ML23296A254

Relief Request 70 – Proposed Alternatives for Pressurizer Lower Shell Temperature Nozzle

ML23296A254
Person / Time
Site:	Palo Verde
Issue date:	10/23/2023
From:	Harbor C Arizona Public Service Co
To:	Office of Nuclear Reactor Regulation, Document Control Desk
References
102-08690-CDH/MSC
Download: ML23296A254 (1)

Text

10 CFR 50.55a A member of the STARS Alliance, LLC Callaway

• Diablo Canyon

• Palo Verde

• Wolf Creek Cary D. Harbor Vice President Regulatory & Oversight Palo Verde Nuclear Generating Station P.O. Box 52034 Phoenix, AZ 85072 Mail Station 7605 Tel: 623.393.7953 102-08690-CDH/MSC October 23, 2023 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001

Subject:

Palo Verde Nuclear Generating Station Unit 1 Docket No. STN 50-528 Renewed Operating License Number NPF-41 Relief Request 70 - Proposed Alternatives for Pressurizer Lower Shell Temperature Nozzle Pursuant to Title 10 of the Code of Federal Regulations (10 CFR) 50.55a, Codes and Standards, paragraph (z)(1), Arizona Public Service Company (APS) requests Nuclear Regulatory Commission (NRC) authorization of Relief Request 70, on the basis that the alternatives provide an acceptable level of quality and safety. APS proposes alternatives to American Society of Mechanical Engineers (ASME) Pressure Vessel Code,Section XI, 2013 Edition, and ASME Code Case N-638-10, Similar and Dissimilar Metal Welding Using Ambient Temperature Machine [Gas Tungsten Arc Welding] GTAW Temper Bead Technique,Section XI, Division 1, dated May 6, 2019, for Palo Verde Nuclear Generating Station (PVNGS), Unit 1, as described in the Enclosure to this letter.

On October 9, 2023, while performing small nozzle inspections during Unit 1 refueling outage, boric acid leakage was found on the area of the weld of the pressurizer lower shell temperature nozzle. As part of the PVNGS corrective action program, an evaluation of the cause of this leakage, including an assessment of the extent of condition, is in progress.

The proposed alternatives are requested for one operating cycle. Authorization is requested for the duration of the PVNGS, Unit 1 Cycle 25, which is currently scheduled to conclude in the Spring of 2025.

A separate relief request will be submitted to justify continued use of the nozzle repair for the life of the plant. This permanent relief request, which will contain the appropriate analyses and justification for the remainder of the plant operating life, will be submitted prior to the end of the upcoming operating cycle.

102-08690-CDH/MSC ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Relief Request 70 - Proposed Alternatives for Pressurizer Lower Shell Temperature Nozzle Page 2 A pre-submittal meeting was held between APS and the NRC staff on October 20, 2023.

APS requests verbal authorization of the relief request by October 27, 2023.

APS makes the following new commitment to the NRC by this letter:

The final one-cycle flaw analytical evaluation, evaluation of repair, and corrosion evaluation will be submitted within 14 days following the end of the current PVNGS, Unit 1 refueling outage.

Should you need further information regarding this letter, please contact Matthew S.

Cox, Licensing Department Leader, at (623) 393-5753.

Sincerely, CDH/MSC/cr

Enclosure:

Relief Request 70 - Proposed Alternatives in Accordance with 10 CFR 50.55a(z)(1) for Pressurizer Lower Shell Temperature Nozzle cc:

J. D. Monninger NRC Region IV Regional Administrator S. P. Lingam NRC NRR Project Manager for PVNGS L. N. Merker NRC Senior Resident Inspector for PVNGS Harbor, Cary (Z16762)

Digitally signed by Harbor, Cary (Z16762)

Date: 2023.10.23 18:01:08 -07'00'

Enclosure Relief Request 70 Proposed Alternatives in Accordance with 10 CFR 50.55a(z)(1) for Pressurizer Lower Shell Temperature Nozzle

Enclosure Relief Request 70 Page i Arizona Public Service Company Palo Verde Nuclear Generating Station - Unit 1 Proposed Alternatives in Accordance with 10 CFR 50.55a(z)(1) for Pressurizer Lower Shell Temperature Nozzle 1.0 ASME CODE COMPONENT AFFECTED 2.0 APPLICABLE CODE EDITION AND ADDENDA 3.0 APPLICABLE CODE REQUIREMENTS 4.0 REASON FOR REQUEST 5.0 PROPOSED ALTERNATIVE AND BASIS FOR USE 6.0 DURATION OF PROPOSED ALTERNATIVE 7.0 PRECEDENTS

8.0 REFERENCES

ATTACHMENT 1: White Paper - Ambient Temperature Temper Bead Elimination of 48-Hour Hold Time From N-888 When Using Austenitic Filler Material

Enclosure Relief Request 70 Page ii LIST OF ACRONYMS Acronym Definition ALJGW As-Left J-Groove Weld APS Arizona Public Service Company ASME American Society of Mechanical Engineers ATTB Ambient Temperature Temper Bead CEA Control Element Assembly EPFM Elastic Plastic Fracture Mechanics GTAW Gas Tungsten Arc Welding HAZ Heat Affected Zone HIC Hydrogen Induced Cracking ISI Inservice Inspection JGW J-Groove Weld LAS Low Alloy Steel LEFM Linear Elastic Fracture Mechanics NDE Nondestructive Examination NRC Nuclear Regulatory Commission OCJ One-Cycle Justification OD Outside Diameter PDI Performance Demonstration Initiative PT (Liquid) Penetrant Testing PVNGS Palo Verde Nuclear Generating Station PWHT Post Weld Heat Treatment PWSCC Primary Water Stress Corrosion Cracking PZR Pressurizer RCS Reactor Coolant System RG Regulatory Guide UT Ultrasonic Testing

Enclosure Relief Request 70 Page 1 1.0 ASME CODE COMPONENT AFFECTED Component:

Pressurizer Lower Temperature Nozzle Code Class:

1 Examination Category:

B-P, American Society of Mechanical Engineers (ASME) Code Section XI Item No.

B15.10, Table IWB-2500-1 (B-P)

==

Description:==

Pressurizer Lower Shell Temperature Nozzle TE101, Nominal Pipe Size 1-inch Interval:

Fourth (June 1, 2019 to July 17, 2028) 2.0 APPLICABLE CODE EDITION AND ADDENDA:

The current edition for the fourth Inservice Inspection (ISI) interval is the American Society of Mechanical Engineers (ASME) Code,Section XI, 2013 Edition (Reference 1).

The Code of Construction for the Pressurizer is the ASME Code Section III, 1971 Edition with Addenda through Winter 1972 (Reference 3).

The repair installation Code of Construction is the ASME Code,Section III, 1974 Edition with Addenda through Winter 1975 (Reference 4).

The fourth ISI interval for PVNGS Unit 1 began on June 1, 2019, and is currently scheduled to end on July 17, 2028.

3.0 APPLICABLE CODE REQUIREMENTS ASME Code,Section XI, 2013 Edition Flaw Removal IWA-4412 states Defect removal shall be accomplished in accordance with the requirements of IWA-4420.

IWA-4421 states Defects shall be removed or mitigated in accordance with the following requirements:

IWA-5250(a)(3) states Components requiring corrective action shall have repair/replacement activities performed in accordance with IWA-4000 or corrective measures performed where the relevant condition can be corrected without a repair/replacement activity.

Enclosure Relief Request 70 Page 2 Flaw Evaluation IWB-3142.1(b) states A component whose visual examination detects the relevant conditions described in the standards of Table IWB-3410-1 shall be unacceptable for continued service, unless such components meet the requirements of IWB-3142.2, IWB-3142.3, or IWB-3142.4.

