ML25043A070
| ML25043A070 | |
| Person / Time | |
|---|---|
| Site: | Palo Verde  |
| Issue date: | 03/25/2025 |
| From: | Tony Nakanishi Plant Licensing Branch IV |
| To: | Heflin A Arizona Public Service Co |
| Orders, William | |
| References | |
| EPID L-2024-LLA-116 | |
| Download: ML25043A070 (13) | |
Text
March 25, 2025 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 72 ALTERNATE EXAMINATION OF VENT NOZZLE OF SAFETY INJECTION TANK (EPID L-2024-LLR-0051)
Dear Adam Heflin:
By letter dated July 31, 2024 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML24213A329), as supplemented by letter dated November 21, 2024 (ML24326A361), Arizona Public Service Company (the licensee) submitted to the U.S. Nuclear Regulatory Commission (NRC) a proposed alternative from the requirements of the American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code),Section XI, at Palo Verde Nuclear Generating Station (Palo Verde), Unit 1.
Specifically, pursuant to Title 10 of the Code of Federal Regulations (10 CFR) 50.55a(a)(z)(1),
the licensee is proposing an alternative to the required flaw characterization of ASME Code Section XI, paragraphs IWC-3420 and IWA-3300 on the basis that the alternative will provide an acceptable level of quality and safety. The licensee is not proposing any alternative to the required successive examinations (IWB/IWC-2420). Relief Request 72 seeks approval to continue to allow the analyzed flaw to remain in place for the remainder of the currently expected plant life for Palo Verde, Unit 1.
As set forth in the enclosed safety evaluation, the NRC has determined that the proposed alternative weld repair for Palo Verde, Unit 1, safety injection tank 1A provides an acceptable level of quality and safety. Accordingly, the NRC staff concludes that the licensee has adequately addressed all regulatory requirements set forth in 10 CFR 50.55a(z)(1). Therefore, the NRC authorizes the use of Relief Request 72 at Palo Verde, Unit 1, for the remainder of plant life, through the initial license extension.
All other ASME Code,Section XI requirements for which relief was not specifically requested and authorized in this proposed alternative remain applicable, including 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 by email at William.Orders@nrc.gov.
Sincerely, Tony T. Nakanishi, Chief Plant Licensing Branch LPL4 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket Nos. STN 50-528,
Enclosure:
Safety Evaluation cc: Listserv TONY NAKANISHI Digitally signed by TONY NAKANISHI Date: 2025.03.25 17:27:31 -04'00'
Enclosure SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELIEF REQUEST 72 ALTERNATE EXAMINATION OF VENT NOZZLE OF SAFETY INJECTION TANK PALO VERDE NUCLEAR GENERATING STATION, UNIT 1 ARIZONA PUBLIC SERVICE COMPANY DOCKET NO. 50-528
1.0 INTRODUCTION
By letter dated July 31, 2024 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML24213A329), as supplemented by letter dated November 21, 2024 (ML24326A361), Arizona Public Service Company (the licensee) submitted to the U.S. Nuclear Regulatory Commission (NRC) a proposed alternative from the requirements of the American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code),Section XI, at Palo Verde Nuclear Generating Station (Palo Verde), Unit 1.
Specifically, pursuant to Title 10 of the Code of Federal Regulations (10 CFR) 50.55a(a)(z)(1),
the licensee is proposing an alternative to the required flaw characterization of ASME Code Section XI, paragraphs IWC-3420 and IWA-3300, on the basis that the alternative will provide an acceptable level of quality and safety. The licensee is not proposing any alternative to the required successive examinations (IWB/IWC-2420). Relief Request 72 seeks approval to continue to allow the analyzed flaw to remain in place for the remainder of the currently expected plant life for Palo Verde, Unit 1.
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.
3.0 TECHNICAL EVALUATION
3.1 Licensees Proposed Alternative ASME Code Components Affected The affected components are the nozzles of the safety injection tank (SIT). They are classified as ASME Code Class 2, Examination Category C-B, Item No. C2.20.
Applicable Code Editions and Addenda The Code of record for the fourth 10-year inservice inspection (ISI) interval for Palo Verde, Units 1, 2, and 3 is the ASME Code,Section XI, 2013 Edition.
