ML071210024
| ML071210024 | |
| Person / Time | |
|---|---|
| Site: | Millstone  |
| Issue date: | 05/03/2007 |
| From: | Chernoff H NRC/NRR/ADRO/DORL/LPLI-2 |
| To: | Christian D Dominion Nuclear Connecticut |
| Hughey J, NRR/DORL, 301-415-3204 | |
| References | |
| TAC MD3379 | |
| Download: ML071210024 (14) | |
Text
May 3, 2007 Mr. David A. Christian Sr. Vice President and Chief Nuclear Officer Dominion Nuclear Connecticut, Inc.
Innsbrook Technical Center 5000 Dominion Boulevard Glen Allen, VA 23060-6711
SUBJECT:
REQUEST FOR APPROVAL TO USE IR-2-47 FOR DISSIMILAR METAL WELD OVERLAYS AS AN ALTERNATIVE REPAIR TECHNIQUE (TAC NO. MD3379)
Dear Mr. Christian:
By letter dated October 17, 2006, as supplemented by letters dated March 28, March 30, and April 23, 2007, Dominion Nuclear Connecticut, Inc. (DNC), submitted to the U.S. Nuclear Regulatory Commission (NRC), Alternative Request IR-2-47, Revision 1 to allow the installation of preemptive weld overlays (PWOL) on pressurizer nozzle dissimilar and similar metal welds at Millstone Power Station Unit 3 (MPS3). The proposed approach is an alternative to the requirements of American Society of Mechanical Engineers Boiler and Pressure Vessel Code,Section XI. The relief is requested pursuant to Title 10 of the Code of Federal Regulations (10 CFR), Section 50.55a(a)(3)(i).
Based upon the review of the information you provided, the NRC concluded that the proposed Alternative Request IR-2-47, Revision 1, will provide an acceptable level of quality and safety.
Therefore, pursuant to 10 CFR 50.55a(a)(3)(i), the NRC staff authorizes the use of Alternative Request IR-2-47, Revision 1, for the installation of PWOL on the dissimilar and similar metal welds of the pressurizer nozzles at MPS3. The effective period of the proposed alternative is the second inservice inspection interval which ends on October 23, 2008. The NRC staffs Safety Evaluation is enclosed. If you have any questions, please contact the project manager, John Hughey at (301) 415-3204.
Sincerely, Mark G. Kowal /ra/ for Harold K. Chernoff Harold K. Chernoff, Chief Plant Licensing Branch I-2 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket No. 50-423
Enclosure:
As stated cc: See next page
- By SE Dated OFFICE LPLI-2/PM LPLI-2/LA LPL1-2/PM CPNB/BC OGC (NLO) LPLI-2/BC NAME GEMiller CSola JHughey TChan JMoore HChernoff (per telecon w/
J.Hughey DATE 05/02/07 05/02/07 05/02/07 *05/01/07 05/02/07 5/3/07 Millstone Power Station, Unit No. 3 cc:
Lillian M. Cuoco, Esquire Mr. Joseph Roy Senior Counsel Director of Operations Dominion Resources Services, Inc. Massachusetts Municipal Wholesale Building 475, 5th Floor Electric Company Rope Ferry Road P.O. Box 426 Waterford, CT 06385 Ludlow, MA 01056 Edward L. Wilds, Jr., Ph.D. Mr. David W. Dodson Director, Division of Radiation Licensing Supervisor Department of Environmental Dominion Nuclear Connecticut, Inc.
Protection Building 475, 5th Floor 79 Elm Street Roper Ferry Road Hartford, CT 06106-5127 Waterford, CT 06385 Regional Administrator, Region I Mr. J. Alan Price U.S. Nuclear Regulatory Commission Site Vice President 475 Allendale Road Dominion Nuclear Connecticut, Inc.
King of Prussia, PA 19406 Building 475, 5th Floor Rope Ferry Road First Selectmen Waterford, CT 06385 Town of Waterford 15 Rope Ferry Road Mr. Chris L. Funderburk Waterford, CT 06385 Director, Nuclear Licensing and Operations Support Charles Brinkman, Director Innsbrook Technical Center Washington Operations Nuclear Services 5000 Dominion Boulevard Westinghouse Electric Company Glen Allen, VA 23060-6711 12300 Twinbrook Pkwy, Suite 330 Rockville, MD 20852 Senior Resident Inspector Millstone Power Station c/o U.S. Nuclear Regulatory Commission P. O. Box 513 Niantic, CT 06357 Mr. J. W. "Bill" Sheehan Co-Chair NEAC 19 Laurel Crest Drive Waterford, CT 06385 Ms. Nancy Burton 147 Cross Highway Redding Ridge, CT 00870 Mr. Evan W. Woollacott Co-Chair Nuclear Energy Advisory Council 128 Terrys Plain Road Simsbury, CT 06070
SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION ALTERNATIVE REQUEST IR-2-47, REVISION 1 MILLSTONE POWER STATION UNIT 3 DOMINION NUCLEAR CONNECTICUT, INC.
