NG-07-0176, Alternative to ASME Section XI Requirements to Use Code Cases N-504-2 and N-638-1 for Weld Overlay Repairs at the Duane Arnold Energy Center
| ML070660482 | |
| Person / Time | |
|---|---|
| Site: | Duane Arnold  |
| Issue date: | 02/24/2007 |
| From: | Vanmiddlesworth G Duane Arnold |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| NG-07-0176, TAC MA7125, TAC MC5173 | |
| Download: ML070660482 (24) | |
Text
FPL Energy Duane Arnold, LLC 3277 DAEC Road Palo, Iowa 52324 FPLEnergy.
Duane Arnold Energy Center February 24, 2007 NG-07-0176 10 CFR 50.55a(a)(3)(i)
U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001 Duane Arnold Energy Center Docket No: 50-331 Op. License No: DPR-49 Alternative to ASME Section XI Requirements to use Code Cases N-504-2 and N-638-1 for Weld Overlay Repairs at the Duane Arnold Energy Center
References:
- 1) Letter, C. Craig (USNRC) to E. Protsch (IES Utilities), "Alternative to the American Society Of Mechanical Engineers Boiler and Pressure Vessel Code Repair Requirements for the Recirculation Line for Duane Arnold Energy Center (TAC NO. MA7125)," dated November 19, 1999 (ML9933004430) and supporting Safety Evaluation (ML9933004460)
- 2) Letter, D. Roberts (USNRC) to W. Levis (PSEG Nuclear, LLC), "Hope Creek Generating Station - Evaluation of Relief Request HC-RR-12-WO1 (TAC NO. MC5173)," dated August 29, 2005 (ML051520177)
Pursuant to 10 CFR 50.55a(a)(3)(i), FPL Energy Duane Arnold requests NRC authorization of an alternative to the requirements of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code,Section XI, Rules for Inservice Inspection of Nuclear Power Plant Components. The Duane Arnold Energy Center (DAEC) Inservice Inspection (ISI) Program complies with the requirements of the ASME Code Section XI, 2001 Edition with Addenda through 2003. This proposed alternative would permit the use of a full structural weld overlay repair for an indication identified in the N2C and N2F recirculation inlet nozzle, safe-end-to-nozzle weld joint (RRC-F002 and RRF-F002). The DAEC is currently in its fourth ten-year ISI interval, which began November 1, 2006 and will end concurrent with the DAEC Operating License on February 21, 2014. Consequently, the requested relief is for the remainder of the current Operating License. Enclosure 1 to this letter contains that request for relief.
NG-07-0176 February 24, 2007 Page 2 of 3 During the current DAEC refueling outage (RFO20), augmented weld inspections were being performed in accordance with the DAEC fourth ten-year interval ISI program. The original scope of examinations included four recirculation riser safe-end-to-nozzle welds and one Core Spray (CS) safe-end-to-nozzle weld. These inspections identified a new indication in the safe-end-to-nozzle weld in the N2F recirculation riser (weld RRF-F002).
The N2F nozzle had a previously-identified indication (sub-surface flaw) in the RRF-F002 weld, in a different location, that was found during the inspections conducted during RFO16 (1999). Consequently, the N2F nozzle was being re-inspected this RFO as an ASME Code required successive examination. A more-detailed history of the examinations of this nozzle weld is included in the Enclosure 2 to this letter.
Due to the identification of the new indication in RRF-F002, the RFO20 inspection scope has been expanded to include two additional F002 recirculation riser safe-end-to-nozzle welds, as well as the other CS safe-end-to-nozzle weld. The remaining two F002 welds had structural overlays applied in 1999 and are not considered susceptible to further cracking and thus, were not inspected. During the expanded scope of inspections, a new indication on the N2C safe-end-to-nozzle weld (RRC-F002) was found. This indication will also require repair prior to resuming power operations. The other F002 weld and CS nozzle weld have had their inspections completed, with no new indications found.
While the determination of the formal root cause is being tracked in the Corrective Action Program (RCE01062), the preliminary assessment is that the indication is due to Stress Corrosion Cracking (SCC).
10CFR50.55a(a)(3)(i) states that proposed alternatives may be used when authorized by the Director of the Office of Nuclear Reactor Regulation provided that the proposed alternatives provide an acceptable level of quality and safety. FPL Energy Duane Arnold hereby requests NRC authorization to use Code Cases N-504-2 and N-638-1, with the exceptions and clarifications noted in Enclosure 1, to perform repair activities on safe-end-to-nozzle welds RRF-F002 and RRC-F002.
The requested relief is similar to that previously granted for the DAEC in 1999 (Reference 1) and to one more recently approved for the Hope Creek Generating Station in 2005 (Reference 2).
FPL Energy Duane Arnold requests approval of this request prior to beginning the weld overlay repair of safe-end-to-nozzle welds RRC-F002 and RRF-F002, currently scheduled for February 26, 2007.
This letter contains no new commitments and no revisions to existing commitments.
NG-07-0176 February 24, 2007 Page 3 of 3 Questions regarding this matter should be directed to Steve Catron, Licensing Manager, at (319) 851-7234.
