Senior Vice President Constellation Energy Generation, LLC President and Chief Nuclear Officer Constellation Nuclear 4300 Winfield Road Warrenville, IL 60555
SUBJECT:
QUAD CITIES NUCLEAR POWER STATION, UNIT 2 - PROPOSED ALTERNATIVE TO THE REQUIREMENTS OF THE ASME CODE (EPID L-2023-LLR-0022)
Dear Mr. Rhoades:
By [[letter::RS-23-058, Submittal of Sixth Inservice Inspection Interval Relief Request I6R-11 Proposed Alternatives for a Temper Bead Weld Repair of the Mating Surfaces of the Reactor Pressure Vessel Head and Shell Flanges|letter dated April 24, 2023]] (Agencywide Documents Access and Management System (ADAMS) Accession No. ML23114A378), as supplemented by letter dated April 28, 2023 (ML23118A312), Constellation Energy Generation, LLC (the licensee), proposed an alternative to the requirements of the American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code),Section XI, Rules for Inservice Inspection of Nuclear Power Plants, Article IWA-4400 for Quad Cities Nuclear Power Station (Quad Cities), Unit 2.
Specifically, pursuant to Title 10 of the Code of Federal Regulations (10 CFR), part 50, section 55a, paragraph (z), subparagraph (1) (10 CFR 50.55a(z)(1)), alternative 16R-11 requests authorization to eliminate the 48-hour hold time after the third tempering layer before performing nondestructive evaluation as required in paragraph 4(a)(2) of Code Case N-638-10, Similar and Dissimilar Metal Welding Using Ambient Temperature Machine Gas Tungsten Arc Welding (GTAW), which is an approved alternative to ASME Code,Section XI, IWA-4400. In addition, alternative request 16R 11 proposes to perform surface examinations in lieu of the required volumetric examination in Code Case N-638-10.
The U.S. Nuclear Regulatory Commission (NRC) staff has reviewed the subject request and concludes, as set forth in the enclosed safety evaluation, that the proposed alternative provides reasonable assurance that the use of progressive dye penetrant examinations of each of the three temper bead layers for the Quad Cities, Unit 2, reactor pressure vessel (RPV) head and shell flanges is a suitable alternative to the volumetric inspection and provides reasonable assurance of structural integrity because sound welds are deposited. In addition, the NRC staff has determined that the elimination of the 48-hour hold time before examining temper bead welding is acceptable because the use of austenitic Alloy 82 filler material that is to be used will provide a level of diffusible hydrogen content in the ferritic base metal heat affected zone will be too low to promote hydrogen-induced cracking. Accordingly, the licensee has adequately addressed the regulatory requirements set forth in 10 CFR 50.55a(z)(1) for an acceptable level of quality and safety with respect to alternative request I6R-11 at Quad Cities, Unit 2. Therefore, the NRC staff authorizes the use of proposed alternative 16R-11 at Quad Cities, Unit 2, for the
repairs to the RPV head and shell flanges expected to be performed during the forced maintenance outage Q2M30.
All other ASME Code,Section XI, requirements for which an alternative was not specifically requested and approved in the proposed alternative remain applicable, including third-party review by the Authorized Nuclear Inservice Inspector.
If you have any questions, please contact the Project Manager, Robert Kuntz at 301-415-3733 or via e-mail at Robert.Kuntz@nrc.gov.
Sincerely, V. Sreenivas, Acting Chief Plant Licensing Branch III Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket No. 50-265
Enclosure:
Safety Evaluation cc: Listserv Digitally signed by Venkataiah Sreenivas Date: 2023.05.08 11:27:19 -04'00'
Enclosure SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO ALTERNATIVE REQUEST 16R-11 CONSTELLATION ENERGY GENERATION, LLC QUAD CITIES NUCLEAR POWER STATION, UNIT 2 DOCKET NO. 50-265
1.0 INTRODUCTION
By [[letter::RS-23-058, Submittal of Sixth Inservice Inspection Interval Relief Request I6R-11 Proposed Alternatives for a Temper Bead Weld Repair of the Mating Surfaces of the Reactor Pressure Vessel Head and Shell Flanges|letter dated April 24, 2023]], (Agencywide Documents Access and Management System (ADAMS) Accession No. ML23114A378) as supplemented by letter dated April 28, 2023 (ML23118A312), Constellation Energy Generation, LLC (the licensee), proposed an alternative to the requirements of the American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code),Section XI, Rules for Inservice Inspection of Nuclear Power Plants, Article IWA-4400 for Quad Cities Nuclear Power Station (Quad Cities), Unit 2.
