ML080580354

Safety Evaluation of Proposed Alternative for the Application of Weld Overlay

ML080580354
Person / Time
Site:	Summer
Issue date:	03/25/2008
From:	Martin R NRC/NRR/ADRO/DORL/LPLII-1
To:	Archie J South Carolina Electric & Gas Co
Martin R, NRR/DORL, 415-1493
References
TAC MD5765
Download: ML080580354 (21)

Text

March 25, 2008 Mr. Jeffrey B. Archie Vice President, Nuclear Operations South Carolina Electric & Gas Company Virgil C. Summer Nuclear Station Post Office Box 88 Jenkinsville, SC 29065

SUBJECT:

VIRGIL C. SUMMER NUCLEAR STATION, UNIT NO. 1, PROPOSED ALTERNATIVE FOR THE APPLICATION OF WELD OVERLAY ON DISSIMILAR METAL WELDS OF PRESSURIZER NOZZLES (TAC NO. MD5765)

Dear Mr. Archie:

By letter dated June 1, 2007, as supplemented by letter dated January 18, 2008, South Carolina Electric and Gas Company (the licensee), submitted a proposed alternative to the requirements of Section XI of the American Society of Mechanical Engineers, Boiler and Pressure Vessel Code (ASME Code), under the provisions of Title 10 of the Code of Federal Regulations (10 CFR),

Part 50, Section 50.55a(a)(3)(i) for the Virgil C. Summer Nuclear Station, Unit 1 (VCSNS). The licensee submitted relief request RR-III-05, as an alternative to the ASME Code, paragraph IWA-4330 for mitigating primary water stress corrosion cracking on dissimilar metal welds of pressurizer nozzles using full structural weld overlays at VCSNS. The request is based on ASME Code Case N-740.

Based on the information submitted, the U.S. Nuclear Regulatory Commission (NRC) staff concludes that Relief Request RR-III-05 will provide an acceptable level of quality and safety.

Therefore, pursuant to 10 CFR 50.55a(a)(3)(i), the proposed alternative RR-III-05 is authorized for the installation of weld overlay on the dissimilar and similar metal welds of the pressurizer nozzles at the VCSNS.

Sincerely,

/RA/

Robert E. Martin, Senior Project Manager Plant Licensing Branch II-1 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket No. 50-395

Enclosure:

Safety Evaluation cc w/encl: See next page

ML080580354 NRR-028 OFFICE NRR/LPL2-1/PM NRR/LPL3-1/LA OGC NRR/LPL2-1/BC NAME RMartin BTully LSubin MWong DATE 3/25/08 3/25/08 3/10/08 3/25/08

SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION ALTERNATIVE FOR THE APPLICATION OF WELD OVERLAY ON DISSIMILAR METAL WELDS OF PRESSURIZER NOZZLES SOUTH CAROLINA ELECTRIC & GAS COMPANY SOUTH CAROLINA PUBLIC SERVICE AUTHORITY VIRGIL C. SUMMER NUCLEAR STATION, UNIT NO. 1 DOCKET NO. 50-395

1.0 INTRODUCTION

By letters dated June 1, 2007 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML071550420), and January 18, 2008 (ADAMS Accession No. ML080230050), South Carolina Electric & Gas (the licensee) submitted Relief Request RR-III-05 as an alternative to the requirements of American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code,Section XI, IWA-4330 for mitigating potential primary water stress corrosion cracking (PWSCC) on dissimilar metal welds (DMW) of pressurizer nozzles using full structural weld overlays at the V.C. Summer Nuclear Station (VCSNS). Relief request RR-III-05 is based on ASME,Section XI, Code Case N-740.

A DMW is defined as a weld that joins two pieces of metals 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 metals that are of the same material. In this alternative, the similar metal weld joins the stainless steel safe end to the stainless steel piping.

The similar metal weld itself is made of stainless steel weld material.

The industry has experienced degradation of the Alloy 82/182 weld material which is susceptible to 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 Alloy 82/182 welds to form a new pressure boundary.

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 U.S. Nuclear Regulatory Commission (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 code of record for the third 10-year ISI interval at VCSNS is the ASME Code,Section XI, 1998 Edition with 2000 Addenda. For ultrasonic examinations, the licensee uses Appendix VIII, Performance Demonstration for Ultrasonic Examinations, of the ASME Section XI, 1998 Edition through 2000 Addenda.

3.0 RELIEF REQUEST RR-III-05 AS PROPOSED BY THE LICENSEE The information in section 3.0 is largely from the licensees submittal.

3.1 Component Affected Pressurizer Nozzle to Spray Line (one location, two welds)

The pressurizer nozzle is carbon steel (P-No. 3) welded to a stainless steel (P-No. 8) safe end, which is, in turn, welded to stainless steel (P-No. 8) pipe. The weld (CGE-1-4503-46) joining the nozzle to safe end is an 82/182 DMW and the weld (CGE-1-4503-45) joining the safe end to pipe is stainless steel. Because of the close proximity of the two welds, a single weld overlay will be installed across both welds.