DOCKET NUMBER 50-423
1.0 INTRODUCTION
By letter dated October 17, 2006 (ML062910122 [Agency Documents Access and Management System accession number]), Dominion Nuclear Connecticut, Inc. (the licensee) requested NRC staff review and approval of Alternative Request IR-2-47 to allow the application of preemptive weld overlays (PWOL) over the pressurizer nozzle dissimilar and similar metal welds at Millstone Power Station Unit 3 (MPS3). The proposed approach is an alternative to the requirements of American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code),Section XI. By letter dated March 28, 2007 (ML070880565), the licensee responded to the NRC staffs request for additional information and revised Alternative Request IR-2-47. The updated version is Alternative Request IR-2-47, Revision 1. By letter dated March 30, 2007 (ML070960355), the licensee submitted Westinghouse report, Millstone Unit 3 Pressurizer, Relief, and Surge Nozzles Structural Weld Overlay Qualification, WCAP-16734-NP (non-proprietary version) and WCAP-16734-P (proprietary version). The reports provide the technical basis of the weld overlay design and associated analyses. By letter dated April 23, 2007 (ML071160239), the licensee provided its response to the NRC staffs request for additional information regarding certain requirements of Alternative Request IR-2-47, Revision 1.
A dissimilar metal weld is defined as a weld that joins two pieces of metals that are not of the same material. The proposed alternative would apply to the dissimilar metal weld that joins the ferritic pressurizer nozzle to the austenitic stainless steel safe end. The dissimilar metal weld material itself is made of nickel-based Alloy 82/182. A similar metal weld joins two pieces of metals that are of the same material. This alternative would also apply to the similar metal weld that joins the stainless steel safe end to the stainless steel piping. The similar metal weld material itself is made of stainless steel.
The industry has experienced degradation of the Alloy 82/182 weld material which is susceptible to primary water stress corrosion cracking (PWSCC) in the pressurized water reactor environment. For the proposed alternative, the weld overlay is a process by which a PWSCC-resistant weld metal is deposited on the outside surface of the susceptible material to form a new pressure boundary.
Enclosure
The licensee stated that without PWOLs, extensive machining would be necessary to perform the ASME Code required ultrasonic testing (UT) of the welds to which this request applies.
Consequently, PWOLs for mitigation of potential PWSCC susceptible areas are scheduled for the cycle 11 refueling outage in the Spring 2007. The licensee does not plan to perform UT examinations of the subject welds prior to weld overlay installation.
2.0 REGULATORY EVALUATION
Pursuant to Title 10 of the Code of Federal Regulations (10 CFR) Section 50.55a(g)(4), ASME Code Class 1, 2, and 3 components (including supports) must meet the requirements, except the design and access provisions and the preservice examination requirements, set forth in the ASME Code,Section XI, Rules for Inservice Inspection (ISI) of Nuclear Power Plant Components, to the extent practical within the limitations of design, geometry, and materials of construction of the components. The regulations require that inservice examination of components and system pressure tests conducted during the first 10-year interval and subsequent intervals comply with the requirements in the latest edition and addenda of Section XI of the ASME Code, incorporated by reference in 10 CFR 50.55a(b), 12 months prior to the start of the 120-month interval, subject to the limitations and modifications listed therein.
Pursuant to 10 CFR 50.55a(a)(3) alternatives to requirements may be authorized by the NRC if the licensee demonstrates that: (i) the proposed alternatives provide an acceptable level of quality and safety, or (ii) compliance with the specified requirements would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety.
The ASME Code of record for the current, second 10-year ISI interval at MPS3 is the 1989 Edition of the ASME Code,Section XI. The ASME Code,Section XI, 1998 Edition, no Addenda, IWA-4000 is used for the MPS3 Section XI repair/replacement program.
