ML20307A017
| ML20307A017 | |
| Person / Time | |
|---|---|
| Site: | Monticello  |
| Issue date: | 11/02/2020 |
| From: | Nancy Salgado Plant Licensing Branch III |
| To: | Conboy T Northern States Power Company, Minnesota |
| Kuntz R | |
| References | |
| EPID L-2020-LLR-0022 | |
| Download: ML20307A017 (10) | |
Text
November 2, 2020 Mr. Thomas A. Conboy 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 - REQUEST FOR ALTERNATIVE FOR CORE SUPPORT STRUCTURE WELD EXAMINATION (EPID L-2020-LLR-0022)
Dear Mr. Conboy:
By letter dated February 14, 2020 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML20045E769), as supplemented by letter dated July 20, 2020 (ADAMS Accession No. ML20202A474), Northern States Power Company, a Minnesota corporation doing business as Xcel Energy (NSPM, the licensee), submitted to the U. S.
Nuclear Regulatory Commission (NRC), a proposed alternative to the inservice inspection (ISI) requirements of the American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code) for the H8, H9, and H10 welds of the core support structure at the Monticello Nuclear Generating Plant (Monticello).
Specifically, pursuant to Title 10 of the Code of Federal Regulations (10 CFR) Part 50, paragraph 50.55a(z)(2), in lieu of performing the required ASME Code successive examinations in ASME Code,Section XI, IWB-2420(b) and (c) for the H8, H9, and H10 welds, alternative request RR-016 proposed to revise the previously authorized alternative in RR-008 (ADAMS Accession No. ML15013A036) for the H8 and H9 welds to remove the 2021 refueling outage from the scope of the authorized alternative and to apply a similar alternative to the H10 welds of the core support structure. The relief requested to use the proposed alternative on the basis that complying with the requirements would result in hardship or unusual difficulty without a compensating increase in quality or safety.
As set forth in the enclosed safety evaluation, the NRC staff determined that complying with the subsequent examinations specified in IWB-2420(b) and (c) of Section XI of the ASME Code would result in hardship or unusual difficulty without a compensating increase in quality or safety. Accordingly, the NRC staff concludes that the request has adequately addressed all of the regulatory requirements set forth in 10 CFR 50.55a(z)(2). Therefore, the NRC staff authorizes alternative request RR-016 at Monticello for the remainder of the fifth 10-year ISI interval expected to end on May 31, 2022.
All other requirements of Section XI of the ASME Code for which an alternative was not specifically requested and approved remain applicable, including third party review by the Authorized Nuclear Inservice Inspector.
T. Conboy If you have any questions, please contact Robert Kuntz at 301-415-3733 or via e-mail at Robert.Kuntz@nrc.gov.
Sincerely, Digitally signed by Joel S.
Joel S. Wiebe Date: 2020.11.02 Wiebe 15:26:34 -05'00' Nancy L. Salgado, Chief Plant Licensing Branch III Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket No. 50-263
Enclosure:
Safety Evaluation cc: Listserv
SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION REQUESTS FOR ALTERNATIVE NO. RR-016 ASME CODE, SECTION XI, EXAMINATION CATEGORY B-N-2 H8, H9, AND H10 WELDS, OF THE CORE SUPPORT STRUCTURE FIFTH 10-YEAR INSERVICE INSPECTION INTERVAL NORTHERN STATES POWER COMPANY - MINNISOTA MONTICELLO NUCLEAR GENERATING PLANT DOCKET NO. 50-263
1.0 INTRODUCTION
By letter dated February 14, 2020 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML20045E769), as supplemented by letter dated July 20, 2020 (ADAMS Accession No. ML20202A474), Northern States Power Company, a Minnesota corporation doing business as Xcel Energy (NSPM, the licensee), submitted to the U. S.
Nuclear Regulatory Commission (NRC), a proposed alternative to the inservice inspection (ISI) requirements of the American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code) for the H8, H9, and H10 welds of the core support structure at the Monticello Nuclear Generating Plant (Monticello).
