ML25148A291
| ML25148A291 | |
| Person / Time | |
|---|---|
| Site: | Callaway  |
| Issue date: | 05/28/2025 |
| From: | Ameren Missouri, Union Electric Co |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML25148A289 | List: |
| References | |
| ULNRC-06949 | |
| Download: ML25148A291 (1) | |
Text
Enclosure 1 to Relief Request 15R-O1 ULNRC-06949 Page 1 of 9 Proposed Alternative In accordance with 10 CFR 50.55a(z)(1)
Alternative Provides Acceptable Level of Quality and Safety 1.
ASME Code Component Affected:
Components:
Reactor Vessel Bottom Head (RVBH) Bottom Mounted Instrumentation (BMI)
Nozzles 57, 48, 35, and 30 Code Class:
ASME Section III Class 1 Examination Category:
B-P, American Society of Mechanical Engineers (ASME) Code Section XI Code Item Number:
B15.10, Table IWB-2500-1 (B-P)
B15.80, Code Case N-722-1, Table 1 There are 58 RVBH BMI nozzles welded to the inside surface of the RV with partial penetration J-groove welds.
===2.
Applicable Code Edition and Addenda===
Callaways Inservice Inspection (151) Program complies with the 2019 Edition of the ASME Boiler and Pressure Vessel Code (BPVC),Section XI.
Callaways fifth 151 interval began December 19, 2024, and ends on December 18, 2036. Callaway moved from the ten-to the twelve-year 151 interval as allowed by ASME Code Case N-92 1.
The Code of Construction for the reactor vessel is ASME Code Section III, 1 971 Edition with Addenda through Winter 1972.
The Code ofConstruction for the reactor vessel BMI nozzle repair modification installation is ASME Code,Section III, 201 5 Edition.
===3.
Applicable Code Requirement===
IWA-441 1, Welding, Brazing, Fabrication, and Installation, states, in part, (e) The requirements of IWA 4600(b) may be used when welding is to be performed without the post-weld heat treatment required by the Construction Code.
IWA-4600, Alternative Welding Methods, states, in part, (b) When post-weld heat treatment is not to be performed, the following provisions may be used.
Section XI Code Case N-638-11, Similar and Dissimilar Metal Welding UsingAmbient Temperature Machine GTA W Temper Bead Technique Section XL Division I Paragraph 4(a)(2) states, in part, When austenitic materials are used, the completed weld shall be nondestructively examined after the three tempering layers (i.e., layers 1, 2, and 3) have been in place for at least 48 hr.
to Relief Request 15R-O1 ULNRC-06949 Page 2 of 9
===4.
Reason for Request===
on May 6, 2025, during Callaways refueling outage (RFO), Quality Control inspectors were performing a visual (VT-2) exam of the reactor vessel bottom head (RVBH) bottom mounted instrument (BMI) nozzles.
During this exam, the inspectors identified dry white residue, resembling boric acid, coming from the annulus between BMI Nozzle 48 and the penetration through the RVBH. (See Figure 4-1.) The area was cleaned, with no active boric acid leak observed, and the condition was entered into Callaways Corrective Action Program.
Additionally, operators transitioned the plant back down to cold shutdown to satisfy reactor coolant system (RCS) operational leakage Technical Specification requirements.
As part of the extent-of-condition investigation, ultrasonic testing (UT) and visual (VT-i) exams were performed on all 58 BMI nozzles. It has been determined, from the analysis of the UT and VT-i data, that four RVBH BMI nozzles require repair. Figure 4-3 depicts the locations of the BMI nozzles. It is noted that Ameren will be submitting a separate relief request proposing an alternative (i.e., one-cycle justification) related to ASME Section XI Code requirements on flaw indications identified in the BMI nozzles, which is not within the scope ofthis relief request.
BMI Nozzle 57: VT-i data of the partial penetration J-groove weld shows linear indications propagating from a pit located at approximately 2700. Stress corrosion cracking (5CC) flaws could not be ruled out.