3.0 PROPOSED ALTERNATIVE REQUEST IR-2-47, REVISION 1 3.1 Component Affected The licensee states that ASME Code components associated with this request are the Class 1 dissimilar metal piping welds with Alloy 82/182 weld metal that operating experience has shown to be susceptible to PWSCC. The components are also scheduled for examination in accordance with the risk-informed inservice inspection (RI-ISI) and the guidelines of MRP-139, Primary System Piping Butt Weld Inspection and Evaluation Guideline, Electric Power Research Institute, 1010087, Proprietary. The five dissimilar metal welds (joining the pressurizer nozzle to safe end) that are scheduled to have full structural PWOLs applied are listed below. The five similar metal welds (joining the safe end to pipe) that are adjacent to the dissimilar metal welds will also be weld overlaid.
- 1. Weld No. 03-X-5551-X-T: 14-inch RCS Safe End-To-Surge Nozzle and adjacent Safe End-To-Pipe (Weld No. RCS-SL-FW-4),
- 2. Weld No. 03-X-5644-A-T: 6-inch Safety Nozzle-To-Safe End Weld at 81° Azimuth and adjacent Safe End-To-Pipe (Weld No. RCS-516-FW-1),
- 3. Weld No. 03-X-5648-B-T: 6-inch Safety Nozzle-To-Safe End Weld at 147° Azimuth and adjacent Safe End-To-Pipe (Weld No. RCS-516-FW-3),
- 4. Weld No. 03-X-5649-C-T: 6-inch Safety Nozzle-To-Safe End Weld at 212° Azimuth and adjacent Safe End-To-Pipe (Weld No. RCS-516-FW-5), and
- 5. Weld No. 03-X-5650-D-T: 6-inch Relief Nozzle-To-Safe End Weld at 278° Azimuth and adjacent Safe End-To-Pipe (Weld No. RCS-513-FW-1).
All subject welds are ASME Code Class 1 welds and located in the Reactor Coolant System (RCS) pressure boundary. The Code Examination Category is R-A, "Risk-Informed Piping Examinations," and Code Item number is R1.15, "Elements Subject to PWSCC."
All pressurizer nozzles are made of (P-3) Alloy steel, SA-508, Class 2. The nozzle-to-safe-end weld and butter material are made of Alloy 82/182 (F-43). Safe ends are made of (P-8) wrought stainless steel (SS), SA-182, Grade F316C. Surge, safety, and relief pipes are made of (P-8) wrought stainless steel.
3.2 Applicable Code Requirements The licensee stated that MPS3 is currently in the second 10-year ISI interval that began on April 23, 1999, and is scheduled to end on October 23, 2008. The ASME Code of record for the current 10-year ISI interval is the 1989 Edition of the ASME Code,Section XI.
The ASME Code,Section XI, 1998 Edition, no Addenda, IWA-4000, is used for the MPS3 repair/replacement program. By letter dated September 13, 2005, the NRC staff approved the use of the 1998 edition of the ASME Code,
Subject:
Millstone Power Station, Unit Nos. 2 and 3:
Request to Use 1998 Edition, with No Addenda, of ASME Section XI for Repair/Replacement Activities (TAC Nos. MC7347 and MC7348) [ADAMS accession number ML052210033].
The ASME Code requirements for which the relief is requested are contained in the following:
(1) 1998 Edition with no Addenda of the ASME Code,Section XI, IWA-4000, and (2) 1995 Edition with the I996 Addenda of the ASME Code,Section XI, Appendix VIII, Supplement 11.
In Alternative Request IR-2-47, Revision 1, the licensee proposes to permit the implementation of scheduled PWOLs at MPS3, as an alternative to the repair/replacement requirements of the ASME Code,Section XI, 1998 Edition with no Addenda, IWA-4000.
Pursuant to 10 CFR 50.55a(g)(6)(ii)(C), the licensee will perform ultrasonic examination of the completed PWOL in accordance with the ASME Code Section XI, 1995 Edition with the 1996 Addenda, Appendix VIII, Supplement 11. The licensee will perform these examinations in accordance with the Performance Demonstration Initiative (PDI) program to satisfy the requirements of Supplement 11 of Appendix VIII to the ASME Code,Section XI. By letter dated January 20, 2006, the NRC approved the use of the PDI program for MPS3 in the review of the licensees Relief Request IR-2-39 (ML053260012).
3.3 Proposed Alternatives and Basis The licensee proposed to use Alternative Request IR-2-47, Revision 1, pursuant to the provisions of 10 CFR 50.55a(a)(3)(i) as alternatives to the above applicable code requirements.
The alternative requirements are based on the industry's experience with weld overlay repairs for flaws suspected or confirmed to be from PWSCC. The licensees basis is discussed below.