The construction code for Palo Verde, Units 1, 2, and 3 is the ASME Code,Section III, 1971 Edition, and 1973 Winter Addenda.
The installation code for Palo Verde, Units 1, 2, and 3 is the ASME Code,Section III, 1974 Edition, and 1975 Winter Addenda.
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Applicable Code Requirement===
ASME Code,Section XI, subarticle IWA-3300 and paragraph IWC-3420, are applicable to any flaws discovered during ISI of a Class 2 component. Specifically, as stated in the licensees submittal dated July 31, 2024:
IWA-3300, Flaw Characterization, states that flaws detected by inservice examinations shall be sized by the bounding rectangle or square for the purpose of description and dimensioning.
IWC-3420, Characterization, states that each detected flaw or group of flaws shall be characterized by the rules of IWA-3300 to establish the dimensions of the flaw(s). These dimensions shall be used in conjunction with the acceptance standards of IWC-3500.
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Reason for Request===
In 2008, the licensee detected a nitrogen leak at Unit 1 SIT 1A. The licensee identified a leakage source at the 2-inch vent line nozzle annulus (between the vent line nozzle and the SIT shell) where leak testing (snoop method) indicated a two bubble per second leak. The licensee stated that ultrasonic testing (UT) of the carbon steel material of the vessel around the nozzle attachment weld (Alloy 82/182) showed no flaws. The licensee concluded that the leak was most likely due to porosity and/or slag inclusions in the Alloy 82/182 weld metal of the pressure boundary weld on the inner diameter of the tank. The licensee repaired the leak by making a partial penetration nozzle attachment weld at the junction of the vent line nozzle and the outside surface of the tank shell. The repair resulted in moving the pressure boundary from the original weld inside of the SIT to the new repair weld at the outside of the SIT and leaving the flaw in place in the original weld at the inside surface of the tank.
The licensee stated that removal of the flaw would require access to the original nozzle weld on the interior of the SIT. The licensee further stated that this would require scaffolding both outside and inside the SIT. According to the licensee, the limited size of the manway and the distance from the manway to the repaired nozzle presented significant personnel safety concerns such as, confined space entry due to the small size of the manway and personnel fall protection due to the 40-foot drop from the weld to the tank bottom. The licensee also considered the difficulty in controlling entry of foreign material from grinding and welding into the SIT.
The licensee stated that ASME Code,Section XI, IWC-3420 and IWA-3300 require that a flaw or group of flaws be characterized to establish the dimensions of the flaw(s) and that these dimensions be used in conjunction with the acceptance standards of IWC-3500, Acceptance Standards. The licensee explained that flaw characterization by UT is not possible due to the geometry of the partial penetration weld. Because the personnel safety concerns and the inability to characterize the flaw, the licensee developed an alternative that postulated a worst-case flaw in the original weld, evaluated its acceptance using IWB-3600, Analytical Evaluation of Flaws, and determined that the defective original weld could be left in place. The postulated flaw is an axial crack in the original Alloy 82/182 weld metal. The licensee stated that this postulated crack represents the worst-case for the most-likely weld discontinuities such as porosity and slag inclusions that caused the leak. The licensee further stated that this weld material and base material are not susceptible to stress corrosion cracking at 140 degrees Fahrenheit (ºF) in a nitrogen environment. In addition, no operating experience has been identified concerning incidents of stress corrosion cracking of Alloy 600/182/82 in an air or nitrogen environment at ambient temperature.
Proposed Alternative The licensee proposed not to perform the required flaw characterization of the original partial penetration weld of the vent nozzle inside the SIT in accordance with the ASME Code,Section XI, IWA-3300 and IWC-3420.
The licensee stated that Relief Request 72 seeks approval to continue to allow the analyzed flaw to remain in place for the remainder of the currently expected plant life for Unit 1.
The licensee stated that it is not proposing any alternative to the required successive examinations of the ASME Code,Section XI, IWB/IWC-2420.