Specifically, pursuant to Title 10 of the Code of Federal Regulations (10 CFR) Part 50, paragraph 50.55a(z)(2), in lieu of performing the required ASME Code successive examinations in ASME Code,Section XI, IWB-2420(b) and (c) for the H8, H9, and H10 welds, alternative relief request (RR)-016 proposed to revise the previously authorized alternative in RR-008 (ADAMS Accession No. ML15013A036) for the H8 and H9 welds to remove the 2021 refueling outage (RFO) from the scope of the authorized alternative and to apply a similar alternative to the H10 welds of the core support structure. The relief requested to use the proposed alternative on the basis that complying with the requirements would result in hardship or unusual difficulty without a compensating increase in quality or safety.
2.0 REGULATORY EVALUATION
The core support structural welds, H8, H9, and H10, at Monticello are ASME Code, Class 1 components, whose ISI are performed in accordance with Section XI, Rules for Inservice Enclosure
Inspection of Nuclear Power Plant Components, of the ASME Code and applicable edition and addenda, as required by 10 CFR 50.55a(g).
Pursuant to 10 CFR 50.55a(g)(4), 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 regulations require that inservice examination of components and system pressure tests conducted during the first 10-year ISI interval and subsequent intervals comply with the requirements in the latest edition and addenda of Section XI of the ASME Code incorporated by reference in 10 CFR 50.55a(a)(1)(ii) 12 months prior to the start of the 120-month interval, subject to the limitations and modifications in 10 CFR 50.55a(b)(2).
Licensees may propose alternatives to the requirements in 10 CFR 50.55a(g)(4) prior to implementation of the alternative, as specified in 10 CFR 50.55a(z).
Based on the above and subject to the following technical evaluation, the NRC staff finds that regulatory authority exists for the licensee to request the alternative and the NRC staff to authorize it.
3.0 TECHNICAL EVALUATION
3.1 Components Affected Code Class: ASME Code,Section XI, Class 1 Examination Category: B-N-2, Welded Core Support Structures and Interior Attachments to Reactor Vessels Item No.: B13.30 Interior attachments beyond beltline region B13.40 Core support structure Component Nos.: Welds H8, H9, and H10 3.2 Applicable ASME Code Edition The ASME Code of record for Monticello for the fifth 10-year ISI interval is the 2007 Edition with the 2008 addenda of the ASME Code,Section XI.
3.3 ASME Code Requirements The 2007 Edition with the 2008 addenda of the ASME Code,Section XI, Table IWB-2500-1, Examination Category B-N-2, Item Nos. B13.30 and B13.40 require examination by visual testing (VT-3) of attachment welds H8, H9, and H10 located within the interior of the Monticello reactor pressure vessel. If the VT-3 examinations reveal relevant indications in these welds, and if analytical evaluation is selected for demonstrating continued operation, ASME Code,Section XI, IWB-3142.4 requires subsequent examinations as specified in IWB-2420(b) and (c).
IWB-2420(b) states:
If a component is accepted for continued service in accordance with IWB-3132.3 or IWB-3142.4, the areas containing flaws or relevant conditions shall be
reexamined during the next three inspection periods listed in the schedule of the Inspection Program of IWB-2400. Alternatively, acoustic emission may be used to monitor growth of existing flaws in accordance with IWA-2234.
IWB-2420(c) states:
If the reexamination required by IWB-2420(b) reveals that the flaws or relevant conditions remain essentially unchanged for three successive inspection periods, the component examination schedule may revert to the original schedule of successive inspections.
3.4 Reason for Proposed Alternative Section 4 of the enclosure to the submittal discussed the indication history of the H8, H9, and H10 welds and the limitations for accessing those welds for ISI examinations. The request stated that the limited access precluded examining all flawed areas of these welds within the lower plenum (below the core support plates), and disassembly/reassembly of surrounding reactor vessel internal components, such as jet pumps and fuel cells, to access all flawed areas for examination would entail additional work, time, and dose, and could damage the jet pump assemblies and other surrounding components. For these reasons, the request stated that complying with the ASME Code required examinations would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety, and the licensee is thus proposing an alternative pursuant to 10 CFR 50.55a(z)(2).
3.5 Proposed Alternative The 2014 alternative request, RR-008 (ADAMS Accession No. ML14064A185), proposed to visually examine all accessible areas (instead of all flawed areas) of the topside and underside of both the H8 and H9 welds during the remaining RFOs at the time within the three periods of the Monticello fifth 10-year lSI interval (i.e., 2015, 2017, 2019, and 2021). The NRC staff authorized RR-008 in as 2015 safety evaluation (SE), ADAMS Accession No. ML15013A036).