3.3.1 The Structural Weld Overlay Assembly The licensee states that the full structural weld overlay satisfies all the structural design requirements of the pipe assuming no strength contributed by the pipe (e.g. as if the pipe were not there). The PWOL will completely cover the existing Alloy 82/182 weld metal and will extend onto the ferritic and austenitic stainless steel material on each end of the weld, including the adjacent safe end-to-pipe (similar metal) weld.
3.3.2 Weld Overlay Design The licensee will design PWOLs as full structural overlays (with an assumed worst-case flaw in the original weld) in accordance with the requirements of Alternative Request IR-2-47, Revision
- 1. The details surrounding the design analysis for the PWOLs are described in Westinghouse report, Millstone Unit 3 Pressurizer, Relief, and Surge Nozzles Structural Weld Overlay Qualification, WCAP-16734-NP which was submitted on March 30, 2007.
3.3.3 Examinations The licensee stated that it will perform all examinations in accordance with Alternative Request IR-2-47, Revision 1, excluding an UT examination after a completed PWOL. The UT examination after a completed PWOL will be performed in accordance with ASME Code Section XI, 1995 Edition with the 1996 Addenda, Appendix VIII, Supplement 11 with the alternatives that are used to comply with the PDI program.
The licensee states that the current configuration of the subject dissimilar metal welds does not permit UT in accordance with the ASME Code Section XI, Appendix VIII, Supplement 10, without extensive machining. Therefore, none of these trimetallic welds [i.e., alloy 82/182 weld metal, low alloy steel nozzle, and stainless steel safe end] will receive a pre-weld overlay UT examination. However, all of the trimetallic weld overlay repairs will receive post-weld overlay UT examinations. Therefore, if any of the trimetallic weld overlay repairs are found with PWSCC or other unacceptable flaws during the post-weld overlay UT examinations, no other dissimilar metal weld examinations will be performed. This is because the total population of similar trimetallic welds [i.e., alloy 82/182 weld metal, low alloy steel nozzle, and stainless steel safe end] that are associated with the pressurizer environment will be repaired and receive a post-weld overlay UT examination during this activity.
3.3.4 Required Activities The licensee stated that it will submit the preliminary results of the evaluations of the effects of changes in applied loads as a result of weld shrinkage prior to entry into Mode 4 from refueling outage (RFO) 11 and will submit the final evaluations within 60 days of the plant restart. If flaw growth in the weld overlay does occur and the acceptance standards of Table IWB-3514-2
cannot be met, or a determination cannot be made to prove that it is not PWSCC, the licensee will repair the flaw and will not use IWB-3600, IWC-3600, or IWD-3600 of the ASME Code,Section XI, to accept these types of flaws [i.e., PWSCC flaws].
The licensee will provide the examination results of the weld overlays, a discussion of any repairs to the overlay material and/or base material, and the reason for the repair within 14 days after the completion of the ultrasonic examination of weld overlay installations.
3.3.5 Contingency For Hot Cracking The licensee states that operating experience has shown the potential for hot cracking in the first layer of nickel alloy weld overlay deposits over stainless steel base materials. The major weld overlay vendors are currently conducting extensive investigations into this phenomenon.
Initial results have shown that higher levels of sulfur in the austenitic stainless steel base metal substrate can cause this hot cracking. The licensee and its vendor believe that the possibility of this hot cracking occurring in the MPS3 pressurizer nozzle PWOLs is remote based on the following: (1) Previous application of a weld overlay on the MPS3 pressurizer spray nozzle conducted in refueling outage 10 in 2005 showed no evidence of hot cracking. (2) Certified material test reports (CMTRs) for all of the stainless steel materials involved in this request have lower sulfur content than the spray nozzle that had a weld overlay applied in 2005. The spray nozzle had a sulfur content of 0.015 [percent]. The CMTRs also indicated lower sulfur content than the stainless steel material in which hot cracking has been found, either in production welds or in mock-up welds including those designed specifically to induce hot cracking. In light of the above, no special operations are recommended for prevention of hot cracking at MPS3. However, because the investigation of this phenomenon is still ongoing and the exact levels of sulfur and other elements or combinations of elements required to preclude hot cracking are unknown, the licensee has stated that it will implement the following contingency:
The licensee will install PWOL on the nozzle safe end with the highest sulfur content. An informational liquid penetrant (LP) examination will be performed on the relevant portion (the stainless steel safe end) of the first layer of the PWOL.