Gary Van Middlesworth Site Vice President, Duane Arnold Energy Center FPL Energy Duane Arnold Enclosures (2) cc: Administrator, Region Ill, USNRC Project Manager, DAEC, USNRC Resident Inspector, DAEC, USNRC
Enclosure 1 to NG-07-0176 Alternative to ASME Section XI Repair Requirements to use Code Cases N-504-2 and N-638-1 for Weld Overlay Repairs at the Duane Arnold Energy Center 1.0 ASME Code Component(s) Affected Code Class: 1
References:
ASME Section XI, 2001 Edition, including and through the 2003 Addenda ASME Section XI, Case N-504-2 ASME Section XI, Case N-638-1 NUREG-0313 Rev 2 Generic Letter 88-01 BWRVIP-75 DAEC Fourth Ten Year ISI Plan - NRC Approved Relief Request NDE-R002, "Relief to use the PDI Program for Implementation of Appendix VIII, Supplement 11 requirements," and Relief Request NDE-R005 "Risked Informed ISI for Class 1 B-F & B-J Welds and Class 2 C-F-2 Welds (ML070090357)
Examination Categories: R-A (B-F)
Item Number: R1.16 (B5.10)
Description:
Alternative Repair for the RRC-F002 and RRF-F002 Recirculation Inlet Nozzle, Safe-end-to-Nozzle Welds Component Numbers: RRC-F002 Recirculation Inlet Nozzle Safe-end Weld RRF-F002 Recirculation Inlet Nozzle Safe-end Weld 2.0 Applicable Code Edition and Addenda ASME Code Section XI, 2001 Edition, including Addenda through 2003.
3.0 Applicable Code Requirement IWA-4421 (a) and IWA-461 1.1(a) require removal of the detected flaw.
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Enclosure 1 to NG-07-0176 IWA-4610(a) requires that the area to be welded shall be pre-heated to 300'F minimum for gas tungsten arc welding (GTAW).
IWA-4610(a) requires that thermocouples shall be used to monitor process temperatures.
IWA-4631 (b) specifies that the surface of the completed weld on the ferritic steel shall not exceed 100 square inches.
IWA-4633.2(c) specifies that the first three layers of the weld shall be deposited with heat inputs within +/-10% of that used in the procedure qualification test. Subsequent layers shall be deposited using heat input equal to or less than that used for layers beyond the third in the procedure qualification.
IWA-4633.2(c) also specifies that at least one layer of weld reinforcement shall be deposited and then this reinforcement shall be removed substantially flush with the surface surrounding the weld.
4.0 Reason for Request The request is based on restoring the structural integrity of the RRC-F002 and RRF-F002 recirculation inlet nozzle safe-end-to-nozzle weld joints using technically sound welding practices and non-destructive examination (NDE), while limiting repair personnel exposure to the maximum extent practical. The following cited Code articles identify the actions that would be required if the repair were conducted in accordance with the Code without exception.
IWA-4421 (a) and IWA-461 1.1(a) require defect removal in this case. The repair cavity would extend through wall since outer diameter (OD) removal would be required.
Internal diameter (ID) removal of the indication would be impractical since it would require the removal of the thermal sleeve and jet pump riser from the reactor interior.
IWA-4610(a) requires the area to be welded shall be pre-heated to 300'F minimum for GTAW. Since the nozzle will remain full of water, establishing the 300°F minimum pre-heat temperature cannot be achieved.
IWA-4610(a) also requires the use of thermocouples to monitor process temperatures.
Due to the personnel exposure associated with the installation and removal of the thermocouples, the nozzle configuration, and because the nozzle will be full of water, a contact pyrometer will be used, in lieu of thermocouples, to verify pre-heat and interpass temperature limits are met.
IWA-4631 (b) specifies the surface of the completed weld on the ferritic steel shall not exceed 100 square inches. Restoring the structural integrity of the safe-end-to-nozzle 2 of 14
Enclosure 1 to NG-07-0176 weld with the weld overlay will require welding on more than 100 square inches of surface on the low alloy steel base material.
IWA-4633.2(c) specifies the first three layers of the weld shall be deposited with heat inputs within +/-10% of that used in the procedure qualification test. Subsequent layers shall be deposited using heat input equal to or less than that used for layers beyond the third in the procedure qualification. Code Case N-638-1 allows for layers beyond the third to exceed the heat input, provided it is in accordance with the procedure qualification records (PQRs).
IWA-4633.2(c) also specifies that at least one layer of weld reinforcement shall be deposited and then this reinforcement shall be removed substantially flush with the surface surrounding the weld. The weld reinforcement will not be removed flush to the surface.
5.0 Proposed Alternative and Basis for Use A full structural weld overlay repair is proposed for the safe-end-to-nozzle weldments.
The nozzle material is SA-508 Class 2 low alloy steel. The safe-end is Alloy 600 SB-166. The existing weld material is Alloy 82, with Alloy 182 buttering.
The weld overlay will be designed consistent with the requirements of NUREG-0313, Revision 2 (which was implemented by Generic Letter (GL) 88-01), Code Case N-504-2, "Alternative Rules for Repair of Classes 1, 2, and 3 Austenitic Stainless Steel Piping," Code Case N-638-1, "Similar and Dissimilar Metal Welding Using Ambient Temperature GTAW Temper Bead Technique," and IWB-3640, ASME Section Xl 2001 Edition, including Addenda through 2003 with Appendix C.
Welder Qualification And Welding Procedures All welders and welding operators will be qualified in accordance with ASME Section IX and any special requirements of ASME Xl or applicable code cases. Qualified personnel under the vendor's (Welding Services Inc. (WSI)) welding program will perform the weld overlay repair.