Pressurizer Nozzle to Safety Valve (three locations, two welds each)

The pressurizer nozzle is carbon steel (P-No. 3) welded to a stainless steel (P-No. 8) safe end, which is, in turn, welded to stainless steel (P-No. 8) pipe. The weld joining the nozzle to safe end is an 82/182 DMW and the weld joining the safe end to pipe is stainless steel. Because of the close proximity of the two welds, a single weld overlay will be installed across both welds. There are three safety valves: A, B and C. The identifications for the DMWs are CGE-1-4501-1, CGE-1-4501-12, and CGE-4501-23 for safety valves A, B, and C, respectively. The identification for the similar metal welds is CGE-1-4501-2, CGE-1-4501-13, and CGE-4501-24 for safety valves A, B, and C, respectively.

Pressurizer Nozzle to Relief Valve (one location, two welds)

The pressurizer nozzle is carbon steel (P-No. 3) welded to a stainless steel (P-No. 8) safe end, which is, in turn, welded to stainless steel (P-No. 8) pipe. The weld (CGE-1-4502-1) joining the nozzle to safe end is an 82/182 DMW and the weld (CGE-1-4502-2) joining the safe end to pipe is stainless steel. Because of the close proximity of the two welds, a single weld overlay will be installed across both welds.

Pressurizer Nozzle to Surge Line (one location, two welds)

The pressurizer nozzle is carbon steel (P-No. 3) welded to a stainless steel (P-No. 8) safe end, which is, in turn, welded to stainless steel (P-No. 8) pipe. The weld (CGE-1-4500A-1) joining the nozzle to safe end is an 82/182 DMW and the weld (CGE-1-4500A-2) joining the safe end to pipe is stainless steel. Because of the close proximity of the two welds, a single weld overlay will be installed across both welds.

3.2 Applicable Code Edition And Addenda The code of record for the third 10-year ISI interval is the ASME Code,Section XI, 1998 Edition with 2000 Addenda. For ultrasonic examinations, the licensee uses Appendix VIII, Performance Demonstration for Ultrasonic Examinations, of the ASME Section XI, 1998 Edition through 2000 Addenda. The original construction code for the pressurizer is the ASME Section III, 1971 Edition through the summer 1971 Addenda, Subsection NB.

3.3 Applicable Code Requirements IWA-4170(b) of the ASME Code,Section XI, requires that repairs and the installation of replacement items be performed in accordance with the Owners Design Specification and the original Construction Code of the component or system. Alternatively, IWA-4170(b) allows for use of later Editions/Addenda of the Construction Code or ASME Section III. IWA-4300 and IWA-4500 provide defect removal and alternative welding methods when the requirements of IWA-4170(b) cannot be met. IWA-4800 requires the performance of preservice examinations based on IWB-2200 for Class 1 components. The ASME Code,Section XI, Table IWB-2500, Categories B-F and B-J, prescribe inservice inspection requirements for Class 1 butt welds. The ASME Code,Section XI, Appendix VIII, Supplement 11 specifies performance demonstration requirements for ultrasonic examination of weld overlays.

3.4 Proposed Alternative And Basis Pursuant to 10 CFR 50.55a(a)(3)(i), the licensee proposes the alternatives to the ASME Code requirements specified in Section 3.3 of this safety evaluation (SE). The proposed alternatives are applicable to the six DMWs identified in Section 3.1 of this SE.

The June 1, 2007, submittal contains several attachments. Attachment I to the June 1, 2007, submittal identifies affected components, ASME Code requirements, proposed alternatives and technical basis. Attachment II provides detailed information of the affected components and a general design drawing. Attachment III contains requirements for the design, crack growth calculation, examination and pressure testing of the weld overlay. Attachment IV contains requirements for the ambient temperature temperbead welding technique and Attachment VI contains the associated technical basis. Attachment V contains a comparison of the ASME Code Case N-504-3 and Appendix Q of ASME Section XI with the proposed alternative of Attachment III.

Attachment VII contains a comparison of ASME Section XI, Appendix VIII, Supplement 11 to that implemented by the performance demonstration initiative (PDI) program. Attachment VIII contains a list of commitments. All these attachments are part of Relief Request RR-III-05. These alternatives are based on the methodology of ASME Section XI, Code Case N-740.

The licensee will perform ultrasonic examinations of the proposed preemptive full structural weld overlays in accordance with Appendix VIII, Supplement 11 of the 1998 Edition, 2000 Addenda of ASME Section XI, which is modified by the PDI program.

The licensee states that the proposed alternative provides a methodology for preventing potential failures due to PWSCC based on the use of filler metals that are resistant to PWSCC (e.g., Alloy 52M), procedures that create compressive residual stress profiles in the original weld, and post-overlay preservice and inservice inspection requirements that ensure structural integrity for the life of the plant. The proposed weld overlays will also meet the applicable stress limits from ASME Section III. Crack growth evaluations for PWSCC and fatigue of any conservatively postulated flaws will demonstrate that structural integrity will be maintained.

The licensee stated that Alloy 52M weld metal has demonstrated sensitivity to certain impurities, such as sulfur, when deposited onto austenitic stainless steel base materials. Therefore, should this condition exist, a butter (transitional) layer of austenitic stainless steel filler metal (308L) will be applied across the austenitic stainless steel base material. The butter layer is simply a base metal build-up of the underlying austenitic stainless steel base material. It is deposited to facilitate installing the weld overlay. The properties of the butter layer will match those of the austenitic stainless steel base material; as such, the butter layer is not part of the weld overlay or credited towards the design thickness of the weld overlay. Additionally, the stainless steel butter layer will not be deposited onto the existing Alloy 82/182 weld or carbon steel base material of the pressurizer nozzle.

