ML24323A214
| ML24323A214 | |
| Person / Time | |
|---|---|
| Site: | Monticello  |
| Issue date: | 11/21/2024 |
| From: | Robert Kuntz Plant Licensing Branch III |
| To: | Hafen S Northern States Power Company, Minnesota |
| Ballard, B | |
| References | |
| EPID L-2024-LLR-0014 | |
| Download: ML24323A214 (1) | |
Text
November 21, 2024 Shawn Hafen Site Vice President Northern States Power Company - Minnesota Monticello Nuclear Generating Plant 2807 West County Road 75 Monticello, MN 55362
SUBJECT:
MONTICELLO NUCLEAR GENERATING PLANT, UNIT NO. 1 - RELIEF REQUEST RR-017, INSERVICE INSPECTION (ISI) IMPRACTICALITY DURING THE FIFTH 10-YEAR INTERVAL (EPID L-2024-LLR-0014)
Dear Shawn Hafen:
By letter dated January 30, 2024 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML24033A295), Northern States Power Company, a Minnesota corporation (NSPM, the licensee), doing business as Xcel Energy, submitted a relief request pursuant to Title 10 of the Code of Federal Regulations (10 CFR), section 50.55a(g)(5)(iii),ISI
[Inservice Testing] program update: Notification of impractical ISI Code requirements, for Monticello Nuclear Generating Plant, Unit 1 (Monticello). The relief request is for nozzle-to-vessel weld examinations performed during the 2019, 2021, and 2023, refueling outages where the required coverage could not be obtained when examined to the extent practical due to plant design. The request is applicable for the fifth 10-year ISI Interval.
Specifically, pursuant to 10 CFR 50.55a(g)(5)(iii), the licensee requested relief from requirements of the American Society of Mechanical Engineers Boiler and Pressure Vessel Code, section XI, division 1 (ASME Code, section XI) for past volumetric inspections performed on specified reactor pressure vessel (RPV) nozzle-to-vessel (NV) welds during the fifth 10-year ISI interval that applies to Monticello. The licensee proposed to use the actual completed weld coverages for the referenced RPV NV welds as an acceptable alternative that provides reasonable assurance of continued structural integrity of the welds.
The U.S. Nuclear Regulatory Commission (NRC) staff has reviewed the subject request and concludes, as set forth in the enclosed safety evaluation, that NSPM has adequately addressed all of the regulatory requirements set forth in 10 CFR 50.55a(g)(5)(iii). The NRC staff further concludes that granting relief pursuant to 10 CFR 50.55a(g)(6)(i) is authorized by law and will not endanger life or property or the common defense and security and is otherwise in the public interest giving due consideration to the burden upon the licensee that could result if the requirements were imposed on the facility. Therefore, the NRC staff grants relief for the limited-scope volumetric examinations that were performed on the subject RPVs N-3C, N4-B, N-10, N2-G, N6, N7-and N4-D NV, welds during either the second or third period of the fifth 10-year ISI interval for Monticello, as approved through the end of the fifth 10-year ISI interval which ended on May 31, 2022.
All other ASME Code, section XI, requirements for which relief was not specifically requested and approved remain applicable, including the third-party review by the Authorized Nuclear lnservice Inspector.
If you have any questions, please contact the Project Manager, Brent Ballard, at 301-415-0680 or by e-mail to Brent.Ballard@nrc.gov.
