ML081360646
| ML081360646 | |
| Person / Time | |
|---|---|
| Site: | Prairie Island  |
| Issue date: | 06/15/2008 |
| From: | Lois James NRC/NRR/ADRO/DORL/LPLIII-1 |
| To: | Wadley M Nuclear Management Co |
| Poole Justin 301-415-2048 | |
| References | |
| TAC MD5868 | |
| Download: ML081360646 (15) | |
Text
June 15, 2008 Mr. Michael D. Wadley Site Vice President Prairie Island Nuclear Generating Plant Nuclear Management Company, LLC 1717 Wakonade Drive East Welch, MN 55089
SUBJECT:
PRAIRIE ISLAND NUCLEAR GENERATING PLANT, UNIT 2 - ALTERNATIVE TO ASME CODE, SECTION XI, STRUCTURAL WELD OVERLAY OF PRESSURIZER SURGE NOZZLE WELD, ALTERNATIVE REQUEST NO. 2-RR-4-8, REVISION 1 (TAC NO. MD5868)
Dear Mr. Wadley:
By letter dated June 25, 2007 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML071760332), as supplemented by letters dated January 15, 2008 (ADAMS Accession No. ML081510906) and May 7, 2008 (ADAMS Accession No. ML081280890), Nuclear Management Company, LLC (the licensee) requested Nuclear Regulatory (NRC) approval, pursuant to Title 10 of the Code of Federal Regulations (10 CFR),
Part 50, Section 50.55a(a)(3)(i), for an alternative to the requirements of American Society of Mechanical Engineers Boiler and Pressure Vessel Code,Section XI. Alternative Request 2-RR-4-8, Revision 1, as proposed, would allow the installation of a full structural weld overlay (FSWOL) on the pressurizer surge line nozzle-to-safe end dissimilar metal and safe end-to-reducer stainless steel butt welds at Prairie Island Nuclear Generating Plant (PINGP)
Unit 2.
The NRC staff has reviewed the licensee=s submittal and determined that Alternative Request 2-RR-4-8, 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 2-RR-4-8, Revision 1, for the installation of a FSWOL on the pressurizer surge line nozzle-to-safe end dissimilar metal and safe end-to-reducer stainless steel butt welds at PINGP Unit 2. The effective period of the proposed alternative is the fourth inservice inspection interval which ends on December 20, 2014.
Sincerely,
/RA/
Lois James, Chief Plant Licensing Branch III-1 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket No. 50-306
Enclosure:
Safety Evaluation cc w/encl: See next page
ML081510906) and May 7, 2008 (ADAMS Accession No. ML081280890), Nuclear Management Company, LLC (the licensee) requested Nuclear Regulatory (NRC) approval, pursuant to Title 10 of the Code of Federal Regulations (10 CFR),
Part 50, Section 50.55a(a)(3)(i), for an alternative to the requirements of American Society of Mechanical Engineers Boiler and Pressure Vessel Code,Section XI. Alternative Request 2-RR-4-8, Revision 1, as proposed, would allow the installation of a full structural weld overlay (FSWOL) on the pressurizer surge line nozzle-to-safe end dissimilar metal and safe end-to-reducer stainless steel butt welds at Prairie Island Nuclear Generating Plant (PINGP)
Unit 2.
The NRC staff has reviewed the licensee=s submittal and determined that Alternative Request 2-RR-4-8, 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 2-RR-4-8, Revision 1, for the installation of a FSWOL on the pressurizer surge line nozzle-to-safe end dissimilar metal and safe end-to-reducer stainless steel butt welds at PINGP Unit 2. The effective period of the proposed alternative is the fourth inservice inspection interval which ends on December 20, 2014.
Sincerely,
/RA/
Lois James, Chief Plant Licensing Branch III-1 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket No. 50-306
Enclosure:
Safety Evaluation cc w/encl: See next page DISTRIBUTION PUBLIC LPL3-1 r/f RidsNrrDorlLpl3-1 RidsNrrPMMChawla RidsNrrLATHarris RidsOGCRp RidsNrrDciCpnb RidsNrrDorlDpr RidsRgn3MailCenter RidsNrrAdes JPoole, NRR CLipa, RGN-III RidsAcrsAcnw&mMailCenter RidsNrrOd JAdams, EDO Region III ADAMS ACCESSION NO.: ML081360646 *Per Memo dated May 19, 2008 ** with changes OFFICE NRR/LPL3-1/PM NRR/LPL3-1/PM NRR/LPL3-1/LA DCI/CPNB OGC NRR/LPL3-1/BC NAME JPoole MChawla THarris TChan*
JBiggins**
LJames DATE 04/ 22/08 06 / 13 /08 05/23/08 5/19/08 06/10 /08 06 / 15 /08
SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION ALTERNATIVE REQUEST 2-RR-4-8 TO ASME CODE, SECTION XI NUCLEAR MANAGEMENT COMPANY, LLC PRAIRIE ISLAND NUCLEAR GENERATING PLANT (PINGP), UNIT 2 DOCKET NO. 50-306
1.0 INTRODUCTION
By letter dated June 25, 2007 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML071760332), as supplemented by letters dated January 15, 2008 (ADAMS Accession No. ML081510906) and May 7, 2008 (ADAMS Accession No. ML081280890), Nuclear Management Company, LLC (NMC, the licensee) requested Nuclear Regulatory (NRC) approval, pursuant to Title 10 of the Code of Federal Regulations (10 CFR),
Part 50, Section 50.55a(a)(3)(i), for an alternative to the requirements of American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code,Section XI, 1998 Edition through the 2000 Addenda. Alternative Request 2-RR-4-8, Revision 1, as proposed, would allow the installation of a full structural weld overlay (FSWOL) on the pressurizer surge line nozzle-to-safe end dissimilar metal and safe end-to-reducer stainless steel butt welds at Prairie Island Nuclear Generating Plant (PINGP) Unit 2.