Basis for Relief The basis for this alternative is 10 CFR 50.55a(a)(z)(1) that allows alternatives to the ASME Code,Section XI in cases where the proposed alternative would provide an acceptable level of quality and safety.
In lieu of fully characterizing/sizing the existing flaw in the original nozzle weld located in the inside surface of the SIT shell, the licensee analyzed a worst-case flaw to demonstrate that the flaw will not affect the structural integrity of the SIT and is acceptable to remain in service for the end of the operating license. The licensees flaw evaluation is based on (a) thermal and mechanical stress analysis of the repair, and (b) flaw tolerance evaluation to ensure that the postulated, worst-case flaw, meets the acceptance criteria of the ASME Code,Section XI.
These analyses are documented in the licensees submittal for Relief Request 39 dated July 11, 2008 (ML082250675).
The first attachment (attachment 1) to the July 11, 2008, submittal presents a thermal and mechanical stress analysis that assumes linear elastic behavior of the configuration after the proposed alternative repair to the SIT vent relief nozzle. The second attachment to the July 11, 2008, submittal performs a flaw tolerance evaluation, which was made up of four subtasks.
First, the analysis determines the bounding through-wall stress distribution based on the stress analysis generated in the first attachment. In the second step, the stress intensity factors are determined as a function of crack depth for the assumed worst-case flaw in the original, partial penetration pressure boundary weld on the inner surface between the shell and the vent relief nozzle. For the third step, the analysis calculates the allowable flaw size based on the ASME Code,Section XI guidelines and the material properties of the shell. Finally, the potential growth of the worst-case flaw due to fatigue loading is accounted for according to the design specifications for the SIT.
The licensee stated in its submittal dated July 31, 2024, that The flaw tolerance evaluation resulted in a postulated final flaw of 0.352 inch (original depth of the partial penetration weld plus projected growth due to fatigue) in the carbon steel material of the [SIT] vessel. The postulated flaw was analyzed in accordance with the [ASME Code,Section XI,] IWB-3600 and shown to be acceptable to IWB-3612, Acceptance Criteria Based on Applied Stress Intensity Factor. The original flaw evaluation demonstrates that the original flaw will remain within acceptable [ASME Code,] Section XI, limits for the expected 40-year plant life (remaining plant operating license plus 20-year life extension).
The licensee stated that it determined that the external new weld was not identified as a bounding location for the environmentally assisted fatigue screening analyses as the weld is not wetted and can be managed for fatigue only. The licensee further stated that the original analysis remains valid, and the flaw is acceptable for a fatigue life of 40 years (repair occurred in 2008), which includes the initial period of extended operation.
The licensee stated that when moving the pressure boundary of a nozzle from the inside to the outside of the tank, the flaw must be characterized and shown not to impact the integrity of the tank or new weld. The licensee further stated that its analysis demonstrates that any flaw in the nozzle or attachment weld will not propagate such that the structural integrity of the new weld or the tank boundary is affected.
Duration of Proposed Alternative The licensee requested that Relief Request 72 be authorized for the remainder of the plant life through initial license extension.
3.2
NRC Staff Evaluation
The NRC staff evaluated the acceptability of Relief Request 72 to ensure that the postulated, worst-case flaw in the original J-groove weld meets the acceptance criteria of the ASME Code,Section XI, IWB-3600, so that the structural integrity of the SIT will be maintained to the end of 60-year operating license. Specifically, the NRC staff reviewed the licensees thermal and mechanical stress analysis of the original weld and new weld, and the flaw tolerance evaluation of the original weld as documented in Relief Request 39.
3.2.1 Background There are four SITs in Palo Verde, Units 1, 2, and 3 that contain borated water and are pressurized with nitrogen. The tank vent nozzle is located on the upper head of the SIT in the nitrogen blanket area. The tank is approximately 41 ft high and 9 ft in diameter and operates at 60-145°F range with 600 pounds per square-inch gauge.
By letter dated July 11, 2008, the licensee submitted for NRC review and approval Relief Request 39, which is the predecessor to the current Relief Request 72. Relief Request 39 contains necessary analyses for the SIT nozzle repair and was applicable to the 40-year plant license. By letter dated July 2, 2009 (ML091700197), the NRC staff approved Relief Request 39 for the 40-year plant license. The NRC staff noted that Relief Request 39 contains the original stress analyses and flaw evaluation that are used to support Relief Request 72. At the time of the Relief Request 39 submittal, the licensee had not applied for Unit 1 license renewal.