The licensee stated that the welding procedure that will be used to deposit the butter layer onto the stainless steel base material will be qualified in accordance with ASME Code,Section IX. The welding procedure qualification ensures that the mechanical properties of the weld butter meet the requirements of the base material. Since the buttering material mechanical properties match the base material, there will be no adverse affect on weld shrinkage relative to the application of the Alloy 52M weld overlay. Additionally, the austenitic stainless steel butter layer will not adversely affect the ability to ultrasonically examine the weld overlay or the base material. The butter layer will be typical in thickness to that applied for the structural weld overlays (~0.080 to 0.100 inches) and will be applied to the base metal with specific welding parameters established in welding procedure qualification and mock-up programs.

Weld Overlay Design and Verification The licensee stated that the fundamental design basis for full structural weld overlays is to maintain the original design margins with no credit taken for the underlying PWSCC-susceptible weldments. The assumed design basis flaw for the purpose of structurally sizing the weld overlays is 360° in circumferential extent and 100 percent through the original wall thickness of the DMWs. Regarding the crack growth analysis for the preemptive full structural weld overlay, the licensee will assume a flaw originating from the inside diameter with a depth of 75 percent and a circumference of 360°. A 75 percent through-wall flaw is the largest flaw that could remain undetected. The licensee will perform a preservice volumetric examination after application of the weld overlay using an ASME Section XI, Appendix VIII (as implemented through PDI) examination procedure. This examination will verify that there is no cracking in the outer 25 percent of the original weld and base material. The preservice examination will also demonstrate that the assumption of a 75 percent through-wall crack is conservative. However, if any crack-like flaws are identified in the upper 25 percent of the original weld or base material by the preservice examination, then the licensee will use the as-found flaw (postulated 75 percent through-wall flaw plus the portion of the flaw in the upper 25 percent) in the crack growth analysis.

The licensee will perform nozzle-specific stress analyses to establish a residual stress profile in each nozzle. Internal diameter weld repairs will be assumed in these analyses that effectively bound any actual weld repairs that may have occurred in the nozzles. The analyses will then simulate application of the weld overlays to determine the final residual stress profiles. Post-weld overlay residual stresses at normal operating conditions will be shown to result in beneficial compressive stresses on the inside surface of the components, assuring that further crack initiation due to PWSCC is highly unlikely.

The licensee will also perform fracture mechanics analyses to predict crack growth for postulated flaws. Crack growth due to PWSCC and fatigue will be analyzed for the original DMW. The crack growth analyses will consider all design loads and transients, plus the post-weld overlay and through-wall residual stress distributions. It will demonstrate that the postulated cracks will not grow beyond the design basis for the weld overlays.

The analyses will demonstrate that applying the weld overlays does not impact the conclusions of the existing nozzle stress reports. The ASME Code,Section III stress and fatigue criteria will be met for regions of the overlays remote from assumed cracks.

The licensee will measure shrinkage during the overlay application. Shrinkage stresses at other locations in the piping systems arising from the weld overlays will be demonstrated not to have an adverse effect on the systems. Clearances of affected supports and restraints will be checked after the overlay repair, and will be reset within the design ranges as required. The licensee will evaluate total added weight on the piping systems due to the overlays for potential impact on piping system stresses and dynamic characteristics. The as-built dimensions of the weld overlays will be measured and evaluated to demonstrate that they meet or exceed the minimum design dimensions of the overlays.

Mechanical Properties The principal reasons to preheat a component prior to repair welding is to minimize the potential for cold cracking. The two cold cracking mechanisms are hydrogen cracking and restraint cracking. Both of these mechanisms occur at ambient temperature. Preheating slows down the cooling rate resulting in a ductile, less brittle microstructure thereby lowering susceptibility to cold cracking. Preheat also increases the diffusion rate of monatomic hydrogen that may have been trapped in the weld during solidification.

As an alternative to preheat, the ambient temperature temperbead welding process utilizes the tempering action of the welding procedure to produce tough and ductile microstructures. Because precision bead placement and heat input control are utilized in the machine gas tungsten arc weld (GTAW) process, effective tempering of weld heat affected zones (HAZ) is possible without applying preheat. Section 2-1 of EPRI Report GC-111050, Ambient Temperature Preheat for Machine GTAW Temperbead Applications, states that the temperbead process is carefully designed and controlled such that successive weld beads supply the appropriate quantity of heat to the untempered HAZ such that the desired degree of carbide precipitation (tempering) is achieved. The resulting microstructure is very tough and ductile. Subarticle IWA-4633.2 of the ASME Code, Sections XI, requires a post-weld soak for the temperbead welding. Performed at 450 degree Fahrenheit (°F) - 550°F for 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> (P-No. 3 base materials), this post-weld soak assists diffusion of any remaining hydrogen from the repair weld. As such, the post-weld soak is a hydrogen bake-out and not a post-weld heat treatment as defined by the ASME Code. Between the 450°F and 550°F temperature ranges, the post-weld soak does not stress relieve, temper, or alter the mechanical properties of the weldment in any manner. Attachment IV of the June 1, 2007, submittal establishes detailed welding procedure qualification requirements for base materials, filler metals, restraint, impact properties, and other procedure variables. The qualification requirements contained in Attachment IV provide assurance that the mechanical properties of repair welds will be equivalent to or superior to those of the surrounding base material.