IWA-3300(a) states, in part, Flaws detected by the preservice and inservice examinations shall be sized IWA-3300(b) states, in part, Flaws shall be characterized in accordance with IWA-3310 through IWA-3390, as applicable IWB-3420 states 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-3522.1 states, in part, A component whose visual examination (IWA-5240) detects any of the following relevant conditions shall meet IWB-3142 and IWA-5250 prior to continued service IWB-3522.1(a) states any throughwall or throughweld, pressure-retaining material leakage from insulated and noninsulated components; IWB-3610(b) states, in part, 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, The sequence of component examinations which was established during the first inspection interval shall be repeated IWB-2420(b) states, in part, 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 ASME Code Case N-722-1, Additional Examinations for PWR Pressure Retaining Welds in Class 1 Components Fabricated With Alloy 600/82/182 Materials Item No. B15.180, Instrumentation Connections, require visual examination each refueling outage with IWB-3522 acceptance standards.

Welding Code Case N-638-10, Similar and Dissimilar Metal Welding Using Ambient Temperature Machine GTAW Temper Bead Technique Code Case N-638-10 (Reference 2) provides requirements for automatic or machine gas tungsten arc welding (GTAW) of Class 1 components without the use of preheat or post-weld heat treatment.

Enclosure Relief Request 70 Page 3 Paragraph 4(a)(2) of Code Case N-638-10 requires the completed weld to be nondestructively 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 />.

4.0 REASON FOR REQUEST Palo Verde Nuclear Generating Station (PVNGS) is in the second period of the fourth 10-year inservice inspection (ISI) interval. On October 9, 2023, while performing small nozzle inspections in support of boric acid walkdowns during refueling outage 1R24, boric acid leakage was found on the area of the weld of the pressurizer lower shell temperature nozzle at PVNGS Unit 1. The pressurizer thermowell nozzle (TE101), is Item No. B15.10 in Table IWB-2500-1, ASME Section XI, and Item No. B15.180 in Table 1, Code Case N-722-1. Nondestructive examination of the weld indicated leakage through the reactor coolant pressure boundary. The examination result constitutes defects in the primary coolant system that are unacceptable under ASME Section XI.

The original Alloy 600 pressurizer lower shell temperature nozzle was pre-emptively replaced 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. Figure 1 provides a sketch of the existing pressurizer lower shell temperature nozzle configuration and approximate location of the leakage indication. The leakage indication discovered on October 9, 2023, is in the Alloy 82 J-groove weld on the outside of the pressurizer shell.

Figure 1: Pressurizer Lower Shell Temperature Nozzle - Existing Configuration A borescope inspection was performed of the original J-groove weld and autogenous outer sleeve weld. The visual inspection showed linear indications along the autogenous weld toe and a rounded indication near the midpoint of the autogenous weld. There are two possible leak paths. The first possible leak path is through a defect at the autogenous weld between the Alloy 690 outer sleeve and the original Alloy 82/182 J-groove weld. RCS coolant could enter the annulus between the outer sleeve

Enclosure Relief Request 70 Page 4 and the bore in the shell and wet the inboard side of the nozzle to pad Alloy 82 J-groove weld.

Since the root of the nozzle to weld pad Alloy 82 J-groove weld is wetted by design, the second possible leak path is through the annulus between the Alloy 690 nozzle and Alloy 690 sleeve. The final leak path likely initiated at the root of the Alloy 82 weld pad to Alloy 82 J-groove weld and propagated through the J-groove weld until through-wall.

Figure 1A provides a depiction of the possible leak paths.

Figure 1A: Pressurizer Lower Shell Temperature Nozzle - Potential Leak Paths The portion of the Alloy 82 weld pad, Alloy 82 J-groove weld, and Alloy 690 nozzle surrounding the defect was removed from the pressurizer and will be destructively examined to determine the actual leak path. A liquid penetrant examination performed after removing all but approximately 1/8 inch of the Alloy 82 weld pad identified a 5/16-inch axial indication in the weld pad material adjacent to the leak. As part of the PVNGS corrective action program, a Level 1 Condition Report will evaluate the cause of the leakage and assess the extent of condition.

The pressurizer contains one (1) thermowell nozzle (TE101) that measures water temperature. The pressurizer design temperature and pressure are 700 degrees Fahrenheit and 2500 pounds per square inch, respectively. The pressurizer operating temperature and pressure are 653 degrees Fahrenheit and 2250 pounds per square inch, respectively. The pressurizer shell material is SA-533, Gr. B, Cl. 1 and the minimum thickness is 5 inches with a 1.379-inch diameter bore. The non-pressure boundary portion of the TE101 nozzle adjacent to Potential Leak Path 2 in Figure 1A has a non-interference fit with respect to the inside diameter of the outer sleeve.

As a result of the leakage indication/defect in the pressurizer lower shell temperature nozzle to weld pad Alloy 82 attachment weld, APS has removed the portion of the nozzle J-groove weld and adjacent weld pad material to preserve the leakage indication/defect for evaluation. The APS design removes the remainder of the entire Alloy 82 weld pad and replaces the existing nozzle J-groove weld and weld pad with a nickel-base weld material with low susceptibility to Primary Water Stress Corrosion Cracking (PWSCC). An Alloy 690 replacement nozzle will be installed.

Enclosure Relief Request 70 Page 5 APS is proposing to remove the existing Alloy 690 nozzle, Alloy 82 nozzle J-groove weld, and Alloy 82 weld pad and use ASME Section XI, Code Case N-638-10, and ASME Section III to apply a new welded pad and J-groove weld on the outer surface of the pressurizer using a nickel-base material with low susceptibility to PWSCC, Nickel-Alloy 52M (SFA-5.14, ERNiCrFe-7A) filler metal. Figure 2 provides a sketch of the planned pressurizer lower shell temperature nozzle repair. The new weld pad will be welded to the OD of the pressurizer shell using the machine Gas Tungsten Arc Welding (GTAW)

Ambient Temperature Temper Bead (ATTB) welding technique, with inert shielding gas.

APS is proposing to attach a nickel-based alloy nozzle to the new nickel-based alloy weld pad with a partial penetration weld using a manual GTAW welding technique and Nickel-Alloy 52M filler metal. The replacement nozzle, weld pad and partial penetration weld materials will be nickel-based materials with low susceptibility to PWSCC.

Figure 2: Pressurizer Lower Shell Temperature Nozzle - Repaired Configuration The PWSCC susceptible weld pad and nozzle J-groove weld material (Alloy 182/82) and the Alloy 690 nozzle installed in 1992 have been completely removed. The Alloy 690 outer sleeve installed in 1992 will remain in place. The original construction inside-vessel J-groove partial penetration attachment weld and the sleeve installed in 1992 will remain in place. A flaw evaluation will demonstrate the acceptability of leaving the original partial penetration J-groove attachment weld, with a maximum postulated flaw, in place for one cycle (see Basis for Flaw Analytical Evaluation below). IWA-4412 contains requirements for the removal of, or 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.

IWA-4412 requires defect removal in accordance with IWA-4420. IWA-3300 requires flaws detected during inservice examinations to be sized. IWB-2420 requires successive examinations of flaws accepted for continued service. IWB-3400 and IWB-

Enclosure Relief Request 70 Page 6 3600 were written with the expectation that volumetric NDE techniques such as Ultrasonic Testing (UT) would be used to determine the flaw size and shape. In support of the flaw evaluation, the ASME Code paragraphs IWB-3420 and IWB-3610(b) require characterization of the flaw in the leaking nozzle. There is not a Performance Demonstration Initiative (PDI) qualified technique to perform NDE of the configuration of the partial penetration J-groove weld and autogenous weld of the Alloy 690 sleeve at the ID 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, a postulated flaw will be evaluated for acceptability for one cycle an alternative is proposed for IWA-4412, IWA-3300, and IWB-2420.

