ML25251A159
| ML25251A159 | |
| Person / Time | |
|---|---|
| Site: | Palo Verde  |
| Issue date: | 09/12/2025 |
| From: | William Orders Plant Licensing Branch IV |
| To: | Heflin A Arizona Public Service Co |
| Orders, William | |
| References | |
| EPID L-2025-LLR-047 | |
| Download: ML25251A159 (1) | |
Text
September 12, 2025 Mr. Adam Heflin Executive Vice President/
Chief Nuclear Officer Mail Station 7605 Arizona Public Service Company P.O. Box 52034 Phoenix, AZ 85072-2034
SUBJECT:
PALO VERDE NUCLEAR GENERATING STATION, UNIT 1. RELIEF REQUEST 76 ALTERNATIVE REPAIR OF PRESSURIZER INSTRUMENT NOZZLE (EPID L-2025-LLR-0047)
Dear Mr. Heflin:
By letter dated April 23, 2025 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML25113A296), as supplemented by letter dated May 22, 2025 (ML25142A405, non-proprietary and ML25142A406, proprietary information), Arizona Public Service Company (the licensee) requested relief from the requirements of the American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code),Section XI, 2013 Edition, and ASME Code Case N-638-10, Similar and Dissimilar Metal Welding Using Ambient Temperature Machine GTAW [Gas Tungsten Arc Welding] Temper Bead Technique,Section XI, Division 1, for Palo Verde Nuclear Generating Station (Palo Verde), Unit 1.
Pursuant to Title 10 of the Code of Federal Regulations (10 CFR) 50.55a(z)(1), Acceptable level of quality and safety, the licensee submitted for U.S. Nuclear Regulatory Commission (NRC) review and approval Relief Request 76 to support a modification/repair of the pressurizer lower instrument nozzle RC-023 (V208) on the basis that the proposed nozzle modification will provide for an acceptable level of quality and safety for Operating Cycle 26.
On April 25, 2025 (ML25118A063), the NRC staff approved Relief Request 76 for one cycle of operation (Cycle 26), by the verbal authorization process.
As set forth in the enclosed safety evaluation, the NRC staff finds that the proposed alternative will provide an acceptable level of quality and safety for the operation of the pressurizer lower instrument nozzle RC-023 (V208). Accordingly, the staff concludes that the licensee has adequately addressed all the regulatory requirements set forth in 10 CFR 50.55a(z)(1).
Therefore, the NRC authorizes the use of Relief Request 76 for one operating cycle (Cycle 26),
which is currently scheduled to conclude in the fall of 2026, at Palo Verde, Unit 1.
All other requirements of ASME Code,Section XI, for which relief was not specifically requested and authorized by the NRC staff remain applicable, including the third-party review by the Authorized Nuclear Inservice Inspector.
If you have any questions, please contact the Palo Verde Project Manager, William Orders, at (301) 415-3329 or via email at William.Orders@nrc.gov.
Sincerely, Tony Nakanishi, Chief Plant Licensing Branch IV Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket No. STN 50-528
Enclosure:
Safety Evaluation cc: Listserv TONY NAKANISHI Digitally signed by TONY NAKANISHI Date: 2025.09.12 11:32:56 -04'00'
Enclosure SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELIEF REQUEST 76 ALTERNATIVE REPAIR OF PRESSURIZER INSTRUMENT NOZZLE ARIZONA PUBLIC SERVICE COMPANY PALO VERDE NUCLEAR GENERATING STATION, UNIT 1 DOCKET NO. 50-528
1.0 INTRODUCTION
By letter dated April 23, 2025 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML25113A296), as supplemented by letter dated May 22, 2025 (ML25142A405, non-proprietary and ML25142A406, proprietary information), Arizona Public Service Company (the licensee) requested relief from the requirements of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (ASME Code),Section XI, 2013 Edition, and ASME Code Case N-638-10, Similar and Dissimilar Metal Welding Using Ambient Temperature Machine GTAW [Gas Tungsten Arc Welding] Temper Bead Technique,Section XI, Division 1, for Palo Verde Nuclear Generating Station (Palo Verde), Unit 1.
Pursuant to Title 10 of the Code of Federal Regulations (10 CFR) 50.55a(z)(1), Acceptable level of quality and safety, the licensee submitted for U.S. Nuclear Regulatory Commission (NRC) review and approval Relief Request 76 to support a modification/repair of the pressurizer lower instrument nozzle RC-023 (V208) on the basis that the proposed nozzle modification will provide for an acceptable level of quality and safety for Operating Cycle 26.