Sincerely, Robert Kuntz, Acting Chief Plant Licensing Branch III Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket No. 50-263
Enclosure:
Safety Evaluation cc: Listserv ROBERT KUNTZ Digitally signed by ROBERT KUNTZ Date: 2024.11.21 11:54:46 -05'00'
Enclosure SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELIEF REQUEST NO. RR-017 REGARDING REACTOR PRESSURE VESSEL NOZZLE-TO-VESSEL WELDS NORTHERN STATES POWER COMPANY MONTICELLO NUCLEAR GENERATING PLANT, UNIT NO. 1 DOCKET NO. 50-263
1.0 INTRODUCTION
By letter dated January 30, 2024, (Agencywide Documents Access and Management System Accession No. ML24033A295), as supplemented by letter dated August 28, 2024, (ML24242A065), Northern States Power Company, a Minnesota Corporation (NSPM or the licensee), doing business as Xcel Energy, requested relief from the volumetric examination coverage requirements of the American Society of Mechanical Engineers Boiler and Pressure Vessel Code, section XI, division 1 (ASME Code, section XI) for past volumetric inspections performed on specified reactor pressure vessel (RPV) nozzle-to-vessel (NV) welds during the fifth 10-year inservice inspection (ISI) interval that applies to Monticello Nuclear Generating Plant, Unit No. 1 (Monticello).
Specifically, pursuant to Title 10 of the Code of Federal Regulations (10 CFR) 50.55a(g)(5)(iii),
the licensee requested relief as presented in relief request (RR)-017 based on its determination that compliance with the subject examination coverage requirements is impractical for certain nozzle-to-vessel weld examinations performed during the 2019, 2021, and 2023, refueling outages because required coverage could not be obtained when examined to the extent practical due to plant design.
2.0 REGULATORY EVALUATION
Pursuant to 10 CFR 50.55a(g)(4), inservice inspection of ASME Code Class 1, 2, and 3 components (including supports) shall meet the requirements, except the design and access provisions and the preservice examination requirements, set forth in the ASME Code, section XI, to the extent practical within the limitations of design, geometry, and materials of construction of the components. The regulation requires that ISI examinations of ASME Code Class 1, 2 and 3 components and system pressure tests conducted during the first 10-year ISI interval and subsequent ISI intervals must comply with the latest edition and addenda of ASME Code, section XI, that was incorporated by reference in 10 CFR 50.55a(a)(1)(ii) 18 months prior to the start of the 120-month (10-year) interval and subject to the limitations and modifications listed therein.
Section 50.55a(g)(5)(iii) of 10 CFR states that if a licensee has determined that conformance with a ASME Code requirement is impractical for its facility, the licensee must notify the U.S.
Nuclear Regulatory Commission (NRC or Commission) and submit, as specified in 10 CFR 50.4, information to support its determination. Determinations of impracticality in accordance with this section must be based on the demonstrated limitations experienced when attempting to comply with the ASME Code requirements during the ISI interval for which the request is being submitted. Requests for relief made in accordance with this section are required to be submitted for approval by the Director of the NRCs Office of Nuclear Reactor Regulation no later than 12 months after the expiration of the initial or subsequent 120-month (10-Year) inspection interval for which relief is sought.
Section 50.55a(g)(6)(i) of 10 CFR states that the Commission will evaluate determinations under paragraph (g)(5) of section 50.55a that ASME Code requirements are impractical. The regulation states that the Commission may grant such relief and may impose such alternative requirements as it determines are authorized by law, will not endanger life or property or the common defense and security and are otherwise in the public interest giving due consideration to the burden upon the licensee that could result if the requirements were imposed on the facility.
Based on the above regulatory requirements, and subject to the following technical evaluation, the NRC staff finds that regulatory authority exists for the licensee to request and the Commission to grant relief and impose such alternative requirements as it determines are authorized by law, will not endanger life or property or the common defense and security, and are otherwise in the public interest giving due consideration to the burden upon the licensee that could result if the requirements were imposed on the facility.
3.0 TECHNICAL EVALUATION
3.1 The Licensees Relief Request (Relief Request No. RR-017 for fifth 10-year ISI Interval)
Applicable Code Edition and Addenda. The licensee states that the applicable version of ASME Code,Section XI, for the fifth 10-year ISI interval of Monticello is the 2007 Edition of ASME Code,Section XI, inclusive of the 2008 Addenda.