BMI Nozzle 48: UT data shows weld fabrication flaws in the partial penetration J-groove weld, which have not propagated into the nozzle tube material. Therefore, the most likely leak path is through the annulus between the outer diameter ofthe BMI nozzle tube and inside surface ofthe RVBH bore hole.
BMI Nozzle 3 5 : UT data shows an indication in the partial penetration J-groove weld, which has propagated into the nozzle tube material. The indications are consistent with stress corrosion cracking (5CC).
BMI Nozzle 30: UT data shows an indication in the partial penetration J-groove weld, which has slightly propagated into the nozzle tube material. The indications are consistent with 5CC. UT data shows that the SSC indication is also located within a grouping ofJ-groove weld fabrication flaws.
As a result ofthe RCS leakage and UT data, Ameren will be performing a half-nozzle repair modification of BMI Nozzles 57, 48, 35, and 30 usingASME Section XI, Code Case N-638-ii, andASME Section III. Figure 4-i depicts the existing configuration ofthe BMI nozzles. The modification will consist ofremoving the lower portion ofthe existing Alloy 600 nozzle, applying an Alloy 52M weld pad on the outer surface ofthe RVBH, and installing a replacement Alloy 690 nozzle with an Alloy 52M partial penetration J-groove weld. Figure 4-2 depicts the planned BMI nozzle modification. The new weld pad will be welded to the outer surface of the RVBH using machine Gas Tungsten Arc Welding (GTAW) Ambient Temperature Temper Bead (ATTB) welding with inert shielding gas. The new Alloy 690 half nozzle will be inserted into the bore and welded to the Alloy 52M weld pad with an Alloy 52M partial penetration J-groove weld using a manual GTAW welding technique. As a result of this modification, the BMI nozzle penetration pressure boundary will move from the original construction partial penetration J-groove weld that is inside the RVBH to the new partial penetration J-groove weld outside the RVBH.
to Relief Request 15R-O1 ULNRC-06949 Page 3 of 9 As stated above, installation of the new weld pad using ATTB welding will be in accordance with ASME Code Case N-63 8-1 1
. The NRC approved Code Case N-63 8-1 1 in Reg. Guide 1
. 1 47, Revision 2 1, 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 (PWHT). Code Case N-638-1 1 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 non destructive examination (NDE). However, for the modification of BMI nozzles 57, 48, 35, and 30, 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 the NDE methods to be performed after completion of the weld modification, without waiting for the 48-hour hold time.
to ULNRC-06949 Relief Request 15R-O1 Page 4 of 9 Figure 4-1 Bottom Mounted Nozzle No.48 Existing Configuration SA-533 J-GROOE WELD AND BLJ1TERING ALLOY 82/12 Dry white residue observed on nozzle 48.
ALLOY GUID TUOE to ULNRC-06949 Relief Request 15R-O1 Page 5 of 9 Figure 4-2 Bottom Mounted Nozzle No.48 Modified Configuration 54533.. GRADE B. CLASS 1 EcISTtr4G BMI NOZZLE ALLOY &JO JG9O4Td WELD ALLOY 52M REPLMEVEll NOZZLE ALLOY 9O THIUBLE GUI TUBE
Enclosure I to ULNRC-06949 Relief Request 15R-O1 Page 6 of 9 Figure 4-3 Bottom Mounted Nozzle Locations
.L1L Lcce Pcrta Pdon1tcr 1C) ArlçI6 Iron arid RVLIS Tth1n I
p.
9
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I ER to Relief Request l5R-O1 ULNRC-06949 Page 7 of 9
===5.
Proposed Alternative and Basis for Use===
Proposed Alternative In accordance with 10 CFR 50.55a, Codes and Standards, paragraph (z)(1), Ameren proposes an alternative to the Section XI Code Case N-638-11 requirement on the basis that the alternative provides an acceptable level of quality and safety. Specifically, Ameren proposes an alternative to Case N-63 8-1 1, Paragraph 4(a)(2),
that requires a 48-hour hold time prior to performing NDE as described below.