On April 25, 2025 (ML25118A063), the NRC staff approved Relief Request 76 for one cycle of operation (Cycle 26), by the verbal authorization process. This safety evaluation (SE) is to document the details of the staffs evaluation as part of the verbal authorization.
2.0 Regulatory Evaluation Adherence to Section XI of the ASME Code is mandated by 10 CFR 50.55a(g)(4), Inservice inspection standards requirements for operating plants, which states, in part, that ASME Code Class 1, 2, and 3 components will meet the requirements, except the design and access provisions and the preservice examination requirements, set forth in the ASME Code,Section XI.
The regulations in 10 CFR 50.55a(z), Alternative to codes and standards requirements, states:
Alternatives to the requirements of paragraphs (b) through (h) of [10 CFR 50.55a]
or portions thereof may be used when authorized by the Director, Office of Nuclear Reactor Regulation. A proposed alternative must be submitted and authorized prior to implementation. The applicant or licensee must demonstrate that: (1) Acceptable level of quality and safety. The proposed alternative would provide an acceptable level of quality and safety; or (2) Hardship without a compensating increase in quality and safety. Compliance with the specified requirements of this section would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety.
Based on the above, and subject to the following technical evaluation, the NRC staff finds that regulatory authority exists for the licensee to request and the NRC to grant the relief requested by the licensee.
3.0 Technical Evaluation 3.1 Relief Request 76 3.1.1 ASME Code Component Affected The affected component is the ASME Code Class 1, pressurizer lower instrument nozzle RC-023 (V208) (nominal pipe size 1 inch). The nozzle is classified under Examination Category B-P, Item No. B15.10, of the ASME Code,Section XI, table IWB-2500-1.
3.1.2 Applicable Code Edition and Addenda The current edition for the fourth inservice inspection (ISI) interval is the ASME Code,Section XI, 2013 Edition.
The Code of Construction for the pressurizer is the ASME Code,Section III, 1971 Edition with Addenda through Winter 1973.
The modification installation Code of Construction is the ASME Code,Section III, 1974 Edition with Addenda through Winter 1975.
The fourth ISI interval for Palo Verde, Unit 1, began on June 1, 2019, and is currently scheduled to end on July 17, 2028.
3.1.3 Applicable Code Requirements The following applicable requirements of the ASME Code,Section XI, 2013 Edition specify that:
Flaw Removal
- IWA-4412 states that Defect removal shall be accomplished in accordance with the requirements of IWA-4420.
- IWA-4421 states, in part, that defects shall be removed or mitigated in accordance with the requirements of IWA-4421(a), (b), (c) and (d).
Flaw Evaluation
- IWA-3300(b) states, in part, that Flaws shall be characterized in accordance with IWA-3310 through IWA-3390, as applicable.
- IWB-3420 states that Each detected flaw or group of flaws shall be characterized by the rules of IWA-3300 to establish the dimensions of the flaws. These dimensions shall be used in conjunction with the acceptance standards of IWB-3500.
- IWB-3610(b) states, in part, that For purposes of evaluation by analysis, the depth of flaws in clad components shall be defined in accordance with Figure IWB-3610-1....
Successive Examinations
- IWB-2420(a) states, in part, that The sequence of component examinations which was established during the first inspection interval shall be repeated.
- IWB-2420(b) states, in part, that 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.
Table 1 of ASME Code Case N-722-1, Additional Examinations for PWR [Pressurized Water Reactor] Pressure Retaining Welds in Class 1 Components Fabricated With Alloy 600/82/182 Materials,Section XI, Division 1, specifies that Inspection Item No. B15.180, Instrumentation Connections, require visual examination each refueling outage with IWB-3522 acceptance standards. Code Case N-722-1 is mandated in 10 CFR 50.55a(g)(6)(ii)(E)(1-4) with conditions.
Welding ASME Code Case N-638-10 provides requirements for automatic or machine GTAW of Class 1 components without the use of preheat or post-weld heat treatment. Paragraph 4(a)(2) of Code Case N-638-10 requires the completed weld to be non-destructively examined after the three tempering layers have been in place for at least 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br />. The NRC staff approved Code Case N-638-10 in Regulatory Guide (RG) 1.147, Inservice Inspection Code Case Acceptability, ASME Section XI, Division 1, Revision 20, December 2021 (ML21181A222).
The NRC staff notes that section 5.0 of the licensees submittal dated April 23, 2025, has referenced additional ASME Code requirements as shown below.
ASME Code,Section III, NB-4620 requires post-weld heat treatment.
ASME Code Case N-749, Alternative Acceptance Criteria for Flaws in Ferritic Steel Components Operating in the Upper Shelf Temperature Range,Section XI, Division1, provides guidance on flaw evaluations. The NRC has conditionally approved the use of Code Case N-749 as shown in RG 1.147, Revision 21 (ML23291A003).