Applicable ISI Interval Dates. The licensee states that the fifth 10-year ISI interval for Monticello began on September 1, 2012, and ended on May 31, 2023.
ASME Code Requirements and NRC-Approved Code Case The licensee states that the ISI criteria in ASME Code, section XI, table IWB-2500-1, Examination Category B-D, Inspection Item No. B3.90, requires the licensee to perform volumetric examinations of the subject RPV NV welds using the examination volume specified in ASME Code, section XI, figure IWB-2500-7(b). The licensee states that this figure shows the weld configuration that is applicable to the subject RPV NV welds at Monticello. However, the licensee indicated that at the start of the fifth 10-year ISI interval for Monticello, it applied ASME Code Case N-613-1, Ultrasonic Examination of Penetration Nozzles in Vessels, Examination Category B-D, Item Nos. B3.10 and B3.90, Reactor Nozzle-to-Vessel Welds, Figs. IWB-2500-7(a), (b), and (c),Section XI, Division 1, to the current licensing basis for the ISI program, as endorsed for use in NRC Regulatory Guide (RG) 1.147, Revision 16. In relation to this, the licensee identifies that ASME Code Case N-613-1 permits use of an alternative, redefined, examination volume for the RPV NV welds in figure 2 of the code case, where the configuration in the code case figure includes the width of the RPV NV weld plus one-half of an inch of the adjacent base metal on each side of the widest part of the weld. This is in contrast to the weld volume in ASME Code, section XI, figure IWB-2500-7(b), which dictates a weld volume covering the width of the weld plus the adjacent base metal on each side of the widest part of the weld equal to one-half of the vessel shell wall thickness. It should be noted that per the criteria in RG 1.147, Revision 20 (as is the latest revision of the RG incorporated by reference into 10 CFR 50.55a), ASME Code Case N-613-1 has been superseded by Code Case N-613-2.
However, the reduced examination volume specified in figure 2 of Code Case N-613-2 is the same as that in figure 2 of ASME Code Case N-613-1 for the subject RPV NV weld examinations.
The licensee also states that ASME Code, section XI, paragraph IWA-2200(c, requires that when VT-1 visual, surface, radiographic, or ultrasonic examination techniques are performed on an ASME Code Class component with a defined surface area or volume, essentially 100 percent of the required surface area or volume shall be examined. The licensee clarified that essentially 100% coverage is achieved when the applicable examinations coverage is greater than 90 percent of the required surface area or volume.
Applicable Components and ISI Interval Dates of Examination.
Consistent with the licensees information, table 3.1-1 below, lists the ASME Code, section XI, Class 1, RPV NV welds that were examined and subject to limited inspections during the years 2019, 2021, and 2023, refueling outages (and the associated ISI inspection period for examinations), the limited examination coverages associated with the ISI inspections of the NV welds and additional details (e.g., applicable ASME Code, section XI, Examination Category and Inspection Item, whether any flaw indications were detected during the performed inspections, and the cause of limited inspection coverage scope).
