ML22073A095
| ML22073A095 | |
| Person / Time | |
|---|---|
| Site: | Arkansas Nuclear  |
| Issue date: | 04/07/2022 |
| From: | Jennifer Dixon-Herrity Plant Licensing Branch IV |
| To: | Entergy Operations |
| Wengert T | |
| References | |
| EPID L-2021-LLR-0084 | |
| Download: ML22073A095 (14) | |
Text
OFFICIAL USE ONLY PROPRIETARY INFORMATION April 7, 2022 The nonpublic version of this document contains proprietary information pursuant to Title 10 of Code of Federal Regulations, Section 2.390. Proprietary information is identified by bold text enclosed within double brackets, as shown here: ((example proprietary text)).
Proprietary information pursuant to Title 10 of Code of Federal Regulations, Section 2.390 has been redacted from the public version of this document. Redacted information is identified by blank space enclosed within double brackets, as shown here: (( )).
ARKANSAS NUCLEAR ONE, UNIT 2 - APPROVAL OF REQUEST FOR ALTERNATIVE FROM CERTAIN REQUIREMENTS OF THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS BOILER AND PRESSURE VESSEL CODE (EPID L-2021-LLR-0084)
LICENSEE INFORMATION Licensee: Entergy Operations, Inc.
Licensee Address: ANO Site Vice President Arkansas Nuclear One Entergy Operations, Inc.
N-TSB-58 1448 S.R. 333 Russellville, AR 72802 Plant Name and Unit: Arkansas Nuclear One, Unit 2 (ANO-2)
Docket No.: 50-368 APPLICATION INFORMATION Submittal Date: November 5, 2021 Submittal Agencywide Documents Access and Management System (ADAMS) Accession No.: ML21309A007 Supplement Dates: November 7, 2021 and December 10, 2021 Supplement ADAMS Accession Nos.: ML21312A017 and ML21344A068 (Package)
Applicable Operating Cycle: Cycle 29, which is scheduled to end in the second quarter of 2023.
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OFFICIAL USE ONLY PROPRIETARY INFORMATION Licensee Proposed Alternative No. or Identifier: Relief Request ANO2-RR-21-002 Applicable Provision: Title 10 of the Code of Federal Regulations (10 CFR),
paragraph 50.55a(z)(1), Acceptable level of quality and safety.
Applicable Code Requirements: American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (ASME Code),Section XI, paragraph IWB-3420 and subparagraph IWB-3132.3; ASME Code,Section III, paragraph NB-5245 and subparagraph NB-5330(b); ASME Code Case N-638-7, Similar and Dissimilar Metal Welding Using Ambient Temperature Machine GTAW [Gas Tungsten Arc Welding] Temper Bead Technique,Section XI, Division 1, paragraphs 1(g) and 2(b).
Applicable Code Edition and Addenda: 2007 Edition through 2008 Addenda of ASME Code Section XI; 1992 Edition of ASME Code,Section III Brief Description of the Proposed Alternative and Basis:
In its proposed alternative submittal dated November 5, 2021, the licensee stated, in part:
While performing ultrasonic (UT) examinations of RVCH [reactor vessel closure head] penetration nozzles in accordance with ASME Code Case N-729-6 (Item No. B4.20), an axial, planar indication was identified in Control Element Drive Mechanism (CEDM) [penetration] Nozzle 46. The indication was located along the outside diameter downhill side of the nozzle in the J-groove weld fillet region.
The licensee proposed to use the repair technique referred to as half-nozzle repair for the subject nozzle. The half-nozzle repair involves machining away the lower section of the Alloy 600 nozzle containing the flaw and welding the remaining portion of the nozzle to the RVCH in order to form a new pressure boundary. The new Alloy 52M weld would also attach a replacement Alloy 690 lower nozzle. This repair technique requires relief from certain aspects of ASME Code,Section XI and Section III.
The licensee proposed to use ambient temperature inner diameter temper bead (IDTB) welding in accordance with ASME Code Case N-638-7. Code Case N-638-7 allows the use of the welding procedure qualified in a previous version of the Code Case. The licensees welding procedure was qualified in accordance with Code Case N-638-4, which is an earlier version of Code Case N-638-7. Specifically, the weld qualification requirements in Code Case N-638-4, paragraph 2.1(a) state that [t]he materials shall be post-weld heat treated to at least the time and temperature that was applied to the materials being welded. The licensees qualified welding procedure does not comply with the simulated post-weld heat treatment (PWHT) requirements of Code Case N-638-4 but does comply with the enhanced and more conservative simulated PWHT requirements in Code Case N-638-7.