In the "L-PI-08-003, Prairie Lsland Nuclear Generating Plant, Unit 2 - Response to RAI Regarding 10 CFR 50.55a, Request for Relief from ASME Section XI Repair and Replacement Requirements: Proposed Alternatives for Application of Structural [[Topic" contains a listed "[" character as part of the property label and has therefore been classified as invalid. to t|January 15, 2008, letter]], the licensee responded to the NRC staffs request for additional information and revised the proposed alternative. The NRC staff evaluated the issues in the proposed alternative in the June 25, 2007, and "L-PI-08-003, Prairie Lsland Nuclear Generating Plant, Unit 2 - Response to RAI Regarding 10 CFR 50.55a, Request for Relief from ASME Section XI Repair and Replacement Requirements: Proposed Alternatives for Application of Structural [[Topic" contains a listed "[" character as part of the property label and has therefore been classified as invalid. to t|January 15, 2008, letter]]s. However, the proposed alternative identified in Enclosure 2 to the "L-PI-08-003, Prairie Lsland Nuclear Generating Plant, Unit 2 - Response to RAI Regarding 10 CFR 50.55a, Request for Relief from ASME Section XI Repair and Replacement Requirements: Proposed Alternatives for Application of Structural [[Topic" contains a listed "[" character as part of the property label and has therefore been classified as invalid. to t|January 15, 2008, letter]], will be the version upon which this safety evaluation is based.
A dissimilar metal butt weld (DMBW) is defined as a weld that joins two pieces of metal that are not of the same material. In the proposed alternative, the dissimilar metal weld joins the ferritic pressurizer nozzle to the austenitic stainless steel safe end. The dissimilar metal weld itself is made of nickel-based Alloy 82/182 material. A similar metal weld joins two pieces of metal that are of the same material. In this alternative, the similar metal weld joins the stainless steel safe end to the stainless steel reducer. The similar metal weld itself is made of stainless steel weld material.
The alternative proposed in Alternative Request 2-RR-4-8, Revision 1, is based on modifications to ASME Code Case N-504-2, Alternative Rules for Repair of Class 1, 2, and 3 Austenitic Stainless Steel Piping,Section XI, Division 1; Code Case N-638-1, Similar and Dissimilar Metal Welding Using Ambient Temperature Machine GTAW [Gas Tungsten-Arc Welding] Temper Bead Technique; and ASME Code,Section XI, Appendix VIII, Supplement 11, Qualification Requirements for Full Structural Overlaid Wrought Austenitic Piping Welds, to the 1998 Edition through 2000 Addenda of ASME Code,Section XI.
2.0 REGULATORY EVALUATION
Pursuant to 10 CFR 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, ARules 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, fourth 10-year ISI interval at PINGP Unit 2 is the ASME Code,Section XI 1998 Edition through the 2000 Addenda.
3.0 PROPOSED ALTERNATIVE REQUEST 2-RR-4-8 3.1 Components Affected The licensee states that the ASME Code components affected are the nozzle-to-safe end Alloy 82/182 DMBW and safe end-to-reducer stainless steel (SS) butt weld on surge line connection to the pressurizer.
The subject welds are ASME Code Class 1 welds 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 [Primary Water Stress Corrosion Cracking] for the surge nozzle-to-safe end weld (weld ID number W-17) and R1.11 Elements Subject to Thermal Fatigue for the safe end-to-reducer weld (weld ID number W-16).
The pressurizer nozzle is SA-206, Grade WCC carbon steel (P1) material, with a 309/308L stainless steel buttering. The nozzle-to-safe-end weld is made of Alloy 82/182 (F-43) material.
The safe end is SA-182, F316L SS (P-8) material. The reducer is A-403, Grade WP, Type 316 SS (P-8) material. The weld joining the safe end and the reducer is 308 SS.
3.2 Applicable Code Requirements The licensee states that PINGP Unit 2 is currently in the fourth 10-year ISI interval that began on December 21, 2004, and is scheduled to end on December 20, 2014. The ASME Code of record for the current 10-year ISI interval is the 1998 Edition through the 2000 Addenda of the ASME Code,Section XI.
The requirements for which the relief is requested are contained in the following: (1) ASME Code 1998 Edition through the 2000 Addenda of the ASME Code,Section XI, IWA-4420 and IWA-4520, which require repair/replacement activities to be performed in accordance with the Owners Requirements and the original Construction Code for the affected component or system, (2) risk informed inservice inspection (RI-ISI) program for PINGP Unit 2 as implemented under request 2-RR-4-5, which was approved by the NRC on January 5, 2006, and (3) ASME Code,Section XI, Appendix VIII, Supplement 11, which specifies the performance demonstration requirements for ultrasonic examination including requirements for ultrasonic testing (UT) procedures, equipment, and personnel for UT of completed weld overlays.