ASME Section XI Examination Category and Inspection Item RPV Nozzle Component ID Number Component Description Percent Weld Coverage Obtained During the UT Inspection Inspection Required by ASME Code Case N-613-1, Figure 2 Cause of Limited Coverage Report Indications RPV NV Welds Subject to Limited Weld Coverages for UT Examinations Performed in Year 2019 (2nd Period, 5th 10-Year ISI Interval Inspections)
Examination Category BD, Inspection Item B3.90 N-3C NV RPV Main Steam Line Nozzle NV Weld 83%
90% by Volume Access limited due to nozzle configuration One (Acceptable Indication)
Examination Category BD, Inspection Item B3.90 N-4B NV RPV Feedwater Inlet Nozzle NV Weld 83%
90% by Volume Access limited due to nozzle configuration None Examination Category BD, Inspection Item B3.90 N-10 NV RPV Standby Liquid Control Nozzle NV Weld 85%
90% by Volume Access limited due to nozzle configuration None RPV NV Welds Subject to Limited Weld Coverages for UT Examinations Performed in Year 2021 (3rd Period, 5th 10-Year ISI Interval Inspections)
Examination Category BD, Inspection Item B3.90 N-2G NV RPV Recirculation Inlet Nozzle NV Weld 78%
90% by Volume Access limited due to nozzle configuration None Examination Category BD, Inspection Item B3.90 N-6A NV RPV Head Spare Nozzle NV Weld 86%
90% by Volume Access limited due to nozzle configuration None ASME Section XI Examination Category and Inspection Item RPV Nozzle Component ID Number Component Description Percent Weld Coverage Obtained During the UT Inspection Inspection Required by ASME Code Case N-613-1, Figure 2 Cause of Limited Coverage Report Indications Examination Category BD, Inspection Item B3.90 N-7 NV RPV Head Vent Nozzle NV Weld 87%
90% by Volume Access limited due to nozzle configuration None RPV NV Welds Subject to Limited Weld Coverages for UT Examinations Performed in Year 2023 (3rd Period, 5th 10-Year ISI Interval Inspections)
Examination Category BD, Inspection Item B3.90 N-4D NV RPV Feedwater Inlet Nozzle NV Weld 83%
90% by Volume Access limited due to nozzle configuration None Table 3.1 RPV NV Welds Within the Scope of RR-017 Summary of Licensees Non-Destructive Examination (NDE) Methods and Results Section 6.2.1 in Enclosure 1 of RR-017 provides a full summary of the NDE methods used to inspect the referenced RPVs N-3C, N-4B, N-10, N-2G, N-6A, N-7, and N-4D NV, welds during the second or third period of the fifth10-year ISI interval. The licensee stated that it performed the required volumetric inspections of these welds using UT (ultrasonic testing) methods, including: (1) the refracted longitudinal wave mode of UT propagation for scans of the examination volume in the radial direct and for the outer 85 percent of the weld volume for parallel scans, and (2) the UT shear wave mode of propagation for the inner 15 percent of the weld volume during parallel scans.
The licensee stated that the volumetric UT scans of the subject RPV NV welds were performed from both the inside surfaces and outside surfaces of the welds to the maximum extent practical within the limitations imposed by the design of the components. The licensee explained that the UT inspections performed on inside surfaces of the welds covered in the inside 15 percent volumes of the welds and achieved 100 percent of the required inner 15 percent weld volumes.
The licensee stated that the UT examinations from the outside surfaces of the welds that were performed cover the outer 85 percent volumes of the welds and that it was these UT examinations from the outside surfaces of the welds that encountered limited weld access.
Specifically, the licensee identified that the referenced RPV NV welds are fully accessible from the inside vessel plate sides of the welds. However, the licensee clarified that, for the outside surfaces, the curvatures of the nozzle forgings and the proximity of the nozzle forgings to the NV welds precluded the licensee from achieving the needed coverages of the specific areas identified in the RR-017 figures as being excluded from the outer 85 percent portions of the weld volumes.
Licensee's Reported Impracticality and Burden Caused by Compliance The licensee indicated that the Monticello construction permit was issued in 1967 and that the Monticello systems and components were designed for construction before the ISI requirements of the ASME Code, section XI, were formalized and published. Since Monticello components were not specifically designed to meet the requirements of ASME Code, section XI, the licensee stated that full compliance with the requirements in ASME Code, section XI, is not feasible or practical within the limits of the Monticello design. The licensee also stated that 10 CFR 50.55a recognizes that there may be limitations to the performance of ISI examinations of ASME Code Class components in accordance with the ASME Code, section XI (including those applicable to ASME Code Class 1 RPV NV welds), as referenced for implementation in accordance with the requirements in the 10 CFR 50.55a(g)(1) and (g)(4) rules. However, the licensee states that 10 CFR 50.55a(g)(5)(iii) permits licensees to request relief from the requirements of ASME Code, section XI, for requirements that are determined to be impractical for implementation at the facility.