Welding Procedure Specification (WPS) WPS 03-43-T-804-102967 (machine GTAW with cold wire feed) for welding SFA-5.14, ERNiCrFe-7A, UNS N06054, F-No. 43 (commercially known as Alloy 52M) will be used.
If repairs to the overlay are required, manual GTAW for welding SFA-5.14, ERNiCrFe-7A, UNS N06054, F-No. 43 (commercially known as Alloy 52M) will be used. In the unlikely event of a through-wall defect, UNS W86152, F No. 43 (commercially known as Alloy 152) will be used to seal any defect if it is greater than 0.125 inch from the P-3 nozzle material before beginning the structural weld overlay using GTAW.
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Enclosure 1 to NG-07-0176 Welding Wire and Electrodes A consumable welding wire, highly resistant to stress corrosion cracking (SCC), was selected for the overlay material. Alloy 52M contains a nominal 30 wt% Cr that imparts excellent resistance to SCC. Where localized repairs are required, Alloy 52M will be used.
Weld Overlay Desigqn The weld overlay will extend around the full circumference of the safe-end-to-nozzle weldment location in accordance with NUREG-0313, Rev. 2, Code Case N-504-2, and GL 88-01. The overlay length will extend across the projected flaw intersection with the outer surface beyond the extreme axial boundaries of the flaw. The design thickness and length has been computed in accordance with the guidance provided in Code Case N-504-2 and ASME Section Xl, IWB-3640, 2001 Edition including Addenda through 2003 and Appendix C. The overlay will completely cover the area of the flaw and other Alloy 182 susceptible material with the highly resistant Alloy 52M weld filler material.
309 Not to Scale 4 of 14
Enclosure 1 to NG-07-0176 To provide the necessary weld overlay geometry, it will be necessary to weld on the low alloy steel nozzle base material. A temper bead welding approach will be used for this purpose following the guidance of ASME Section Xl Code Case N-638-1, "Similar and Dissimilar Metal Welding Using Ambient Temperature Machine GTAW Temper Bead Technique." This Code Case provides for machine GTAW temper bead weld repairs to P-No. 3, Group No. 3, nozzle base material at ambient temperature. The temper bead approach was selected because temper bead welding supplants the requirement for post-weld heat treatment (PWHT) of the heat-affected zone (HAZ) in welds on low alloy steel material. Also, the temper bead welding technique produces excellent toughness and ductility as demonstrated by welding procedure qualification in the HAZ of welds on low alloy steel materials, and, in this case, results in compressive residual stresses on the inside surface, which assists in inhibiting SCC. This approach provides a comprehensive weld overlay repair and increases the volume under the overlay that can be examined.
The overlay length conforms to the guidance of Code Case N-504-2, which satisfies the stress requirements.
Examination Requirements NUREG-0313, Rev. 2, and Code Case N-504-2, specify ultrasonic test (UT) using methods and personnel qualified in accordance with ASME Section Xl, Appendix I. The UT techniques to be used for the final post-weld examination have been qualified through the Electric Power Research Institute (EPRI) NDE Center, which satisfies the requirements of ASME Section Xl, Appendix I. Furthermore, NUREG-0313 states that the UT to be performed in accordance with the requirements of the applicable Edition and Addenda of ASME Section Xl. ASME Section Xl, 2001 Edition including Addenda through 2003 is the Code of record for the DAEC fourth 10-year Inservice Inspection Interval. Therefore, the acceptance criteria that will be used for the UT will be IWB-3130, "Inservice Volumetric and Surface Examinations," and ASME Section Xl Non-mandatory Appendix Q, "Weld Overlay Repair of Class 1, 2, and 3 Austenitic Stainless Steel Piping Weldments," as clarified under Exceptions to Code Case N-638-1 Paragraph 4.0(b). In addition, an NRC-approved relief request (NDE-R002) for the DAEC (ML070090357) allows the use of the Performance Demonstration Initiative (PDI)
Program for implementation of Appendix VIII, Supplement 11 requirements for the examination of piping welds with overlays.
The examination requirements for the weld overlay repair are summarized in Table 1.
No final post-weld examinations will be performed until 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> has elapsed after completion of welding. This is required to detect any possible hydrogen-induced cracking that may occur in the low alloy steel nozzle HAZ.
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Enclosure 1 to NG-07-0176 TABLE 1 Examination Requirements Exam Description Method Technique Reference PDI Qualified As Found Flaw Detection Auto UT Implementing IWB-3514.4 ASME Section XI Appendix VIII Pre-weld UT Thickness Manual UT 0 N-504-2 IWA-4611.1 (a)
Surface Prior to Welding PT (Visible) Penetrant N-504-2(c)
(Visble)Penerant N-638-1,4.0(a)
IWA-4634 IW-54634j Color Contrast Final Weld Overlay Surface PT (Visible) Penetrant N-504-20)
(Visble)Penerant N-638-1 ,4.0(b)
IWA-4634 Final Weld Overlay for Thickness UT 0 N-504-2(j)
N-638-1,4.0(b)
PDI Qualified IWA-4634 Final Weld Overlay and Outer Implementing IWB-3514.4 25% of the Underlying Wall UT ASME Section XI N-504-2(j)
Thickness Volumetric Pre-service Appendix VIII RR- N-638-1,4.0(b)
NDE-002 Appendix Q 6 of 14
Enclosure 1 to NG-07-0176 Pressure Testing The completed repair shall be given a system leakage test in accordance with ASME Section Xl, IWA-5000, since the pressure boundary has not been penetrated (no leakage has occurred). In the event an unexpected through wall defect is identified, either before or during the repair, an additional exception from the hydrostatic pressure test requirements defined in Code Case N-504-2 will be needed. A system leakage test will be performed in accordance with ASME Section Xl, IWA-5000 of the 2001 Edition with the 2003 Addenda. Precedence for use of a leak test at normal operating temperature and pressure, in lieu of a hydrostatic test, has been set with Code Case N-416-1, which has been incorporated in ASME Section Xl starting with the 1998 Edition, 1999 Addenda.