The current alternative request, RR-016, proposed to revise the previously authorized alternative in RR-008 for welds H8 and H9. Specifically, the request proposed to apply the examinations and monitoring conducted in the 2013, 2015, 2017, and 2019, RFOs, as an acceptable alternative to IWB-2420(b) and IWB-2420(c). The request proposed no examinations of the H8 and H9 welds during the 2021 refueling outage as was specified in RR-008.
Additionally, the request proposed to include the H10 welds in RR-016 because they are similar in location, examination processes, histories, and risks as the H8 and H9 welds. The request proposed no further examinations of the H10 welds at azimuthal locations 30°, 90°, 120°, 300°,
330°, and 350°.
3.6 Basis for Proposed Alternative Section 5 of the enclosure to the submittal discussed the basis for the proposed alternative. In the summary, the request stated the proposed alternative is based on: (1) hardship of examination via the jet pumps, or by disassembly of jet pumps or fuel assemblies and other internal components, which the licensee related to concerns of high dose and potential damage to the jet pumps, as discussed in Section 3.4 of this SE; (2) no changes in the indications detected in the H8, H9, and H10, welds; (3) structural evaluation of these welds, considering
crack growth that supports fewer examinations; and (4) continued implementation of Online Noble Metal Chemistry (OLNC).
3.7 Duration of Alternative The request proposed to apply the alternative for the remainder of the fifth 10-year ISI interval of the Monticello ASME Code,Section XI, ISI program expected to end on May 31, 2022.
3.8 Staff Evaluation The NRC staff reviewed the reasons for requesting an alternative pursuant to 10 CFR 50.55a(z)(2) provided in Section 4 of the enclosure to the submittal. The staff noted that these are the same reasons proposed in RR-008 and determined that the same limited access conditions to the flawed areas that existed in RR-008 still exist. Therefore, the staff finds that the additional visual examination of the H8 and H9 welds in 2021 per the NRC-approved request in RR-008 to fulfill IWB-2420(b) of the ASME Code and additional visual examination of the H10 welds would be subject to the same hardship and risks with respect to high dose and damage to surrounding components without adding safety value. The staff also finds that additional visual examinations would not guarantee meaningful results because the oxide layer build up, due to online noble metal injection around the indications, can preclude clear images of the indication as the licensee reported in the 2017 RFO examination of the H10 welds.
Accordingly, the staff finds the licensees reasons for its request described in Section 4 of the enclosure, meet the intent of 10 CFR 50.55a(z)(2) in that a hardship of complying with the ASME Code requirements (in this case the requirements on successive examinations) exists without a compensating increase in quality or safety.
The NRC staff reviewed the bases for the proposed alternative discussed in Section 5 of the enclosure to the submittal. The request reiterated the reasons of hardship of accessing the H8, H9, and H10, welds via the jet pumps and fuel cells, which the NRC staff evaluated above. The request also discussed how structural integrity of these welds are maintained without having to further examine them for the remainder of the fifth 10-year ISI interval. The NRC staffs evaluation of the structural integrity assessment provided in the request is provided in the following sections.
H8 and H9 welds The request stated that the H8 and H9 welds are made of Inconel 82/182 and join the horizontal shroud support plate to the inside surface of the reactor pressure vessel bottom head (H9 weld) and to the outside surface of the core shroud (H8 weld). The shroud support plate (along with other reactor internals) maintains adequate coolant flow during normal operations, seismic disturbances, and design basis accidents. Specifically, the shroud support plate is part of the envelope of the core coolant flow to maintain two thirds core height in the event of a loss of coolant accident in the recirculation line. Also, together with the shroud legs, the shroud support plate provides lateral support for the core shroud, which contain the fuel cell assemblies. Thus, since the H8 and H9 welds hold the shroud support plate in place, their structural integrity plays a safety role by maintaining adequate coolant flow to the core and structural alignment and integrity of the fuel cell assemblies.