The licensee states that the UT examination limitations for the subject RPV NV welds are due to inherent geometric contours associated with the nozzle design, as illustrated in the attachments that were included in the RR-017 submittal. The licensee reports that its limited coverage examinations were conducted to the extent practical within the design constraints of the subject RPV NV weld components and that full compliance with the examination coverage requirements in ASME Code, section XI, paragraph IWA-2200(c), and use of the alternate RPV NV weld volume defined in Figure 2 of ASME Code Case N-613-1, would require modification or replacement of these components where geometric contour is inherent to the component design.
Based on these criteria, the licensee states that full compliance with the volumetric inspection requirements for RPV NV welds in ASME Code, section XI, table IWB-2500-1, Examination Category B-D, Inspection Item B3.90, and the corresponding weld volume coverage criteria in ASME Code, section XI, paragraph IWA-2200(c) (as linked to the weld volumes defined in figure 2 of Code Case N-613-1), is impractical for the design configurations of RPVs N-3C, N-4B, N-10, N-2G, N-6A, N-7, and N-4D, NV welds that were inspected during the second or third period of the fifth 10-year ISI interval for Monticello.
Licensee's Proposal to Demonstrate Structural Integrity of the Welds In RR-017, the licensee requested relief on the achieved weld volume coverages for RPVs N-3C, N-4B, N-10, N-2G, N-6A, N-7, and N-4D NV, welds on the basis that the required examination coverage of essentially 100 percent by volume criterion is impractical due to the physical obstructions and limitations imposed by the design, geometries and materials of construction. The licensee proposed to use the actual completed weld coverages (in the range of 79 percent - 85 percent achieved weld coverage by volume) as an acceptable alternative that provides reasonable assurance of continued structural integrity of the referenced RPV NV welds.
The licensee supported its request through inclusion of tables A, B, and C, in Enclosure,, of RR-017, which summarize the specific RPV NV welds that were inspected and subject to a limited weld coverages during the years 2019, 2021, and 2023, inspections, and the results of the UT inspections. The licensee also provided inspection scan result figures for the referenced RPV NV nozzles, as included in Enclosure, Attachment 2, of RR-017 (with the figures defining the actual RPV NV weld volumes achieved when compared to the minimum volume defined and required for the subject NV welds in figure 2 of Code Case N-613-1).
3.2 NRC Staff Evaluation The NRC staff evaluated the licensee's determination in accordance with the ISI impracticality provisions of 10 CFR 50.55a(g)(5)(iii) in order to confirm that achievement of the volumetric inspection weld coverage requirements specified in ASME Code, section XI, paragraph IWA-2200(c) or the alternate RPV NV weld volume defined in figure 2 of ASME Code Case N-613-1 are impractical for the RPV nozzle welds specified in RR-017. The staffs review addressed the following information:
(a) information describing examination limitations and figures showing component geometry, which limited UT transducer access for achieving 100 percent examination coverage of the examination volume defined for inspection in Figure 2 of ASME Section XI Code Case N-613-1; (b) figures illustrating UT scan directions and examination coverages for the inner 15 percent and outer 85 percent of the examination volume; (c) summaries of limited-scope examination results (i.e., relevant indications or lack thereof) in Tables A, B, and C of RR-017; (d) component material flaw information and operational experience (OpE) that is relevant to the structural integrity of the welded joints.