3.1.4 Reason for Request The licensee stated that Palo Verde, Unit 1, is in the second period of the fourth 10-year ISI interval. During the Palo Verde, Unit 1 refueling outage 25 (1R25), the licensee preemptively replaced the Alloy 82 pressure retaining weld material with Alloy 52M weld material in the Unit 1 pressurizer instrument nozzles because the Alloy 82 weld is susceptible to primary water stress corrosion cracking (PWSCC). The licensee stated that it did not detect any age-related flaws or any pressure boundary leakage on the six instrument nozzle welds prior to the modification.
Relief Request 76 applies to modification of the lower instrument nozzle RC-023 (V208). The licensee preemptively replaced the original Alloy 600 pressurizer lower instrument nozzle in 1992 with an Alloy 690 nozzle, an Alloy 690 outer sleeve, an Alloy 82 weld pad, and an Alloy 82 nozzle-to-weld pad J-groove weld. For the 2025 modification, the licensee planned to remove the existing Alloy 690 nozzle, Alloy 82 J-groove weld, and Alloy 82 weld pad. The licensee planned to install a new weld pad and J-groove weld on the outer surface of the pressurizer bottom head surface using Alloy 52M filler metal. The new weld pad will be welded to the outer surface of the pressurizer bottom head using machine GTAW ambient temperature temper bead (ATTB) welding, with inert shielding gas. The licensee planned to attach an Alloy 690 nozzle to the Alloy 52M weld pad with a partial penetration weld using a manual GTAW welding technique and Alloy 52M filler metal.
During the modification, the licensee observed moisture between the Alloy 690 outer sleeve (corrosion liner) and the low alloy steel bottom head. The presence of moisture in the annulus indicated a possible leak path through the autogenous weld of the corrosion liner and the original J-groove weld. Subsequently, the licensee performed a borescope inspection of the autogenous outer sleeve weld to confirm the presence of a crack in the autogenous weld. The licensee identified an approximate 1/2 to 3/4-inch linear indication in the autogenous weld and an approximate 3/16 to 1/4-inch linear indication on the toe of the autogenous weld.
The licensee stated in its letter dated April 23, 2025, that, The linear indications along with the presence of moisture between the corrosion liner and the bottom head indicate a crack in the autogenous weld or a possible crack in the original J-groove weld remnant. Due to the possibility of a crack in the original J-groove weld a crack growth will be performed of a postulated worst-case (largest) crack in the [original] J-groove weld which bounds indications found in the autogenous weld.
The licensee also stated in its letter dated April 23, 2025, that, A flaw evaluation will demonstrate the acceptability of leaving the original partial penetration J-groove attachment weld [on the inside surface of the pressurizer],
with a maximum postulated flaw, in place for the life of the pressurizer. IWA-4412
[of the ASME Code, Section XI] requires the removal of, or the reduction in size of defects. The postulated flaw in the original J-groove weld will not be removed; therefore, an alternative is proposed for these requirements.
ASME Code,Section XI, IWA-4412 requires defect removal in accordance with IWA-4420.
Subarticle IWA-3300 requires flaws detected during inservice examinations to be sized.
Paragraph IWB-2420 requires successive examinations of flaws accepted for continued service.
Subarticles IWB-3400 and IWB-3600 assume that the detected flaw size and shape are based on ultrasonic testing. In support of the flaw evaluation, IWB-3420 and IWB-3610(b) require characterization of the flaw. The licensee stated that the performance demonstration initiative qualified technique does not exist to perform ultrasonic examination of the partial penetration J-groove weld and autogenous weld at the inside diameter surface of the pressurizer that can be used to accurately characterize the location, orientation, or size of a potential flaw in the original J-groove weld. Therefore, the licensee will evaluate a postulated flaw for acceptability for one cycle of operation and proposed the alternative to address the requirements of IWA-4412, IWA-3300, and IWB-2420.
The licensee stated that the ASME Code,Section III, NB-4620 requires all welds to be post-weld heat treated except as otherwise permitted in article NB-4622.7. The licensee is installing a welded pad using ATTB welding in accordance with ASME Code Case N-638-10 with austenitic filler materials within 1/8-inch of or on ferritic materials without the requirement for preheat or post-weld heat treatment. Code Case N-638-10 requires that the three tempering weld layers be in place for at least 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> prior to performance of surface and volumetric examination. The licensee stated that liquid penetrant and ultrasonic acceptance examinations may be performed before the 48-hour period ends. As documented in the white paper in attachment 1 to Relief Request 76, by letter dated April 23, 2025, the licensee stated that technical justification for austenitic filler materials has been developed to allow non-destructive examination (NDE) methods to be performed after completion of the weld modification, without waiting for the 48-hour hold time.