NB-4620 requires all welds to be post-weld heat treated except as otherwise permitted in NB-4622.7. In lieu of this approach, APS is proposing to install a welded pad using ATTB welding in accordance with ASME Code Case N-638-10. The NRC has approved Code Case N-638-10 to allow ATTB welding 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 (Reference 2) 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 NDE. Liquid penetrant and ultrasonic acceptance examinations will be performed before the 48-hour period ends. Technical justification for austenitic filler materials has been developed to allow NDE methods to be performed after completion of the weld repair, without waiting for the 48-hour hold time.

5.0 PROPOSED ALTERNATIVE AND BASIS FOR USE A. Planned Repair Steps The planned steps of the proposed repair are described below. This plan may change due to varying conditions present during the work. Changes to the plan detailed below will remain in compliance with applicable Code requirements.

1. Remove existing Alloy 82 weld pad and Alloy 690 nozzle, etch ferritic surface to verify all Alloy 82 weld metal has been removed

2. Roll expand the Alloy 690 outer sleeve in the penetration bore

3. Install foreign material exclusion plug into the outer sleeve in the penetration bore

4. Install/attach capacitor discharge studs to the pressurizer shell for welding and boring tools

5. Perform liquid penetrant examination on the outer surface of the pressurizer shell in preparation for the new weld pad

6. Install the Alloy 690 weld dam

7. Deposit the Alloy 52M weld pad, in accordance with Code Case N-638-10

8. Perform post weld grinding of the weld pad

9. Perform dimensional inspection of weld pad

Enclosure Relief Request 70 Page 7

10. Perform a liquid penetrant examination of weld pad

11. Ultrasonic Testing (UT) of weld pad

12. Machine the weld pad to remove the weld dam and create final weld bore

13. Measure and perform a liquid penetrant examination of the final bore

14. Machine, inspect, and perform a liquid penetrant examination of the replacement nozzle

15. Machine J-groove weld preparation in weld pad

16. Inspect and perform a liquid penetrant examination of the J-groove

17. Remove foreign material exclusion plug

18. Replacement nozzle installation and welding

19. Inspect and perform a progressive liquid penetrant examination of the nozzle weld (1/2 thickness at approx. 0.31 inches and full thickness at approx. 0.62 inches)

20. Remove capacitor discharge studs attached to pressurizer shell

21. Install and weld replacement thermowell into new Alloy 690 nozzle

22. Inspect and perform liquid penetrant examination of the thermowell weld

23. Perform liquid penetrant examination of pressurizer shell at capacitor discharge stud attachment locations in accordance with Section III Note that Figures 1 and 2 are provided to assist in clarifying the above description and are not intended to provide design information.

The repair discussed in the above plan will be performed in accordance with ASME Code Section XI, 2013 Edition. Additionally, NDE is planned as described in the list of operations above and will meet the acceptance criteria of ASME Code Section III, 1974 Edition with addenda through Winter 1975. The new weld pad is examined with liquid penetrant testing (PT) and UT examinations following completion as required by ASME Code Case N-638-10. These examinations will verify there are no unacceptable indications (cracking or fabrication defects) in the newly installed weld pad or original base metal material. The UT examination will be in accordance with ASME Section III, NB-5244 and include the weld deposit, the fusion zone and the parent metal beneath the weld to assure freedom from lack of fusion and laminar defects. The new Alloy 690 nozzle to weld pad Alloy 52M partial penetration weld will be liquid penetrant examined in accordance with ASME Code Section III, NB-5245 at half thickness and full thickness after the fillet weld is complete. ISI examination requirements will be in a subsequent submittal that will request use extension of the alternatives for the remainder of plant operating life.

Enclosure Relief Request 70 Page 8 B. Proposed Alternatives In accordance with 10 CFR 50.55a, Codes and Standards, paragraph (z)(1), APS proposes alternatives to the requirements specified in Section 3 above on the basis that performing the alternatives stated below provide an acceptable level of quality and safety.

A design analysis is being performed in accordance with the design requirements of ASME Code Section III, 2013 Edition. The analysis will confirm that the new nozzle will not eject from the pressurizer under design conditions. The new design will be reconciled to the original construction code and address design and transient loads to ensure all code requirements are met. A one-cycle evaluation of the repair will be submitted to the NRC. Refer to the Summary of Commitments in Table 8-1 for timing of the submittal of the evaluation of the repair.

In addition to the ASME Code rules the following alternatives are proposed:

Flaw Removal and Flaw Evaluation As an alternative to flaw removal or reduction in size, of the original J-groove weld on the inner surface of the pressurizer shell, to meet the applicable acceptance standards per IWA-4412, APS proposes to implement an OD repair of the pressurizer lower shell temperature nozzle utilizing an OD weld pad and replacement full nozzle as described in the repair of the nozzle penetration.

As an alternative to performing the NDE required to characterize a flaw under IWB-3420 and IWB-3610(b) in the pressurizer lower shell temperature nozzle penetration, APS proposes analyzing a maximum postulated flaw that bounds the range of flaw sizes that could exist in the original J-groove weld. See "Basis for Flaw Analytical Evaluation below.

Welding In lieu of NB-4620, APS is installing a welded pad using ATTB welding in accordance with ASME Code Case N-638-10. The NRC has approved ASME Code Case N-638-10 to allow ATTB welding of dissimilar materials.

Examination (liquid penetrant surface and UT volumetric) of the completed weld pad will be performed in accordance with ASME Section III acceptance criteria after the weld pad has been prepared for NDE and dimensionally inspected.

Pursuant to 10 CFR 50.55a(z)(1), APS proposes an alternative to ASME Section XI and ASME Code Case N-638-10. An alternative is proposed to the requirements of N-638-10, Paragraph 4(a)(2), that requires a 48-hour hold time prior to performing NDE. APS intends to perform weld repair(s) with austenitic filler material in accordance with the ATTB welding technique of Code Case N-638-10, with one exception. As an alternative to performing the required NDE at least 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> after the three tempering layers have reached ambient temperature, APS proposes to perform the NDE methods after completion of the weld repair. The bases for these alternatives are provided below.

Enclosure Relief Request 70 Page 9 APS plans to apply weld repair of the pressurizer shell by using austenitic Nickel-Alloy 52M (SFA-5.14, ERNiCrFe-7A) filler material over the repair area. The weld repair will be a minimum of three (3) layers per the temper bead rules in Code Case N-638-10. The ATTB technique of ASME Section XI, Code Case N-638-10 has been approved by the NRC in Regulatory Guide (RG) 1.147, Revision 20.

C. Basis for Flaw Analytical Evaluation The assumptions of IWB-3600 of ASME Section XI for analytical flaw evaluation are that cracks are fully characterized in accordance with IWB-3420 in order to compare the calculated parameters to the acceptable parameters addressed in IWB-3500.

There are no qualified UT examination techniques for examining the original nozzle-to-pressurizer shell J-groove weld. Therefore, since it is not possible to characterize the flaw geometry that may exist therein, it is conservatively assumed that the as-left condition of the remaining J-groove weld includes a flaw extending through the entire Alloy 82 J-groove weld and buttering.

Since hoop stresses in the J-groove weld are generally higher than axial stresses at the nozzle penetration, the preferential direction for cracking is axial relative to the pressurizer (PZR) shell. Therefore, a radial-axial flaw (radial with respect to the nozzle axis) in the Alloy 82 J-groove weld and buttering is postulated and would propagate by PWSCC through the weld and buttering to the interface with the low alloy steel PZR material. Any growth of the postulated as-left flaw into the PWSCC resistant low alloy steel would be by fatigue crack growth under cyclic loading conditions.