Applicable ASME Code,Section XI, Figure or ASME Code Case Figure Used to Define the Minimum Weld Volumes Required for Subject RPV NV Welds within the Scope of RR-017 Examination Item No. B3.90 of the ASME Code, section XI, table IWB-2500-1, Examination Category B-D, requires that essentially 100 percent (i.e., at least a 90 percent weld coverage by volume) be achieved for the applicable RPV NV weld volume that is defined in ASME Code, section XI, figures IWB-2500-7(a), IWB-2500-7{b), or IWB-2500-7(c), for the applicable NV weld configuration, or alternatively for the alternate volume that is defined in either ASME Code, section XI, Code Case N-613-1 or N-613-2. Thus, the NRC staff noted that the licensee's citation and use of ASME Code, section XI, Code Case N-613-1, as the applicable ASME code case providing the minimum alternate weld volume for the specified welds does not impact the basis for requesting weld volume coverage impracticality in the RR-017 relief request.
Therefore, the NRC staff finds that the licensee's use of ASME Code, section XI, Code Case N-613-1, figure 2, for establishing the required examination volumes of the for the RPV NV welds in RR-017 is acceptable.
Applicable Volumetric Examination Limitations The NRC staff reviewed RR-017, relative to the licensees basis for claiming impracticality with the ability to achieve the minimum weld coverage criteria set in figure 2 of ASME Code Case N-613-1.
The NRC staff noted that RR-017 identifies that the licensee performed the required UT inspections of the subject RPV NV welds in accordance with the ISI requirements defined in ASME Code, section XI, table IWB-2500-1, Examination Category B-D, Item B3.90. The staff confirmed that the RR-017 request appropriately identifies that the subject RPV NV welds received the required examinations to the extent practical within the limited access of the component design, and that the UT inspections performed on the inner 15 percent portions of the weld volumes achieved 100 percent coverage of the required inside portion inspection volumes. The NRC staff verified that the RR-017 request appropriately explains that the examination volumes subject to the configurational limitations of the NV weld designs (and subject to the claims of ISI impracticality in RR-017) involved the 85 percent outer portions of the weld volumes for outside surface UT inspections performed in both the radial and circumferential scanning directions.
Enclosure, Attachment 2, in the RR-017 request includes scanning figures for the following RPV NV welds:
RPV N3-C main steam line nozzle NV weld (limited UT inspection in Year 2019)
RPV N4-B feedwater inlet nozzle NV weld (limited UT inspection in Year 2019)
RPV N10 standby liquid control nozzle NV weld (limited UT inspection in Year 2019)
RPV N2-G recirculation inlet nozzle NV weld (limited UT inspection in Year 2021)
RPV N6-A RPV head spare nozzle NV weld (limited UT inspection in Year 2021)
RPV N7 RPV head vent nozzle NV weld (limited UT inspection in Year 2021)
RPV N4-D feedwater inlet nozzle NV weld (limited UT inspection in Year 2023)
The NRC staff noted that the figures included cross-sectional diagrams of the subject welds that depicted: (1) the configurations of the welds including the cross sections of the welds and configurations and curvature radii of the inner blend radius sections of the associate RPV nozzles, along with portions of the RPV base metal associated of the weld configurations; and (2) the required examination volumes of the welds as established required by figure 2 of ASME Code, section XI, Code Case N-613-1, including the small portions of the nozzle inner blend radii sections and RPV shell base metal that are required to be within the scope of the required examination volume. The staff noted that these inspection scan figures illustrated the amount of examination coverage that was achieved for the two orthogonal scan directions (perpendicular and parallel to the weld) for both the inner 15 percent and the outer 85 percent of the required examination volume. Based on its review of these figures, the NRC staff verified that the longitudinal wave UT transducer scans from the outside surfaces of the welds, for both radial and circumferential scans performed from the outside surfaces, would not be able to achieve significant coverage of the examination volume because the surfaces of the nozzle forgings adjoined to the subject welds (which include a curved nozzle blend radius section of the nozzle) are contoured in a manner that limits the amount of coverage that would be capable of being achieved using the longitudinal wave UT transducers applied to the outside surfaces of the welds.