3.3 Proposed Alternatives and Basis The licensee proposes Alternative Request 2-RR-4-8, Revision 1, for the installation of FSWOL at PINGP Unit 2 on the pressurizer surge nozzle-to-safe end Alloy 82/182 DMBW and safe end-to-reducer SS butt weld, as an alternative to the requirements contained in the above mentioned ASME Code references and NRC-approved relief request. The weld overlay is to be installed and examined in accordance with Code Cases N-504-2, N-638-1 and ASME Section XI, Nonmandatory Appendix Q, Weld Overlay Repair of Class 1, 2 and 3 Austenitic Stainless Steel Piping Weldments, as modified in Alternative Request 2-RR-4-8, Revision 1, under the provisions of 10 CFR 50.55a(a)(3)(i). Installation to the above requirements is in lieu of requirements in the Construction Code, and examination is in lieu of requirements in the RI-ISI program.
The licensee will perform ultrasonic examination of the completed FSWOL in accordance with the Performance Demonstration Initiative (PDI) program implementation of ASME Code Section XI, 1998 Edition through the 2000 Addenda, Appendix VIII, Supplement 11. ASME Code,Section XI, Appendix Vlll, Supplement 11 cannot be used without modification for nondestructive examination (NDE) qualifications of a FSWOL.
3.4 Duration of the proposed Request The licensee states that the FSWOL is currently planned for installation during the PlNGP Unit 2 fall 2008 refueling outage as a preventive measure against flaw development in the DMBW.
The proposed alternative is requested for the design life of the FSWOL, as determined by the required evaluation in Paragraph (g) of Code Case N-504-2 and the corresponding requirements in Appendix Q. The licensee proposed that the ISI requirements beyond the current interval will be evaluated and established by the PlNGP Unit 2 RI-ISI Program.
4.0 NRC STAFF EVALUATION The licensees proposed Alternative Request 2-RR-4-8, Revision 1, is based on Code Cases N-504-2, and N-638-1. The NRC staff has endorsed Code Cases N-504-3, and N-638-1 in Regulatory Guide (RG) 1.147, Revision 15. RG 1.147 requires that the ASME Code,Section XI, Appendix Q, be used when Code Case N-504-3 is used. The NRC staff evaluated the acceptability of the licensees proposed alternative based on the requirements of Code Cases N-504-3 and N-638-1. The NRC staff endorsed the use of Code Case N-504-3 in RG 1.147, Revision 15, Inservice Inspection Code Case Acceptability, ASME Section XI, Division 1, during the same time period it was reviewing the licensees submittal. Therefore, some of the
issues that the NRC staff raised during the review of the licensees submittal were based on Code Case N-504-3 and the discussion below will contain references to Code Case N-504-2 as well.
of Alternative Request 2-RR-4-8, Revision 1, provides requirements for the weld overlay design and examination. The requirements of Enclosure 2 follow Code Case N-504-3 and Code Case N-638-1. Tables 2 and 3 of Alternative Request 2-RR-4-8, Revision 1, contain proposed modifications to the requirements of these code cases. When implementing Code Case N-638-1, RG 1.147, Revision 15, requires that UT examinations be demonstrated for the repaired volume using representative samples which contain construction type flaws and that the acceptance criteria of NB-5330 of Section III edition and addenda approved in 10 CFR 50.55a apply to all flaws identified within the repaired volume. ASME Code,Section XI, Appendix Q requires the use of procedures and personnel qualified in accordance with Appendix VIII. Table 4 of Alternative Request 2-RR-4-8, Revision 1, contains proposed modifications to the requirements of ASME Code,Section XI, Appendix VIII.
4.1 General Requirements The licensee stated that the overlay is a pre-emptive measure addressing PWSCC concerns identified by the industry for welds associated with Alloy 600/82/182 components exposed to pressurized-water reactor primary coolant. NMC has concluded that the application of a FSWOL to the PlNGP Unit 2 pressurizer surge nozzle DMBW is the most appropriate course of action in order to ensure RCS pressure boundary integrity. The FSWOL of the nozzle-to-safe end DMBW would interfere with the future examination of the SS safe end-to-reducer weld due to the close proximity of the two welds. Therefore, the FSWOL will extend from the ferritic carbon steel nozzle across both butt welds to the SS pipe reducer to allow examination of both welds.
The licensee has proposed to design the FSWOL in accordance with the design and repair methodologies of Code Case N-504-2 and provisions of Nonmandatory Appendix Q will be followed with modifications. These modifications and their bases are summarized in Table 2 of Alternative Request 2-RR-4-8, Revision 1. All other applicable requirements not listed in Table 2 will be met as described in Code Case N-504-2 and Nonmandatory Appendix Q.
Weld overlay examination, preservice of the completed repair, and ISI will be performed in accordance with Code Case N-504-2, ASME Section XI, Appendix Q, Subarticles Q-4100, Q-4200 and Q-4300, and ASME Section XI, Appendix VIII, Supplement 11 with the modifications noted in Section 5.3 and Table 4 of Alternative Request 2-RR-4-8, Revision 1. These examinations will meet the applicable code and code case requirements as modified by the request.