Pre-heat and PWHT Requirements Pre-heat and PWHT are typically required for welding on low alloy steel material. ASME Section III specifies PWHT on P-No. 3, Group No. 3, base materials unless temper bead welding is performed under limited restrictions (area and depth limits). ASME Section Xl, 2001 Edition including Addenda through 2003, specifies 300°F minimum pre-heat be used for temper bead welding. PWHT cannot be performed and the pre-heat requirements would necessitate draining the reactor pressure vessel (RPV) and a portion of the recirculation system piping. This would result in unacceptable radiation dose rates. Therefore, consistent with ALARA practices and prudent utilization of outage personnel, the RPV will not be drained for this activity. The nozzle and connected piping will be full of water.
Alternatives to Code Case N-504-2 Code Case N-504-2 Applicability to Nickel Based Austenitic Steel Code Case N-504-2 was prepared specifically for austenitic stainless steel material. An alternate application for nickel based austenitic materials (Alloy 52M) is needed due to the specific materials and configuration of the existing nickel based alloy weld and buttering (Alloy 82 and Alloy 182).
Exception to Code Case N-504-2, Requirement (b)
Code Case N-504-2, Requirement (b) requires the weld overlay shall be low carbon (0.035% maximum) austenitic stainless steel. A nickel-based filler is required and Alloy 52M has been selected to be used.
Exception to Code Case N-504-2, Requirement (e)
Code Case N-504-2, Requirement (e) requires the first two layers of the weld overlay to have a ferrite content of at least 7.5 FN (Ferrite Number). These measurements will not be performed for this overlay since the nickel alloy filler is a fully austenitic material.
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Enclosure 1 to NG-07-0176 Exception to Code Case N-504-2, Requirement (h)
Code Case N-504-2, Requirement (h) specifies that a system hydrostatic test shall be performed in accordance with IWA-5000 if the flaw penetrates the pressure boundary.
In the event the flaw becomes through wall, leak testing only, in accordance with ASME Section XI, IWA-5000, will be performed.
Alternatives to Code Case N-638-1 Exception to Code Case N-638-1 Paragraph 1.0(a)
Code Case N-638-1 paragraph 1.0(a) specifies that the maximum weld area on the finished surface shall be 100 square inches. Restoring the structural integrity of the safe-end-to-nozzle weld with the weld overlay will require welding on more than 100 square inches of surface on the low alloy steel base material.
Exception to Code Case N-638-1 Paragraph 4.0(b)
Code Case N-638-1 paragraph 4.0(b) specifies that the final weld surface and the band around the area (1.5T width or 5 inches, whichever is less) shall be examined using surface and ultrasonic methods when the completed weld has been at ambient temperature for at least 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br />. The UT shall be in accordance with ASME Section Xl Appendix I. Full UT of the 1.5T band will not be performed.
Exception to Code Case N-638-1 Paragraph 4.0(c)
Code Case N-638-1 paragraph 4.0(c) specifies that the area from which weld-attached thermocouples have been removed shall be ground and examined using a surface examination method. Thermocouples will not be used. Calibrated pyrometers will be utilized to monitor pre-heat & interpass temperatures.
Basis For The Alternatives IWA-4421(a) and IWA-4611.1(a) require defect removal in this case. The repair cavity would extend through wall since OD removal would be required. The ID is inaccessible due to the thermal sleeve. Therefore, the flaw will not be removed. Structural weld overlays covering flaws are permitted by Code Case N-504-2, provided the necessary weld overlay geometry is used. Therefore, this alternative provides an acceptable level of quality and safety.
IWA-4610(a) requires the area to be welded shall be pre-heated to 300°F minimum for GTAW. Since the nozzle will remain full of water, establishing the 300'F minimum pre-heat temperature cannot be achieved. Code Case N-638-1, paragraph 1.0(b) provides for machine GTAW temper bead weld repairs to P-No. 3, Group No. 3, nozzle base material at ambient temperature. The ambient temperature temper bead approach was 8 of 14
Enclosure 1 to NG-07-0176 selected because temper bead welding supplants the requirement for PWHT of the HAZ in welds on low alloy steel material. Also, the temper bead welding technique produces excellent toughness and ductility, as demonstrated by welding procedure qualification, in HAZ of welds on low alloy steel materials. Welding procedure qualifications have been successfully performed using Alloy 52M welds on P-No. 3, Group No. 3, base material using the ambient temperature temper bead technique.
Therefore, this alternative provides an acceptable level of quality and safety.
IWA-461 0(a) also requires the use of thermocouples to monitor process temperatures.