Thus, based on its review, the NRC staff confirmed that the diagrams of the subject RPV NV welds and associated RPV nozzle inner blend radius sections in the figures provided sufficient demonstration that the contours of the nozzle blend radius sections had created an obstruction (and limitation), such that the face of the longitudinal wave UT transducer could not be maintained flush against the outside surface of the nozzle inside radius section or the portion of the weld metal immediately adjacent to the nozzle inside radius. Thus, consistent with the licensees claim, the staff confirmed that such obstructions in the component design would preclude the transmission of adequate sound energy from the longitudinal wave into the portions of the RPV nozzle inside radius and NV weld metal (of the subject RPV NV weld) that are obstructed by the configuration contour of the RPV nozzle design. The NRC staff verified that UT shear wave scans performed during circumferential scans from the inside surfaces of the nozzles and adjacent NV welds were performed on flat surfaces and thus were capable of achieving the required full (i.e., essentially, 100 percent) coverage of the inner 15 percent of the examination volume of the component configurations (including the portions of the nozzle, NV weld, and adjacent RPV shell material) subject to the ASME Code, section XI, Inspection Item B3.90, inspection requirements along with a reasonable amount of coverage (about 80 percent) of the outer 85 percent of the examination volume. Thus, the NRC staff verified that the specified examination coverage limitations were restricted to those UT inspections that were performed on the outer 85 percent of the examined weld volumes. Based on its review of this information, the NRC staff determined that the licensee has provided adequate demonstration that the geometric contours of the subject RPV NV weld joints prevented the performance of adequate UT scans from the outside surfaces of the subject NV welds and the associated nozzle blend radii sections, thereby, limiting the volumetric examination coverages for the subject RPV NV welds to coverages in the range of 78 percent to 87 percent of examination volumes required for inspection in accordance with figure 2 of ASME Code, section XI, Code Case N-613-1.
Limited-Scope Examination Results and Aging Management Operating Experience RR-017 states that the licensee did not identify any relevant indications as a result of the UT examination inspections that were performed in either year 2019, 2021, or 2023. The NRC staff determined that this statement in RR-017 would require further clarifications and reconciliation from the licensee, given that table A of RR-017 indicates that one flaw indication was detected in the N-3C RPV main steam line nozzle NV weld as part of the UT inspections that were performed in 2019, and issued a request for confirmation of information (RCI) by email dated August 5, 2024 (ML24218A228).
In the supplement dated August 28, 2024 (ML24242A065), the licensee confirmed one relevant indication was detected during the 2019 examination of the N-3C NV weld and determined to be acceptable per ASME Code, section XI, paragraph IWB-3512-1. The licensee amended the statement in the RR-017 submittal to indicate that no unacceptable flaws were identified in the subject RPV NV welds. Based on the conformance of the flaw indication with the maximum flaw size allowed by ASME Code, section XI, paragraph IWB-3512-1, the NRC staff finds that it will be acceptable for the licensee to schedule the next required reinspection of the N3-C main steam line nozzle NV weld (including the flaw in the weld) during the sixth 10-year ISI interval for Monticello and that consecutive reinspections of the N3-C main steam line nozzle NV weld did not need to be included in the set of RPV NV weld inspections that were scheduled and performed during the years 2021 and 2023 examinations.