Hydrogen Cracking Hydrogen cracking is a form of cold cracking. It is produced by the action of internal tensile stresses acting on low toughness HAZs. The internal stresses are produced from localized build-ups of monatomic hydrogen. Monatomic hydrogen forms when moisture or hydrocarbons interact with the welding arc and molten weld pool. The monatomic hydrogen can be entrapped during weld solidification and tends to migrate to transformation boundaries or other microstructure defect locations. As concentrations build, the monatomic hydrogen will recombine to form molecular hydrogen, thus generating localized internal stresses at these internal defect locations. If these stresses exceed the fracture toughness of the material, hydrogen-induced cracking will occur. This form of cracking requires the presence of hydrogen and low toughness materials. It is manifested by intergranular cracking of susceptible materials and normally occurs within 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> of welding.

Subarticle IWA-4600 of the ASME Code,Section XI, establishes elevated preheat and post-weld soak requirements. The elevated preheat temperature of 300°F increases the diffusion rate of hydrogen from the weld. The post-weld soak at 450°F was also established to bake-out or facilitate diffusion of any remaining hydrogen from the weldment. However, while hydrogen cracking is a concern for shielded metal arc welding (SMAW), which uses flux covered electrodes, the potential for hydrogen cracking is significantly reduced when using the machine GTAW welding process.

The machine GTAW welding process is inherently free of hydrogen. Unlike the filler used in the SMAW process, GTAW welding filler metals do not rely on flux coverings, which may be susceptible to moisture absorption from the environment. Conversely, the GTAW process utilizes dry inert shielding gases that cover the molten weld pool from oxidizing atmospheres. Any moisture on the surface of the component being welded will be vaporized ahead of the welding torch. The vapor is prevented from being mixed with the molten weld pool by the inert shielding gas that blows the vapor away before it can be mixed. Furthermore, modern filler metal manufacturers produce wires having very low residual hydrogen. This is important because filler metals and base materials are the most realistic sources of hydrogen for automatic or machine GTAW temperbead welding. Therefore, the potential for hydrogen-induced cracking is greatly reduced by using the machine GTAW process.

In the unlikely case that hydrogen cracking occurs, nondestructive examination (NDE) of the weldment will be performed at least 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> after completing the third layer of the weld overlay, thereby providing assurance that the cracking would be identified.

Cold Restraint Cracking Cold cracking generally occurs during cooling at temperatures approaching ambient temperature.

As stresses build under a high degree of restraint, cracking may occur at defect locations. Brittle microstructures with low ductility are subject to cold restraint cracking. However, the ambient temperature temperbead process is designed to provide a sufficient heat inventory so as to produce the desired tempering for high toughness. Because the machine GTAW temperbead process provides precision bead placement and control of heat, the toughness and ductility of the HAZ will typically be superior to the base material. Therefore, the resulting structure will be appropriately tempered to exhibit toughness sufficient to resist cold cracking.

Commitments The licensee will submit the following information to the NRC within fourteen days from completing the final ultrasonic examinations of the completed weld overlays:

1. Weld overlay examination results including a listing of indications detected. The recording criteria of the ultrasonic examination procedure to be used for the examination of the VCSNS pressurizer overlays requires that all indications, regardless of amplitude, be investigated to the extent necessary to provide accurate characterization, identity, and location. Additionally, the procedure requires that all indications, regardless of amplitude, that cannot be clearly attributed to the geometry of the overlay configuration be considered flaw indications.

2. Disposition of indications using the standards of ASME Section XI, IWB-3514-2 and/or IWB-3514-3 criteria and, if possible, the type and nature of the indications. The ultrasonic examination procedure requires that all suspected flaw indications are to be plotted on a cross-sectional drawing of the weld and that the plots should accurately identify the specific origin of the reflector.

3. A discussion of any repairs to the weld overlay material and/or base metal and the reason for the repairs.

4. A stress analysis summary demonstrating that the pressurizer nozzles will perform their intended design functions after the 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 III, are satisfied. The stress analysis will also include results showing that the requirements of IWB-3000 of the ASME Code,Section XI, are satisfied. The results will show that the postulated crack including its growth in the nozzles will not adversely affect the integrity of the overlaid welds. This information will be submitted to the NRC prior to entry into Mode 4 start-up from VCSNSs seventeenth refueling outage.

Proposed Alternative to ASME Section XI Appendix VIII, Supplement 11 Appendix VIII, Supplement 11 of the 1998 Edition, 2000 Addenda of ASME Section XI specifies requirements for performance demonstration of ultrasonic examination procedures, equipment, and personnel used to detect and size flaws in full structural overlays of wrought austenitic piping welds. The licensee modified the Appendix VIII, Supplement 11 qualification requirements by the proposed alternatives in the Performance Demonstration Initiative (PDI) program as indicated in Attachment VII of this request because the industry cannot meet the requirements of Appendix VIII, Supplement 11. Therefore, the PDI initiatives to ASME Section XI Appendix VIII, Supplement 11 as described in Attachment VII will be used for qualification of ultrasonic examinations used to detect and size flaws in the preemptive full structural weld overlays of this request.