Due to the personnel exposure associated with the installation and removal of the thermocouples, the nozzle configuration, and because the water in the line containing the nozzle will not be drained, thermocouples will not be used to verify that pre-heat and interpass temperature limits are met. In lieu of thermocouples, a contact pyrometer will be used to verify pre-heat temperature and interpass temperature compliance with the WPS requirements. The use of a contact pyrometer provides equivalent temperature monitoring capabilities and is recognized as acceptable calibrated measuring and test equipment (M&TE). Therefore, this alternative provides an acceptable level of quality and safety.
IWA-4631(b) specifies the surface of the completed weld on the ferritic steel shall not exceed 100 square inches. Restoring the structural integrity with the weld overlay of the safe-end-to-nozzle weld will require welding on more than 100 square inches of surface on the low alloy steel base material. If this limit were maintained, the length of weld overlay extension on the nozzle base material would be limited to approximately 2.25 inches, including the taper. This distance could be justified as sufficient to provide load redistribution from the weld overlay back into the nozzle without violating ASME III stress limits for primary local and bending stresses, and secondary and peak stresses.
However, this length would not permit a complete UT of the outer 25% of the nozzle and safe-end thickness as specified by Code Case N-504-2. The overlay will extend to the transition taper of the low alloy steel nozzle so that qualified UT of the required volume can be performed. Therefore, this alternative provides an acceptable level of quality and safety.
Code Case N-432 has always allowed temper bead welding on low alloy steel nozzles without limiting the temper bead weld surface area. The two additional conditions required by N-432, that are not required by Code Case N-638, are that temper bead welds have pre-heat applied and that the procedure qualification be performed on the same specification, type, grade, and class of material. As previously discussed, elevated pre-heat necessitates draining of the RPV and a portion of the recirculation system piping. This would result in unacceptable radiation dose rates.
The ASME Code committees have recognized that the 100 square inches restriction on the surface area is unnecessarily limiting and Code Case N-638-3 has been issued to increase the surface area limit to 500 square inches. The code case attempts to combine the features of Code Case N-432 and N-638 into a single code case. The supporting analysis for the code case (EPRI Technical Report 1008454, "Proposed 9 of 14
Enclosure 1 to NG-07-0176 Code Case, Expansion of Temper Bead Repair") concluded that the residual stresses are not detrimentally changed by increasing the surface area of the repair and increasing the HAZ tempering is unaffected by the weld overlay application. Therefore, this alternative provides an acceptable level of quality and safety.
IWA-4633.2(c) specifies the first three layers of the weld shall be deposited with heat inputs within +/-10% of that used in the procedure qualification test. Subsequent layers shall be deposited using heat input equal to or less than that used for layers beyond the third in the procedure qualification. Code Case N-638-1 allows for layers beyond the third to exceed the heat input provided it is in accordance with the PQRs. Therefore, this alternative provides an acceptable level of quality and safety.
IWA-4633.2(c) also specifies that at least one layer of weld reinforcement shall be deposited and then this reinforcement shall be removed, to be substantially flush with the surface surrounding the weld. The weld overlay is austenitic and thus, there is no need to remove the final layer. Also, overlays, by definition, cannot be substantially flush with the surrounding surface. Overlays are permitted per Code Case N-504-2. The toe of the weld on the low alloy steel nozzle shoulder will be indexed between layers such that proper HAZ tempering will result. Therefore, this alternative provides an acceptable level of quality and safety.
Code Case N-638-1 is approved (with one limitation) for generic use in Regulatory Guide (RG) 1.147, Revision 14, and was developed for both similar and dissimilar metal welding using ambient temperature machine GTAW temper bead technique. The welding methodology of Code Case N-638-1 will be followed for the overlay, whenever welding within the 0.125-inch minimum distance from the low alloy steel nozzle base material.
Code Case N-504-2 is approved (with one limitation) for generic use in RG 1.147, Revision 14, and was developed for welding on and using austenitic stainless steel material. An alternate application for nickel-based and low alloy steel materials is proposed due to the specific configuration of this weldment. The weld overlay proposed is austenitic material having a mechanical behavior similar to austenitic stainless steel.
It is also compatible with the existing weld and base materials.
The methodology of Code Case N-504-2 is to be followed, except for the following:
Exception to Code Case N-504-2, Requirement (b)
Code Case N-504-2, Requirement (b) requires the weld overlay shall be low carbon (0.035% maximum) austenitic stainless steel.
A consumable welding wire highly resistant to SCC was selected for the overlay material. This material, designated as UNS N06054, F-No. 43, is a nickel based alloy weld filler material, commonly referred to as Alloy 52M and will be deposited using the machine GTAW process with cold wire feed. Alloy 52M contains about 30 wt%
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Enclosure 1 to NG-07-0176 chromium, which imparts excellent corrosion resistance to the material. By comparison, Alloy 82 is identified as a SCC-resistant material in NUREG-0313 Revision 2 and contains nominally 20 wt% chromium, while Alloy 182 has a nominal chromium content of 15 wt%. With its higher chromium content than Alloy 82, Alloy 52M provides an even higher level of resistance to SCC consistent with the requirements of the Code Case.
Therefore, this alternative provides an acceptable level of quality and safety.
Exception to Code Case N-504-2, Requirement (e)
Code Case N-504-2, Requirement (e) requires the first two layers of the weld overlay to have a ferrite content of at least 7.5 FN (Ferrite Number).