Any FSWOL preservice ultrasonic indications characterized as weld flaws in the FSWOL weld metal that exceed the acceptance standards of IWB-3514-2, will be removed or reduced to an acceptable size. Indications located in the outer 25 percent of the base metal that are characterized as cracks will meet the design analysis requirements as specified in Appendix Q, Q-3000 of ASME,Section XI.
4.1.1 Contingency for Hot Cracking The licensee states that during recent DMBW overlay activities, where use of ERNiCrFe-7A (Alloy 52M) and ERNiCrFe-7 (Alloy 52) has been used for the filler metal, flaws in the first layer have occurred in the portion of the overlay deposited on the austenitic stainless steel portions (safe ends, pipe etc.) of the assemblies in some cases. The characteristics of the flaws described above are indicative of hot cracking. This phenomenon has not been observed on the ferritic steel or ENiCrFe-3 (Alloy 182) DMBWs.
The licensee states that further studies have determined that this problem may occur when using Alloy 52M filler metal on austenitic stainless steel materials with high sulfur content.
The licensee further states that limited tests and evaluations recently performed by AREVA concluded that welding with Alloy 52M on stainless steel base material with 0.020 weight percent sulfur results in cracking while welding on stainless steel base materials with less than 0.010 weight percent have resulted in no cracking.
To reduce the susceptibility of hot cracking occurrence due to welding Alloy 52M on the stainless steel base materials with high sulfur, NMC will apply a barrier layer prior to FSWOL on all stainless steel components associated with the PlNGP Unit 2 pressurizer surge nozzle with sulfur content greater than 0.010 weight percent, and may opt to apply a barrier layer on all stainless steel components. The barrier layer will use ER309L on the stainless steel and Alloy 82 on the stainless steel near the dissimilar metal weld to stainless steel fusion zone only.
Paragraph (e) of Code Case N-504-3 requires specific delta ferrite content in the weld layer when austenitic stainless steel weld metal is used. Paragraph (e) of N-504-3 requires weld reinforcement layers to have a deposited delta ferrite content of at least 7.5 ferrite number (FN, also expressed in percentages). This delta ferrite requirement is applicable to stainless steel weld overlays that are subject to intergranular stress corrosion cracking (IGSCC).
The NRC staff notes that IGSCC is a credible degradation mechanism for austenitic stainless steels in oxidizing boiling-water reactor environments. However, there is no industry experience to suggest that IGSCC is a concern for pressurized-water reactors as the case for PINGP. In general, adverse environmental conditions in pressurized-water reactors have been reduced and are not conducive for IGSCC.
Further, the NRC staff notes that the applied austenitic stainless steel buffer layer will not be included in the structural weld overlay thickness. The delta ferrite requirements of Code Case N-504-3, paragraph (e) only apply to the structural layers of a weld overlay. They do not apply to nonstructural weld layers. Conversely, paragraphs (c) and (d) of the code case do apply to nonstructural weld layers but do not include any delta ferrite requirements. Therefore, if nonstructural layers for a weld overlay subject to IGSCC do not require delta ferrite testing, then a nonstructural stainless steel buffer layer (for an Alloy 52M weld overlay) that is not subject to IGSCC does not require delta ferrite testing.
The NRC staff finds that the minimum delta ferrite requirement of Code Case N-504-3, paragraph (e), is not applicable to the applied austenitic stainless steel buffer layer. The NRC staff also finds that the licensee will deposit the buffer layer on the pipe in accordance with
ASME Code,Section IX qualified welding procedure specification(s) and perform liquid penetrant examination of the barrier layer surface and volumetric examination of the barrier layer will be included in the final ultrasonic examination of the overlay.
4.2 Crack Growth Considerations and Design The licensee has proposed to apply the weld overlay in accordance with the requirements of ASME Section XI, Code Case N-504-2, as modified in Alternative Request 2-RR-4-8, Revision 1.
Table 2 of the request identifies the proposed modifications, which are 1) use of a nickel alloy weld material to mitigate the potential of PWSCC, 2) elimination of the requirement in Section XI, Appendix Q to satisfy Construction Code and Owners Requirements when applying the overlay, and 3) performance of a system leakage test in accordance with IWA-5000 in lieu of a hydrostatic test.
For the design of the FSWOL, the licensee conservatively assumes that a 360 degree circumferential through-wall flaw is present in the original Alloy 82/182 weld and in the original SS weld. Thus, the FSWOL will extend around the full circumference of the nozzle-to-safe end weld location and the safe end-to-reducer weld location as required by ASME Code Case N-504-3. The weld reinforcement material is to be Alloy 52M, or equivalent, and applied as a FSWOL to the existing ferritic carbon steel nozzle, austenitic SS safe end, SS pipe reducer, and the Alloy 82/182 and austenitic SS weld material. Repairs, if required, may be based on Alloy
Use of the Alloy 52/52M material for the FSWOL is acceptable as the licensee has stated that the FSWOL will be sized to meet all structural requirements without considering the structural component of the existing Alloy 82/182 and SS weldments. The licensee further states that Alloys 52M and 52 contain about 30 percent Cr (significantly higher than Alloy 82/182), imparting excellent PWSCC resistance, and that the austenitic nickel alloy weld overlay shall consist of at least two weld layers deposited from a filler material with a Cr content of at least 28 percent.