3.1.5 Proposed Alternative As an alternative to the ASME Code,Section XI, requirements listed in section 3.1.3 of this SE for flaw removal, flaw evaluation and flaw characterization of flaws in the original J-groove weld in the inner surface of the pressurizer bottom head, the licensee proposed analyzing a maximum postulated flaw that bounds the range of flaw sizes that could exist in the original J-groove weld.
As an alternative to the ASME Code,Section XI, required successive examination listed in section 3.1.3 of this SE, the licensee proposed to forgo successive examinations of the original J-groove weld. To support its alternative, the licensee proposed to use the fracture mechanics analysis to demonstrate that the flaws in the original J-groove weld are acceptable for one cycle of operation based on the requirements of ASME Code Case N-749.
In lieu of post-weld heat treatment required by ASME Code,Section III, NB-4620, the licensee proposed to install a welded pad using ATTB welding in accordance with ASME Code Case N-638-10, which does not require post-weld heat treatment. Also, in lieu of performing the required NDE at least 48-hours after the three tempering layers have reached ambient temperature in accordance with Code Case N-638-10, the licensee proposed to perform the NDE after completion of the weld modification.
The licensee stated in the current submittal that it will submit the final one-cycle flaw analytical evaluation, evaluation of modified configuration, and corrosion evaluation within 14 days following the end of the current Palo Verde, Unit 1 refueling outage. The NRC staff verified that the licensee subsequently submitted the final one-cycle flaw analytical evaluation, evaluation of modified configuration, and corrosion evaluation in a supplement dated May 22, 2025.
3.1.6 Duration of Proposed Alternative The licensee requested authorization of Relief Request 76 for the duration of the Palo Verde, Unit 1, Cycle 26, which is currently scheduled to conclude in the fall of 2026.
3.2
NRC Staff Evaluation
The licensees modification to nozzle RC-023 (V208) involves alternatives to various requirements of the ASME Code,Section XI, and ASME Code Case N-638-10. In support of the proposed nozzle modification, the licensee provided a technical basis that includes the design analysis, flaw evaluation, basis for elimination of the 48-hour hold, corrosion evaluation, loose parts evaluation, basis for eliminating successive examinations, and reactor coolant system (RCS) leak detection assessment. The NRC staff evaluated various topics of the licensees technical basis as follows.
Design Analysis The licensee evaluated nozzle RC-023 (V208) in accordance with the design requirements of ASME Code,Section III. The licensee calculated primary stress, primary plus secondary stress, and cumulative fatigue usage in accordance with the 2013 Edition of the ASME Code;Section III, Subsection NB, for the modification nozzle; new J-groove weld; weld pad; and affected region of the pressurizer lower head. The licensees design analysis demonstrates that all primary stresses, primary plus secondary stresses and fatigue criteria, are satisfied per NB-3200 for at least one cycle of operation. The design analysis confirms that the new nozzle will not eject from the pressurizer under design conditions.
The NRC staff reviewed the licensees design analysis of the RC-023 (V208) replacement nozzle and determined that the primary stress, primary plus secondary stress, and cumulative fatigue usage of the pressurizer lower instrument nozzle RC-023 (V208) have complied with all corresponding design criteria of the ASME Code,Section III, Subsection NB. The staff further determined, based on the licensees design analysis, that the new nozzle will not eject from the pressurizer under design conditions.
Flaw Evaluation The licensee evaluated the condition of the as-left J-groove weld of the 2025 modification for one additional cycle of operation based on the linear-elastic fracture mechanics (LEFM) method in accordance with ASME Code,Section XI, IWB-3610, and the elastic-plastic fracture mechanics (EPFM) method of ASME Code Case N-749. The NRC staff confirmed that the licensee considered all applicable conditions associated with Code Case N-749 as stated in RG 1.147, Revision 21.
The flaw evaluation requirement of ASME Code,Section XI, IWB-3600, presumes that cracks are fully characterized in accordance with IWB-3420. The licensee stated that qualified ultrasonic testing examination techniques for examining the original, as-left, J-groove weld at the nozzle-to-pressurizer bottom head are not available. As such, the licensee assumed that the as-left condition of the remaining J-groove weld includes flaws extending through the entire J-groove weld and buttering in its flaw evaluation.