The composition of nickel-based Alloy 52M is such that delta ferrite does not form during welding, because Alloy 52M welds are 100% austenitic and contain no delta ferrite due to the high nickel composition (approximately 60 wt% nickel). Consequently, delta ferrite measurements will not be performed for this overlay. Therefore, this alternative provides an acceptable level of quality and safety.
Exception to Code Case N-504-2, Requirement (h)
Code Case N-504-2, Requirement (h) specifies that a system hydrostatic test shall be performed in accordance with IWA-5000 if the flaw penetrates the pressure boundary.
Leak testing in accordance with ASME Section Xl (2001 Edition with the 2003 Addenda), IWA-5000, will be performed. Precedence for use of a leak test at normal operating temperature and pressure in lieu of a hydrostatic test has been set with Code Case N416-1 that has been incorporated in ASME Section Xl beginning in the 1998 Edition with the 1999 Addenda. Therefore, this alternative provides an acceptable level of quality and safety.
Exception to Code Case N-638-1 Paragraph 1.0(a)
Code Case N-638-1 paragraph 1.0(a) specifies that the maximum weld area on the finished surface shall be 100 square inches. Restoring the structural integrity with the weld overlay of the safe-end-to-nozzle weld will require welding on more than 100 square inches of surface on the low alloy steel base material. The weld overlay will cover approximately 180 square inches of the low alloy steel nozzle.
Code Case N-432 allows temper bead welding on low alloy steel nozzles without limiting the temper bead weld surface area. The two additional conditions required by N-432, that are not required by Code Case N-638-1, are that temper bead welds have pre-heat applied and that the procedure qualification be performed on the same specification, type, grade and class of material. As previously discussed, elevated pre-heat necessitates draining of the RPV and a portion of the recirculation system piping.
By removing the water in the pipe, nozzle area, and (in vessel) inlet riser a large amount of shielding is removed. The radiation dose rates at the weld overlay location would 11 of 14
Enclosure 1 to NG-07-0176 increase, thereby significantly increasing personnel dose.
The ASME Code committees have recognized that the 100 square inches restriction on the surface area is unnecessarily limiting and Code Case N-638-3 has been issued to increase the surface area limit to 500 square inches. The code case attempts to combine the features of Code Case N-432 and N-638 into a single code case. The supporting analysis for the code case is found in EPRI Technical Report 1008454, "Expansion of Temperbead Repair: Proposed Code Case," which concluded that the residual stresses are not detrimentally changed by increasing the surface area of the repair and increasing the HAZ tempering is unaffected by the weld overlay application.
The technical basis that justifies exceeding 100 square inches of surface area for repair welds is found in EPRI Technical Report 1003616, "Additional Evaluations to Expand Repair Limits for Pressure Vessels and Nozzles." This technical report describes an ANSYS Finite Element Analysis (FEA) conducted on the Nine Mile Point - Unit 2 feedwater nozzle weld overlay repair. The analysis consisted of modeling the welding processes for both thermal and mechanical respects. The two overlays were modeled; one was 100 square inches, the other was extended to blend into the nozzle radius to achieve greater than 100 square inches surface area repair currently permitted by the ASME Code requirements. Comparison of the residual stresses of the two overlays showed that the effect of extending the overlay to the nozzle radius minimally impacted the residual stress profile and, in some cases, slightly increased the beneficial compressive stresses on the nozzle inner diameter. Therefore, this alternative provides an acceptable level of quality and safety.
Exception to Code Case N-638-1 Paragraph. 4.0(b)
Code Case N-638-1 Paragraph 4.0(b) specifies that the final weld surface and band area (1.5T width or 5 inches, whichever is less) shall be examined using surface and ultrasonic methods when the completed weld has been at ambient temperature for at least 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br />. The UT shall be in accordance with ASME Section XI, Appendix I.
Surface exams will be performed. IWA-4634 requires UT of the weld only. Any laminar flaws in the weld overlay will be evaluated in accordance with ASME Section XI Non-mandatory Appendix Q, Paragraph Q-4100, except, as allowed by IWB-3132.3, any flaws that exceed the acceptance standards of Table IWB-3410-1 are acceptable for continued service, without repair, if an analytical evaluation, performed in accordance with IWB-3600, meets the acceptance criteria of IWB-3600. Full UT of the 1.5T band will not be performed. The weld overlay will extend into the blend radius of the nozzle beyond the length required by Code case N-504-2 for structural reinforcement. This extension onto the blend radius eliminates a stress riser on the nozzle and provides additional OD surface area for UT examination of the defect area. UT examination on the nozzle beyond the overlay will not provide any information regarding the area of the defect that required repair. Additionally, such UT would likely be unsatisfactory when applied to the nozzle blend radius, where the toe of the weld overlay resides. The UT return signal would be difficult to obtain and to interpret. Alternatively, surface examination will assure that no defects have been created at the toe of the weld overlay. Therefore, this alternative provides an acceptable level of quality and safety.
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Enclosure 1 to NG-07-0176 Exception to Code Case N-638 Paragraph. 4.0(c)
Code Case N-638-1 paragraph 4.0(c) specifies that the area from which weld-attached thermocouples have been removed, shall be ground and examined using a surface examination method. Due to the personnel exposure associated with the installation and removal of the thermocouples, the nozzle configuration, and because the nozzle will be full of water, thermocouples will not be used to verify that the pre-heat and interpass temperature limits are met. In lieu of thermocouples, a contact pyrometer will be used to verify pre-heat temperature and interpass temperature compliance with the WPS requirements. Therefore, this alternative provides an acceptable level of quality and safety.