The first layer of weld metal deposited may not be credited toward the required weld overlay structural design thickness, unless the as-deposited diluted first layer applied over the austenitic base material, austenitic weld metal and ferritic carbon steel base material contains at least 24 percent Cr. If the first diluted layer is credited, the licensee will ensure the 24 percent Cr content by either chemical analysis of the production weld or a representative coupon taken from a mockup prepared in accordance with the production welding procedure specification. If taken from a mockup, the welding parameters demonstrated on the mockup to achieve 24 percent Cr on the first layer will be used for the production weld first layer.
The NRC staff finds that (1) the use of the Alloy 52M and Alloy 52 for repairs acceptable, as it contains adequate chromium content (28 percent -31.5 percent as specified in ASME Section II) to achieve a high degree of resistance to PWSCC, (2) the weld overlay will be designed as a FSWOL consistent with the requirements of ASME Code Case N-504-2 and Section XI Nonmandatory Appendix Q, (3) the FSWOL thickness and length will be designed according to the guidance provided in Code Case N-504-3, and (4) the FSWOL design assumes there is no contribution to structural integrity from the original section of pipe, and all other applicable requirements not addressed in Table 2 of Alternate Request 2-RR-4-8, Revision 1, will be met as described in Code Case N-504-3 and Appendix Q.
ASME Section XI, Appendix Q first appeared in the 2005 Addenda of the ASME Code. In Article Q-3000 subpart (b), the design of the overlay was specified to satisfy the requirements of the Construction Code and Owners Requirements in accordance with IWA-4221. The intent of the
overlay installation is to reduce the size of a flaw to an acceptable size by the application of additional structural material and the weld overlay, while leaving the original flaw in service.
Also, the weld overlay will create favorable compressive stresses in the inner diameter region of the pipe to limit the crack growth. The Construction Code and Owners Requirements require the removal of the flaw.
4.3 Proposed Ambient Temperature Temper Bead Welding Application of the FSWOL requires welding on the ferritic steel nozzle material with Alloy 52M.
Temperbead welding will be used for this purpose using Code Case N-638-1. Code Case N-638-1 describes the process for welding similar and dissimilar metals using ambient temperature machine GTAW temperbead method. GTAW will be performed in accordance with Code Case N-638-1, with some modifications. As noted previously, Code Case N-638-1 was conditionally accepted for use in NRC RG 1.147, Revision 15, and was developed for welding similar and dissimilar metals using the ambient temperature machine GTAW temperbead technique. In accordance with Code Case N-638-1, the area around the weld must be at least 50ºF.
The methodology of Code Case N-638-1 will be followed for welding on ferritic material where the Construction Code requires post-weld heat treatment, with modifications. The proposed modifications to Code Case N-638-1 specifically address the draining requirement, weld area limit of 100 square inches, ultrasonic examination thickness, 48-hour post-weld hold criteria prior to NDE, and the use of nonattached temperature measuring devices for temperbead welding.
These specific changes and the basis justifying each proposed modification to the methodologies specified in Code Case N-638-1 are addressed in Table 3 of Alternative Request 2-RR-4-8, Revision 1. All other applicable requirements not addressed in Table 3 of Alternate Request 2-RR-4-8, Revision 1, will be met as described in Code Case N-638-1.
Code Case N-638-1 was written for situations when it is impractical to drain the component or impractical for radiological reasons. The licensee has stated that as part of plant refueling, the surge nozzle and associated piping will be drained below the overlay location for brief periods during FSWOL installation.
The area of the FSWOL covering the ferritic base material for the PlNGP Unit 2 surge nozzle, is approximately 140 square inches which will exceed the 100 square inches limit of surface covered by the FSWOL imposed by Code Case N-638-1. The licensee proposes that the maximum ferritic carbon steel base material covered by the FSWOL will be limited to 300 square inches. As basis for exceeding the 100 square inches, the licensee cites Electric Power Research Institute (EPRl) Technical Report 1003616, Additional Evaluations to Expand Repair Limits for Pressure Vessels and Nozzles, which provided justification for weld overlay areas up to 500 square inches. In addition, the licensee states that finite element analyses of the ambient temperature temper bead repair process show that residual stresses are not detrimentally affected by increasing the allowable repair area, and in fact, the analyses show that in some cases, increasing the allowable repair area improves the residual stress distribution in the weldment. The licensee also cites that the most recent revision of Code Case N-638 permits a repair area up to 500 square inches.
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 presentation slides entitled, Bases for 500 Sq. In. Weld Overlay Over Ferritic Material, were provided to the NRC staff in a public meeting on January 10, 2007 (ADAMS Accession No. ML070470565).
Based on a review of the information provided, the NRC staff finds that the 300 square inch weld area limit over the ferritic base metal is acceptable because the EPRI finite element analyses demonstrate that the stresses of a nozzle with a 500 square inch weld area will not adversely affect the integrity of the pressurizer nozzle.