If indications of hot cracking are identified, as determined by the welding engineer, the suspect nickel alloy weld metal will be completely removed along with any indications in the underlying base metal of the stainless steel safe end.
Complete removal of the nickel alloy weld metal will be verified by acid etch and the resulting cavity shall be LP inspected. Then a weld build-up will be applied to the cavity using stainless steel weld material for the bulk of the build-up with the final tie-in to the existing nickel alloy weld being performed using nickel alloy weld metal. This weld build-up will serve two purposes: (1) restore the stainless steel base metal to its original contour, and (2) provide an improved base layer for application of the PWOL that will reduce the susceptibility to hot cracking.
The licensee states that this application of a weld build-up is not part of the PWOL and will be performed in accordance with the requirements of MPS3 Section XI Repair/Replacement program, and ASME Section Ill, which meets the requirements of the ASME Code Section XI, I998 edition with no addenda, IWA-4000.
3.3.6 Duration of the Proposed Request The licensee states that Alternative Request IR-2-47, Revision 1, will be applied for the duration of, up to and including, the last outage of the current, second 10-year ISI interval. Inservice examination requirements for PWOLs will be applied in accordance with the alternative requirements of Enclosure 1 to Alternative Request IR-2-47, Revision 1. The inservice examination requirements are also applicable to the weld overlay previously installed on the dissimilar metal weld (Weld No. 03-X-5641-E-T) joining the pressurizer spray nozzle to safe end.
4.0 NRC STAFF EVALUATION Alternative Request IR-2-47, Revision 1, consists of 3 parts: (1) The technical basis for the weld overlay as discussed above, (2) Enclosure 1, Alternative Requirements for Dissimilar Metal Weld Overlays, and (3) Mandatory Appendix I, Ambient Temperature Bead Welding. of Alternative Request IR-2-47, Revision 1, provides requirements for the weld overlay design and examination. The requirements of Enclosure 1 follow Code Case N-740-1 Dissimilar Metal Weld Overlay for Repair of Class 1, 2, and 3 ItemsSection XI, Division 1 of the ASME Code,Section XI. Mandatory Appendix I to Enclosure 1 follows Code Case N-638-2, which is an updated version of N-638-1.
The NRC staff has endorsed Code Cases N-504-2, Alternative Rules for Repair of Class 1, 2, and 3 Austenitic Stainless Steel Piping Section XI, Division 1, and N-638-1, Similar and Dissimilar Metal Welding Using Ambient Temperature Machine GTAW [gas tungsten arc welding] Temper Bead Technique Section XI, Division 1. Regulatory Guide 1.147, Revision 14, states that the ASME Code,Section XI, Appendix Q, Weld Overlay Repair of Class 1, 2, and 3 Austenitic Stainless Steel Piping Weldments, be used when Code Case N-504-2 is used.
The NRC staff has not endorsed Code Cases N-740, N-740-1, and N-638-2. Therefore, the NRC staff used the requirements of Code Cases N-504-2, N-638-1, and Appendix Q to evaluate Alternative Request IR-2-47, Revision 1.
The NRC staff evaluated the updated Alternative Request IR-2-47, Revision 1. However, the NRC staffs safety evaluation also includes a discussion of the original Alternative Request IR-2-47 as part of the review process.
4.1 General Requirements Code Case N-504-2 and/or Appendix Q of the ASME Code,Section XI, require certain specification and surface condition of the applicable base metal (carbon steel, stainless steel, and Alloy 82/182) and weld overlay filler metal (Alloy 52M), and the chromium content of the weld overlay deposits. Section 1.0 of Enclosure 1 of Alternative Request IR-2-47, Revision 1, provides the corresponding requirements.
In the March 28, 2007, letter, the licensee stated that the procedure qualification records (PQRs) for qualification of the temper bead portion of the welds and for overlay of the buttered P3 nozzle material were performed using ERNiCrFe-7 (Alloy 52) or ERNiCrFe-7A (Alloy 52M) filler metal. The licensee stated further that both have 28.0 to 31.5 percent chromium, the same as the 52M filler metal that will be used for the production overlays. As discussed in
section 3.3.5 above, the licensee will deposit a sacrificial layer on the base metal as a contingency plan to prevent hot cracking that could result from high sulfur content in austenitic stainless steel base material. This sacrificial layer will be approximately 1/32-inch to 1/16-inch in thickness and will not be used as part of the weld overlay design thickness. This sacrificial layer will cover the nozzle, the nozzle- to-safe-end weld, the safe end, the safe end to pipe weld, and the pipe.