3.5 Duration Of The Proposed Request The licensee stated that this alternative repair is requested for the third 10-year ISI interval.

4.0 STAFF EVALUATION As stated above, Relief Request RR-III-05 is based on Code Case N-740. The NRC staff has not yet endorsed Code Case N-740 and cannot use Code Code N-740 to evaluate Relief Request RR-III-05. However, the NRC staff has endorsed Code Case N-504-3, Alternative Rules for Repair of Class 1, 2, and 3 Austenitic Stainless Steel Piping Section XI, Division 1. The NRC staff has also endorsed Code Case N-638-1, Similar and Dissimilar Metal Welding Using Ambient Temperature Machine GTAW [gas tungsten arc welding] Temperbead Technique Section XI, Division 1. The NRC staff endorsed the use of Code Case N-504-3 in Regulatory Guide 1.147, Revision 15, Inservice Inspection Code Case Acceptability, ASME Section XI, Division 1, during the same period that it was reviewing the VCSNS submittal. The requirements between Code Cases N-504-2 and N-504-3 are similar. In accordance with Regulatory Guide 1.147, Revision 15, the ASME Code,Section XI, Appendix Q, Weld Overlay Repair of Class 1, 2, and 3 Austenitic Stainless Steel Piping Weldments, shall be used when Code Case N-504-3 is used. Therefore, to evaluate Relief Request RR-III-05, the NRC staff has used Code Cases N-504-3, N-638-1, and Appendix Q to the ASME Code,Section XI.

4.1 General Requirements The general requirements for the overlay design are provided in Section 1 of Attachment III to the June 1, 2007, submittal. Attachment V of the June 1, 2007, submittal provides a comparison of Code Case N-504-2 and Appendix Q of ASME Section XI with the proposed alternative of Attachment III. The NRC staff finds that the general requirements in Section 1 of Attachment III are consistent with the requirements of Code Case N-504-3 and/or Appendix Q of the ASME Code,Section XI, with the following exceptions.

Code Case N-504-3 is only applicable to defect in austenitic stainless steel piping and not to nickel-alloy steel piping. However, paragraph 1.0(a) of Attachment III of the June 1, 2007, submittal proposes the weld overlay be applied to nickel alloy 82/182 pipe welds in addition to austenitic stainless steel welds. The staff finds this exception acceptable because the proposed alternative is specifically prepared to address the repair of nickel-based Alloy 82/182 welds and austenitic stainless welds based on welding procedure qualifications.

As stated in the licensees Attachment V, According to paragraph (e) of Code Case N-504-2, as supplemented by Appendix Q, the weld reinforcement shall consist of at least two weld layers having as-deposited delta ferrite content of at least 7.5 [ferrite number] FN. The first layer of weld metal with delta ferrite content of at least 7.5 FN shall constitute the first layer of the weld reinforcement that may be credited toward the required thickness. Alternatively, first layers of at least 5 FN provided the carbon content is determined by chemical analysis to be less than 0.02 percent. Paragraph (e) of Code Case N-504-3 is applicable to austenitic stainless steel filler metal and not to nickel alloy filler metal. Paragraph 1.0(d) of Attachment III of the June 1, 2007, submittal proposes to use the chromium content of the weld overlay as an acceptance criterion.

The proposed weld overlay uses nickel Alloy 52M filler metal instead of austenitic stainless steel filler metals. As stated in Attachment V, The first layer of nickel Alloy 52M deposited weld metal may be credited toward the required thickness provided the portion of the layer over the austenitic base material, austenitic weld, and the associated dilution zone from an adjacent ferritic base material contains at least 24 percent chromium. The chromium content of the deposited weld metal may be determined by chemical analysis of the production weld or from a representative coupon taken from a mockup prepared in accordance with the weld procedure specification for the production weld. The licensee may not take credit for the first weld layer toward the required thickness unless it has been shown to contain at least 24 percent chromium. This is a sufficient amount of chromium to prevent PWSCC. The licensee stated that Section 2.2 of EPRI Technical Report MRP-115, 1006696, Crack Growth Rates for Evaluating PWSCCC of Alloy 82, 182, and 132 Welds, states the following: The only well explored effect of the compositional differences among the weld alloys on PWSCC is the influence of chromium. Buisine, et al. evaluated the PWSCC resistance of nickel-based weld metals with various chromium contents ranging from about 15 percent to 30 percent chromium. Testing was performed in doped steam and primary water. Alloy 182, with about 14.5 percent chromium, was the most susceptible. Alloy 82 with 18-20 percent chromium took three or four times longer to crack. For chromium contents between 21 and 22 percent, no stress corrosion crack initiation was observed... The NRC staff finds that the licensee provided acceptable acceptance criteria for the chromium content of the weld overlay filler metal. The NRC staff also finds that paragraph (e) of N-504-3 does not apply to alloy 52M weld metal because nickel-based alloys such as alloy 52M have no delta ferrite and measuring delta ferrite per paragraph (e) is not appropriate.

The staff questioned whether the delta ferrite content requirement of Code Case N-504-3, paragraph (e) is applicable to the buffer layer because it uses austenitic stainless steel weld metal.