The use of overlay filler material that provides excellent resistance to SCC develops an effective barrier to flaw extension. Also, temper bead welding techniques produce excellent toughness and ductility in the weld HAZ low alloy steel materials, and in this case, results in compressive residual stresses on the inside surface that help to inhibit further SCC. The design of the overlay for the safe-end-to-nozzle weldment uses methods that are standard in the industry. There are no new or different approaches in this overlay design which would be considered either first-of-a-kind or inconsistent with previous approaches. The overlay will be designed as a full structural overlay in accordance with Code Case N-504-2. The temper bead welding technique that will be implemented in accordance with Code Case N-638-1 will produce a tough, ductile, corrosion-resistant overlay.
Use of Code Cases N-504-2 and N-638-1 has been accepted in RG 1.147, Revision 14, with the following limitations as providing an acceptable level of quality and safety.
Code Case N-504-2 Limitation The provisions of Section Xl, Non-mandatory Appendix Q, "Weld Overlay Repair of Class 1, 2, and 3 Austenitic Stainless Steel Piping Weldments," must also be met, as noted in RG 1.147.
The DAEC will meet the associated requirements contained in this non-mandatory Appendix Q.
Code Case N-638-1 Limitation UT examinations shall be demonstrated for the repaired volume using representative samples which contain construction type flaws. The acceptance criteria of NB-5330 of Section III edition and addenda approved in 10CFR50.55a apply to all flaws identified in the repair volume The DAEC will implement this limitation.
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Enclosure 1 to NG-07-0176 FPL Energy Duane Arnold concludes that the alternative repair approach described above presents an acceptable level of quality and safety to satisfy the requirements of 10CFR50.55a(a)(3)(i).
6.0 Duration of Proposed Alternative The DAEC is currently in its fourth ten-year ISI interval, which began November 1, 2006 and will end concurrent with the DAEC Operating License on February 21, 2014.
Consequently, the requested relief is for the remainder of the current Operating License.
7.0 Precedents The observed flaws at DAEC are consistent with the documented SCC observed at DAEC in 1999 on the safe-end-to-nozzle welds N2B and N2D. Similar flaws have been observed at other BWRs, including Perry, Nine Mile Point - Unit 2, Susquehanna - Unit 1, and more recently at Hope Creek Generating Station.
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Enclosure 2 to NG-07-0176 Supplemental Information on Duane Arnold Energy Center Recirculation Nozzle to Safe End Welds RRC-F002 and RRF-F002 Recirculation Piping Safe End Design The function of the N2 nozzles is to connect the recirculation system inlet piping to the RPV. The design includes a safe-end and thermal sleeve connecting the external recirculation piping to the internal jet pump riser (see Figures 1 and 2).
The existing safe-end-to-nozzle weld is Alloy 82 and connects the Alloy 600 SB-166 safe-end to the SA-508 Class 2 low alloy steel nozzle, buttered with Alloy 182. A portion of the original Alloy 82/182 safe-end-to-nozzle weld remains on the nozzle side as a result of installing a modified safe-end with an integrally-attached thermal sleeve in 1978.
This is an ASME Section II, Part C, SFA-5.14 ERNiCr-3 UNS N06082 (commercially known as Alloy 82) weld, that connects an approximately 13.12 inch OD by 11.00 inch ID Alloy 600 SB-166 safe-end to the SA-508 Class 2 low alloy steel nozzle, buttered with ASME Section II, Part C, SFA-5.11 ENiCrFe-3, UNS W86182 (commercially known as Alloy 182).
DAEC ISI Program The DAEC has implemented the Risked-Informed ISI Program using the methodology in EPRI TR-112657 Rev B-A, in accordance with RG 1.178. The Risked-Informed implementation started in the third 10-year interval in the second period. A Relief Request (NDE-R005) was approved granting continuation of the Risk-Informed program for the current fourth 10-year interval (ML070090357).
The RRC-F002 and RRF-F002 safe-end-to-nozzle welds were classified as Category R-A (B-F), Item No. R1.16 (B5.10) welds.
Failure Risk Ranking Failure Consequence Risk Category Mechanism Potential TT (IGSCC) High (High) Medium High 2(2)
RRF-F002 was ranked as a Category 2 with a consequence ranking of High and degradation mechanisms of thermal fatigue and IGSCC. This means that RRC-F002 and RRF-F002 are in the sample where 25% of the welds are required to be examined once every 10 years. RRF-F002 was part of the 25% population. RRC-F002 was not chosen as part of the 25% population.
Current Inspection Results During RFO20, ISI UT examinations of the dissimilar weld metal joint at the N2F recirculation inlet nozzle, safe-end-to-nozzle weld (RRF-F002) was examined, as an 1 of 7
Enclosure 2 to NG-07-0176 ASME Code required successive examination. The RRF-F002 weld has a sub-surface indication identified during a previous inspection (RFO16). This was the third, and last, successive examination. This indication has not changed.
The welds were examined with an ASME Section XI, Appendix VIII qualified, Electric Power Research Institute (EPRI) - Performance Demonstration Initiative (PDI) procedure. The inspection was performed using encoded, manual, phased-array UT.