The licensee has detected unidentified leakage into the drywell at a current rate of approximately 3.3 gallon/minute in the Quad Cities, Unit 2, drywell floor drains. As a result, the licensee has decided to perform a maintenance outage to locate and repair the leakage. The licensee has not yet performed primary containment entry to positively confirm the source of the leakage. Based on the analysis of primary containment temperature and leakage monitoring system data, it is postulated that the O-rings which seal the reactor pressure vessel (RPV) head-to-shell flange may have failed with the potential for steam cutting of the adjacent flange surface(s). The licensee is proposing to use Alternative Request 16R-11 to perform a repair a leak to the mating surfaces between the RPV head and shell flanges during a forced maintenance outage. The licensee stated that the alternative is being submitted as a contingency repair plan prior to entering the maintenance outage, in anticipation that this is the source of the leakage. The licensee plans to take a mold of the affected areas to determine the areas and depth of the repair necessary. Repair options will be chosen based upon the discovery and sizing of the affected areas.
Pursuant to Title 10 of the Code of Federal Regulations (10 CFR) 50.55a(z)(1), the licensee submitted alternative 16R-11 requesting the U.S. Nuclear Regulatory Commission (NRC) approval to eliminate the 48-hour hold time after the third tempering layer before performing nondestructive evaluation (NDE) as required in paragraph 4(a)(2) of NRC-approved Code Case N-638-10, Similar and Dissimilar Metal Welding Using Ambient Temperature Machine Gas Tungsten Arc Welding (GTAW), which is an approved alternative to ASME Code,Section XI, IWA-4400. Code Case N-638-10 supports ambient temperature temper bead (ATTB) welding using an automatic or machine GTAW process to repair certain materials. In addition, the alternative proposes to perform surface examinations in lieu of the required volumetric examination in Code Case N-638-10. The licensee has concluded that the proposed alternative will provide an acceptable level of quality and safety in accordance with 10 CFR 50.55a(z)(1).
2.0 REGULATORY EVALUATION
2.1 Regulatory Requirements 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 Director, Office of Nuclear Reactor Regulation. A proposed alternative must be submitted and authorized prior to implementation. The applicant or licensee must demonstrate:
(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 grant relief and the use of the proposed alternative.
3.0 TECHNICAL EVALUATION
3.1 Alternative Request 16R-11 3.1.1 ASME Code Components Affected RPV Head Flange RPV Shell Flange Materials RPV Head and Shell - SA-302 Grade B RPV Head and Shell Flanges - SA-336 3.1.2 Applicable Code Edition and Addenda The current code of record for Quad Cities, Unit 2, is the ASME Code,Section XI, 2017 Edition.
Quad Cities, Unit 2, is currently in its sixth 10-year inservice inspection (ISI) program interval which began on April 2, 2023, and will end on April 1, 2033.
3.1.3 Applicable Code Requirements The licensee implements ASME Code Case N-638-10 for Quad Cities, Unit 2, within the stations sixth interval ISI program. Code Case N-638-10 is approved in Table 1 of Regulatory Guide (RG) 1.147 Inservice Inspection Code Case Acceptability, ASME Section XI, Division 1, Revision 20, (ML21181A222). This Code Case is an approved alternative to ASME Code,Section XI, IWA-4400, to use an automatic or machine GTAW temper bead technique to repair certain materials.
The proposed repair(s) will be performed by ATTB welding in accordance with ASME Code Case N-638-10. Paragraph 4(a)(2) of Code Case N-638-10 requires the final NDE be performed at least 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> after the completion of the third tempering layer. In accordance with paragraph 4(a)(2), examination of the welded region shall include both volumetric and surface examination methods.
3.1.4 Proposed Alternative As an alternative to the requirement of Code Case N-638-10, paragraph 4(a)(2), which requires a 48-hour hold time after the third tempering layer prior to performing NDE, the licensee intends to perform weld repairs with austenitic Alloy 82 filler material in accordance with the ATTB welding technique of Code Case N-638-10 with two exceptions. As an alternative to performing the required NDE at least 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> after the completed weld has reached ambient temperature, the licensee is proposing to perform the NDE methods without waiting for the required 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br />.
Additionally, the licensee proposes to perform progressive surface examinations in lieu of the required volumetric examination in Code Case N-638-10.
3.2
NRC Staff Evaluation
The licensee is proposing to perform emergent weld repairs(s) at Quad Cities, Unit 2, using ATTB welding. The licensee stated that the affected areas needing repair will be mechanically prepared to support the necessary welding. The licensee stated that the repair activities will be to remove surface induced defects, reweld/build up the mating surface, and re-machine to allow for proper sealing surfaces for the O-rings as designed. The depth of the repair work should be constrained to the cladding thickness and possibly a limited amount of low alloy steel flange material beneath the cladding.
Welding will be performed using the machine GTAW process with Alloy 82 filler material. The licensee stated that a minimum of three weld layers will be applied for each repair. In lieu of performing the surface and volumetric examination on the completed weld at least 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> after the completion of the third tempering layer, the licensee is proposing to perform the NDE after each layer without waiting for the required 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br />.