In its January 18, 2008 letter, the licensee responded that the statements considering the ferrite content would apply if the structural weld overlay were composed of austenitic stainless steel instead of nickel alloy 52M. The application of an austenitic stainless steel layer may be applied as a buffer layer to seal the base material if required. The application of an austenitic stainless steel buffer, if used, will not be credited to the structural weld overlay thickness required in the design. Thus, the ferrite content requirements of Code Case 504-3, paragraph (e) do not apply.

The buffer layer, if applied, will not be applied to the carbon steel nozzle nor the dissimilar metal weld.

4.2 Crack Growth Considerations and Design The requirements for the overlay design and the crack growth calculation are provided in Section 2 of Attachment III of the June 1, 2007, submittal. Attachment V of the submittal provides a comparison of the proposed alternative of Attachment III to Code Case N504-2 and Appendix Q of ASME Section XI. The NRC staff finds that the information in Section 2 of Attachment III is consistent with the requirements of Code Case N-504-3 and/or Appendix Q of the ASME Code,Section XI, with the following exceptions.

Paragraph (f) of N-504-3 and paragraph Q-3000 of Appendix Q require certain flaw sizes to be assumed in the weld overlay design and crack growth calculation. The licensee stated that because the weld overlays are being installed preemptively and not as a repair, postulated flaws are being assumed in the proposed alternative. As stated in Attachment V, regarding the crack growth analysis, a flaw originating from the inside diameter, with a depth of 75 percent, and a circumference of 360° will be assumed. The size of the assumed flaws shall be projected to the end of the design life of the overlay. Crack growth, including both stress corrosion and fatigue crack growth, shall be evaluated in the materials in accordance with subarticle IWB-3640 of the ASME Code,Section XI.

Section 2 of Attachment III to the June 1, 2007, submittal proposes that if the flaw is at or near the boundary of two different materials, evaluation of flaw growth in both materials is required.

A 75 percent through-wall flaw is the largest flaw that could remain undetected. As stated in Attachment V, A preservice volumetric examination will be performed after application of the weld overlay using an ASME Code Section XI, Appendix VIII (as implemented through PDI) examination procedure. This examination will verify that there is no cracking in the upper 25 percent of the original weld and base material. The preservice examination will also demonstrate that the assumption of a 75 percent through-wall crack is conservative. However, if any crack-like flaws are identified in the upper 25 percent of the original weld or base material by the preservice examination, then the as-found flaw (postulated 75 percent through-wall flaw plus the portion of the flaw in the upper 25 percent) will be used for the crack growth analysis. This requirement is more conservative than that of N-504-3 and the ASME Code,Section XI, Appendix Q, paragraph Q-3000. With regard to the overlay design, flaws are considered to be 100 percent through the original weld for the entire circumference and no structural credit is taken for the original weld.

The NRC staff finds that the weld overlay design and crack growth calculations in Relief Request RR-III-05 are either consistent with or more conservative than the requirements of Code Case N-504-3 and/or Appendix Q to the ASME Code,Section XI. Therefore, Section 2 of Attachment III of the proposed alternative is acceptable.

4.3 Examination and Inspection Section 3 of Attachment III provides requirements for acceptance, preservice, and inservice examinations of the installed weld overlay. Attachment V provides a comparison to Code Case N504-2 and Appendix Q of ASME Code,Section XI. The NRC staff finds that the information in Section 3 of Attachment III is consistent with the requirements of Code Case N-504-3 and Appendix Q of the ASME Code,Section XI, with the following exceptions:

Acceptance Examination Paragraph 4.0(b) of Code Case N-638-1 requires that the ultrasonic examination be conducted at 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> after the completed weld overlay is cooled to ambient temperature. The purpose of the 48-hour hold after the weld overlay cooled to ambient temperature is to allow sufficient time for hydrogen cracking to occur (if it is to occur) after welding such that the weld overlay could be repaired prior to placing it in service. However, paragraphs 3.0(a)(2) and (a)(3) of Attachment III of the June 1, 2007, submittal proposes that the non-destructive examination be conducted 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> after completing the third temperbead layer of the weld overlay when ambient temperature temperbead welding is used. This proposal will allow the weld to be examined earlier than is required by Code Case N-638-1.

The staffs concern is on the potential for hydrogen cracking or cold cracking resulting from welding. The staff identified four potential contributors to cracking. They are changes in the microstructure of the base metal, sources for hydrogen introduction, tensile stress and temperature, and diffusivity and solubility of hydrogen in the P-3 material (i.e., pressurizer nozzles).

As presented in the licensees Attachment I,Section VI.A.3.f, the licensees technical basis is Electric Power Research Institute (EPRI) report 1013558, "Repair and Replacement Applications Center: Temperbead Welding Applications - 48 Hour Hold Requirements for Ambient Temperature Temperbead Welding" (ADAMS Accession No. ML070670060). The data in the EPRI report is based on testing performed on SA-508, Class 2 low-alloy ferritic steels, which is the material of the VCSNS pressurizer nozzles.

After evaluating all of the issues relevant to hydrogen cracking such as microstructure of susceptible materials, availability of hydrogen, applied stresses, temperature, and diffusivity and solubility of hydrogen in steels, EPRI concluded that there appears to be no technical basis for waiting the 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> after cooling to ambient temperature before beginning the NDE of the completed weld. There should be no hydrogen present, and even if it were present, the temperbead welded component should be very tolerant of the moisture. EPRI also notes that over 20 weld overlays and 100 repairs have been performed using temperbead techniques on low alloy steel components over the last 20 years. During this time, there has never been an indication of hydrogen cracking by the NDEs performed after the 48-hour hold or by subsequent inservice inspections.