4.2 Crack Growth Considerations and Design Code Case N-504-2 and/or Appendix Q of the ASME Code,Section XI, provide requirements for the overlay design and the crack growth calculation. Section 2 of Enclosure 1 of Alternative Request IR-2-47, Revision 1, provides the corresponding requirements.
The proposed weld overlay is designed to contain the assumed flaw in the underlying base material or original weld as follows: (a) 100 percent through-wall for the entire circumference, or (b) 100 percent through-wall for 1.5 inches or the combined width of the weld plus buttering, whichever is greater, in the axial direction for the entire circumference. In addition to the design flaws, the proposed alternative also requires crack growth calculations for as-found flaws and postulating flaws in the base metal to verify that flaws in the base metal will not grow to unacceptable size.
Paragraph 2(a) of Enclosure 1 of Alternative Request IR-2-47 states that flaw characterization and evaluation requirements shall be based on the as-found flaw. In Section 4.3 of Alternative Request IR-2-47, the licensee stated that it will not perform UT examination on the dissimilar and similar metal welds prior to weld overlay installation. This implies that the information regarding the as-found flaw(s) will not be available. The NRC staff observes that without conducting an UT examination of the dissimilar and similar metal welds prior to weld overlay installation, the condition of the original welds may not be known after the weld overlay installation. This is because UT is qualified to detect only outer 25 percent of the original weld (or base metal) after weld overlay installation. The condition of the remaining inner 75 percent of the original weld would not be known.
In the March 28, 2007, letter, the licensee revised Section 2(a) of Enclosure 1 of Alternative Request IR-2-47, Revision 1, to require flaw assumptions for the crack growth calculations for various prior-installation examination situations. Paragraph 2(a)(2)(b) requires that if no UT examination is performed prior to the mitigative weld overlay, a postulated 360-degree circumferential flaw 75 percent through the original wall thickness will be used for the crack growth calculations. Similarly, an axial flaw 75 percent through the original wall thickness set at 1.5 inches or the combined width of the weld, whichever is greater will also be used for the crack growth calculations. Any actual flaw that exceeds the depth of these assumptions would be detectable by the qualified post weld overlay UT examinations. The NRC staff finds that Paragraph 2(a)(2)(b) is acceptable because the assumed flaw size for the crack growth calculation is conservative.
During the review the NRC staff expressed concerns regarding Paragraphs 2(a)2(c) and 2(a)2(d) because they are not consistent with Code Case N-504-2 and Appendix Q of the ASME Code,Section XI. By letter dated April 23, 2007, the licensee stated that paragraphs 2(a)2(c) and 2(a)2(d) are used only if an examination is performed prior to the application of the PWOL. The licensee further stated that an examination is not being performed prior to the
application of the PWOL for the MPS3 weld repair configuration and that the requirements of paragraph 2(a)2(b) would be used instead. The NRC staff finds the licensee's response to paragraphs 2(a)2(c) and 2(a)2(d) to be acceptable.
Paragraphs g(2) and g(3) of Code Case N-504-2 require evaluations of residual stresses and flaw growth of the repaired weldments. Similar evaluations are required in Section 2 of Alternative Request IR-2-47. Paragraph 2(b)(7) of Alternative Request IR-2-47 states that the effects of any changes in applied loads, as a result of weld shrinkage from the entire overlay on other items in the piping system shall be evaluated. The licensee stated that the analysis of the effects of changes in applied loads as a result of weld shrinkage will address the requirements of Subarticles NB-3200 and NB-3600 of the ASME Code Section Ill. The report will also include the crack growth calculations to demonstrate that crack growth in the weld overlay or base metal is acceptable and residual stress distribution in the weld overlay and original weld demonstrate favorable stress distribution. The licensee will submit the preliminary results of the evaluations prior to entry into Mode 4 from refueling outage 11 and will submit the final evaluations within 60 days of the plant restart. The NRC staff finds that the licensee will provide timely stress analyses of the nozzles as a result of PWOL consistent with paragraphs g(2) and g(3) of Code Case N-504-2 , which is acceptable.
4.3 Examination and Inspection Code Case N-504-2 and/or Appendix Q of the ASME Code,Section XI, require certain acceptance, preservice, and inservice examinations of the installed weld overlay. Section 3 of of Alternative Request IR-2-47, Revision 1, provides corresponding requirements.