Ameren is installing a weld pad using ATTB welding in accordance with ASME Case N-638-11. The NRC has approved ASME Case N-63 8-1 1 in Reg. Guide 1. 147, Revision 2 1, to allow ATTB welding of dissimilar materials. The weld pad will consist of a minimum ofthree (3) layers ofAustenitic Nickel-Alloy 52M (SFA 5. 14, ERNiCrFe-7A) filler material in accordance with the temper bead requirements in Case N-63 8-1 1.
Liquid penetrant surface and UT volumetric examination 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. In lieu of 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 been installed, Ameren proposes to perform the NDE methods after completion of the weld pad.
See Basis for Elimination ofthe Ambient 48-Hour Hold Time below.
Planned Repair Steps The planned steps ofthe proposed repairs ofBMI Nozzles 57, 48, 35, and 30 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.
Sever nozzle and thimble guide tube.
2.
Clean and perform surface and volumetric exams ofweld pad region.
3.
Machine to thin existing Alloy 600 nozzle, as specified in final implementing drawings.
4.
Roll expand the existing Alloy 600 nozzle (optional).
5.
Machine original RVBH bore to remove the lower portion ofthe existing Alloy 600 nozzle.
6.
C1eanRVBH bore.
7.
Clean weld dam and install in RVBH bore.
8.
Deposit new Alloy 52M weld pad, as specified in final implementing drawings.
9.
Perform surface and volumetric exams on new Alloy 52M weld pad.
- 10. Install wedge dam.
- 11. Machine and clean final RVBH bore.
Enclosure I to Relief Request 15R-O1 ULNRC-06949 Page 8 of 9
- 16. Perform progressive surface exams as the J-groove weld is deposited, to include a final surface exam.
1 7. Prep end ofthimble guide tube.
1 8. Weld replacement Alloy 690 nozzle to thimble guide tube and thimble guide tube to coupling.
Basis for Elimination ofthe Ambient 48-Hour Hold Time Elimination of the 48-hour hold is based on Attachment 1 and is from a white paper presented at a 2023 ASME Pressure and Vessels & Piping Conference, titled, PVP 2023-107489: Elimination of the 48-hour Hold for Ambient Temperature Temper Bead Welding with Austenitic Weld Metal, which was developed for the proposed change to ASME Code Case N-888-1. Although Code Case N-888-l is not approved in Reg.
Guide 1. 147, Revision 2 1, it has been approved by the ASME Section XI Standards Committee. Code Case N-888 is the culmination of temper bead Code Cases that have been produced thus far, combining requirements from N-638, N-839, and Appendix I in Code Cases such as N-740 and N-754, etc. Therefore, justification is also applicable to the planned use ofCode Case N-638-l 1 at Callaway Plant Unit 1 (with this exception).
Conclusion The ATTB 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-638-11 provisions allow for ambient temperature temper bead welding with no PWHT. However, the 48-hour hold prior to performing the final weld acceptance NDE has remained a Code Case requirement. Attachment 1 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-l 1 is not necessary prior to examination ofthe weld as HIC is not considered credible for the described application.
===6.
Duration of Proposed Alternative===
The reliefrequest will remain in effect until Callaways end ofplant life.
===7.