Given the emergent nature of the PVNGS Unit 1 pressurizer lower shell temperature nozzle repair, there is not sufficient time to perform a detailed plant specific life of repair finite element analysis for the as-left J-groove weld (ALJGW) flaw during this Refueling Outage. Therefore, instead, the life of repair as-left J-groove weld flaw analyses performed in 2010 for PVNGS Units 1, 2, and 3 pressurizer lower shell temperature nozzles is used as the basis to demonstrate that the flaw is acceptable from the time of initial repair in 1992 up to one additional cycle following the repair in 2023. The existing 2010 J-groove weld flaw analyses for the repaired pressurizer lower shell temperature nozzle determined a repair weld life based on linear elastic fracture mechanics (LEFM) and elastic plastic fracture mechanics (EPFM) analyses for 60 years of plant operation following the 1992 repair.

The flaw evaluation for the current 2023 repair is performed for one cycle of operation in accordance with IWB-3610 (LEFM method), as well as per Case N-749 (EPFM method). Case N-749 is considered with all applicable conditions stated in Table 2 of RG 1.147, Revision 20. The one cycle justification (OCJ) flaw evaluation is based on the following:

a. A review of the existing 2010 LEFM analysis developed for the PVNGS Units 1, 2, and 3 pressurizer lower shell temperature nozzles is performed and concludes that the analysis remains valid and bounds the current PVNGS Unit 1 pressurizer lower shell temperature nozzle repair OCJ.

Specifically, the evaluation reviews the differences between the ASME Section XI Code years used for the LEFM analysis and the Code year applicable for the current repair and concludes that the equation to calculate the critical material

Enclosure Relief Request 70 Page 10 fracture toughness (KIC) is identical and also the fatigue crack growth formulas are identical. Therefore, both ASME Section XI Code years are considered equivalent.

Additionally, the LEFM analysis considers the following loads for the fatigue crack growth calculation: internal pressure, weld residual stresses, and all applicable thermal transient stresses. A projected 60-year number of cycles was used and therefore, bounding the fatigue plus corrosion crack growth for OCJ in terms of transient number of cycles. The existing 2010 J-groove weld flaw analyses for the repaired pressurizer lower shell temperature nozzle determined a repair weld life based on linear elastic fracture mechanics (LEFM) and elastic plastic fracture mechanics (EPFM) analyses for 60 years of plant operation following the 1992 repair.

Furthermore, the LEFM analysis was applicable to all three units at PVNGS and used a bounding Reference Temperature for Nil Ductility Transition (RTNDT) value for analysis which is a conservative value for PVNGS Unit 1 pressurizer lower shell temperature nozzle.

Finally, the OCJ evaluation also considers the differences in the repair configurations. The pressurizer lower shell temperature nozzle repair in 1992 inserted a sleeve in the nozzle bore that was welded to the ALJGW and also to the repair J-groove weld (JGW) on the PZR OD. For the current repair, the sleeve is cut short about 0.5 inch from the PZR OD location and a roll expansion is performed to prevent the remnant sleeve from becoming a loose piece during plant operation. Existence of the sleeve imposes stresses onto the JGW location primarily due to the difference in thermal expansion coefficients between the PZR low alloy steel base metal and the stainless-steel sleeve.

When the sleeve is cut short and attached to the bore with the roll expansion, the sleeve length is reduced and so are the thermal stresses on the JGW due to the sleeve. Therefore, the results from the previous repair configuration analysis bound the current repair configuration.

Results from the bounding LEFM analysis indicated the initial and final flaw sizes on the pressurizer lower shell temperature nozzle ALJGW exceeded the LEFM ASME Section XI IWB-3610 criterion for several transients requiring a subsequent EPFM analysis to be performed as discussed in item b below.

b. A review of the existing 2010 EPFM analysis developed for the PVNGS Units 1, 2, and 3 pressurizer lower shell temperature nozzles is performed and concludes that the analysis remains valid and bounds the current PVNGS Unit 1 pressurizer lower shell temperature nozzle repair OCJ.

The EPFM evaluation was performed in 2010, prior to the approval of Code Case N-749. As such, the EPFM analysis used the best method available at that time, i.e., ASME Section XI, Appendix K. However, more conservative safety factors (SF) were selected as compared to those allowed in Code Case N-749:

SF = 3.0 and 1.5 for primary and secondary loads, respectively, while the corresponding SF allowed by Code Case N-749 are 2.0 and 1.0 (Section 3.1(a) in N-749). Therefore, the results from the EPFM analysis are conservative and bound the current repair OCJ in terms of safety factors.

Enclosure Relief Request 70 Page 11 Additionally, it was verified that the NRC requirements on Code Case N-749 and the conditions stated in Table 2 of RG 1.147, Revision 20, are satisfied in the EPFM analysis in terms of the applicability of the EPFM method. Specifically, all load conditions that exceeded the LEFM criteria were confirmed to be above the temperature limit Tc1, below which the LEFM method must be applied.

c. Section 3.1(c) in Code Case N-749 additionally requires the evaluation of primary stress limits per NB-3000 assuming a local area reduction of the pressure retaining membrane that is equal to the area of the flaw. This criterion was not considered in the EPFM analysis developed for the PVNGS Units 1, 2, and 3 pressurizer lower shell temperature nozzles. Therefore, the OCJ for PVNGS Unit 1 pressurizer lower shell temperature nozzle further evaluates the primary stress limits of the repaired configuration considering a final flaw depth and width for fatigue plus corrosion flaw growth through the next cycle. To evaluate the requirement, article NB-3228.1 of Section III of the ASME Code is utilized. NB-3228.1 states that the limits on General Membrane Stress Intensity (NB-3221.1), Local Membrane Stress Intensity (NB-3221.2),

and Primary Membrane plus Primary Bending Stress Intensity (NB-3221.3) need not be satisfied at a specific location if it can be shown by limit analysis that the specified loadings do not exceed two-thirds of the lower bound collapse load. The yield strength of the material to be used in these calculations is 1.5Sm. Per NB-3112.1(a), the design pressure shall be used in showing compliance with this limit.

D. Basis for Elimination of the Ambient 48-Hour Hold Time Elimination of the 48-hour hold is based on Attachment 1 of this Enclosure, which is a white paper based on PVP 2023-107489, Elimination of the 48-hour Hold for Ambient Temperature temper Bead Welding with Austenitic Weld Metal. Removal of the 48-hour hold is supported by the white paper that was developed for the proposed change to ASME Code Case N-888-1. Although this ASME Case is not approved in RG 1.147, it has been approved by the ASME Section XI Standards Committee. Since Code Case N-888 is the culmination of temper bead code cases that have been produced over the years, combining requirements from N-638, N-839, and Appendix I in cases such as N-740 and N-754, etc., the justification is also applicable to the planned use of Code Case N-638-10 at PVNGS Unit 1.

E. Corrosion Evaluation A corrosion evaluation will be performed to consider potential material degradation due to the repair of the pressurizer lower shell temperature nozzle. The repair will result in the pressurizer low alloy steel being exposed to the reactor coolant. The corrosion evaluation will address general corrosion, crevice corrosion, and galvanic corrosion of the exposed low-alloy steel. The review will also evaluate PWSCC potential for Alloy 52M. The corrosion evaluation will be submitted to the NRC.

Refer to Summary of Commitments in Table 8-1 for timing of submittal of the corrosion evaluation.

F. Loose Parts Evaluation Given the original pressurizer lower shell temperature nozzle J-groove weld will not be removed, APS completed a loose parts evaluation to assess the potential for J-

Enclosure Relief Request 70 Page 12 groove weld fragments, or the outer sleeve entering the pressurizer during power operation. The most probable scenario is that any fragments of the weld or pieces of the sleeve will drop down into the pressurizer lower head and remain there.