Acceptance Examination Section 3(a) of Enclosure 1 of Alternative Request IR-2-47, Revision 1, requires surface examinations of installed PWOL and the acceptance criteria of NB-5300 of the ASME Code,Section III. Section 3(a) also requires ultrasonic examinations of the installed weld overlay to assure adequate fusion and to detect fabrication defects. The required examination surface and volume are defined in Figure 1 of the alternative request. Any planar indication found in the weld overlay that is rejected by IWB-3514-2 will be removed. The proposed acceptance criteria are consistent with Code Case N-504-2 and Appendix Q of the ASME Code,Section XI, except Paragraph 3(a)(3).
Paragraph 3(a)(3) of Enclosure 1 of Alternative Request IR-2-47, Revision 1, requires that for planar indications outside this examination volume, the nominal wall thickness shall be t2" as shown in Fig 1(c) for volumes A-E-H-D and F-B-C-G . . .. Figure 1(c) is shown in Enclosure 1 of Alternative Request IR-2-47, Revision 1. Volumes A-E-H-D and F-B-C-G are defined in Figure 1(c) and refer to the portion of the weld overlay that is 0.5 inch away from the original weld. The NRC staff notes that UT is not qualified to examine the inner 75 percent of the base metal after weld overlay installation. Therefore, the t2" dimension, which is the weld overlay thickness plus pipe thickness, should not be a parameter for the acceptance criteria of IWB-3514-2 because the t2" dimension includes the 75 percent depth of the base metal. If the t2" dimension were used, larger flaws would be allowed to remain in service in the weld overlay.
By letter dated April 23, 2007, the licensee responded to a NRC staff request to justify the proposed "t2" dimension. The licensee stated that it is recognized that PWSCC is not a concern in the pressurizer nozzle or safe end that is included for PWOL in Alternative Request
IR-2-47, Revision 1. The licensee stated further that there is no presumptive need to assume flaws in these base metals. Therefore, the full t2" dimension can be used for the volumes identified in Figure 1(c) to accept flaws in PWOL in accordance with Table IWB-3514-2. The NRC staff agrees with the licensee that the pressurizer nozzles that are made of low alloy steel and safe ends that are made of stainless steel have not had a history of PWSCC. However, the NRC staff does not agree with the above argument if the base metal is made of material that is susceptible to PWSCC such as Alloy 600 material. The NRC staff notes that the purpose of the weld overlay is to repair the dissimilar metal weld. Volumes A-E-H-D and F-B-C-G as defined in Figure 1(c), pertain to the portion of the weld overlay that is not required to support the pressure boundary function of the weld overlay or base metal. If larger flaws are allowed to remain in service in this portion of the weld overlay, the structural integrity of the portion of the weld overlay that covers the original weld will not be adversely affected.
Therefore, the NRC staff finds that the use of the t2" dimension is acceptable since the base metal of the pressurizer nozzle and safe ends at MPS3 are not susceptible to PWSCC.
Preservice Examination Section 3(b) of Enclosure 1 of Alternative Request IR-2-47, Revision 1, requires a preservice examination of the installed weld overlay and the upper (outer) 25 percent of the original pipe wall thickness by UT. The required examination volume is defined in Figure 2 of Enclosure 1 of Alternative Request IR-2-47, Revision 1.
As stated in Paragraph 3(b)(2) of Enclosure 1 of Alternative Request IR-2-47, Revision 1, if a flaw is detected in the outer 25 percent of the base metal (or original weld) during the preservice examination, the as-found flaw size would be used for the crack growth evaluation.
The NRC staff does not believe this is a conservative assumption for the crack growth calculation if the original weld was not examined prior to installing PWOL. The current ultrasonic examination is qualified only to detect flaws in the outer 25 percent of the pipe base metal after a weld overlay is applied. With the limited UT qualification, the condition of the inner 75 percent of the pipe base metal would not be known. A conservative assumption would be to assume existence of a crack of 75 percent through-wall depth in the inner 75 percent pipe base metal. The 75 percent depth flaw should be added to the depth of the crack found in the outer 25 percent of the pipe base metal. This worst-case crack (the sum of two flaws) should be used to calculate crack growth. .
In the March 28, 2007, letter, the licensee responded to an NRC staff request to justify use of the as-found flaw size for the crack growth calculation that paragraph 3(b)(2) has been expanded and is included as 3(b)(3) in Enclosure 1 of Alternative Request IR-2-47, Revision 1.
The licensee stated that if flaws are found in the outer 25 percent of the existing base metal or original weld and cannot be determined to be isolated from the inside diameter of the existing base material or weld, the flaw depth will be conservatively sized by adding the thickness of the remaining 75 percent of the original existing base metal or weld thickness to the through-wall dimension for any flaw growth calculations performed.