Paragraph NB-5245 of ASME Code, Section Ill, 1992 Edition, specifies progressive surface examination of partial penetration welds for the acceptance of the repair. The construction code requires progressive surface examination, in lieu of volumetric examination, because volumetric examination is not practical for the conventional partial penetration weld configurations. Since the subject repair weld is suitable for UT examination and the weld is accessible from both the OFFICIAL USE ONLY PROPRIETARY INFORMATION
OFFICIAL USE ONLY PROPRIETARY INFORMATION top and bottom sides, the licensee proposed to use the combination of UT and liquid penetrant (PT) examinations. In addition, preservice and inservice inspections will be performed on the repaired nozzle, as described in the licensees supplemental letter dated November 7, 2021.
ASME Code, Section Ill, NB-5330(b) states that [i]ndications characterized as cracks, lack of fusion, or incomplete penetration are unacceptable regardless of length. In comparison, an artifact of ambient temperature temper bead welding exists as an anomaly at the triple points of the repair weld. There are two triple points in the repair weld, as shown in Figure 2 of the enclosure to the proposed alternative dated November 5, 2021. The anomaly consists of an irregularly shaped very small void. Mockup testing was performed in similar proposed alternatives that were previously approved by the U.S. Nuclear Regulatory Commission (NRC) staff. The testing verified that the anomalies are common and do not exceed 0.10 inches in depth and are assumed to exist, for purposes of analysis, around the entire bore circumference at the triple point elevation.
The licensee stated that, given the emergent nature of the repair during Refueling Outage 2R28, there was not sufficient time to perform the detailed life of repair finite element analyses for the triplet point anomaly. The licensee performed a comparative analysis for the triple point anomaly in comparison with the previous life of repair analysis that was previously approved by the NRC staff for the St. Lucie Plant by letter dated June 17, 2003 (ADAMS Accession No. ML031700111), among others.
In its analysis, the licensee determined that the ANO-2 RVCH involves higher fracture toughness, smaller inner radius and lower numbers of transient cycles for the analysis period, while the other parameters such as transient pressure and temperature conditions are comparable. Therefore, the licensee confirmed that the previous reference analysis is bounding for the ANO-2 case in terms of material fracture toughness, applied loading and transient cycle numbers for crack growth analysis. Since the previous life of repair analysis for the triple point anomaly resulted in a life longer than 20 years, the licensee demonstrated that the triple point anomaly of the ANO-2 weld repair is acceptable for one operating cycle of 18 months.
Similarly, the licensee performed a comparative analysis for the as-left J-groove weld flaw in comparison with the previous life of repair analysis that was approved by the NRC staff for the St. Lucie Plant by letter dated June 17, 2003. Since the life of repair analysis for the similar repair is bounding for the ANO-2 case and results in a repair life longer than 10 years, the licensee determined that the as-left J-groove weld flaw is acceptable for one operating cycle.
The applicable ANO-2 operating cycle (Cycle 29) for the proposed alternative is scheduled to end in the second quarter of 2023. Additional details of the licensees request are described in the licensees proposed alternative submittal dated November 5, 2021, as supplemented by letters dated November 7, 2021 and December 10, 2021.
The proposed alternative identifies a regulatory commitment that, if required, the licensee will submit a revised alternative request to extend the design life of the repair weld for at least one inservice inspection interval (10 years) and to support ANO-2s return to service following Refueling Outage 2R29. The NRC staff notes that such a future request for using the repair weld beyond one operating cycle will be subject to another safety evaluation by the NRC staff.
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OFFICIAL USE ONLY PROPRIETARY INFORMATION REGULATORY EVALUATION Regulatory Basis: 10 CFR 50.55a(z)(1)
The NRC regulations in 10 CFR 50.55a(z), Alternatives to codes and standards requirements, state that [a]lternatives 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 licensee must demonstrate that its request meets one of two criteria: (1) the proposed alternative would provide an acceptable level of quality and safety in accordance with paragraph (z)(1); 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 in accordance with paragraph (z)(2).
The licensee submitted the request on the basis that the proposed alternative would provide an acceptable level of quality and safety in accordance with 10 CFR 50.55a(z)(1). Based on the above regulations, the NRC staff concludes that regulatory authority exists to authorize an alternative to ASME Code,Section III and Section XI, as requested by the licensee.
TECHNICAL EVALUATION During the ANO-2 Refueling Outage 2R28 (fall 2021), the licensee performed UT examinations on RVCH penetration nozzles in accordance with ASME Code Case N-729-6, Alternative Examination Requirements for PWR [Pressurized Water Reactor] Reactor Vessel Upper Heads with Nozzles Having Pressure-Retaining Partial-Penetration Welds,Section XI, Division 1 (Item No. B4.20). An axial, planar indication was identified in CEDM penetration Nozzle 46, which is an ASME Code Class 1 component. The indication was located along the outside diameter of the nozzle at the toe of J-groove weld and downhill side of the nozzle.