The request stated that extensive analysis was performed on the H8 and H9 welds, citing a structural evaluation package by Structural Integrity Associates (SIA) in 2014 (ADAMS Accession No. ML14064A186, Reference 3 of the enclosure to the submittal, the same
evaluation included as an enclosure in RR-008). The NRC staff reviewed the analyses of the H8 and H9 welds in the 2014 SIA structural evaluation during the review of alternative request RR-008. SIA first analyzed the H8 and H9 welds in 2011 and updated the analyses in 2013.
Two flaw profiles in the H8 and H9 welds were considered in the 2014 SIA structural evaluation.
The first flaw profile (Case 1) assumed through-wall flaws (fully cracked, no remaining ligament) in the uninspected regions of the H8 and H9 welds, and 1/2 remaining ligament in the inspected regions. The second flaw profile (Case 2) assumed a surface flaw, 75 percent of the support plate thickness deep, from the bottom side of the H8 and H9 welds along the entire circumferential lengths of the welds.
Growth of the through-wall flaws in the length (circumferential) direction was accounted for in Case 1. The NRC staff has determined that even though growth in the depth direction was not considered for both Cases 1 and 2, the structural evaluation for the H8 and H9 welds is acceptable because the acoustic loads were conservatively doubled in the limiting load conditions (Level C/D in Tables 5 and 6 of the 2014 SIA structural evaluation). The NRC staff also notes that the assumed flaw profiles in Cases 1 and 2 are relatively large to account for growth in the depth direction. The request stated that the subject welds are mitigated against flaw growth and flaw initiation with the implementation of OLNC in 2013, citing BWRVIP [Boiling Water Reactor Vessel and Internal Project]-190, BWR Water Chemistry Guidelines, and BWRVIP 2019-025 2019 BWR Water Chemistry Guidelines Interim Guidance. The staff was not clear if BWRVIP-190 and/or BWRVIP 2019-025 have incorporated the guidance in letter dated January 24, 2018 (ADAMS Accession No. ML18033A323) regarding the use and crediting of all form of hydrogen water chemistry (HWC) mitigation. This guidance states in part that BWR units that use and credit all forms of HWC shall meet the conditions and limitations of BWRVIP-62-A. In the case of plants utilizing OLNC, this means they shall meet the Category 3a NMCA [noble metal chemistry addition] parameters and implementation steps (including platinum loading) of Tables 3-5 and 3-8 [of proprietary report BWRVIP-62-A, BWR Vessel and Internals Project, Technical Basis for Inspection Relief for BWR Internal Components with Hydrogen Injection, May 2019]. The supplement to the submittal confirmed that Monticello meets the Category 3a NMCA parameters and implementation steps (including platinum loading) of Tables 3-5 and 3-8 of BWRVIP-62-A per the January 24, 2018 guidance. The NRC staff finds the response acceptable and therefore determined that HWC mitigation is effective at Monticello.
The request stated that successive examinations of the H8 and H9 welds during four subsequent RFOs (in 2013, 2015, 2017, and 2019) demonstrated no apparent changes to the relevant indications detected in 2011 in these welds. The request also stated that in 2019 it examined 37 percent and 38 percent of the topsides of H8 and H9 welds, respectively, with no relevant indications. The NRC staff finds these topside examinations acceptable since they meet the minimum required unflawed topside lengths of the H8 and H9 welds calculated in the 2014 SIA structural evaluation for Monticello.
Lastly, the request addressed the potential of the relevant indications in the H9 weld extending into reactor pressure vessel base metal by stating that none of the indications extended beyond the boundary of the H8 and H9 welds. The NRC staff finds this acceptable.
Given no apparent changes in the H8 and H9 welds for four examinations after initial detection, the conservatism in the H8 and H9 weld structural evaluation and growth considerations, the NRC staff finds that the evaluation of structural integrity provided in the request for the H8 and H9 welds acceptable and, therefore, not examining the H8 and H9 welds in 2021 is reasonable (see proposed alternative summarized in Section 3.5 of this SE).
H10 welds The request stated that the H10 welds are made of Inconel 182 and join the 14 shroud support legs to the bottom of the core shroud. The shroud support legs provide primarily vertical support to the core shroud, and together with the shroud support plate, provide some lateral for the core shroud. Thus, since the H10 welds are one set of welds that hold the shroud support legs in place, their structural integrity plays a safety role by maintaining structural alignment and integrity of the fuel cell assemblies within the core shroud.