By letter dated December 15, 2008 (ML083510612), the NRC staff received the initial license renewal application for Palo Verde Units 1, 2 and 3. By letter dated April 22, 2011, the NRC staff approved the initial license renewal application for Palo Verde, Units 1, 2 and 3, as documented in NRC report, NUREG-1961, Safety Evaluation Report Related to the License Renewal of Palo Verde Nuclear Generating Station, Units 1, 2 and 3: Docket Numbers 50-528, 50-529, and 50-530, Arizona Public Service Company (ML11095A011).
Relief Request 72 focuses on the analysis of the original weld located in the inner diameter of the SIT, not on the new repaired weld on the outer diameter of the SIT. The NRC staff noted that even though the original weld is no longer part of the pressure boundary, the licensee is required to inspect the original weld periodically in accordance with the ASME Code,Section XI, because the flaw remains in service. Relief Request 72 is requesting relief from periodic inspection of the original weld.
Instead of performing periodic inspection to characterize the flaw in the original weld, the licensee has assumed that the original weld on the inner diameter is completely cracked. Given this worst-case flaw in the original weld (and the potential for it to grow into the SIT 1A shell),
the licensee stated that the original weld will not affect the structural integrity of the SIT and the new weld on the outer diameter of the SIT will provide acceptable levels of quality and safety.
The licensees analyses involve a two-step process. The first step is to perform thermal and mechanical analyses to generate stresses at the original J-groove weld and to show that the new weld is acceptable. The second step is to apply the stresses in the original weld to calculate the flaw growth and determine the acceptability of the flaw in the original weld as part of the flaw tolerance calculation.
3.2.2 Thermal and Mechanical Stress Analysis The licensee constructed a finite element model of a portion of the SIT shell, including the affected vent relief nozzle, the original J-groove weld, and the new pressure boundary weld. The NRC staff observed that for the mechanical stress analysis, the licensee applied unit internal pressure, unit force loadings, and unit moment loadings on the finite element model. For the pressure loadings, the licensee applied a unit pressure of 1,000 pounds per square inch (psi) on the interior surfaces of the vent nozzle and on the annulus region of the nozzle penetration, assuming the original J-groove weld is flawed. The licensee also applied a tensile axial pressure as an end-cap load to the top free end of the nozzle. For the force loadings, the licensee applied a unit force of 1,000 pounds (lbs) in each of the X, Y, and Z directions on the top free end of the nozzle to simulate mechanical force loads acting on the nozzle (i.e., Fx = Fy = Fz = 1,000 lbs).
For the moment loadings, the licensee applied a unit moment of 1,000 in-lbs in each of the three orthogonal X, Y and Z axes on the free end of the nozzle to simulate mechanical piping moment loads acting on the nozzle (i.e., Mx = My = Mz =1,000 in-lbs). The mechanical loads are analyzed at a uniform body temperature of 70oF.
For the thermal analysis, the licensee applied a steady state body temperature of 200oF design temperature on the entire finite element model with a reference temperature of 70oF.
To obtain stresses from the finite element model, the licensee defined eight (Paths 1-8) linearized through-wall stress paths for the stress and fatigue evaluations, and one through-wall hoop stress path is defined for the crack growth evaluation (Path 9). The linearized and mapped stress results from the finite element analyses for Paths 1 through 9 are used in the flaw tolerance evaluation to calculate flaw growth.
The NRC staff determined that the licensees mechanical stress analysis is acceptable because it used appropriate loading conditions on the affected vent nozzle and SIT shell. The NRC staff further determined that the licensees thermal stress analysis is acceptable because a higher temperature (design temperature) of 200oF was used on the entire finite element model than the operating temperature of 140oF which was shown in attachment 1 of the Relief Request 39 submittal dated July 11, 2008.