The licensee proposed to perform a half-nozzle repair of the subject penetration nozzle. By letter dated November 5, 2021, as supplemented by letter dated November 7, 2021, the licensee requested approval of the proposed alternative to the ASME Code repair and replacement requirements. On November 9, 2021 (ADAMS Accession No. ML21314A121), the NRC staff verbally authorized the use of the proposed alternative for operating Cycle 29, which is scheduled to end in the second quarter of 2023.
The licensee submitted analyses on the repair weld triple point anomaly and as-left J-groove weld flaw by letter dated December 10, 2021. This section documents the NRC staffs technical evaluation of the previously verbally-approved proposed alternative and the associated supplements for the use of the repair weld for one 18-month operating cycle (i.e., Cycle 29).
1.0 REPAIR WELDING Use of ASME Code Case N-638-7 As described in Section 5.1, Welding Requirements, of the proposed alternative dated November 5, 2021, the licensee proposed to perform the half-nozzle repair on penetration Nozzle 46 using the ambient temperature temper bead welding process of ASME Code Case N-638-7. The NRC staff noted that the guidance in Regulatory Guide (RG) 1.147, Revision 19, Inservice Inspection Code Case Acceptability, ASME Section XI, Division 1, OFFICIAL USE ONLY PROPRIETARY INFORMATION
OFFICIAL USE ONLY PROPRIETARY INFORMATION dated October 2019 (ADAMS Accession No. ML19128A244) allows the use of Code Case N-638-7 for temper bead welding with a condition that the NRC staff imposed regarding the demonstration of UT examination. Specifically, the condition states that [d]emonstration of ultrasonic examination of the repaired volume is required using representative samples that contain construction-type flaws.
The licensee addressed the condition in Section 7.1, VHP [Vessel Head Penetration] Weld Qualification Mockup UT Acceptance, of the proposed alternative. The licensee explained that Framatome, in support of many similar modifications, has performed demonstrations using IDTB weld mockups since the VHP modifications at Oconee Nuclear Station in 2001. The licensee also indicated that a procedure demonstration took place during the 2010 Davis-Besse Nuclear Power Station control rod drive mechanism repair campaign (ADAMS Accession No. ML102571569). The licensee further clarified that the same mockup was used for the procedure demonstration of UT examination for the Shearon Harris Nuclear Power Plant VHP nozzle modifications, as approved by the NRC staff (ADAMS Accession Nos. ML13238A154 and ML18283B544).
In addition, the licensee provided the following details of the mockup for the UT demonstration.
The IDTB weld half-nozzle repair mockup containing reflectors has been used to simulate construction-type flaws applicable to the weld process, which satisfies the condition on ASME Code Case N-638-7 in RG 1.147, Revision 19. The mockup contains a series of electrical-discharge machining (EDM) notches at the triple point to simulate the triple point anomaly at various depths into the nozzle wall and cracking at the IDTB weld to low alloy steel interface. It also contains flat bottom holes drilled from the mockup outside diameter so that the hole face is normal to the nozzle surface to simulate the under-bead cracking and the lack of bond or fusion throughout the weld volume. The examination procedure demonstrated the ability to detect a weld fabrication triple point anomaly extending 0.05 inch and greater into the weld.
The licensee used a nickel-chromium-iron alloy calibration block that contains a series of EDM notches at nominal depths of 10, 20, 50, and 75 percent deep from both inside diameter and outside diameter surfaces in both the axial and circumferential orientations. The block also contains 1/4-thickness (1/4T), 1/2T, and 3/4T deep end-drilled holes and side-drilled holes that are used for calibration.
The NRC staff finds that the use of the IDTB weld mockup for the half-nozzle repair and the associated UT demonstration are acceptable because the mockup contains various construction-type flaws and, therefore, it meets the condition on ASME Code Case N-638-7 specified in RG 1.147, Revision 19. The NRC staff also finds that the licensees use of the temper bead weld mockup is consistent with that used in the demonstration of UT examination for the previous half-nozzle repair activities at other nuclear power plants, as approved by the NRC staff.
Based on the evaluation above, the NRC staff finds that the licensees use of ASME Code Case N-638-7 for the repair welding process is acceptable because: (1) the use of Code Case N-638-7 has been approved in RG 1.147, Revision 19; and (2) the licensee satisfies the condition regarding the demonstration of UT examination that is specified in RG 1.147, Revision 19.