The licensee has proposed the use of non-attached temperature measuring devices in lieu of weld-attached thermocouples. Code Case N-638-1 does not specifically state that weld-attached thermocouples have to be used, but requires any areas from which weld-attached thermocouples have been removed to be ground and examined using a surface examination method. The licensee has proposed the use of non-attached type devices, such as contact pyrometers, which will enable manual recording of process temperatures. The licensee also stated that the instruments will be calibrated in accordance with approved calibration and control program requirements. The NRC staff finds the use of the non-attached temperature measuring devices to be acceptable as they will be calibrated and will allow for manual monitoring of process temperatures.
The licensee has stated that the conditions imposed by RG 1.147 for the use of Code Case N-638-1 are not applicable in this application. RG 1.147 requires that UT examinations be demonstrated for the repair volume using representative samples which contain construction type flaws, and that the acceptance criteria of NB-5330 of Section III edition and addenda approved in 10 CFR 50.55a, apply to all flaws identified within the repaired volume. The licensee states that Code Case N-638-1 applies to any type of welding where a temper bead technique is to be used. It is not specifically written for a weld overlay repair. Code Case N-504-3 and Appendix Q are applicable for weld overlay repairs. Therefore, the volumetric acceptance criteria for the required structural overlay weld metal and the outer 25 percent of the base material volume below the structural overlay weld metal will be per Appendix Q, Subarticle Q-4100(c). Any repairs that are made to the carbon steel base material prior to the application of the weld overlay will follow the PINGP Section XI Repair/Replacement Program. The NRC staff agrees that the acceptance criteria for the volumetric examinations of the structural overlay weld metal and the outer 25 percent of the base material volume below the structural overlay weld metal should be in accordance with Appendix Q.
The licensee has proposed to design the FSWOL in accordance with the design and repair methodologies of Code Case N-504-2 and provisions of Nonmandatory Appendix Q with modifications. No modifications were made in regard to flaw evaluation. Code Cases N-504-2 and N-504-3 have no differences with respect to flaw evaluations applicable to the licensees proposed alternative. Therefore, flaw evaluations will be conducted in accordance with Code Case N-504-3 and ASME Appendix Q, which the NRC has found to be acceptable as identified in RG 1.147, Revision 15.
4.4 Examination and Inspection The licensee stated that the PlNGP Unit 2 pressurizer nozzle-to-safe end weld was most recently examined in November 2006. The weld was examined per ASME Section XI, Appendix VIII, Supplement 10, using qualified procedures, personnel, and equipment. The examination had a composite coverage of 94.1 percent (91 percent axial scans, 97.3 percent circumferential scan). Materials Reliability Program (MRP) document MRP-139, Materials Reliability Program:
Primary System Piping Butt Weld Inspection and Evaluation Guidelines (MRP-139) requires at least 90 percent coverage for the axial scans. This PlNGP Unit 2 examination met the ASME Section XI and MRP-139 requirements for exam coverage. No PWSCC indications were detected.
The licensee stated that the weld overlay examination, preservice examination of the completed repair, and ISIs will be performed in accordance with Code Case N-504-2, ASME Section XI,
Appendix Q, Subarticles Q-4100, Q-4200 and Q-4300, and ASME Section XI, Appendix VIII, Supplement 11 modified to comply with the PDI as identified in Alternative Request 2-RR-4-8, Revision 1. The modifications pertain to sample sets, types of flaws, spacing of flaws, grading units, and test administration for the personnel, procedure, and equipment performance demonstrations.
Code Cases N-504-3 and N-638-1 and/or Appendix Q of the ASME Code,Section XI, require certain acceptance, preservice, and inservice examinations of the installed weld overlay. The licensee has stated that the design and repair methodologies of Code Case N-504-2 and provisions of Nonmandatory Appendix Q will be followed with modifications. These modifications and their bases are summarized in Table 2 of Alternate Request 2-RR-4-8, Revision 1. The licensee has also stated that specific changes and the basis justifying each proposed modification to the methodologies specified in Code Case N-638-1 are addressed in Table 3 of Alternate Request 2-RR-4-8, Revision 1. All other applicable requirements not listed in Table 2 or Table 3 of Alternate Request 2-RR-4-8, Revision 1, will be met as described in Code Cases N-504-2, N-638-1 and Appendix Q.
The licensee will perform ultrasonic examination of the completed FSWOL in accordance with the PDI program implementation of ASME Code Section XI, 1998 Edition through the 2000 Addenda, Appendix VIII, Supplement 11. ASME Code,Section XI, Appendix Vlll, Supplement 11 cannot be used without modification for NDE qualifications of a FSWOL.
EPRI created the PDI program to implement performance demonstration requirements contained in Appendix VIII of Section XI of the ASME Code. To this end, the PDI program has developed a program for qualifying equipment, procedures, and personnel in accordance with the UT criteria of Appendix VIII, Supplement 11. Prior to the Supplement 11 program, EPRI was maintaining a performance demonstration program (the precursor to the PDI program) for weld overlay qualification under the Tri-party Agreement with the NRC, BWR Owners Group, and EPRI, in the NRC letter dated July 3, 1984 (ADAMS Legacy Accession No. 8407090122). Later, the NRC staff recognized the EPRI PDI program for weld overlay qualifications as an acceptable alternative to the Tri-party Agreement in its letter dated January 15, 2002, to the PDI Chairman (ADAMS Accession No. ML020160532).