The request stated that the H10 welds in all 14 shroud support legs were assumed conservatively cracked to a depth of 31.2 percent of the H10 weld length. The NRC staff reviewed the description of this assumed conservative cracking in the 2014 SIA structural evaluation and noted that it means that 31.2 percent of the weld length at each of the 14 legs can be cracked and still meet the ASME Code structural margins. The NRC staff notes that the methodology used in the 2014 SIA structural evaluation is similar to the methodology in proprietary report BWRVIP-38, BWR Vessel and Internals Project: BWR Shroud Support Inspection and Flaw Evaluation Guidelines, September 1997, which received an NRC SE by letter dated July 24, 2000 (ADAMS Legacy No. 9909220057). The NRC staff also notes that in the 2014 SIA structural evaluation for Monticello, the H8 and H9 welds were assumed fully cracked (i.e., the applied loads were taken up by the H10 welds). The NRC staff determined that this is a conservative assumption.
The request stated that 8 of 14 H10 welds (at the 10°, 60°, 150°, 170°, 190°, 210°, 240°, and 270° azimuths) received the three successive examinations required by IWB-2420(b) without apparent changes to their previously reported indications, five H10 welds (at the 30°, 90°, 120°,
330°, and 350° azimuths) received two successive examinations without apparent changes to their previously reported indications, and one H10 weld received one successive examination for an indication found in 2013. This one H10 weld, at the 300° azimuth, also had a separate indication, found in 2009, which was reexamined in 2011, 2013, and 2017 with no apparent changes from its initial identification in 2009. The NRC staff finds that the conservatisms in the structural evaluation of the H10 welds discussed in the previous paragraph, together with the conservatism in crack growth discussed in the next paragraph, would adequately address cracking concerns in the five H10 welds that received only two successive examinations and in the one H10 weld at the 300° azimuth.
The H10 welds are in the lower plenum like the H8 and H9 welds and, therefore, are also mitigated against flaw growth and initiation with implementation of OLNC in 2013. The growth period to 31.2 percent of the H10 weld length, considering the growth of 31.9 years to 40 percent of the H10 weld length due to HWC K-independent crack growth per the 2014 SIA analysis would be (31.2/40)*31.9, which comes out to approximately 25 years. Normally, crack size varies exponentially with time, but in this case, the HWC K-independent crack growth is a constant, so the scaling of the number years above is valid. The NRC staff noted there is some conservatism in this crack growth estimate because, as described in the 2014 SIA structural evaluation, the growth was calculated with two assumed crack tips, i.e., the cracks are assumed on two opposite sides of each H10 weld. The request stated that the observed relevant indications are small compared to 31.2 percent of the H10 weld length; given this, the estimated growth period of 25 years reasonably accounts for potential growth of the indications in the structural evaluation.
Given no apparent changes in the H10 welds, the conservatisms in the structural evaluation of these welds, and the crack growth considerations in these welds, the NRC staff finds that the
evaluation provided in the request for structural integrity of the H10 welds acceptable, and that therefore, having no further examinations for the H10 welds at the 30°, 90°, 120°, 300°, 330°,
and 350° azimuths is reasonable (see proposed alternative summarized in Section 3.5 of this SE).
4.0 CONCLUSION
As set forth above, the NRC staff determined that complying with the subsequent examinations specified in IWB-2420(b) and (c) of Section XI of the ASME Code would result in hardship or unusual difficulty without a compensating increase in quality or safety. Accordingly, the NRC staff concludes that the licensee has adequately addressed all of the regulatory requirements set forth in 10 CFR 50.55a(z)(2). Therefore, the NRC staff authorizes alternative request RR-016 at Monticello for the remainder of the fifth 10-year ISI interval expected to end on May 31, 2022.
All other requirements of Section XI of the ASME Code for which an alternative was not specifically requested and approved remain applicable, including third party review by the Authorized Nuclear Inservice Inspector.
Principal Contributor: David Dijamco, NRR Date: November 2, 2020
ML20307A017 *via e-mail OFFICE NRR/DORL/LPL3/PM NRR/DORL/LPL3/LA* NRR/DMLR/MVIB/BC* NRR/DORL/LPL3/BC*
NAME RKuntz SRohrer HGonzalez NSalgado (JWiebe for)
DATE 11/2/2020 11/2/2020 10/27/2020 11/2/2020