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OFFICIAL USE ONLY PROPRIETARY INFORMATION Use of Peening on the Final Layer of the Repair Weld Paragraph 1(g) of ASME Code Case N-638-7 states that [p]eening may be used, except on the initial and final layers. In comparison, the licensee proposed to perform rotary peening on the final layer to provide further assurance that the modified nozzle configuration is resistant to primary water stress corrosion cracking (PWSCC).
The licensee discussed that the residual compressive stress layer applied by the rotary peening has been shown to greatly reduce PWSCC initiation and that, recognizing these benefits, the ASME Code Committee revised Code Case N-638 (i.e., revision to N-638-8) to allow use of peening processes that are designed to reduce residual surface tensile stresses on the final layer of the weld.
With respect to the peening for the repair weld, the NRC staff noted the following: (1) Code Case N-638-8, paragraph 1(h) allows peening on the final layer of the repair weld for compressive residual surface stresses; (2) the same provision is included in Code Case N-638-10; and (3) the recent proposed rule for ASME Code Cases issued on February 2, 2021(86 FR 7820) identifies N-638-10 as an acceptable Code Case without a condition.
Accordingly, the NRC staff finds that the licensees use of peening on the final layer of the repair weld for compressive residual surface stresses is consistent with the staff position in the recent proposed rule for ASME Code Cases cited above and, therefore, is acceptable.
Simulation of PWHT in the Welding Process Qualification The licensee indicated that paragraph 2(b) of ASME Code Case N-638-7 permits the use of welding procedures qualified in accordance with previous revisions of the Code Case.
Accordingly, the licensee proposed to use the welding procedure that was qualified in accordance with Code Case N-638-4, which is an earlier version of N-638-7.
The licensee explained that the proposed alternative involves a deviation from the weld qualification provisions of ASME Code Case N-638-4. Specifically, Code Case N-638-4, paragraph 2.1(a) states that [t]he materials shall be post-weld heat treated to at least the time and temperature that was applied to the materials being welded. In contrast, the licensee used the provision in Code Case N-638-7, paragraph 2.1(a), which states, Prior simulated PWHT of the test assembly is neither required nor prohibited. However, if used, the simulated PWHT shall not exceed the time or temperature already applied to the base material to be welded.
The licensee stated that the simulated PWHT for weld procedure qualification per ASME Code Case N-638-4, paragraph 2.1(a) specifies the longer or at least equal PWHT time for weld qualification test specimens compared to the actual PWHT time. This provision regarding the longer PWHT simulation time makes passing the impact testing requirements of the weld procedure qualification less difficult, as discussed in Section 3.0 of the Electric Power Research Institute Report 1025169, Welding and Repair Technology Center: Welding and Repair Technical Issues in ASME Section XI, December 2012.
As discussed above, the NRC staff finds that the licensees use of the welding qualification process in ASME Code Case N-638-4 with the exception for the simulated PWHT time is acceptable because: (1) Code Case N-638-7 allows the use of the temper bead weld process that was qualified in accordance with a previous version of the Code Case; (2) the licensee uses the weld process qualified in accordance with Code Case N-638-4 with one deviation; (3) the OFFICIAL USE ONLY PROPRIETARY INFORMATION
OFFICIAL USE ONLY PROPRIETARY INFORMATION deviation (i.e., the simulated PWHT time and temperature not exceeding those applied to the base material) is in fact consistent with the provision in Code Case N-638-7; and (4) Code Case N-638-7 is approved for use in RG 1.147, Revision 19, without a condition regarding the simulated PWHT time and temperature in the welding qualification.
2.0 ACCEPTANCE EXAMINATION OF THE WELD MODIFICATION With respect to the acceptance examination of IDTB weld repair, the licensee stated that NB-5245 of ASME Code, Section Ill, 1992 Edition, specifies progressive surface examination of partial penetration welds. The licensee explained that the construction code requires progressive surface examination in lieu of volumetric examination because volumetric examination is not practical for conventional partial penetration weld configurations.
The licensee also stated that the repair weld of the proposed alternative is suitable for UT examination (a volumetric examination method) and is accessible from both the top and bottom sides, as illustrated in Figures 4 through 8 in the proposed alternative dated November 5, 2021.
Accordingly, the licensee proposed to use both PT and UT examinations, as further discussed below.
For the acceptance of the modified configuration of the repaired nozzle, the licensee proposed to perform volumetric examination using UT in accordance with ASME Code Case N-638-7, paragraphs 4(a)(2) and 4(a)(4). The NRC staff noted that the Code Case provisions include volumetric examination on the repair weld and as previously discussed, Code Case N-638-7 has been approved for use in RG 1.147, Revision 19. Therefore, the NRC staff finds that the licensees use of the Code Case provisions regarding volumetric examination using UT is acceptable. The condition imposed on the use of N-638-7 (demonstration of UT examination) is evaluated in Section 1.0 of this safety evaluation.