An ASME Technical Basis Paper (ADAMS Accession No. ML070790679) to support the proposed revision to the 48-hour hold time requirement indicates that the introduction of hydrogen to the ferritic HAZ is limited to the first weld layer because this is the only weld layer that makes contact with the ferritic base material. The Technical Paper states that while the potential for the introduction of hydrogen to the ferritic HAZ is negligible during subsequent weld layers, these layers provide a heat source that accelerates the dissipation of hydrogen from the ferritic HAZ in non-water backed applications. The Technical Basis Paper concludes that there is sufficient delay time to facilitate the detection of potential hydrogen cracking when NDE is performed 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 weld layer.

Furthermore, the solubility of hydrogen in austenitic weld materials such as Alloy 52M is much higher than that of ferritic materials while the diffusivity of hydrogen in austenitic materials is lower than that of ferritic materials. As a result, hydrogen in the ferritic HAZ tends to diffuse into the austenitic weld metal, which has a much higher solubility for hydrogen. This diffusion process is enhanced by heat supplied in subsequent weld layers.

Based on this information, the licensee proposed to revise the 48 hour5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> hold time from the current requirement of performing NDE 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> after the finished weld overlay reaches ambient temperature to the revised requirement of performing NDE 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> after the third temperbead weld layer is completed.

On the basis of information submitted, the staff finds that it is not necessary to wait until 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> after the completed overlay has reached ambient temperature because any delayed hydrogen cracking, were it to occur, is expected to occur within the 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> following completion of the third temperbead weld layer. Therefore, the staff concludes that NDE of the weld overlay 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 weld layer is acceptable.

The acceptance standards in paragraph 3.0(a)(3)(i) of Attachment III of the June 1, 2007, submittal are identical to paragraph Q-4100(c)(1) of the ASME Code,Section XI, Appendix Q, except that paragraph 3.0(a)(3)(i) includes the additional limitation that the total laminar flaw shall not exceed 10 percent of the weld surface area and that no linear dimension of the laminar flaw area exceeds 3.0 inches. The licensee stated that the proposed changes provide additional conservatism to the weld overlay examination and to reduce the size of the un-inspectable volume beneath a laminar flaw. The staff agrees with the licensee that the proposed condition will limit the number of laminar flaws and the flaws in the un-inspectable volume underneath a laminar flaw in the weld overlay. Therefore, the staff finds that paragraph 3.0(a)(3)(i) is acceptable.

Preservice Examination As stated in the licensees Attachment V, the acceptance standards in paragraph 3.0(b)(2) of Attachment III to the June 1, 2007, submittal are identical to paragraph Q-4200(b) except for the following requirement: In applying the acceptance standards, wall thickness, tw, shall be the thickness of the weld overlay. The licensee stated that this provision is to clarify that the nominal wall thickness of Table IWB-3514-2 shall be considered the thickness of the weld overlay.

The acceptance standards were originally written for the welds identified in IWB-2500. Because IWB-2500 does not address weld overlays, this clarification was provided to avoid any potential confusion. The licensee stated that defining the weld overlay thickness as the nominal wall thickness of Table IWB-3514-2 has always been the practice since it becomes the new design wall of the piping or component nozzle. The staff notes that using the weld overlay thickness as a parameter for the acceptance standards of Table IWB-3514-2 is more conservative than using the pipe wall thickness because the proposed requirement limits the size of flaws to remain in service. The staff finds that applying wall thickness, tw, of the weld overlay in the acceptance standards of IWB-3514-2 is conservative and, therefore, is acceptable.

Inservice Examination Paragraph 3(c)(3) of Attachment III to the June 1, 2007, submittal proposes that flaws identified as PWSCC cannot be accepted by the analytical evaluation of subarticle IWB-3600 of the ASME Code,Section XI. Paragraph Q-4300(c) of Appendix Q to the ASME Code,Section XI, does not have this requirement. The staff finds this requirement is more conservative than Appendix Q of the ASME Code,Section XI because it limits the size and reduces the number of PWSCC flaws in the weld overlay in service. Therefore, this requirement is acceptable.

4.4 Ambient Temperature Temperbead Welding Attachment IV of the June 1, 2007, submittal specifies requirements for the ambient temperature temperbead welding, which are consistent with requirements of Code Case N-638-1 with the following exceptions. Attachment VI provides the technical basis for the proposed alternatives to Code Case N-638-1.

Code Case N-638-1, paragraph 1.0(a) limits the maximum area of an individual weld covering ferritic base metal to 100 square inches, whereas paragraph 1.0(b) of Attachment IV to the June 1, 2007, submittal proposes to limit the surface area to 300 square inches. The industrys technical basis for a larger weld surface area than the 100-square-inch area 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." Additional justification provided in the presentation slides entitled, "Bases for 500 Square Inch Weld Overlay Over Ferritic Material," was provided to the NRC staff in a public meeting held on January 10, 2007, in (ADAMS Accession No. ML070470565). The industry provided results of finite element analyses demonstrating that the stresses of a nozzle with the 300-square inch weld area will not adversely affect the integrity of the pressurizer nozzle. Based on a review of the information provided, the staff finds that the proposed 300-square-inch weld area limit over the ferritic base metal is acceptable.