The licensee stated that the uphill and downhill regions of the as-left J-groove weld at the pressurizers spherical bottom head experience the highest hoop stresses at the nozzle penetration. Based on this stress distribution, the licensee identified the radial direction relative to the bottom head surface as the preferential path for crack propagation. Accordingly, the licensee postulated a radial-axial flaw (radial with respect to the nozzle axis) within the J-groove weld and buttering. The licensee indicated that such a flaw would propagate through the J-groove weld and buttering by PWSCC and continue into the low alloy steel of the pressurizer bottom head due to fatigue under cyclic loading conditions.
According to the licensee, there is not sufficient time to complete the detailed plant-specific life of repair finite element analysis for the as-left J-groove weld flaw during the 1R25 refueling outage. Therefore, the licensee used the flaw analyses for the life of repair J-groove weld performed in 2010 for Palo Verde, Units 1, 2, and 3 pressurizer instrument nozzles as the basis to demonstrate that the flaw in nozzle RC-023 (V208) is acceptable from the start of Unit 1 up to one additional cycle following the modification in 2025. The duration of the as-left J-groove weld nozzle RC-023 (V208) modification in the one-cycle flaw analyses is equivalent to 41 years from the initial commercial operation in 1986 to 2027, which will be a few months beyond the end of one operating cycle in fall 2026. As a comparison, the as-left J-groove weld flaw analyses performed in 2010 for the repaired instrument nozzle are for 60 years of plant operation following the 1992 repair. The 2010 flaw analyses consist of LEFM and EPFM analyses To demonstrate that the 2010 LEFM and EPFM analyses are applicable to the 2025 nozzle modification, the licensee reviewed the differences between the edition of the ASME Code,Section XI used for the LEFM analysis (the 2001 Edition with 2003 Addenda in the 2010 analysis and the 2013 Edition for the current 2025 modification). The licensee concluded that the equations to calculate the critical material fracture toughness (K1C) and the fatigue crack growth formula are identical. Therefore, the licensee concluded that both editions of the ASME Code,Section XI, are considered equivalent. The NRC staff reviewed both editions of the aforementioned ASME Code,Section XI, and concurs that they are equivalent with respect to the LEFM analysis.
The licensee stated that the initial flaw size in the 2010 LEFM analysis is assumed to be at the interface of the susceptible Alloy 600 weld butter material with the pressurizer bottom head material. The flaw propagates into the pressurizer low alloy steel base metal through fatigue crack growth only. The 2010 flaw analysis calculated a fatigue crack growth for a maximum operating period of 60 years. The 2010 flaw analysis uses the fatigue crack growth rate for low alloy steel material from the 2001 Edition with 2003 Addenda of the ASME Code,Section XI. For the 2010 LEFM analysis, the licensee considered the following loads for the fatigue crack growth calculation: internal pressure, weld residual stresses, and all applicable thermal transient stresses. The licensee used a projected 60-year number of transient cycles, which bound the fatigue plus corrosion crack growth for the one cycle operation.
The 2010 LEFM analysis uses a bounding reference temperature for nil-ductility transition (RTNDT) value for the Palo Verde pressurizer bottom head material and is applicable for the Unit 1 bottom head material.
The licensee considered the differences in the weld modification configurations between the 2025 modification and the analysis performed in 2010 for the 1992 repair. The nozzle repair in 1992 inserted a sleeve in the nozzle bore that was welded to the as-left J-groove weld and to the repair J-groove weld on the pressurizer outside diameter. The licensee explained that for the 2025 modification, the sleeve is cut short by about 0.5 inches from the pressurizer outer surface and a roll expansion is performed to prevent the remnant sleeve from becoming a loose part during the plant operation. The licensee stated that the existence of the sleeve imposes stresses onto the J-groove weld location primarily due to the difference in thermal expansion coefficients between the pressurizer low alloy steel base metal and the Alloy 690 sleeve. When the sleeve is cut short and attached to the bore with the roll expansion during the 2025 modification, the thermal stresses on the as-left J-groove weld due to the sleeve are reduced.
As such, the NRC staff determined that based on the results of the 2010 flaw analysis, the 1992 nozzle repair configuration bound the 2025 modified nozzle configuration.
The licensee reported that the 2010 LEFM analysis for the 1992 repair indicates that the initial and final flaw sizes in the as-left J-groove weld exceeded the acceptance criterion of ASME Code,Section XI, IWB-3610. As a result, the licensee had to perform an EPFM analysis in 2010 to accept the 1992 nozzle repair as discussed below.
The licensee stated that the 2010 EPFM analysis for the 1992 Palo Verde, Units 1, 2, and 3 pressurizer bottom head instrument nozzles remains valid and bounds the one cycle of operation for the Unit 1 nozzle RC-023 (V208) modification in 2025. The licensee stated that it performed the EPFM evaluation in 2010 before the NRC approval of ASME Code Case N-749.