3.2.3 Flaw Tolerance Evaluation The licensee performed the fracture mechanics analysis to demonstrate that a postulated nozzle corner crack in the interior of the SIT vessel shell meets the acceptance criteria of ASME Code,Section XI. The licensees fracture mechanics evaluation consists of the following steps:
(a) Determine the bounding through-wall stress distributions based on the finite element stress analysis results as discussed above.
(b) Derive the stress intensity factors for a postulated flaw in the original partial penetration weld at the SIT inside surface based on stresses obtained in the stress analyses.
(c) Calculate the allowable flaw size for the vent relief nozzle based on the guidelines of the ASME Code,Section XI.
(d) Calculate the growth of the postulate flaw to the end-of-evaluation period based on fatigue.
(e) Compare the final flaw size to the allowable flaw sizes for the vent relief nozzle.
As indicated in the licensees flaw tolerance calculation, the SIT vessel shell minimum thickness is 1.87 inches, and cladding thickness is 1/8 inches. The nozzle outside and inside diameters of the affected nozzle are 2.5 inches and 1.939 inches, respectively. The design internal and external pressures are 700 psi and 100 psi, respectively. The operating internal and external pressures are 610 psi and 5 psi, respectively. The material of the SIT shell is SA-516 Grade 70; the vent relief nozzle is Alloy 600; and the SIT vessel shell cladding is stainless steel.
The licensee stated that it inspected the base material of the SIT shell and did not find defects.
However, for the fracture mechanics evaluation, the licensee assumed that a nozzle corner flaw exists in the original Alloy 82/182 J-groove weld that attaches the vent nozzle to the inside surface of the SIT shell. This postulated crack represents the worst-case for the most likely weld discontinuities such as porosity and slag inclusions that caused the leak. As such, the licensee assumed an initial flaw size equal to the full cross-section of the existing J-groove weld and is axially oriented with respect to the vent nozzle. The licensee also assumed the postulated nozzle corner crack to grow congruently in the SIT shell, keeping its same shape and a constant aspect ratio. The licensee used the total number of specified full pressure cycles transients, which correspond to a 40-year plant life (20-year remaining life plus 20-year life extension) for crack growth calculations.
The NRC staff noted that the applied stress intensity factor for the final flaw size should be less than the allowable material fracture toughness to demonstrate the crack stability. The licensee used the method in the ASME Code,Section XI, IWB-3600 to perform its flaw evaluation. The vent nozzle is classified as an ASME Code, Class 2 component. As such, the flaw evaluation should follow the flaw acceptance criteria of the ASME Code,Section XI, IWC-3610. However, the 2013 Edition of the ASME Code,Section XI, IWC-3610 states that the acceptance criteria for ferritic material are in the course of preparation and that the acceptance criteria in IWB-3610 may be used. The NRC staff finds that the licensee is permitted to use IWB-3610 to perform flaw evaluation because the SIT is made of ferritic material.
For normal conditions, IWB-3612 requires a safety factor of 10 on the material fracture toughness to derive the allowable material fracture toughness to resist the postulated flaw. That is, KI KIc / 10 where, KI is the maximum applied stress intensity factor. The KIc is the material fracture toughness based on crack initiation for the corresponding crack-tip temperature. The NRC staff noted that the method used to estimate fracture toughness of the shell steel from the measured Charpy impact properties was chosen based on the assumption that the steel in service would be operating in the toughness transition region where brittle fracture could occur.
In reality, the service temperature is high enough to ensure ductile behavior and a higher fracture toughness for the shell could be justified. However, to be conservative, using the lower bound SIT shell material toughness (KIc = 100 ksiin), the licensee determined an allowable material fracture toughness of KI = KIc = 100 / 10 = 31.6 ksiin. This is the allowable material fracture toughness that limits the maximum applied stress intensity factor of the postulated flaw.
Based on an allowable stress intensity factor of 31.6 ksiin, the licensee derived an allowable flaw size of 1.44 inches.
The NRC staff determined that the licensee followed the acceptance criteria of IWB-3612 of the ASME Code,Section XI and used lower bound SIT shell material fracture toughness to derive an allowable flaw size of 1.44 inches. Therefore, the allowable flaw size is acceptable.