In addition, the licensee described further details of the UT examination as follows. The UT examination system is capable of scanning from cylindrical surfaces. The scanning is performed using a 0 degrees (°) longitudinal-wave (L-wave) transducer, 45° L-wave transducers in two opposed axial directions, and 70° L-wave transducers in two opposed axial directions as well as 45° L-wave transducers in two opposed circumferential directions. The repair weld receives 100 percent examination coverage. Additionally, the low alloy steel extending to 0.25 inches beneath the weld into the low alloy steel base material (as shown in Figure 3 of the proposed alternative) will be examined using the 0° L-wave transducer searching for evidence of under bead cracking and lack of fusion in the heat-affected zone. The acceptance criteria of the UT examination are specified in ASME Code,Section III, NB-5330. In the UT examination for the weld modification acceptance, the examination of the repair volume is also extended to include 1 inch of Alloy 600 nozzle material above the weld and 1 inch of Alloy 690 material below the weld.
In addition to the UT examination, the licensee proposed to perform surface PT examination on the entire weld. The licensee explained that the final examination of the new weld and immediate surrounding region would be sufficient to verify that defects have not been induced in the ferritic low alloy steel RVCH base material due to welding to the extent practical. The licensee also indicated that the acceptance criteria of NB-5350 in ASME Code Section III, 1992 Edition, apply to the surface examination.
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OFFICIAL USE ONLY PROPRIETARY INFORMATION The NRC staff finds that the licensees proposal to use the UT and PT examinations is acceptable because: (1) the UT and PT examinations are consistent with the provisions in ASME Code Case N-638-7, as approved in RG 1.147, Revision 19; (2) the examination volume is sufficient to confirm the integrity and acceptability of the modification associated with the repair weld; and (3) the acceptance criteria are consistent with those of the construction code requirements (ASME Code,Section III). The licensees approach for the preservice and inservice inspections are evaluated in the following section.
3.0 PRESERVICE AND INSERVICE INSPECTIONS The proposed alternative, as supplemented by letter dated November 7, 2021, describes the preservice and inservice inspections for penetration Nozzle 46, including the repair weld. The licensee stated that the preservice and inservice inspections of the nozzle comply with ASME Code Case N-729-6, as approved by the NRC in 10 CFR 50.55a. Specifically, the preservice and inservice inspections would include performing volumetric examination in accordance with Code Case N-729-6, Table 1, Item B4.20, as modified by 10 CFR 50.55a(g)(6)(ii)(D).
The licensee also stated that ASME Code Case N-729-6 requires the examination region to extend 1.5 inches beyond partial penetration welds with an incidence angle of 0 degrees with the penetration axis. In lieu of the volumetric examination region that extends 1.5 inches above and 1.5 inches below the J-groove weld shown in Figure 2 of N-729-6, the licensee proposed an alternative examination volume that would extend up to the outer surface of the head (greater than 1.5 inches above the repair weld), including the rotary peened surfaces, and 1 inch below the repair weld as shown in the revised Figure 9 of the licensees supplemental letter dated November 7, 2021.
The NRC staff finds that the proposed examination volume extended above the repair weld is acceptable because: (1) the examination volume of the repair weld extends up to the outer surface of the RVCH, which is greater than the examination volume specified in N-729-6 (i.e.,
1.5 inches above the weld); (2) the examination volume is effective in revealing any relevant indications in the Alloy 600 nozzle within the RVCH and in assessing potential leakage paths; and (3) the extended examination volume fully covers the peened area, which in turn covers the roll-expanded area that may involve a potential increase in PWSCC susceptibility resulting from the roll expansion and associated cold work.
The licensee explained that the examination coverage below the repair weld will be less than the 1.5-inch requirement in ASME Code Case N-729-6 due to the geometric limitations of the repaired nozzle, but the examination coverage would extend a minimum of 1 inch below the weld and would obtain the maximum examination volume practical. The licensee also provided justification for why the examination coverage of 1 inch below the repair weld is considered sufficient as follows: (1) the replacement nozzle material (Alloy 690) in the repair is resistant to PWSCC; (2) the new pressure boundary weld (Alloy 52M) is also resistant to PWSCC; and (3) the replacement nozzle portion, which is subject to the reduced examination coverage below the repair weld, is a non-pressure boundary area.
The NRC staff finds that the proposed examination volume below the repair weld is acceptable because: (1) the examination volume of the repair weld extends a minimum of 1 inch; and (2) the excluded region from the examination coverage of ASME Code Case N-729-6 (i.e.,
between 1 inch and 1.5 inches below the repair weld) is the replacement nozzle portion OFFICIAL USE ONLY PROPRIETARY INFORMATION
OFFICIAL USE ONLY PROPRIETARY INFORMATION fabricated of material more resistant to PWSCC (Alloy 690) than the original material (Alloy 600) and would not be part of the reactor coolant pressure boundary after the weld repair.