As such, the 2010 EPFM analysis used the best method available at that time (i.e., ASME Code,Section XI, appendix K). The licensee stated that the final flaw size used in the 2010 EPFM evaluation for 60 years of fatigue crack growth bounds the projected flaw size applicable to the one cycle operation considering fatigue crack growth plus corrosion through the next cycle for nozzle RC-023 (V208). The 2010 EPFM analysis used the J-T instability analysis approach described in ASME Code,Section XI, appendix K, figure K-4330-1 to predict crack instability (i.e., when the applied J-T line intersects the appropriate J-T material curve). The licensee reported that with safety factors of 3.0 on primary pressure loads and 1.5 on secondary loads, the applied J values for the initial and final flaw size are below the J-T material curve intersection points as shown in the 2010 EPFM analysis. The licensee explained that potential remnant cracking in the as-left J-groove weld is acceptable in accordance with the 2010 flaw evaluation. The licensee concluded that the results from the 2010 EPFM analysis bound the 2025 nozzle modification for one cycle operation at Palo Verde, Unit 1, in terms of stability of the postulated large flaw in the as-left J-groove weld.
In addition to the EPFM analysis, the licensee performed a limit load analysis to demonstrate the structural integrity of the pressurizer bottom head considering the nozzle and the degraded as-left J-groove weld. ASME Code Case N-749 requires the evaluation of primary stress limits per the ASME Code,Section III, NB-3000, assuming a local area reduction of the pressure retaining membrane that is equal to the area of the flaw. As such, for the 2025 nozzle modification, the licensee performed the limit load analysis to satisfy the requirements of IWB-3610(d)(2), which demonstrates the primary stress limits of Article NB-3000 are met. The licensee evaluated the primary stress limits of the modified weld configuration considering a final flaw depth and width equivalent to 41 years of fatigue plus corrosion flaw growth for the one cycle operation for Palo Verde, Unit 1, bottom head instrument nozzle. The NRC staff determined that the licensee has demonstrated that the primary stress requirements of the 2013 ASME Code,Section III, Article NB-3000 are met by the limit load analysis considering a local area reduction of the pressure retaining membrane of the nozzle opening that includes the area of the as-left J-groove weld and a bounding flaw sized for 41 years total of flaw growth.
The NRC staff verified that the 2010 EPFM analysis has satisfied the provisions of ASME Code Case N-749, and the associated conditions in RG 1.147, Revision 21. In addition, the staff noted that the licensees 2010 flaw evaluation was part of technical basis for the Palo Verde, Unit 1, Relief Request 70 regarding the repair of a pressurizer lower shell nozzle. The staff approved Relief Request 70 in an SE dated September 9, 2024 (ML24197A199). The staff determined that the licensees 2010 LEFM and EPFM analyses are applicable and bound the 2025 Unit 1 pressurizer nozzle RC-023 (V208) modification because (a) the 2010 LEFM and EPFM methodologies are applicable to the nozzle RC-023 (V208) modification in 2025; (b) the 2010 LEFM and EPFM analyses qualify the 1992 nozzle repair for 60 years of service, which bound the 41 years of service for nozzle RC-023 (V208); and (c) the 2010 LEFM and EPFM analyses follow the flaw evaluation requirements of the ASME Code,Section XI, which are applicable to the 2025 nozzle modification.
Basis for Elimination of the 48-Hour Hold Relief Request 76 presents a white paper developed from technical paper PVP 2023-107489, Elimination of the 48-hour Hold for Ambient Temperature Temper Bead Welding with Austenitic Weld Metal, as a basis to eliminate the 48-hour hold requirement. This 48-hour delay (between welding completion, cooling to ambient temperature, and the final NDE of the fully welded component) is intended to allow time for the detection of delayed hydrogen cracking, which is known to occur within 48-hours after the weldment reaches ambient temperature. However, the white paper provides data showing that when austenitic weld metal is used (as in the case of Palo Verde, Unit 1, nozzle repair) the diffusible hydrogen content in the heat affected zone of the ferritic base metal is too low to promote hydrogen-induced cracking. Therefore, the 48-hold time is not necessary. The NRC staff has previously found the technical basis in the white paper acceptable and has approved similar alternatives requested by other licensees to eliminate the 48-hour hold for the pipe and nozzle repairs. In this case, the staff determined that eliminating the 48-hour hold for the examination of the weld does not affect the structural integrity of the nozzle RC-023 (V208) modification because hydrogen-induced cracking is not expected to occur in the proposed nozzle repair at Palo Verde, Unit 1 pressurizer, based on the data shown in the white paper. Accordingly, the elimination of the 48-hold is acceptable for the nozzle RC-023 (V208) modification.