3.2.4 Fatigue Crack Growth Analysis The licensee assumed that the postulated flaw in the original J-groove weld grows into the SIT shell wall due to cyclic fatigue loading. The applicable cyclic loadings specified in the design specification of the SIT are the full pressure cycles, hydrostatic tests, and blowdown tests. The licensee used the methodology of ASME Code,Section XI, Appendix A, to analyze the crack growth into the SIT shell by fatigue mechanism. The fatigue crack growth rate for the ferritic steel SIT material is a function of the range of the applied stress intensity factor. The licensee used the reference fatigue crack growth curves in ASME Code,Section XI, Appendix A, for low alloy steels in air environments for the fatigue crack growth evaluation.
The licensee used the stress intensity factors and the 40-year design life transient cycles (20-year remaining life in the current 40-year operating license plus additional 20 years of life extension) to calculate fatigue crack growth for a postulated corner crack. The licensee assumed the initial flaw depth of 0.351 inches as the depth of the remnant J-groove weld at the SIT inside surface. The NRC staff noted that this is a conservative postulated flaw size for the fatigue crack growth analysis because the depth of the postulated flaw is set to a value equivalent to the entire depth of the original inside partial penetration weld used in the design of the vent valve nozzle. In the fatigue crack growth analysis, the licensee used appropriate scale factors based on the internal pressure levels to determine the stress intensity factors due to the full pressure cycles and the hydrostatic and blowdown tests. The licensee stated that because there is less than one occurrence per year for the tests, the licensee used one occurrence per year for the hydrostatic test and blowdown test in the analysis. The licensees fatigue crack growth analysis resulted in a final flaw depth of 0.352 inches after 40 years of plant operation.
The NRC staff noted that the final flaw depth of 0.352 inches is less than the allowable flaw depth of 1.44 inches. Based on the NRC staffs review of the fatigue flaw growth analysis, the staff finds that the analysis demonstrates that the structural integrity of the SIT vent valve nozzle will be maintained through the end of first renewed operating period for the unit.
3.2.5 Inspection The NRC staff recognizes that the relief request proposes not to inspect the original partial penetration weld of the vent nozzle of the SIT. However, the NRC staff requested that the licensee discuss whether it will inspect the new pressure boundary weld at the outside diameter of the SIT. In its response to the NRCs request for additional information (RAI) dated November 21, 2024, the licensee stated that examinations under Item Nos. C2.21 and C2.22 in the ASME Code,Section XI, table IWC-2500-1, Examination Category C-B, are not applicable to the SIT nozzle with the pressure retaining weld moved to the outside surface of the SIT. The licensee stated that it has not performed surface or volumetric examinations since 2008 for the SIT nozzle to shell weld.
The licensee further stated that the SIT nozzle is attached to a 2-inch nominal pipe size (NPS) vent line piping, which is exempt from examination per ASME Code,Section XI, IWC-1221(a)(1), which excludes piping 4-inch and smaller from examination. The licensee explained that the examinations prescribed under Examination Category C-F-1 and Code Case N-716-1, Alternative Classification and Examination Requirements,Section XI, Division 1, are not applicable to the vent nozzle. The NRC staff noted that the latest version of Code Case N-716 that the NRC has approved is N-716-3 as shown in Regulatory Guide 1.147, Revision 21, Inservice Inspection Code Case Acceptability ASME Section XI, Division 1.
However, the NRC staff noted that there is no conflict in the provisions between N-716-1 and N-716-3 in terms of the classification of the subject vent nozzle. As such, the NRC staff determined that it is acceptable that the licensee referenced Code Case N-716-1. In addition, the licensee stated that examinations under Item Nos. C2.21 and C2.22 are not applicable to the vent nozzle, as referenced in Note 5 to table IWC-2500-1, Examination Category C-B.
The NRC noted that Note 5 to table IWC-2500-1, Examination Category C-B refers nozzle weld examination to Examination Category C-F. Table IWC-2500-1, Note 1, to Examination Category C-F-1 specifies that Requirements for examination of welds in piping NPS 4 (DN 100) apply to PWR [pressurized-water reactor] high pressure safety injection and auxiliary feedwater systems in accordance with the exemption criteria of IWC-1220. ASME Code,Section XI, IWC-1221(a)(1) exempts from inspection welds that are attached to piping less than NPS 4 inch.