Several requirements of Code Case N-638-1 are not relevant to the proposed weld overlay. For example, Code Case N-638-1, paragraph 1.0(a) requires that the depth of the weld not be greater than one-half of the ferritic base metal thickness. Paragraph 2.1(b) requires that the pressurized environment be duplicated in the procedure qualification test assembly. Paragraph 2.1(c) requires that the effects of irradiation on the properties of the materials be considered. Paragraph 2.1(h) requires the performance of Charpy V-notch testing of the ferritic weld metal of the procedure qualification test coupon. Paragraph 3.0(c) requires the deposition and removal of at least one weld reinforcement layer for similar materials (i.e., ferritic materials). None of these requirements apply to the proposed weld overlay because of the applied weld metal, overlay design, and installation method. Therefore, the staff agrees with the licensee that these requirements are not applicable to Relief Request RR-III-05.

Code Case N-638-1, paragraph 2.1(j) specifies acceptance criteria for Charpy V-notch tests of the HAZ. Paragraph 2.1(j) of N-638-1 requires that the average values of the three HAZ impact tests be equal to or greater than the average values of the three unaffected base metal tests.

Paragraph 2.1(g) of Attachment IV of the June 1, 2007, submittal proposes that the acceptance criteria for Charpy V-notch testing of the HAZ be based on average lateral expansion values. This change clarifies the intent of paragraph 2.1(j) of N-638-1 and aligns its Charpy V-notch acceptance criteria with that of ASME Code Section III, NB-4330, Impact Test Requirements, ASME Code Section XI, IWA-4620, Temperbead Welding of Similar Materials, and ASME Code Section XI, IWA-4630, Temperbead Welding of Dissimilar Materials. The staff finds the proposed paragraph 2.1(g) of Attachment IV of the June 1, 2007, submittal acceptable.

Code Case N-638-1, paragraph 3.0, does not address monitoring or verification of welding interpass temperatures. However, paragraph 3.0(e) of Attachment IV to the June 1, 2007 submittal proposes that the preheat and interpass temperature be measured using a contact pyrometer. In the first three layers, the interpass temperature will be measured every three to five passes. After the first three layers, interpass temperature measurements will be taken every six to ten passes for the subsequent layers. The proposed interpass temperature controls and measurements are based on field experience with depositing weld overlays. The interpass temperature beyond the third layer has no impact on the metallurgical properties of the low alloy steel HAZ. The staff finds that the proposed interpass temperature controls and measurements are an improvement to N-638-1 to monitor the proper heat input during welding and, therefore, are acceptable.

4.5 Performance Demonstration Initiative Program The U.S. nuclear utilities created the PDI program to implement performance demonstration requirements contained in Appendix VIII of Section XI of the ASME Code. To this end, EPRI has developed a program for qualifying equipment, procedures, and personnel in accordance with the ultrasonic testing 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, as discussed in the NRC letter dated July 3, 1984 (ADAMS 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 the 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. PDI 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 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 the ASME Code,Section XI, Appendix VIII, Supplement 11 as shown in Attachment VII of the June 1, 2007, submittal. The NRC staff concludes that the justifications for the differences are acceptable and the PDI program provides an acceptable level of quality and safety. Therefore, the proposed PDI program is acceptable for use to meet requirements of Supplement 11 of Appendix VIII to the ASME Code,Section XI.

The NRC staff finds that the requirements of Relief Request RR-III-05 are consistent with the provisions of Code Cases N-504-3 and N-638-1 and Appendix Q of the ASME Code,Section XI.

Therefore, the proposed Relief Request RR-III-05 is acceptable.

5.0 CONCLUSION

The NRC staff has reviewed the licensees submittal and determined that Relief Request RR-III-05 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 Relief Request RR-III-05 for weld overlay of the dissimilar and similar metal welds of the pressurizer safety valve, relief valve, spray line, and surge line nozzles for the third 10-year ISI interval at VCSNS.

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: John Tsao, NRR Date: March 25, 2008

Virgil C. Summer Nuclear Station cc:

Mr. R. J. White - Nuclear Coordinator S.C. Public Service Authority c/o Virgil C. Summer Nuclear Station Post Office Box 88, Mail Code 802 Jenkinsville, SC 29065 Resident Inspector/Summer NPS c/o U.S. Nuclear Regulatory Commission 576 Stairway Road Jenkinsville, SC 29065 Chairman, Fairfield County Council Drawer 60 Winnsboro, SC 29180 Mr. Henry Porter, Assistant Director Division of Waste Management Bureau of Land & Waste Management Dept. of Health & Environmental Control 2600 Bull Street Columbia, SC 29201 Mr. Thomas D. Gatlin, General Manager Nuclear Plant Operations South Carolina Electric & Gas Company Virgil C. Summer Nuclear Station Post Office Box 88, Mail Code 300 Jenkinsville, SC 29065 Mr. Bruce L. Thompson, Manager Nuclear Licensing South Carolina Electric & Gas Company Virgil C. Summer Nuclear Station Post Office Box 88, Mail Code 830 Jenkinsville, SC 29065 Ms. Kathryn M. Sutton Morgan, Lewis & Bockius LLP 111 Pennsylvania Avenue, NW.

Washington, DC 20004