Corrosion Evaluation The licensee evaluated general corrosion, crevice corrosion, galvanic corrosion, stress corrosion cracking, and hydrogen embrittlement of the exposed pressurizer bottom head. The licensee also evaluated potential corrosion mechanisms for the Alloy 690 and Alloy 52M used in the modification and any replacement Type 304 instrumentation piping that is connected to nozzle RC-023 (V208).
The licensee presumed that the existing flaw in the original as-left weld of nozzle RC-023 (V208) causes reactor coolant to leak into the annulus between the sleeve and the pressurizer bore. The reactor coolant in the annulus may cause corrosion in the bore of the pressurizer bottom head. The licensee considered various corrosion degradation mechanisms inside the bore of the pressurizer bottom head. The licensee used a corrosion rate for the time periods of plant start up, plant operating, and plant shutdown. The licensee used the corrosion rate as an input into other analyses to establish the integrity of the component in the repaired configuration for one operating cycle (i.e., the ASME Code,Section III analysis and the flaw evaluation). The licensee stated that the general corrosion rate for the pressurizer base materials, previously calculated in the existing corrosion evaluation, is not expected to change based on the indication in the as-left J-groove weld. As such, it is not expected that the current Section III analysis or flaw evaluation (limit load analysis) conclusions will be challenged for a period of one operating cycle.
The licensee stated that the design life of the pressurizer upper and lower instrument nozzle modifications is the remainder of the 60-year licensed operational life of the plant. The licensee further stated that for the exposed low alloy steel of the pressurizer shell, many corrosion mechanisms are not expected to be a concern for the 60-year plant life. As such the NRC staff determined that corrosion will not be a concern for one operating cycle. The staff determined that the licensee used an adequate general corrosion rate for the pressurizer shell material based on industry data and operating experience. The staff noted that wrought Alloy 690 and Alloy 52M filler metal have been shown by extensive testing and in-reactor operating experience to have a low susceptibility to PWSCC and is not expected to be susceptible to other forms of corrosive degradation in the PWR environment, such as general corrosion, crevice corrosion, and galvanic corrosion. Thus, corrosion of Alloy 690 and Alloy 52M is not expected to be a concern for the life of the modifications.
The NRC staff determined that the licensee has appropriately evaluated the impact of the nozzle modification on the corrosion of the pressurizer bottom head material for one cycle of operation. The staff finds that the corrosion of the pressurizer bottom head material will not be significant for the one cycle of operation and therefore, the nozzle modification is acceptable with respect to the corrosion of the pressurizer bottom head.
Loose Parts Evaluation The licensee evaluated the loose parts as part of nozzle RC-023 (V208) modification, including the impact of loose parts such as J-groove weld fragments or the outer sleeve entering and remaining in the pressurizer. The licensee stated that in the unlikely event that the loose parts exit the pressurizer, they could be transported through the RCS. The licensee evaluated the potential effects of the loose parts on steam generator tubes and on the fuel and control elements (e.g., control element assembly (CEA) motion, flow blockage and fretting). The licensee stated that the loose parts are not expected to cause damage to steam generator tubes. Should loose parts reach the lower reactor vessel head region, the loose parts would not be expected to lodge between the CEA and CEA guide tube such that CEA motion would be impeded. The licensee stated that the possibility of CEA binding, though remote, would be able to be cleared by maneuvering the CEA. Therefore, the fragments are unlikely to result in flow blockage. The licensee also stated that even in the case where flow blockage occurs, nucleate boiling performance would not be degraded. Additionally, according to the licensee, in the unlikely event fretting were to occur, the effect of fuel reliability would be small and technical specification (TS) RCS activity limits would not be exceeded.
The NRC staff determined that the licensee has adequately evaluated various scenarios of the impact of the loose parts. Based on the licensees loose parts evaluation, the staff finds that the consequences of the loose parts affecting the plant safety or proper operation are minimal for one cycle of operation.
Basis for Eliminating Successive Examinations The NRC staff noted that IWB-2420(b) of the ASME Code,Section XI, states, in part, that 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. The licensee proposed no successive examinations of the as-left J-groove weld. As an alternative, the licensee proposed to use its fracture mechanics analyses based on the requirements of ASME Code Case N-749, demonstrating that the subject flaw(s) in the as-left J-groove weld is acceptable for one cycle of operation. The licensees analyses consider crack growth under all applicable operating conditions, including transients, for a period extending through Palo Verde, Unit 1, Operating Cycle 26 (41 years of cumulative plant operation). The licensee stated that it will continue to perform VT-2 examinations for evidence of pressure boundary leakage on the exterior surface of the weld pad as part of the existing boric acid corrosion control program (BACCP).