4.0 REPAIR WELD TRIPLE POINT ANOMALY ANALYSIS ASME Code, Section Ill, NB-5330(b) states that indications characterized as cracks, lack of fusion, or incomplete penetration are unacceptable regardless of length. With respect to this design code requirement, the proposed alternative addressed the repair weld triple point anomaly.
The licensee stated that the triple point anomaly is an artifact of ambient temperature temper bead welding at the triple point. As illustrated in Figure 2 of the proposed alternative, there are two triple points in the proposed weld repair. The upper triple point is the point in the repair weld where the low alloy steel RVCH base material, the Alloy 600 nozzle, and the Alloy 52M repair weld intersect. The lower triple point is the point in the repair weld where the low alloy steel RVCH base material, the Alloy 690 replacement nozzle, and the Alloy 52M repair weld intersect.
The licensee also explained that the anomaly consists of an irregularly shaped very small void.
The licensee further stated that mockup testing verified that the anomalies are common and do not exceed 0.10 inches in through-wall extent and are assumed to exist, for purposes of analysis, around the entire bore circumference at the triple point elevation.
In the proposed alternative, the licensee indicated that, given the emergent nature of the ANO-2 penetration nozzle repair, there was not sufficient time to perform a detailed plant-specific life of repair finite element analysis for the triple point anomaly during Refueling Outage 2R28. Such an analysis would address a finite element analysis of potential fatigue crack from the triple point for the lifetime of the repair weld. Instead, the licensee provided a summary of previous life of repair triple point analyses performed for other nuclear power plants, which were approved by the NRC staff (ADAMS Accession Nos. ML20365A001, ML19136A386, ML031700111 and ML18283B544).
The previous NRC staff-approved triple anomaly analyses are summarized as follows: (1) a fracture mechanics analysis is performed for the repair weld design configuration to provide justification in accordance with ASME Code,Section XI for operating with the postulated triple point anomaly; (2) the initial flaw size for the triple point anomaly analysis is 0.10 inches, consistent with examination results; (3) the outermost nozzle penetration is modeled because the applied loading conditions at the location are representative and bounding relative to all the other locations in the RVCH; and (4) the crack growth analysis determines the future flaw size and concludes that it is acceptable for the stated life, which is typically longer than 20 years.
In addition, the licensee provided a comparative analysis on triple point anomaly in its supplemental letter dated December 10, 2021. In the comparative analysis, the licensee compared the triple point analysis parameters of ANO-2 weld repair with those of a previously approved weld repair case to demonstrate that the previous triple point analysis results are bounding for the ANO-2 weld repair and, therefore, the triple point anomaly of ANO-2 is acceptable for at least one operating cycle in comparison with the previously approved analysis for the St. Lucie Plant by letter dated June 17, 2003. The licensees analysis is further summarized and evaluated below.
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OFFICIAL USE ONLY PROPRIETARY INFORMATION The previous triple point anomaly analysis is based on the final flaw size at a predicted repair weld life of more than 20 years. In comparison, the ANO-2 one-cycle justification for triple point anomaly requires evaluation for only one 18-month operating cycle. Therefore, the licensee confirmed that the previous analysis is sufficient to bound the ANO-2 triple point anomaly analysis in terms of the analysis period (i.e., at least 20 years versus 18 months).
The licensee also confirmed that the transient pressure and temperature conditions for crack growth analysis are equivalent between the previously approved analysis and ANO-2 analysis.
The licensee further compared the transient cycles for the applicable number of years (i.e., the analysis periods) and demonstrated that the associated cycles considered in the previous analysis are conservative than the ANO-2 analysis case.
In addition, the licensee demonstrated that the RVCH hoop stress in the previous analysis is bounding for that in the ANO-2 analysis in consideration of the RVCH dimensions and operating pressure levels between the compared analyses. The licensee further confirmed that the other nozzle geometry dimensions (e.g., nozzle thickness and outer diameter) are comparable between the previously approved analysis and ANO-2 analysis such that these dimensions do not affect the comparative analysis results.
With respect to the RVCH steel fracture toughness, the licensee compared the reference nil-ductility transition temperature (RTNDT) of the RVCH steel, which is used to determine the steel fracture toughness between the previous analysis and ANO-2 analysis. The licensee confirmed that the RTNDT of the previously approved analysis is greater than that of the ANO-2 RVCH steel and, therefore, demonstrated that the RVCH steel fracture toughness of the previously approved analysis is more limiting than that of the ANO-2 RVCH steel.
The NRC staff also noted that the previously approved analysis for triple point anomaly meets the following acceptance criteria of ASME Code, Section XI: (1) IWB-3612 (a flaw is acceptable if the applied stress intensity factor at the final flaw size is less than the fracture toughness at the crack tip temperature of the material with appropriate safety factors); (2) IWB-3642 (sufficient loading capacity with full 360° radial-circumferential postulated flaw); and (3) IWB-3643 (flaws with depth greater than 75 percent of the wall thickness are unacceptable).
The NRC staff finds that the licensees one-cycle justification for the triple point anomaly is acceptable because: (1) the triple point anomaly analyses of half-nozzle repairs that previously have been approved by the NRC staff involve a significantly longer analysis period (greater than 20 years) compared to the analysis period of the ANO-2 weld repair (18 months); (2) the previous triple point anomaly analyses confirmed the structural integrity of the repaired nozzle and adjacent RVCH in accordance with the acceptance criteria of ASME Code; and (3) the licensees comparative analysis confirms that the analysis parameters of the previously approved case is representative of or bounding for the ANO-2 triple point anomaly analysis (e.g., RVCH and nozzle dimensions, stress levels, transient loading, and transient cycle numbers) and, therefore, the ANO-2 weld repair is acceptable in terms of the triple point anomaly analysis.
5.0 AS-LEFT J-GROOVE WELD FLAW EVALUATION The licensee also addressed the as-left J-groove weld flaw evaluation. The provisions of IWB-3600 of ASME Code,Section XI for analytical flaw evaluation assume that cracks are fully characterized in accordance with IWB-3420. The licensee indicated that there are no qualified OFFICIAL USE ONLY PROPRIETARY INFORMATION
OFFICIAL USE ONLY PROPRIETARY INFORMATION UT examination techniques available for examining the original nozzle-to-RVCH J-groove welds. Due to the impracticality to characterize the flaw geometry that may exist in the J-groove weld, the licensee conservatively assumed that the as-left condition of the J-groove weld involved a flaw extending through the entire Alloy 82/182 J-groove weld including the weld buttering.
The licensee further explained that the preferential direction for cracking would be radial as a result of the dominant hoop stresses in the J-groove weld. Accordingly, a radial crack in the J-groove weld would propagate into the low alloy steel by fatigue crack growth under cyclic loading because the low alloy steel is resistant to PWSCC.
The NRC staff finds that the licensees approach and assumptions for the as-left J-groove weld flaw evaluation are acceptable because of the following: (1) since no UT examination technique is currently available to characterize the flaw size in the J-groove weld, the licensee conservatively postulated that the flaw extends throughout the entire J-groove weld; (2) the postulation of the axial-radial crack growth direction is reasonable because the hoop stress is dominant near the nozzle and the axial-radial crack orientation is consistent with the licensees nondestructive examination results; and (3) the low alloy steel RVCH is resistant to PWSCC so that fatigue crack growth is the relevant crack growth mechanism for the RVCH in the flaw evaluation.
In the proposed alternative the licensee also indicated that, given the emergent nature of the ANO-2 penetration nozzle repair, there was not sufficient time to perform a detailed plant-specific life of repair finite element analysis for the as-left J-groove weld flaw during Refueling Outage 2R28. Instead, as noted above, the licensee provided a summary of life of repair as-left J-groove weld flaw analyses performed at other nuclear power plants that were previously approved by the NRC. The licensee noted that the NRC staff-approved J-groove weld flaw analyses for similar weld repairs determined a repair weld life based on linear elastic fracture mechanics analysis, which is significantly longer than the one operating cycle for the licensees proposed alternative.
In addition, the licensee provided a comparative analysis on the as-left J-groove weld flaw in its supplemental letter dated December 10, 2021. In the comparative analysis, the licensee compared the J-groove weld flaw analysis parameters of ANO-2 weld repair with those of a previously approved weld repair analysis in order to demonstrate that the previous flaw analysis results are bounding for the ANO-2 weld repair and, therefore, the J-groove weld flaw analysis of ANO-2 is acceptable at least for one operating cycle.
Specifically, the licensee compared the transient cycles for the applicable number of years (i.e.,
the analysis periods) and demonstrated that the associated cycles considered in the previous analysis are bounding for the ANO-2 J-groove weld analysis. In addition, the licensee demonstrated that the RVCH hoop stress in the previous analysis is bounding for that in the ANO-2 analysis in consideration of the RVCH dimensions and operating pressure levels between the compared analyses. The licensee further confirmed that the previous analysis bounds the ANO-2 flaw analysis (( )). The licensee also demonstrated that the other nozzle geometry dimensions (e.g., nozzle thickness and outer diameter) are comparable between the previous analysis and ANO-2 analysis such that these dimensions do not affect the comparative analysis results.