In its response to the RAI dated November 21, 2024, the licensee stated that although it does not plan on performing surface or volumetric examinations on the new vent line nozzle weld, it performs a system leakage test each inspection period in accordance with Examination Category C-H of ASME Code,Section XI, table IWC-2500-1. The NRC staff noted in its RAI that the system leakage test requires the licensee to perform a visual VT-2 examination for leakage in accordance with IWA-5240 and IWC-5220. As such, the licensee has performed a visual examination of the new vent nozzle weld.
3.2.6 Defense-in-Depth Measures Without surface or volumetric examination of the vent nozzle, the NRC staff requested that the licensee provide defense-in-depth measures to ensure structural integrity of the repaired SIT vent nozzle. In its response to the RAI dated November 21, 2024, the licensee stated that each SIT is equipped with one wide range pressure transmitter and two narrow range pressure transmitters to provide indication and/or alarms in the Control Room.
The licensee stated that Palo Verde, Unit 1 SIT 1A has the following three transmitters:
1JSIAPT0331: Wide-range pressure transmitter that provides pressure indication in the Control Room, Remote Shutdown Panel, and Emergency Response Facility Data Acquisition and Display System (ERFDADS).
1JSINPT0332: Narrow-range pressure transmitter that provides pressure indication in the Control Room and Plant Computer and supplies high-and low-pressure alarms.
1JSIAPT0333: Narrow-range pressure transmitter that provides pressure indication in the Control Room and Plant Computer and supplies high-and low-pressure alarms.
The licensee stated that these instrument loops are used for monitoring SIT pressure and occurrences of the low-pressure alarms that would alert the operators of a low-pressure condition that necessitates the appropriate action per the alarm response procedure. The licensee further stated that instances of SIT nitrogen leakage have been identified during an operating cycle by trending indicated pressure and the frequency of low-pressure alarms compared to that of the other SITs.
The NRC staff finds that although surface and volumetric examinations are exempted for the vent line nozzle by ASME Code,Section XI, the licensee has pressure sensors that monitor internal pressure of the SIT and transmit signals to the control room. In case of abnormal pressure events, the operators can take corrective actions based on the alarms in the control room panel.
In summary, the NRC staff has determined that the proposed alternative provides for an acceptable level of quality and safety for SIT 1A at Palo Verde, Unit 1 based on: (1) the licensees performance of a stress analysis and flaw tolerance evaluation that demonstrated that a postulated worst-case flaw in the original J-groove weld will not challenge the structural integrity of the SIT shell at the vent nozzle through the end of the first renewed operating license (initial license extension); (2) continued performance of periodic system leakage testing in accordance with the ASME Code,Section XI; and (3) continued performance of SIT internal pressure monitoring using pressure sensors located in the control room of the unit.
4.0 CONCLUSION
As set forth above, the NRC has determined that the proposed alternative weld repair for Palo Verde, Unit 1 SIT 1A provides an acceptable level of quality and safety. Accordingly, the NRC staff concludes that the licensee has adequately addressed all regulatory requirements set forth in 10 CFR 50.55a(z)(1). Therefore, the NRC authorizes the use of Relief Request 72 at Palo Verde, Unit 1, for the remainder of plant life, through the initial license extension.
All other ASME Code Section XI requirements for which relief was not specifically requested and authorized in this proposed alternative remain applicable, including third party review by the Authorized Nuclear Inservice Inspector Principal Contributors: John Tsao, NRR James Medoff, NRR Omar Khan, NRR Date: March 25, 2025
- concurrence via email OFFICE NRR/DORL/LPL4/PM NRR/DORL/LPL4/LA*
NRR/DNRL/NVIB/BC*
NAME WOrders PBlechman ABuford DATE 3/19/2025 3/20/2025 3/24/2025 OFFICE NRR/DORL/LPL4/PM*
NRR/DORL/LPL4/BC*
NAME WOrders TNakanishi DATE 3/24/2025 3/25/2025