The PDI program is routinely assessed by NRC staff for consistency with the current ASME Code and proposed changes. The PDI program does not fully comport with the existing requirements of Supplement 11. EPRI presented the differences at public meetings in which the NRC participated (Memorandum from Donald G. Naujock to Terence Chan, Summary of Public Meeting Held January 31 - February 2, 2002, with PDI Representatives, March 22, 2002 (ADAMS Accession No. ML010940402), and Memorandum from Donald G. Naujock to Terence Chan, Summary of Public Meeting Held June 12 through June 14, 2001, with PDI Representatives, November 29, 2001 (ADAMS Accession No. ML013330156)). Based on the discussions at these public meetings, the NRC staff determined that the PDI program provides an acceptable level of quality and safety.
The NRC staff evaluated the differences between the PDI program and Supplement 11 identified in Table 4 of Alternative Request 2-RR-4-8, Revision 1. The NRC staff concludes that the differences are acceptable and that the examination under the PDI program provides an acceptable level of quality and safety and, therefore, the proposed alternative to Supplement 11 is acceptable.
4.4.1 Acceptance Examination
Code Case N-638-1 specifies that a band around the area to be welded be examined using surface and ultrasonic methods when the completed weld has been at ambient temperature for 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br />. The band is defined as an area around the area to be welded equal to 1 1/2 times the component thickness or 5 inches, whichever is less. For the overlay, this would be a band 1 1/2 times the thickness or 5 inches, whichever is less, from the toe of the weld overlay on the ferritic material. The licensee has stated that the final weld overlay shall be examined in accordance with Code Case N-504-2 and Nonmandatory Appendix Q, as modified, and that the examination of the weld overlay covering the ferritic base material and examination of the adjacent ferritic base material shall be performed no sooner than 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> after completion of the third temperbead layer over the ferritic base material.
Code Case N-504-3 and Appendix Q require the weld overlay and the adjacent base material for at least 1/2 inch from each side of the weld to be examined using the liquid penetrant method.
The licensee states that the examination of the band area after 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> is for the detection of delayed heat affected zone cracking that may occur due to inadvertent introduction of deleterious monatomic hydrogen into a hardened base metal that was not adequately tempered.
The heat affected zone of the ferritic material will be limited to a small area around the first Alloy 52M layer. This area will be within a 1/2 inch of the first Alloy 52M layer. Therefore, examination of at least 1/2 inch from each side of the weld using liquid penetrant method provides an acceptable level of quality and safety.
The 48 hour5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> hold time after the weld reaches ambient temperature is to allow adequate time for any dissolved hydrogen to propagate through the weld. In January of 2007, EPRI made a presentation to the NRC (ADAMS Accession No. ML070470552), regarding the 48 hour5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> hold time for NDE after temper bead welding. EPRI presented information to support changing the start of the hold time from completion of the temper bead welding to completion of the third temper bead layer. The information indicated the low amount of hydrogen available for absorption in the ferritic base material, the higher solubility of hydrogen in the austenitic nickel alloy weld filler material than the carbon steel nozzle, and the diffusion of hydrogen during subsequent non-temperbead weld overlay layers should allow the hydrogen to escape from the ferritic material and permit the hold time before NDE to begin after the third temperbead layer is completed. This information is further supported by EPRI Report 1013558, Temperbead Welding Applications 48-hour Hold Requirements for Ambient Temperature Temperbead Welding. Starting the hold time from the completion of the third temperbead layer will provide adequate time for any delayed hydrogen cracking to manifest and be detected by surface examination. Therefore, the proposal provides an acceptable level of quality and safety.
Code Case N-504-3 requires a hydrostatic test in accordance with IWA-5000 to be conducted if a flaw penetrates the original pressure boundary prior to welding, or if any evidence of the flaw penetrating the pressure boundary is observed during the welding operation. A hydrostatic test of the primary system at normal operating temperature would test the system at 1.02 times the normal operating pressure. In lieu of a hydrostatic test, the licensee has proposed to perform a system leakage test in accordance with ASME Section XI, IWA-5000. Performance of a system leakage test is consistent with the ASME Code Section XI, 1998 Edition with 2000 Addenda and Code Case N-416-3 which was approved in RG 1.147, Revision 15. For a system leakage test to be used Code Case N-416-3 and Section XI, 1998 Edition with 2000 Addenda require the NDE methodologies and acceptance criteria of the 1992 Edition of Section III shall be met prior to return to service. However, the NRC staff has no objection if the NDE of the subject weld overlay is performed using the methodologies and acceptance criteria of Appendix Q. Appendix Q was developed partly because there was not sufficient guidance provided in Section III or Section XI for the application of weld overlays. Therefore, the NRC staff finds that the performance of a system leakage test along with the NDE of Appendix Q will provide an acceptable level of quality and safety.
4.4.2 Preservice Examination Code Case N-504-3 and ASME Section XI, Appendix Q requires liquid penetrant and ultrasonic examination of the completed weld repair, and that the acceptance standards of Table IWB-3514-2 shall apply. Appendix Q specifically states that the preservice examination acceptance standards are to be satisfied. Both Code Case N-504-3 and Appendix Q require that examinations be performed in the outer 25 percent of the underlying pipe wall/base material.
As the licensee has not proposed any modifications to these preservice examination requirements, and the licensee has stated that all other applicable requirements not listed in Tables 2 of Alternative Request 2-RR-4-8, Revision 1, will be met as described in Code Case N-504-3 and Appendix Q, the NRC staff has determined that the proposed examinations meet the requirements and will provide an acceptable level of quality and safety.