OFFICIAL USE ONLY PROPRIETARY INFORMATION
OFFICIAL USE ONLY PROPRIETARY INFORMATION The licensee also confirmed that the RTNDT of the previous analysis is greater than that of the ANO-2 RVCH steel and, therefore, demonstrated that the RVCH steel fracture toughness of the previous analysis is more limiting than that of the ANO-2 RVCH steel.
The NRC staff noted that the previously approved analysis for the J-groove weld flaw meets the acceptance criteria of ASME Code,Section XI, IWB-3612, which states that a flaw is acceptable if the applied stress intensity factor at the final flaw size is less than the fracture toughness at the crack tip temperature of the material with appropriate safety factors.
The NRC staff finds that the licensees one-cycle justification for the J-groove welds flaw evaluation is acceptable because of the following: (1) the previous flaw growth analyses of similar half-nozzle repair, which have been approved by the NRC staff, involve a significantly longer analysis period that the analysis period of the ANO-2 weld repair; (2) the previous flaw analyses confirmed the structural integrity of the RVCH in accordance with the acceptance criteria of ASME Code; and (3) the licensees comparative analysis confirms that the analysis parameters of the previous analysis is representative of or bounding for the ANO-2 flaw analysis (e.g., RVCH and penetration dimensions, stress levels, transient loading, and transient cycle numbers) and, therefore, demonstrates that the ANO-2 weld repair is acceptable in terms of the as-left J-groove weld flaw analysis.
6.0 CORROSION EVALUATION The licensee explained that the IDTB nozzle repair leaves a small portion of low alloy steel in the nozzle bore area exposed to reactor coolant. General corrosion of the exposed low alloy steel of the RVCH can occur between the repair weld and the original J-groove weld that is located below the repair weld. The licensee also stated that an evaluation was performed for similar prior repairs for the potential corrosion concerns at the low alloy steel wetted surface, as reviewed and approved by the NRC staff (e.g., ADAMS Accession Nos. ML031700111, ML18283B544 and ML19136A386). Further, the NRC staff notes that no issues of corrosion that could have challenged the structural integrity of the head materials have been identified in subsequent examinations of these IDTB-repaired penetration nozzles.
In addition, the licensee discussed that, due to the depletion of oxygen, tight geometry, and lack of primary coolant system flow at the exposed low alloy steel, general corrosion will significantly decrease after a period of time following return to the service. The licensee also explained that, as the surface of the low alloy steel passivates, the long-term corrosion rate is expected to be negligible.
The NRC staff finds that the licensees evaluation provides reasonable assurance that the effect of general corrosion is insignificant because the general corrosion rate in the tight bore gap region is expected to significantly decrease as iron oxides build up on the steel bore surface, thereby filling in the tight gap between the nozzle outer diameter surface and RVCH bore surface.
CONCLUSION As set forth above, the NRC staff determines that proposed alternative, as described in the licensees letter dated November 5, 2021, as supplemented by letters dated November 7, 2021, and December 10, 2021, for the use of the temper bead welding repair on ANO-2 RVCH penetration Nozzle 46 is acceptable on the basis that the proposed alternative provides an OFFICIAL USE ONLY PROPRIETARY INFORMATION
OFFICIAL USE ONLY PROPRIETARY INFORMATION acceptable level of quality and safety. The NRC staff finds that the proposed alternative will provide reasonable assurance of the structural integrity and leak tightness of the repaired nozzle for one cycle of operation. Accordingly, the NRC staff concludes that the licensee has adequately addressed the regulatory requirements in 10 CFR 50.55a(z)(1). Therefore, the NRC staff authorizes the use of the proposed alternative for one operating cycle (Cycle 29), which is scheduled to end in the second quarter of 2023 at ANO-2.
All other requirements in ASME Code,Section III and Section XI, for which relief or an alternative was not specifically requested and approved as part of this subject request, remain applicable, including third-party review by the Authorized Nuclear Inservice Inspector.
Principal Contributors: S. Min J. Collins Date: April 7, 2022
/RA/
Jennifer L. Dixon-Herrity, Chief Plant Licensing Branch IV Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation cc Redacted/Public version only (ADAMS Accession No. ML22073A095): Listserv OFFICIAL USE ONLY PROPRIETARY INFORMATION
Proprietary ML22073A094 Nonproprietary ML22073A095 *by e-mail OFFICE NRR/DORL/LPL4/PM* NRR/DORL/LPL4/LA* NRR/DNRL/NPHP/BC* NRR/DORL/LPL4/BC*
NAME TWengert PBlechman MMitchell JDixon-Herrity DATE 3/21/2022 3/21/2022 3/8/2022 4/7/2022