As a result of this examination, a circumferentially-oriented indication was identified in RRF-F002. The indication appears to be planar with a length of approximately 7 inches and a depth of 56% through-wall.
Due to finding the above indication, the original examination scope was expanded to include the other susceptible safe-end-to-nozzle welds in the recirculation and Core Spray nozzles. Because the N2B and N2D nozzles had previously been repaired with full structural overlays in 1999 (ML9933004460), those nozzles are no longer considered to be susceptible and were excluded from the expanded scope. Those examinations found a new indication in the F002 weld in the N2C nozzle. This indication is circumferentially-oriented and appears to be planar with a length of approximately 6 inches and a depth of 74% through wall.
The above flaw in each weld is not believed to extend through wall as verified by no observed leakage of the entire OD weld surface and adjacent areas. Estimates on indication depth provided from information available from the detection and length sizing examination data indicates that the flaw size estimates would exceed the acceptance criteria stated in IWB-3514-2.
Suspected DeQradation Mechanism Even though the root cause evaluation has not been completed, experience at the same weld joint on the N2B & D Nozzles (RRB-F002 and RRD-F002) at DAEC in 1999 and at other Boiling Water Reactors (BWRs), it appears that the cause of the flaw is most likely due to stress corrosion cracking (SCC).
Hydrogen Water Chemistry (HWC) was implemented in 1987. The injection rate for HWC is at 2.6 standard cubic feet per minute (scfm) that results in an Electro-Chemical Potential (ECP) of below -230 pMho/cm. NobleChem TM was initially implemented in 1996, with follow-up injections in 1999 and 2005.
In addition, no stress improvement (either mechanical or inductive heat) has been used on these welds.
Inspection History Below is the history of examinations for the safe-end-to-nozzle weld RRF-F002 during the second and third inspection intervals. It is important to note that the examination history includes examinations from both the Augmented Inspection Program (Generic Letter 88-01) and the ASME Code Program.
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Enclosure 2 to NG-07-0176 Examination History of Safe-end to Nozzle Weld RRF-F002 Examination Date Type of Examination Results (Automated/Manual) 1987 Manual ID Geometry (counterbore) 1988 Manual Dissimilar Material Interface ID Geometry 1990 Automated 600 L-wave recorded root GE Smart UT System and inside surface geometry from both sides of weld 450 L-wave recorded root geometry on upstream side also acoustic interface from downstream side 450 shear recorded root geometry on upstream side 1996 Automated 450 Shear Wave recorded GE Smart 2000 System weld root geometry from the upstream side.
450 L-wave recorded weld root geometry from upstream side.
600 L-wave recorded weld root geometry from both sides.
1999 Automated 450 RL recorded ID surface GE Smart 2000 System geometry 600 RL recorded root and ID surface geometry 70' RL recorded acoustic interface, clad interface
- 1999 Supplement Automated 450 RL recorded root After portion of crown GE Smart 2000 System geometry removal 4" to 15" clockwise 600 RL recorded root from Top-Dead Center geometry and lack of fusion 70' RL recorded lack of fusion 2001 Manual No Change in previously recorded indication 3 of 7
Enclosure 2 to NG-07-0176 Examination Date Type of Examination Results (Automated/Manual) 2005 Automated 450 RL recorded root geometry 450 Shear recorded root geometry 600 RL recorded weld repair area, ID geometry and Interface 2007 Encoded Manual Phased 1 - Linear Ind. 7" long Array TRL 90 & TRL 270 - ID Geometry (Root) 1 - Weld Defect 15mm long (same as 2005 data)
- This examination put the weld in a Successive Examination Schedule Examination History of Safe-end to Nozzle Weld RRC-F002 Examination Date Type of Examination Results (Automated/Manual) 1983 Manual 360° impedance mismatch 1985 Manual ID & OD Geometry ID Counter-bore 1987 Manual ID Geometry 1988 Manual Dissimilar. Material.
Interface, ID Geometry 1992 Manual 450 RL recorded non-relevant indications, ID surface geometry 600 RL recorded non-relevant indications and acoustic interface.
1995 Automated 450 RL recorded ID surface GE Smart 2000 System geometry 600 RL recorded root and ID surface geometry 1999 Automated 450 RL recorded ID surface GE Smart 2000 System geometry 60' RL recorded root and ID surface geometry 700 RL recorded acoustic interface 2007 Encoded Manual Phased 1 - Linear Ind. 6" long Array TRL 90 & TRL 270 - ID Geometry (Root) 4 of 7
Enclosure 2 to NG-07-0176 Similar BWR Experience The observed flaw at DAEC is consistent with the documented SCC observed at DAEC in 1999 on safe-end-to-nozzle welds. Similar flaws have been observed at other BWRs, including Perry, Nine Mile Point - Unit 2, Susquehanna - Unit 1, and more-recently Hope Creek Generating Station.
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Enclosure 2 to NG-07-0176 FIGURE 1 Lifting Eye,
- Hold Downs
- Inlet Elbow J-et Pump Nozzle
. Mixing Assembly
- Core Shroud Wedge and Restrainer
- Slip Fit
- Core Plate
- Pressure Sensing
- Diffuser
- Pressure Sensing Inst.
Tap (Jet Pump Nos.
1, 5, 9 and 13)
- Core Shroud Baffle Plate 6 of 7
Enclosure 2 to NG-07-0176 FIGURE 2 7 of 7