Precedents===
Arizona Public Service Company (APS) letter, Palo Verde Nuclear Generating Station, Unit 1 Relief Request 76 Proposed Alterative for Pressurizer Lower Shell Nozzle, dated April 23, 2025 (ADAMS Accession No. ML25113A296)
Arizona Public Service Company (APS) letter, Palo Verde Nuclear Generating Station, Unit 1 Relief Request 70 Proposed Alternative for Pressurizer Lower Shell Temperature Nozzle, dated October 23, 2023 (ADAMS Accession No. ML23296A254) to Relief Request 15R-O1 ULNRC-06949 Page 9 of 9 0
NRR E-mail Capture,
Subject:
Palo Verde Unit 1 Verbal Authorization of Relief Request 70, Proposed Alternatives for Pressurizer Lower Shell Temperature Nozzle (EPID L-2023-LLR-0057), dated October 27, 2023 (ADAMS Accession No. ML23303A011) 0 NRC Safety Evaluation letter, Palo Verde Nuclear Generating Station, Unit 1 Re: Relief Request 70 Proposed Alternatives for Pressurizer Lower Shell Temperature Nozzle (EPID L-2023-LLR-0057), dated September 9, 2024 (ADAMS Accession No. ML24197A199)
Energy Harbor Nuclear Corporation letter, Beaver Valley Power Station, Unit No. 2 10 CFR 50.55a Request 2-TYP-4-RV-06 for Alternative Repair Methods for Reactor Pressure Vessel Head Penetrations, dated April 28, 2023 (ADAMS Accession No. ML23118A381) 0 NRR E-mail Capture,
Subject:
Beaver Valley Unit 2 Verbal Authorization of Alternative Request, 2-TYP-4-RV-06 (EPID: L-2023-LLR-0021), dated May 9, 2023 (ADAMS Accession No. ML23129A312)
Constellation Energy Generation, LLC letter, Submittal ofEmergency ReliefRequest 15R-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) o NRR E-mail Capture,
Subject:
Verbal Approval for NMP1 Proposed Alternative Weld Overlay N2E Safe-end to nozzle DM weld (EPID L-2023-LLR-00l 1), dated March 31, 2023 (ADAMS Accession No. ML23090A130) 8.
References ASME Code,Section XI, Rules for Inspection and Testing of Components of Light-Water-Cooled Plants, Division 1, 2019 Edition.
ASME Code, Case N-63 8-11, Similar and Dissimilar Metal Welding Using Ambient Temperature Machine GTAW Temper Bead Technique,Section XI, Division 1, dated August 2, 2019.
Attchment 1 to Attachment I to Relief Request 15R-O1 Enclosure to ULNRC-06949 Page 1 of 15 Ambient Temperature Temper Bead-Elimination of 48-Hour Hold Time from N-888 When using Austenitic Filler Material White Paper I
. 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.
Attchment 1 to Attachment I to Relief Request 15R-O1 Enclosure to ULNRC-06949 Page 2 of 15 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 I
. 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. I or P-No. 3 materials and their associated welds or welds joining P-No. 8 or P-No. 43 materials to P-No. I 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-hours 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.
to Enclosure to ULNRC-06949 Attchment 1 to Relief Request 15R-O1 Page 3 of 15 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 Cliff 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. Luci&
Mid-Wall RVH Repair (2) 2004 Palo Verde 22 Heater Sleeve Repairs -Pads (34) 2004 Susquehanna Unit 12 Recirculation inlet and outlet nozzles 2004 HopeCreek1 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 NI Nozzle 2005 SONGS 32 Heater Sleeve Repairs -Pads (29) 2005 Three Mile Island1 SWOL (I) 2005 St. Luci&
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 2 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 Canyo&
SWOL (6) 2008 Diablo Canyo&
SWOL (4) 2008 Seabrook1 SWOL (4) 2009 Three Mile Island1 SWOL (I) 2009 Three Mile Island1 Full Nozzle with Structural Pad (I)
Attchment 1 to to Relief Request 15R-O1 Enclosure to ULNRC-06949 Page 4 of 15 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) 201 1 Hatch4 Nozzle WOL (1) 201 1 Talen Energy Corporation4 N5 core spray nozzles 201 1 Monticello4 Emergent WOL (1) 2011 Three Mile Island4 TMI PZR Spray Nozzle (1) 2011 Doel1 SWOL(1) 2011 Tihange1 SWOL(1) 2011 St. Luci&
1/2 Nozzle with Structural Pad (30) 2012 North Anna4 SG Nozzle WOLS (3) 2012 Palo Verde4 Small Bore CL Nozzles WOL 2012 Grand GuIr 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 NSA 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) 201 5 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
Attchment 1 to Attachment I to Relief Request 15R-O1 Enclosure to ULNRC-06949 Page 5 of 15
- 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 Xl 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-hours 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.
Attchment I to to Relief Request 15R-O1 Enclosure to ULNRC-06949 Page 6 of 15 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-hours 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 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].