The NRC determined that it is acceptable that the licensee does not perform successive examinations of the flaw(s) in the as-left J-groove weld through one cycle of operation because the licensee has performed adequate flaw evaluations demonstrating that (a) the existing flaw in the as-left J-groove weld will not affect the structural integrity of the pressurizer bottom head for one operating cycle, (b) the potential loose parts from the as-left J-groove weld will not affect the safe operation of the plant, and (c) the licensee will perform VT-2 visual examinations of the exterior surface of the pressurizer bottom head when conducting system leakage tests in accordance with Examination Category B-P of the ASME Code,Section XI, table IWB-2500-1.
RCS Leak Detection Assessment The licensee describes the various methods used to identify RCS leakage at Palo Verde, Unit 1, in a timely manner. The licensee stated that the pressure boundary leakage associated with the original pressurizer TE-101 nozzle that was repaired under Relief Requests 70 and 73, was identified by routine outage-based ISI and BACCP walkdowns, and not the RCS leak detection systems, as the identified leak was extremely small.
The licensee stated that it inspects pressurizer instrument nozzles during refueling outages in accordance with BACCP procedures. These inspections are the primary method of detecting small amounts of leakage that are below the TS thresholds. TS 3.4.16, RCS Leakage Detection Instrumentation, establishes the limiting condition for operation (LCO) for instrumentation credited for detecting leakage from the RCS during power operations.
The licensee stated that multiple instrument locations are used to identify the location of leakage sources. The safety significance of RCS leakage varies widely depending on its source, rate, and duration. Quickly separating the identified leakage from the unidentified leakage provides quantitative information to the operators allowing them to take corrective action should leakage occur that is detrimental to the safety of the facility and the public.
If RCS leakage occurs, the licensee would review the RCS leakage quantity against the TS associated RCS leakage criteria. Depending on the source identified, a shutdown could be required in accordance with TS LCO 3.4.14 that has the following specific limits: (a) No pressure boundary LEAKAGE, (b) 1 gpm [gallons per minute] unidentified LEAKAGE, (c) 10 gpm identified LEAKAGE, and (d) 150 gallons per day primary to secondary LEAKAGE through any one steam generator (SG).
The licensee explained that if unidentified leakage increases greater than 0.10 gpm above the normal, steady-state value for a given plant condition during the performance of the routine RCS water inventory balance, administrative procedures would require that the controls and actions for monitoring RCS leakage under the BACCP be implemented. The licensee would investigate the increased leakage and can shut down the unit in a controlled manner prior to a nozzle failure, if unacceptable increased leakage were to occur.
The NRC staff determined that the licensee has implemented various measures for monitoring potential leakage from nozzle RC-023 (V208) during normal operation and refueling outages, such as the RCS leakage detection systems, BACCP inspections, visual examination during RCS system leakage tests, and other outage-related ISIs, are adequate for detecting potential leakage through the repaired weld pad. If leakage does occur at the nozzle, the licensee is required to adhere to the leakage limits in TS 3.4.14 and perform corrective actions within the specified completion time. Therefore, the NRC staff finds that the performance monitoring of nozzle RC-023 (V208) is acceptable.
Summary Based on its evaluation, the NRC staff finds that proposed alternative provides reasonable assurance of the structural integrity of the modified pressurizer lower instrument nozzle RC-023 (V208) for one cycle of operation.
4.0 CONCLUSION
As set forth above, the NRC staff determines that proposed alternative will provide an acceptable level of quality and safety for the operation of the pressurizer lower instrument nozzle RC-023 (V208). Accordingly, the staff concludes that the licensee has adequately addressed all the regulatory requirements set forth in 10 CFR 50.55a(z)(1). Therefore, the NRC authorizes the use of Relief Request 76 for one operating cycle, Cycle 26, which is currently scheduled to conclude in the fall of 2026, at Palo Verde, Unit 1.
All other requirements of ASME Code,Section XI, for which relief was not specifically requested and authorized by the NRC staff remain applicable, including the third-party review by the Authorized Nuclear Inservice Inspector.
Principal Contributors: John Tsao, NRR David Dijamco, NRR Date: September 12, 2025
- via eConcurrence NRR-028 OFFICE NRR/DORL/LPL4/PM*
NRR/DORL/LPL4/LA*
NRR/DNRL/NVIB/BC*
NAME WOrders PBlechman ABuford DATE 9/8/2025 9/10/2025 9/11/2025 OFFICE NRR/DORL/LPL/BC*
NAME TNakanishi DATE 9/12/2025