4.4.3 Inservice Examination
, Table 2 of Alternative Request 2-RR-4-8, Revision 1, requires inservice examinations be conducted ultrasonically with the examination volume defined in Fig. Q-4300-1.
The frequency of examinations will be in compliance MRP-139, with the additional requirement of at least one ultrasonic examination within 10 years of the FSWOL application. Additionally by letter dated May 7, 2008, the licensee agreed that if indications are found in the pre-application, ultrasonic examination, the first inservice examination will be performed during the first or second outage following FSWOL application. The MRP-139 guidance for ISI goes beyond current ASME Code inspection requirements for PINGP, Unit 2. The NRC staff finds that the inservice examination requirements in the May 7, 2008, letter, and Enclosure 2, Table 2 of Alternative Request 2-RR-4-8, Revision 1, are consistent with or more conservative than the ASME Code,Section XI, Appendix Q.
However, the NRC staff notes that revision of the ASME Code requirements is ongoing for DMBWs, including those mitigated by FSWOL. Once the revised requirements are required by NRC regulatory action, the licensees proposed alternative inspection frequency may no longer be acceptable per regulatory requirements, and relief from the revised ASME Code requirements would be necessary to be submitted to the NRC and approved to continue under the re-inspection schedule of the licensees proposed alternative.
4.5 Required Activities The licensee stated that it will, prior to Mode 4 start-up, submit to the NRC a stress analysis summary demonstrating that the pressurizer nozzle will perform its intended design functions after the full structural weld overlay installation. The stress analysis report will include results showing that the requirements of NB-3200 and NB-3600 of the ASME Code, Section Ill are satisfied. The results will show that the postulated crack including its growth in the nozzle will not adversely affect the integrity of the overlaid welds.
The licensee committed to provide, within 14 days after the completion of the ultrasonic examination of weld overlay installation, the weld overlay examination results including a list of indications detected, disposition of the indications using the standards of ASME Section XI, Appendix Q, the type and, if possible, the nature of the indications, a discussion of any repairs to the weld overlay material and/or base metal, and the reason for the repair.
5.0 CONCLUSION
The NRC staff has reviewed the licensee=s submittal and determined that Alternative Request
2-RR-4-8, 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 2-RR-4-8, Revision 1, for the installation of a FSWOL of the dissimilar metal weld of the pressurizer surge nozzle weld, safe end, and safe end-to-reducer weld at PINGP, Unit 2, for the design life of the repair as determined by paragraph (g) of Code Case N-504-2.
The NRC staff finds that the ISI frequency identified in Enclosure 2, Table 2 of Alternative Request 2-RR-4-8, Revision 1 is acceptable as it is based on current industry mandatory ISI guidance and Code Case N-504-3 and Appendix Q, and therefore pursuant to 10 CFR 50.55a(a)(3)(i), the NRC staff authorizes the use of the licensees proposed alternative for the fourth ISI interval at PINGP, Unit 2, which is scheduled to be complete on December 20, 2014.
However, the NRC staff notes that revision of the ASME Code requirements is ongoing for DMBWs, including those mitigated by FSWOL. Once the revised ASME Code requirements are required by regulatory action, and if the licensees proposed alternative inspection frequency is no longer acceptable per the revised regulatory requirements, the re-inspection schedule of the licensees proposed alternative would no longer remain acceptable for the fourth ISI interval at PINGP, Unit 2. In such case, the licensee will need to follow the revised ISI requirements.
All other ASME Code,Section XI requirements for which relief was not specifically requested and approved in this relief request remain applicable, including third-party review by the Authorized Nuclear Inservice Inspector.
Principal Contributor: Keith Hoffman, DCI/CPNB Date: June 15, 2008
Prairie Island Nuclear Generating Plant, Units 1 and 2 cc:
Peter M. Glass Assistant General Counsel Xcel Energy Services, Inc.
414 Nicollet Mall Minneapolis, MN 55401 Manager, Regulatory Affairs Prairie Island Nuclear Generating Plant Nuclear Management Company, LLC 1717 Wakonade Drive East Welch, MN 55089 Manager - Environmental Protection Division Minnesota Attorney General=s Office 445 Minnesota St., Suite 900 St. Paul, MN 55101-2127 U.S. Nuclear Regulatory Commission Resident Inspector's Office 1719 Wakonade Drive East Welch, MN 55089-9642 Administrator Goodhue County Courthouse Box 408 Red Wing, MN 55066-0408 Commissioner Minnesota Department of Commerce 85 7th Place East, Suite 500 St. Paul, MN 55101-2198 Tribal Council Prairie Island Indian Community ATTN: Environmental Department 5636 Sturgeon Lake Road Welch, MN 55089 Nuclear Asset Manager Xcel Energy, Inc.
414 Nicollet Mall, R.S. 8 Minneapolis, MN 55401 Dennis L. Koehl Chief Nuclear Officer Nuclear Management Company, LLC 700 First Street Hudson, WI 54016 Joel P. Sorenson Director, Site Operations Prairie Island Nuclear Generating Plant Nuclear Management Company, LLC 1717 Wakonade Drive East Welch, MN 55089