In addition, the licensee stated that after completion of each layer, a surface exam will be performed in accordance with the rules of ASME Code,Section III. The licensee is requesting the use of progressive surface exams as an alternative to the requirement of N-638-10 to perform a volumetric examination of the welded region. Dye penetrant test (PT) examination will be performed on 100 percent of each layers surface. Surface exams will be performed progressively, as the weld build up is applied through to the finished machined surface. Each layer will satisfy the PT acceptance criteria for welds of ASME Code,Section III, NB-5300. The adjacent base material will satisfy the surface examination acceptance criteria for base material of ASME Code,Section III, NB-2500. All NDE will be performed by qualified examiners with a procedure qualified in accordance with ASME Code,Section XI.
The NRC staff issued request for additional information (RAI-1) requesting the licensee provide additional information regarding how thickness, geometry, and orientation of the reactor flanges prevent volumetric examinations. In addition, the staff requested the licensee to discuss the maximum extent to which the volumetric and surface examinations of the mating surfaces between the reactor pressure vessel head and the shell flanges can be performed in accordance with Code Case N-638-10, paragraph 4(a)(2).
By letter dated April 28, 2023, the licensee provided a response to RAI-1. In its response, the licensee stated that based on the configuration of the anticipated area of the flange which may need to be repaired and the surface from which the ultrasonic test (UT) examination would be performed, no parallel surfaces exist which would allow the examiner to either establish a back wall or use a back wall response for monitoring to determine if responses would need to be recorded. The licensee stated that the reactor head configuration does not allow examination from the outside surface looking in towards the area that may need to be repaired and based on surface geometry from the O-ring grooves, examination cannot be performed from the flange surface. The licensee provided a figure to support the above statement. Physical geometry, extreme and varied thicknesses, and interferences such as stud holes or landings would prevent shear wave UT or other volumetric examinations from being used from the mating surface or from either perpendicular surface (i.e., flange sides). The licensee stated that UT shear or surface wave applications nor near surface defects in the weld repair areas would not provide an increase in detection of service or weld induced defects greater than the proposed progressive surface examination alternative.
The licensee also provided justification for not performing a radiographic test (RT). The licensee provided a figure to show that there is no practical place to put the source to perform an achievable RT image. In addition, the licensee stated that based on the thickness of the repair are being significantly thinner than the overall thickness of the flange, the possibility of having adequate resolution to identify or characterize any indications in the repair would not be achievable.
4.0 CONCLUSION
As set forth above, the NRC staff has determined that complying with the alternative requirements described in the licensees request referenced above would result in an acceptable level of quality and safety. Based on the information provided by the licensee, the staff agrees that volumetric (UT or RT) examination will not provide meaningful results because of the physical geometry, thicknesses and interferences from stud holes or landings. The staff concludes that progressive PT examinations on each layer of the temper bead welds which satisfy the acceptance criteria of ASME Code,Section III, NB-5300, and of the adjacent base material which satisfy the acceptance criteria of ASME Code,Section III, NB-2500, will ensure that the repaired areas maintain an acceptable level of quality and safety. The proposed alternative provides reasonable assurance that the use of progressive PT examinations of each of the three temper bead layers for the Quad Cities RPV head and shell flanges is a suitable alternative to the volumetric (UT or RT) inspection and provides reasonable assurance of structural integrity because sound welds are deposited.
In addition, the NRC staff has determined that the elimination of the 48-hour hold time before examining temper bead welding is acceptable because the use of austenitic Alloy 82 filler material that is to be used will provide a level of diffusible hydrogen content in the ferritic base metal heat affected zone will be too low to promote hydrogen-induced cracking (HIC). In addition, the temper bead welding will be in accordance with Code Case N-638-10 and will be applied with the machine gas tungsten arc welding process which produces welds with very low diffusible hydrogen when compared to the shielded metal arc welding process which uses a flux. Therefore, HIC is not likely to occur, making the 48-hour hold time requirement unnecessary. In addition, the NRC staff is unaware of any instances of HIC out of the hundreds of welds that have been completed by the nuclear industry using any of the revisions of Code Case N-638.
Accordingly, the NRC staff concludes that the licensee has adequately addressed the regulatory requirements set forth in 10 CFR 50.55a(z)(1). In reviewing the proposed alternative, the NRC staff concludes that the alternative will provide an acceptable level of quality and safety. The NRC staff authorizes the use of proposed alternative 16R-11 at Quad Cities, Unit 2, for the repairs to the RPV head and shell flanges expected to be performed during the forced maintenance outage Q2M30.
All other ASME Code,Section XI, requirements for which an alternative was not specifically requested and approved in the proposed alternative remain applicable, including third-party review by the Authorized Nuclear Inservice Inspector.
Principal Contributors: Eric Reichelt, NRR Mat Burton, NRR Date: May 8, 2023
ML23125A061 OFFICE NRR/DORL/LPL3/PM NRR/DORL/LPL3/LA NRR/DNRL/NPHP/BC NRR/DORL/LPL3/BC(A)
NAME RKuntz SRohrer MMitchell VSreenivas DATE 5/5/2023 5/8/2023 5/5/2023 5/8/2023