For the inspection results of the remaining subject RPV NV welds that did not have reportable flaw indications, the NRC noted that the inner 15 percent of the weld examination volume (for which 100 percent coverage was achieved) has a relatively greater potential for detecting age-related degradation when compared to the outer 85 percent of the examination volume (where the examination coverage was restricted), as the inner 15 percent is adjacent to the RPV interior cladding and is in close proximity to the interior wetted surface of the RPV. However, the staff observed that the likelihood of service-induced degradation resulting from new flaw formations is still very low even for the inner 15 percent of the weld volume, given that all pressure-retaining materials are made from low alloy steel materials. Industry operating experience with ISI of these types of full penetration RPV welds (as made for alloy steel filler metals) shows that the known material aging mechanisms (generally neutron embrittlement and accumulation of fatigue cycles) are very unlikely to result in the formation of new flaws in the welds. The NRC staff notes that low alloy steel weld filler metals used to fabricate the subject pressure-retaining RPV NV welds are not considered to be susceptible to the formation of new flaws by stress corrosion cracking (SCC) or irradiation-assisted stress corrosion cracking (IASCC) mechanisms. Instead, industry operating experience demonstrates that SCC or IASCC is more commonly known to occur in either sensitized stainless steel weld filler metal materials or nickel alloy weld filler metal materials (which are not used for the fabrication of the subject RPV NV welds). Cracking due to metal fatigue is also very unlikely since compliance with fatigue cumulative usage factor acceptance criteria, as per the applicable design code, is required for the duration of the renewed licensed operating term of the plant. Therefore, the NRC staff determined that the licensee's reported limited examination coverages for the subject RPV NV welds provide reasonable assurance of structural integrity for these components, given that the licensee has demonstrated that: (1) it has been able to achieve at least a 78 percent weld coverage for all volumetric inspections performed on the subject RPV NV welds during the fifth 10-year interval, and (2) any flaw indications detected in the subject RPV welds have been dispositioned as being acceptable for further service per the flaw standards of ASME Code, section XI, paragraph IWB-3512-1 (i.e., for the minor but acceptable flaw indication by Code that was detected in the RPV N3-C main steam line NV weld as a result of the UT inspection performed on the weld in year 2019).
NRC Staff Technical Evaluation Summary Based on the evaluation above, the NRC staff finds that the licensee has adequately demonstrated the impracticality associated with achieving essentially 100 percent examination coverage of the subject RPV NV welds based on the demonstrated limitations it experienced when attempting to comply with the Code, section XI, weld volume requirements, as defined in figure 2 of ASME Code Case N-613-1. Further, the NRC staff finds that the licensee's limited-scope volumetric examinations that were already performed on the subject welds for the fifth 10-year ISI interval provide reasonable assurance of structural integrity for the subject components.
4.0 CONCLUSION
As set forth above, the NRC staff determines that compliance with ASME Code, section XI, requirement for essentially 100 percent volumetric examination of the subject RPV NV welds at Monticello is impractical for the licensee. The NRC staff also determines that the licensee's limited-scope volumetric examinations that were already performed on the subject RPV NV welds during the second and third periods of the fifth 10-year ISI interval provide reasonable assurance of structural integrity for the subject weld components. Accordingly, the NRC staff concludes, that the licensee has adequately addressed all of the regulatory requirements set forth in 10 CFR 50.55a(g)(5)(iii). The NRC staff further concludes that granting relief pursuant to 10 CFR 50.55a(g)(6)(i) is authorized by law and will not endanger life or property or the common defense and security and is otherwise in the public interest giving due consideration to the burden upon the licensee that could result if the requirements were imposed on the facility.
Therefore, the NRC staff grants relief for the limited-scope volumetric examinations that were performed on the subject RPVs N-3C, N4-B, N-10, N2-G, N6, N7-and N4-D NV welds during either the second or third period of the fifth 10-year ISI interval for Monticello, as approved through the end of the fifth 10-year ISI interval which ended on May 31, 2023.
All other ASME Code, section XI, requirements for which relief was not specifically requested and approved remain applicable, including the third-party review by the Authorized Nuclear lnservice Inspector.
Principal Contributor: J. Medoff, NRR Dated: November 21, 2024
ML24323A214 OFFICE NRR/DORL/LPL3/PM NRR/DORL/LPL3/LA NRR/DNRL/NVIB/BC NRR/DORL/LPL3/BC(A)
NAME BBallard SRohrer ABuford RKuntz DATE 11/8/2024 11/19/2024 10/25/2024 11/21/2024