In the unlikely event that the loose parts are able to exit the pressurizer, they could be transported through the reactor coolant system (RCS). The potential effects on steam generator tubes and on the fuel and control elements [control element assembly (CEA) motion, flow blockage and fretting] were evaluated. These pieces are not expected to cause damage to steam generator tubes. Fragments of J-groove weld originating in the lower reactor vessel (RV) head region have been evaluated. If loose parts should reach the lower RV 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 possibility of CEA binding, though remote, may occur when attempting to place a CEA in the fully withdrawn position, but would be able to be cleared by maneuvering the CEA. The fragments that may unlikely result in flow blockage that would not degrade departure from nucleate boiling. In the unlikely event fretting were to occur, the effect of fuel reliability would be extremely small and not result in exceeding Technical Specification RCS activity limits.

G. Conclusion The as-left J-groove weld flaw evaluation will demonstrate that the postulated flaw is acceptable from the time of initial repair in 1992 through one additional cycle following the repair being performed during the current Unit 1 refueling outage (i.e.,

1R24). The one-cycle justification of the flaw evaluation will be submitted to the NRC within 14 days of the end of the current refueling outage, see Table 8-1 for Summary of Commitments.

The temper bead technique is an effective tool for performing repairs on carbon and low alloy steel (P-No. 1 and P-No. 3) materials. Code Case N-638-10 provisions allow for ambient temperature temper bead welding with no post weld heat treatment. However, the 48-hour hold prior to performing the final weld acceptance NDE has remained a Case requirement. Attachment 1 of this Enclosure summarizes the technical basis to eliminate the 48-hour delay for examining temper bead welding when using austenitic filler materials. The data and testing performed shows that when austenitic weld metal is used the level of diffusible hydrogen content in the ferritic base metal heat affected area (HAZ) is too low to promote hydrogen-induced cracking (HIC). Therefore, the 48-hour hold requirement in Code Case N-638-10 is not necessary prior to examination of the weld as HIC is not considered credible.

Based on the above, in accordance with 10 CFR 50.55a(z)(1), APS has concluded that the proposed alternatives for flaw evaluation and eliminating the 48-hour hold provide an acceptable level of quality and safety as discussed above.

The timing for submittal of analyses and evaluations that support the alternatives is provided in Table 8-1.

6.0 DURATION OF PROPOSED ALTERNATIVE Authorization is requested for the duration of the PVNGS, Unit 1 Cycle 25, which is currently scheduled to conclude in the Spring of 2025. A separate relief request will be

Enclosure Relief Request 70 Page 13 submitted to justify continued use of the nozzle repair for the life of the plant. This permanent relief request, which will contain the appropriate analyses and justification for the remainder of the plant operating life, will be submitted prior to the end of the upcoming operating cycle.

7.0 PRECEDENTS The following relief request was previously approved to eliminate the 48-hour hold time specified in Code Case N-638-10:

NRC verbal authorization on May 9, 2023 [Agencywide Documents Access and Management System (ADAMS) Accession No. ML23129A312] for Beaver Valley, Unit 2 relief request 2_TYP-4-RV-06 (ADAMS Accession No. ML23118A381).

Letter from David Gudger (Constellation Energy Generation, LLC) to U.S. NRC, "Submittal of Emergency Relief Request I5R-11 Concerning the Installation of a Weld Overlay on Reactor Pressure Vessel Recirculation Inlet Nozzle N2E Safe End-to-Nozzle Dissimilar Metal Weld (32-WD-208)," dated March 24, 2023 (ADAMS Accession No. ML23083B991).

The following relief requests were previously approved for the flaw analytical evaluation:

NRC approval via verbal authorization on November 6, 2020 (ADAMS Accession No. ML20314A028) for Peach Bottom Atomic Power Station, Unit 2. The NRC Safety Evaluation was subsequently issued on April 23, 2021 (ADAMS Accession No. ML21110A680).

NRC verbal authorization on April 15, 2012, for Quad Cities, Unit 2 (ADAMS Accession No. ML12107A472). The NRC Safety Evaluation was subsequently issued on January 30, 2013 (ADAMS Accession No. ML13016A454).

NRC approval via a verbal authorization on May 17, 2017, for Limerick, Unit 2 (ADAMS Accession No. ML17137A307). The NRC Safety Evaluation was subsequently issued on August 14, 2017 (ADAMS Accession No. ML17208A090).

8.0 REFERENCES

1. ASME Code,Section XI, "Rules for Inservice Inspection of Nuclear Power Plant Components," 2013 Edition.

2. ASME Code, Case N-638-10, Similar and Dissimilar Metal Welding Using Ambient Temperature Machine GTAW Temper Bead Technique,Section XI, Division 1, dated May 6, 2019.

3. ASME Code,Section III, Nuclear Power Plant Components, 1971 Edition including Addenda through Winter 1972.

4. ASME Code,Section III, Nuclear Power Plant Components, 1974 Edition including Addenda through Winter 1975.

Enclosure Relief Request 70 Page 14 Table 8-1 Summary of Commitments Commitment Committed Date of Outage Commitment Type One-Time Action (Yes/No)

Programmatic (Yes/No)

The final one-cycle flaw analytical evaluation, evaluation of repair, and corrosion evaluation will be submitted within 14 days following the end of the current Palo Verde Nuclear Generating Station (PVNGS), Unit 1 refueling outage.

Within 14 days following the end of the current PVNGS Unit 1 refueling outage.

Yes No

Enclosure Relief Request 70 Ambient Temperature Temper Bead-Elimination of 48-Hour Hold Time From N-888 When Using Austenitic Filler Material

Enclosure Relief Request 70 1

Ambient Temperature Temper Bead-Elimination of 48-Hour Hold Time from N-888 When using Austenitic Filler Material White Paper

1. Introduction and Background In welding, the presence of hydrogen in the weld metal or heat affected zone (HAZ) can cause hydrogen-induced cracking (HIC) occurring phenomena that occurs after the weldment has cooled to at or near room temperature. HIC is largely dependent upon three main factors, diffusible hydrogen, residual stress and susceptible microstructure. There are many theories on the mechanism for HIC, however, it is well understood that HIC requires simultaneous presence of a threshold level of hydrogen, a susceptible brittle microstructure and tensile stress. Additionally, the temperature must be in the range of 32 to 212°F (0 to 100°C).

Elimination of just one of these four contributing factors will prevent HIC. [1]

Two early overlay (WOL) repairs involving temper bead welding were applied to two core spray nozzle-to-safe end joints at the Vermont Yankee boiling water reactor (BWR) in 1986 to mitigate intergranular stress corrosion cracking [2]. To avoid post weld heat treatment, temper bead was deployed when installing the repair overlay on the low alloy steel SA-508 Class 2 (P-No. 3 Group 3) reactor pressure vessel nozzle. This early application of temper bead welding required elevated preheat and a post weld hydrogen bake.

As the industry experienced an increased need for temper bead welding the requirement for preheating and post weld bake made temper bead welding complicated. EPRI responded to the industry concern and conducted studies that demonstrated that repair to low alloy steel pressure vessel components could be made without the need for preheat or post weld bake

[3,4]. As a result of these studies the preheat and post weld bake requirements were not included in Case N-638 for ambient temperature temper bead welding with machine GTAW.

Deployment of the ambient temperature temper bead technique has been highly successful for many years with no evidence of HIC detected by nondestructive examination (NDE). During the past twenty years, many temper bead weld overlay repairs were successfully performed on BWRs and PWRs using ambient temperature temper bead technique, as illustrated in Table

1. The operating experience shows that with hundreds of ambient temperature temper bead applications, there has not been a single reported occurrence of hydrogen induced cracking.

Case N-888 is the culmination of temper bead code cases that have been produced over the years, combining requirements from N-638, N-839, and Appendix I in cases such as N-740 and N-754, etc. Case N-888 applies to temper bead of P-No. 1 or P-No. 3 materials and their associated welds or welds joining P-No. 8 or P-No. 43 materials to P-No. 1 or P-No 3 materials.

Additionally, Case N-888 provides provisions to allow for ambient temperature preheat with no post weld bake. However, the post weld 48-hour hold at ambient temperature has remained as a requirement in N-888. This 48-hour delay between welding completion and cooling to ambient temperature and the final nondestructive examination (NDE) of the fully welded component is intended to assure detection of delayed hydrogen cracking that is known to occur up to 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> after the weldment is at ambient temperature.

The post weld 48-hour delay following cooling to ambient temperature has resulted in a considerable cost burden to utilities. As there are significant economic advantages associated with eliminating the 48-hour hold time and immediately performing NDE following the completed weld, it is important to determine the technical advantages and disadvantages of making such a change.

Enclosure Relief Request 70 2

Table 1:

Successfully Implemented Repairs Completed Using Temper Bead Technique from 2002-2021 Date Plant Component (Qty.)

2002 Oconee1 Mid-Wall RVH Repair (15) 2002 ANO1 Mid-Wall RVH Repair (6) 2002 Oyster Creek2 Recirculation outlet nozzle (1) 2002 Peach Bottom Units 2 & 32 Core spray, recirculation outlet, and CRD return nozzles 2002 Calvert Cliff2 Heater Sleeve Repairs (Pads) (~50) 2002 Oconee1 Mid-Wall RVH Repair (2) 2002 Davis-Besse1 Mid-Wall RVH Repair (5) 2002 Millstone1 Mid-Wall RVH Repair (3) 2003 Palo Verde 12 Heater Sleeve Repairs -Pads (36) 2003 Pilgrim2 Core spray nozzle and CRD return nozzle 2003 TMI Unit 12 Hot leg and Surge line nozzle 2003 Ringhals1 1/2 Nozzle with Structural Pad (2) 2003 Crystal River1 1/2 Nozzle with Structural Pad (3) 2003 South Texas1 1/2 Nozzle with Structural Pad (2) 2003 Millstone1 Mid-Wall RVH Repair (8) 2003 St. Lucie1 Mid-Wall RVH Repair (2) 2004 Palo Verde 22 Heater Sleeve Repairs -Pads (34) 2004 Susquehanna Unit 12 Recirculation inlet and outlet nozzles 2004 Hope Creek1 SWOL (1) 2004 Palisades1 Mid-Wall RVH Repair (2) 2004 Point Beach1 Mid-Wall RVH Repair (1) 2004 ANO1 Mid-Wall RVH Repair (1) 2005 Palo Verde 32 36 Heater Sleeve Repairs - Pads (36) 2005 ANO2 Mid Wall heater sleeve repair 2005 Waterford2 Mid Wall heater sleeve repair 2005 Calvert Cliffs Unit 22 Hot Leg Drain and Cold Leg Letdown Nozzles 2005 DC Cook Unit 12 Pressurizer Safety Nozzle 2005 TPC Kuosheng2 N1 Nozzle 2005 SONGS 32 Heater Sleeve Repairs -Pads (~29) 2005 Three Mile Island1 SWOL (1) 2005 St. Lucie1 Mid-Wall RVH Repair (3) 2006 SONGS 22 Heater Sleeve Repairs -Pads (~30) 2006 Davis Besse2 Hot and Cold Leg 2006 SONGS 22 Pressurizer Nozzles (6) 2006 Millstone 32 Pressurizer Nozzles (6) 2006 SONGS 32 Pressurizer Nozzles (6) 2006 Oconee 12 Pressurizer Nozzles (6) 2006 Beaver Valley 22 Pressurizer Nozzles (6) 2006 Byron 23 Pressurizer Nozzles (6) 2006 Wolf Creek3 Pressurizer Nozzles (6) 2006 McGuire2 Pressurizer Nozzles (6) 2006 DC Cook1 SWOL (4) 2007 Callaway3 Pressurizer Nozzles (6) 2007 St. Lucie1 SWOL (4) 2007 Crystal River1 SWOL (4) 2007 Three Mile Island1 SWOL (4) 2007 North Anna1 SWOL (4) 2008 Prairie Island1 SWOL (1) 2008 Diablo Canyon1 SWOL (6) 2008 Diablo Canyon1 SWOL (4) 2008 Seabrook1 SWOL (4) 2009 Three Mile Island1 SWOL (1) 2009 Three Mile Island1 Full Nozzle with Structural Pad (1)

Enclosure Relief Request 70 3

Date Plant Component (Qty.)

2009 Crystal River1 SWOL (1) 2009 Palisades1 Mid-Wall RVH Repair (2) 2010 Oconee4 U3 Letdown WOL (1) 2010 Krsko1 SWOL (5) 2010 Tihange1 SWOL (1) 2010 Davis-Besse1 Mid-Wall RVH Repair (24) 2011 Hatch4 Nozzle WOL (1) 2011 Talen Energy Corporation4 N5 core spray nozzles 2011 Monticello4 Emergent WOL (1) 2011 Three Mile Island4 TMI PZR Spray Nozzle (1) 2011 Doel1 SWOL (1) 2011 Tihange1 SWOL (1) 2011 St. Lucie1 1/2 Nozzle with Structural Pad (30) 2012 North Anna4 SG Nozzle WOLS (3) 2012 Palo Verde4 Small Bore CL Nozzles WOL 2012 Grand Gulf4 Reactor Vessel Nozzle Contouring and N6 Weld Overlay 2012 Doel1 SWOL (1) 2012 Calvert Cliffs1 Mid-Wall Przr Heater Repair (119) 2012 Quad Cities1 1/2 Nozzle with Structural Pad (1) 2012 Harris Nuclear Plant1 Mid-Wall RVH Repair (4) 2013 Farley4 Unit 2 FAC Pipe Replacement and WOL 2013 Oconee4 Hot/Cold Leg Small Bore Alloy 600 2013 Hope Creek4 Emergent N5A WOL 2013 Three Mile Island1 SWOL (1) 2013 Palo Verde1 1/2 Nozzle with Structural Pad (1) 2013 Harris Nuclear Plant1 Mid-Wall RVH Repair (2) 2015 Harris Nuclear Plant1 Mid-Wall RVH Repair (3) 2015 Hatch4 N4A WOL 2015 Millstone4 2" Drain WOL 2015 Hatch4 Recirc (N2) WOL 2016 Harris Nuclear Plant1 Mid-Wall RVH Repair (4) 2017 Fitzpatrick4 RHR WOL 2017 Limerick1 1/2 Nozzle with Structural Pad (1) 2018 Waterford4 Emergent Drain Nozzle WOLs (2) 2018 Palisades1 Mid-Wall RVH Repair (3) 2018 Doel1 Mid-Wall RVH Repair (16) 2018 Harris Nuclear Plant1 Mid-Wall RVH Repair (1) 2018 Brunswick1 SWOL (2) 2020 Peach Bottom1 1/2 Nozzle with Structural Pad (1) 2020 Palisades1 Mid-Wall RVH Repair (2) 2021 Oconee4 Complex nozzle pads on RCS piping 2021 ANO-21 Mid-Wall RVH Repair (1)

Notes: Operating experience provided by Steve McCracken (EPRI), Darren Barborak (EPRI, formerly with AZZ), and Travis Olson (Framatome)

(1) Framatome (2) Unknown (3) PCI (4) AZZ Specialty Welding

Enclosure Relief Request 70 4

2. Objective The objective of this white paper is to provide technical justification to eliminate the 48-hour delay when using austenitic filler materials in the temper bead welding process for P-No. 1 and P-No. 3 ferritic materials. The industry and regulatory technical concerns related to this change are examined and the technical bases for changing the requirements for the 48-hour delay are presented. Discussion from white paper for Ambient Temperature Temper bead Weld Overlay Gas Tungsten Arc Welding by Hermann and Associates [9] are included in this white paper.

If adopted, it is expected that the change in the 48-hour delay requirement will become part of a revision to the current ASME Section XI Case N-888 that currently allows for ambient temperature temper bead repairs but requires 48-hour delay after the initial three temper bead layers prior to final NDE.

3. Technical Issues Related to the 48-Hour Delay The reasons for performing the final NDE after the 48-hour delay is the recognition that alloy steels can become susceptible to HIC. There are two primary weld cracking mechanisms of concern for low alloy steels during cooling or after reaching ambient temperature. These are cold cracking of high restraint geometries (weld shrinkage-induced) and hydrogen induced cracking (HIC), often referred to as hydrogen delayed cracking. Cold cracking occurs immediately as the weldment cools to ambient temperature. In contrast, HIC can occur immediately during cooling to ambient temperature or up to 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> after reaching ambient temperature. Cold cracking that occurs with high restraint weldments would therefore be detected by NDE performed immediately after the weldment is complete.

EPRI studies [4] have indicated that cold cracking occurs under conditions of high geometrical restraint especially where low toughness HAZs are potentially present.

Restraint mechanisms can occur either hot (resulting in intergranular or interdendritic cracking), or cold (resulting in transgranular cracking of material having marginal toughness).

Cold cracking occurs immediately as the weld deposit cools to ambient temperature. Proper joint design, appropriate welding procedures and bead sequences, are practical solutions that avoid critical cold cracking conditions. This form of cracking is addressed effectively by the ASME code guidance including welding procedure qualification testing and by in-process and or post-weld inspections.

The other form of cracking at ambient temperature, which is the focus of this white paper, is HIC. This cracking mechanism manifests itself as intergranular cracking of prior austenite grain boundaries and in contrast to cold cracking generally occurs during welding, but also up to 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> after cooling to ambient temperature. It is produced by the action of internal tensile stresses acting on low toughness HAZs (generally characterized by inadequate tempering of weld related transformation products). The most widely accepted theory suggests that the internal stresses will be produced from localized buildup of monatomic hydrogen. Monatomic hydrogen can be entrapped during weld solidification, and will tend to migrate, over time, to prior austenite grain boundaries or other microstructure defect locations. As concentrations build, the monatomic hydrogen will recombine to form molecular hydrogen, thus generating highly localized internal stresses at these internal defect locations. Monatomic hydrogen is produced when moisture or hydrocarbons interact with the welding arc and molten weld pool.

The concerns with and driving factors that cause hydrogen induced cracking have been identified. These issues are fundamental welding and heat treatment issues related to temper bead welding, requiring a technical resolution prior to modification of the current ASME Code Cases N-888 by the ASME Code and the technical community. Specific concerns relate to the

Enclosure Relief Request 70 5

following issues:

-Microstructure

-Sources for Hydrogen Introduction

-Diffusivity and Solubility of Hydrogen In the following discussion of this white paper each of these factors is briefly described to provide insight into the impact and proper management of these factors that cause HIC.

4. Discussion of Technical Issues Related to the 48-Hour Delay Microstructure:

C-Mn and low alloy steels can have a range of weld microstructures which is dependent upon both specific composition of the steel and the welding process/parameters used. Generally, untempered martensitic and untempered bainitic microstructures are the most susceptible to hydrogen cracking. These microstructures are produced when rapid cooling occurs from the dynamic upper critical (Ac3) transformation temperature [1]. Generally, a critical hardness level necessary to promote hydrogen cracking is on the order of Rc 35 for materials with high hydrogen and Rc 45 for low level of hydrogen. Maintaining hardness levels below these thresholds generally avoids hydrogen cracking [1].

EPRI has examined in detail the effects of welding on the hardening of low alloy steels. The microstructure evaluations and hardness measurements discussed in EPRI reports [4, 5, 6]

have described the effects of temper bead welding on the toughness and hardness of P-No.

3 materials. The research results have illustrated that the microstructure in the low alloy steel (P-No. 3) beneath the temper bead WOL in the weld HAZ consists of a structure that is tempered martensite or tempered bainite and has maximum hardness at a distance of 2 to 3 mm (80 to 120 mils) beneath the surface of the order of 280 to 300 KHN (28 to 30Rc) or lower. The research outlines that the microstructure resulting from temper bead welding is highly resistant to HIC. Additionally, hardness would not be a concern provided there are adequate hydrogen controls are in place.

Furthermore, materials having face-centered-cubic (FCC) crystal structures such as austenitic stainless steels (300 series) and nickel base alloys such as Inconel are not susceptible to hydrogen induced cracking. The reason is that FCC atomic structures have ample unit cell volume space to accommodate atomic (diffusible) hydrogen. It is noted that the diffusion of hydrogen at a given temperature is slightly higher in body-centered-cubic (BCC) materials, ferritic steels, than it is in FCC austenitic materials. The FCC crystal structure has increased capacity to strain significantly without cracking (ductility) providing acceptable levels of toughness capable of resisting HIC. The inherent ability to deform and accommodate diffusible hydrogen are the reasons austenitic stainless steel and nickel base coated electrodes do not have low hydrogen designators that are found for ferritic weld materials [6]. Since the ferritic HAZ is in a tempered condition and an FCC filler material is used, a susceptible microstructure susceptible to HIC is highly unlikely.

Presence/sources of Hydrogen:

Hydrogen can be introduced into the weld from several sources. These include 1) hydrogen in the original base material, 2) moisture in electrode coatings and fluxes, 3) organic contaminants (grease or oils), 4) hydrogen in the shielding gas and 5) humidity in the atmosphere.

The reduction of diffusible hydrogen in temper bead and non-temper bead weldments begins

Enclosure Relief Request 70 6

with implementing low hydrogen weld practices. These practices originate with Federal requirements that nuclear utilities control special processes such as welding and design and fabricate components to various codes and standards. These requirements, when followed, will effectively eliminate the contamination, and minimize the environment pathways.

Cleanliness of surfaces to be welded are mandated by Code and subsequently implemented via adherence to sound welding programs. The controls and requirements for cleanliness of the welded surface at nuclear utilities significantly reduce the likelihood of hydrogen entering the weld from surface contamination. Furthermore, repair and replacement applications typically deal with components that have been at operating temperatures above 390ºF (200ºC) for many years and any hydrogen present in the base material would have diffused from the steel and escaped to the atmosphere. Thus, surface contaminants and the base materials are not expected to be a significant source of diffusible hydrogen.

For SMAW, main pathway for diffusible hydrogen to enter the weldment will be the electrode coating. Welding programs primarily maintain low moisture in electrode coatings through procurement via an approved supplier, controlled storage conditions, and conservative exposure durations. The conservative exposure duration and coatings that resist moisture uptake minimize the amount of additional moisture in the coated electrode taking into consideration that moisture uptake is a function of time, temperature, and relative humidity.

Extensive testing by the EPRI Welding and Repair Technology Center shows there is an extremely low probability of HIC with H4 and H4R electrodes. EPRI performed diffusible hydrogen analysis per AWS A4.3 via gas chromatography on thirteen commercially available electrodes. Electrodes with AWS E7018, E8018 and E9018 from multiple vendors exposed at 27°C at 80% relative humidity (HR) for exposure times from 0 to 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />. Many of the electrodes did not have R moisture resistant coating.

Figure 1 shows EPRI diffusible hydrogen test results for the thirteen lots of low hydrogen electrodes. All H4R electrodes exhibited < 16ml/100g of diffusible hydrogen at 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> of exposure. Figure 3 shows that new electrodes without exposure have < 2ml/100g diffusible hydrogen. Only one of the electrodes tested at the extremely aggressive 27°C and 80%

Relative Humidity (HR) 72-hour exposure had diffusible hydrogen > 4 ml/100g. This demonstrates that exposure limits in the field of 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> or less is adequate to assure electrodes maintain the H4R limit. Ferritic electrodes were verified to have less than 4ml/100g diffusible hydrogen [6]. Testing verifies that ambient temperature is acceptable, post weld hydrogen bakeout is not needed, and a 48-hour hold at ambient temperature prior to performing final NDE is unnecessary and diffusible hydrogen levels will be below any susceptibility threshold that supports HIC.

For GTAW, EPRI performed studies investigating the diffusion of hydrogen into low alloy pressure vessel steels [4]. Due to the little information published at the time, EPRI decided to generate experimental data that would provide information on the levels of diffusible hydrogen associated with GTAW welding. The experimentation included individual sets of diffusible hydrogen tests as follows:

1. determination of diffusible hydrogen levels for the GTAW process under severe welding and environmental conditions simulating (or exceeding) repair welding conditions which may be expected in a nuclear plant.

2. measurement of diffusible hydrogen levels for various shieling gas dew point temperatures

3. examination of diffusible hydrogen levels for modern off-the-shelf filler wires,

Enclosure Relief Request 70 7

Discussion of these items can be found in the EPRI documents and will not be reiterated in this report. The results demonstrate that introducing hydrogen is unlikely with the GTAW process. The typical hydrogen content for the GTAW process is less than 1.0mL/100g.

Therefore, hydrogen cracking is extremely unlikely.

Figure 1. Results of EPRI diffusible hydrogen testing at 27°C 80% Relative Humidity (HR) for zero to 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> of exposure [6]

Enclosure Relief Request 70 8

Diffusivity and Solubility of Hydrogen Diffusivity and solubility of hydrogen in ferritic, martensitic, and austenitic steels is an important factor to consider. Materials having face-centered-cubic (FCC) crystal structures such as austenitic stainless steels (300 series) and nickel base Inconels generally are not considered to be susceptible to hydrogen delayed cracking as discussed in the microstructure section, above. Additionally, due to the temperatures expected during the welding of the temper bead layers, and during the welding of any non-temper bead layers, the temperature should be sufficient for the hydrogen to diffuse out of the HAZ, either escaping the structure or diffusing into the austenite, where it can be held in much greater quantities. The diffusion rate is clearly from the ferrite to the austenite and whatever hydrogen remains will reside in the austenite, which has little to no propensity to hydrogen related cracking.

Use of fully austenitic weld metal on ferritic base material is a technique that has been used for decades to install welds on ferritic base materials with high potential of HIC. Austenitic filler materials are used in applications where preheat or post weld bake out is not possible because hydrogen (H+) has high solubility, Figure 3, and low diffusivity, Figure 4, in austenite relative to other phases and acts as a trap for hydrogen to prevent HIC. Figure 3 show the solubility of hydrogen in -Fe and -Fe.

Note that -Fe is at the saturation limit at ~4ml/100g of hydrogen. At temperatures above ~1700° C the solubility of hydrogen in austenite (-Fe) is nearly five times that of ferrite (-Fe). The benefit regarding HIC is the hydrogen stays in the austenite and is not available to promote HIC. Figure 4 shows the overall difference in hydrogen diffusion between ferritic and austenitic materials. The diffusion of hydrogen in ferritic material is orders of magnitude greater compared to austenite.

Again, the obvious advantage regarding HIC prevention is the hydrogen is slow to diffuse out of the austenitic material. When comparing how hydrogen behaves in Figure 2. Graph showing slight increase of diffusible hydrogen after exposure of 24 and 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> [6]

Enclosure Relief Request 70 9

ferritic versus austenitic weldments the hydrogen stays within the austenitic material whereas in ferritic welds, it tends to diffuse into the base material. For a weld made with ferritic electrodes, the H+ is absorbed in the molten weld puddle and as the weld solidifies, it transforms from austenite to ferrite and the H+ is rejected and diffuses into the HAZ of the base material. When the HAZ transforms from austenite to martensite, the H+ becomes trapped in the brittle microstructure and causes cracking, Figure 5. However, with an austenitic electrode, H+ is absorbed in the molten weld puddle and there is no solid state transformation in the solidified weld metal so the H+ stays in the austenitic weld material. No diffusion of the H+ into the brittle martensite, thus avoiding the possibility of HIC, Figure 6. Schematics in Figure 5 and Figure 6 are adapted from Lippold and Granjon as shown in draft chapters 2

& 4 for Temper Bead Welding Process in Operating NPPs, International Atomic Energy Agency, [1, 8].

Figure 3 - Hydrogen solubility in ferritic and austenitic materials as a function of temperature

Enclosure Relief Request 70 10 Figure 4 - Diffusion Coefficient of hydrogen in ferritic and austenitic materials as a function of temperature

Enclosure Relief Request 70 11

5. Conclusion The temper bead technique has become an increasingly effective tool for performing repairs on carbon and low alloy steel (P-No. 1 and P-No. 3) materials.

Case N-888 provisions allow for ambient temperature temper bead welding with no post weld bake. However, the 48-hour hold at ambient temperature prior to performing the final weld acceptance NDE has remained a requirement. This white paper summarizes the technical basis to eliminate the 48-hour delay for temper bead welding when using austenitic filler materials. The data and testing by EPRI and other researchers show that when austenitic weld metal is used the level of diffusible hydrogen content in the ferritic base metal HAZ is too low to promote HIC. The 48-hour hold requirement in Case N-888 can therefore be removed.

Lastly, field experience applying austenitic filler materials to hundreds of dissimilar metal weld overlays using the ambient temperature temper bead procedures has never experienced hydrogen delayed cracking nor would it be expected. The reason is simply that the final diffusible hydrogen content is low -

Figure 5 - Hydrogen movement with ferritic electrodes [8]

Figure 6 - Hydrogen movement with austenitic electrodes [8]

Enclosure Relief Request 70 12 well below any threshold level that would be required for hydrogen induced cracking. Table 1 outlines the last 20 years of temper bead weld repairs in the nuclear industry with no reported occurrence of HIC when using austenitic weld metal.

6. References

1. Welding metallurgy and Weldability, 2015, chapter 5, Hydrogen Induced Cracking -

John Lippold

2. Inconel Weld-Overlay Repair for Low-Alloy Steel Nozzle to Safe-End Joint, EPRI Palo Alto, CA: 1991. NP-7085-D.

3. ASME Case N-638, Similar and Dissimilar Metal Welding Using Ambient Temperature Machine GTAW Temper Bead Techniques,Section XI Division 1, September 24, 1999.

4. Ambient Temperature Preheat for Machine GTAW Temperbead Applications, EPRI Palo Alto, 1998. GC-111050.

5. Temperbead Welding Repair of Low Alloy Pressure Vessel Steels: Guidelines, EPRI Palo Alto, CA: 1993. TR-103354. 1993.

6. Welding and Repair Technology Center: Shielded Metal Arc Temper Bead Welding, EPRI Palo Alto, CA: 2015. 3002005536.

7. 2021 ASME Boiler & Pressure Vessel Code,Section XI Rules for Inservice Inspection of Nuclear Power Plant Components, Division 1.

8. S.L. McCracken and N. Mohr, Draft Chapters 2 and 4 prepared for: Temper Bead Welding Process in Operating NPPs, International Atomic Energy Agency, Vienna, 2022.

9. Repair and Replacement Applications Center: Temperbead Welding Applications 48-Hour Hold Requirements for Ambient Temperature Temperbead Welding, EPRI, Palo Alto, CA:

2006.1013558