Attchment 1 to Attachment I to Relief Request 15R-O1 Enclosure to ULNRC-06949 Page 7 of 15 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 I 20 mils) beneath the surface of the order of 280 to 300 KHN (28 to 3ORc) 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 I) 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 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
Attchment 1 to to Relief Request 15R-O1 Enclosure to ULNRC-06949 Page 8 of 15 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 390°F (20000) 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 E701 8, 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 ofthe 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
< l6mlIlOOg 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 < 2m111 OOg 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 mlIlOOg. 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 4mlIlOOg diffusible hydrogen [6]. Testing verifies that ambient temperature is acceptable, post weld hydrogen bakeout is not needed, and a 48 hour5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> 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,
Attchment I to to Relief Request 15R-O1 Enclosure to ULNRC-06949 Page 9 of 15 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 theGTAW process is less than 1.OmL/lOOg. Therefore, hydrogen cracking is extremely unlikely.
14 4
1 I
l2 10 Region Considered Low
I Hydrogen Code Case N-839 Exposure Limits i
6 8 hour9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />s-E7OXX and 4 hour4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />s-E8OXX/E9OXX
_i_
+
i m
2 4--
0 0
10 20 30 40 50 60 70 80 Exposure Time (hr) e Vendor A E7018 Vendor A E7018 OVendor B E7018-H4R QVendor B E7018-H4R XVendor C E7018-R
- Vendor D E8018-C3H4
+ Vendor E E8018-C3H4 Vendor C E8018-C3H4 XVendor C E8018-NMH4
- Vendor D E9018-B3H4R
- Vendor F E9018-B3H4R A Vendor C E9018-B3H4R
- Vendor C E9018-B3 80%
Figure 1. Results of EPRI diffusible hydrogen testing at 27°C 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]
to Enclosure to ULNRC-06949 0) 00 E
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Attchment 1 to Relief Request 15R-01 Page 10 of 15 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]
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 Vendor
Attchment 1 to Attachment I to Relief Request 15R-O1 Enclosure to ULNRC-06949 Page 1 1 of 15 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 (W) 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 a-Fe and y-Fe. Note that a-Fe is at the saturation limit at -4ml/1OOg of hydrogen. At temperatures above 17OOO C the solubility of hydrogen in austenite (y-Fe) is nearly five times that of ferrite (a-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 ofthe austenitic material. When comparing how hydrogen behaves in 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 W 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].
to Enclosure to ULNRC-06949 C)
C0 E
00 C0 C) 0 Attchment 1 to Relief Request 15R-O1 Page 12 of 15 25 20 15 Liquid Fe 10 5
a Fe 1000 1500 2000 Temperature (°C)
Figure 3
- Hydrogen solubility in ferritic and austenitic materials as a function of temperature
Attachment I to Enclosure to ULNRC-06949 a)
Cl) c\\j S
C) 4-G)
C) a)0 C) 0 U) a)>0 Attchment 1 to Relief Request 15R-O1 io-3 1O6 io 1O8 Temperature, °C Figure 4
- Diffusion Coefficient of hydrogen in ferritic and austenitic materials as a function of temperature to Enclosure to ULNRC-06949 Attchment 1 to Relief Request 15R-O1 Page 14 of 15
-J__
I p
martensite (M) austenite (y-Fe)
- 5. Conclusion The temper bead technique has become an increasingly effective tool for performing repairs on carbon and low alloy steel (P-No. I 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 well below any threshold level that would be required for hydrogen induced cracking. Table I 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.
hydrogen crack Figure 5
- Hydrogen movement with ferritic electrodes [8]
I martensite (M) austenite (y-Fe)
Figure 6
- Hydrogen movement with austenitic electrodes [8]
Attchment 1 to Attachment I to Relief Request 15R-O1 Enclosure to ULNRC-06949 Page 15 of 15
- 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: I 991
. NP-7085-D.
- 3. ASME Case N-638, Similar and Dissimilar Metal Welding Using Ambient Temperature Machine GTAW Temper Bead Techniques, Section Xl Division I, September 24, 1999.
- 4. Ambient Temperature Preheat for Machine GTAW Temperbead Applications, EPRI Palo Alto, I 998. GC-1 I I 050.
- 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 Xl 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: 2006.101 3558