The licensee stated that it will only use the actual UT-determined through-wall dimension in the crack growth analysis for those flaws that do not encroach on the interface between the outer 25 percent of the original base metal or weld thickness and the inner 75 percent of the same existing materials and that can be determined by the qualified UT examination to not be
connected to the interface between the outer 25 percent and the inner 75 percent of the base metal or weld.
The NRC staff finds that the revised paragraph 3(b)(3) adequately addresses the flaw size to be used in the crack growth calculation. Therefore, the proposed preservice examination requirements in paragraph 3(b) of Enclosure 1 of Alternative Request IR-2-47, Revision 1, are acceptable.
Inservice Examination Section 3(c) of Enclosure 1 of Alternative Request IR-2-47, Revision 1, provides requirements for ultrasonic inservice examinations with examination volume defined in Figure 2 of the alternative request.
Paragraph 3(c)(3) of Alternative Request IR-2-47 allows flaws in PWOLs to be accepted by IWX-3640 of the ASME Code,Section XI. The NRC staff would accept the acceptance criteria of IWB-3600 for the flaw in the weld overlay if the flaw growth is caused by fatigue which should likely be insignificant. However, flaw growth by PWSCC could be significant and the NRC staff would find such a growth mechanism in PWOL unacceptable. In the March 28, 2007, letter, the licensee revised Alternative Request IR-2-47 to include the following requirement in Section 4.3.1 of Alternative Request IR-2-47, Revision 1:
If flaw growth in the weld overlay does occur, and the acceptance Standards of IWB-3514-2 cannot be met, or a determination cannot be made to prove that it is not PWSCC, the licensee will repair the flaw and will not use IWB-3600, IWC-3600, or IWD-3600 to accept these types of flaws.
The NRC staff finds the above requirement acceptable because the licensee will appropriately evaluate or repair identified flaws.
4.4 Mandatory Appendix 1 - Ambient Temperature Temper Bead Welding Code Case N-638-1 provides requirements for ambient temperature temper bead welding.
Mandatory Appendix 1 to Enclosure 1 of Alternative Request IR-2-47, Revision 1, is based on Code Case N-638-2. The major difference between the two documents is discussed below.
Paragraph 1(b) of Mandatory Appendix 1 to Enclosure 1 of Alternative Request IR-2-47, Revision 1, states that the maximum area of the weld overlay based on the finished surface over the ferritic base material shall be 500 square inches. Code Case N-638-1 allows only 100 square inches over the ferritic base material. The NRC staff asked the licensee to justify the 500-square-inch surface area. In the March 28, 2007, letter, the licensee responded that the ASME has indicated that the inside diameter compressive stress levels remain essentially the same between the 100-square-inch weld area and 500-square-inch weld area in relation to weld overlay applications. The March 28, 2007, letter also included by reference justifications regarding the bases for the 500 square inch weld overly area previously presented to the NRC.
The licensee also stated that additional justification is provided in EPRl Report 1014351, "Repair and Replacement Applications Center: Topical Report Supporting Expedited NRC Review of Code Cases for Dissimilar Metal Weld Overlay Repairs, December 2006.
Based on a review of the information provided, the NRC staff finds that the 500-square-inch weld area limit over the ferritic base metal is acceptable because the licensee has provided results of finite element analysis demonstrating that the stresses of a nozzle with the 500-square-inch weld area will not adversely affect the integrity of the pressurizer nozzle.
The NRC staff finds that the requirements of Alternative Request IR-2-47, Revision 1, with the associated Enclosure 1 and Mandatory Appendix 1 are consistent with the intent of the provisions of Code Cases N-504-2 and N-638-1 and Appendix Q of the ASME Code,Section XI. Therefore, the proposed alternative is acceptable.
5.0 CONCLUSION
The NRC staff has reviewed the licensees submittal and determined that Alternative Request IR-2-47, Revision 1, will provide an acceptable level of quality and safety. Therefore, pursuant to 10 CFR 50.55a(a)(3)(i), the NRC staff authorizes the use of the Alternative Request IR-2-47, Revision 1, for the PWOL of the dissimilar and similar metal welds of the pressurizer surge line safety valve, and relief valve nozzles at MPS3. The effective period of Alternative Request IR-2-47, Revision 1, is the second 10-year inservice inspection interval which ends on October 23, 2008.
Principal Contributor: John Tsao Date: