ML062970101
| ML062970101 | |
| Person / Time | |
|---|---|
| Site: | Watts Bar, Sequoyah  |
| Issue date: | 10/20/2006 |
| From: | Thompson R Tennessee Valley Authority |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| TAC MD2381, TAC MD2382, TAC MD2383 | |
| Download: ML062970101 (11) | |
Text
Tennessee Valley Authority, 1101 Market Street, Chattanooga, Tennessee 37402-2801 October 20, 2006 10 CFR 50.55a U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, D.C. 20555-0001 Gentlemen:
In the Matter of
)
Docket Nos. 50-327 Tennessee Valley Authority) 50-328 50-390 SEQUOYAH NUCLEAR PLANT (SQN) UNITS 1 AND 2 AND WATTS BAR NUCLEAR PLANT (WBN) UNIT I - PREEMPTIVE WELD OVERLAYS ON ALLOY 600 PRESSURIZER NOZZLE-TO-PIPE WELDS AND ASSOCIATED ALTERNATIVE REPAIR TECHNIQUES - GENERIC REQUEST FOR RELIEF G-RR TVA RESPONSE TO NRC'S REQUEST FOR ADDITIONAL INFORMATION (RAI).
(MD2381, MD2382, MD2383)
This responds to NRC's RAI dated August 29, 2006, and incorporates the results of a telephone conference on October 2, 2006, as described below. TVA's original relief request is dated June 16, 2006.
G-RR-1 included commitments to provide a design report and final structural integrity evaluation on weld overlays. In a telephone call on October 2, 2006, among Doug Pickett, Ed Andruszkiewicz and Jason Paige (NRC), and Rob Brown (TVA), NRC stated that G-RR-1 would only be applicable to the refueling outage cycles specified below due to the uncertainty of future code changes on weld overlays that precluded approval beyond one refueling cycle. The next refueling cycles at SQN are Unit 1, Cycle 15 and Unit 2, Cycle 14, and the current WBN outage is for Unit 1, Cycle 7.
Also, NRC agreed that the contents of this response would supersede prior commitments made by TVA's letter of June 16, 2006.
Recently, during WBN Unit 1, Cycle 7, full coverage of examinations on dissimilar pressurizer welds found no recordable indications that require overlays to repair flaws, therefore negating the need for relief for WBN. As a result, TVA withdraws G-RR-1 A04eq Printed on reced paper
U.S. Nuclear Regulatory Commission Page 2 October 20, 2006 applicability for WBN Unit I Cycle 7 and the Second 10-Year Inservice Inspection Interval and the related WBN commitments.
Enclosure I provides TVA's response to the subject RAI that is applicable to weld overlays planned for SON Units I and 2. Enclosure 2 provides a list of new SON commitments that supersede prior commitments If you have any questions, please contact Rob Brown at (423) 751-7228.
Sincerely, Russell Thompson Acting Manager, Corporate Nuclear Licensing and Industry Affairs Enclosure cc (Enclosure):
U.S. Nuclear Regulatory Commission Sam Nunn Atlanta Federal Center 61 Forsyth Street, SW, Suite 23T85 Atlanta, Georgia 30303-8931 NRC Senior Resident Inspector U.S. Nuclear Regulatory Commission Sequoyah Nuclear Plant 2600 Igou Ferry Road Soddy Daisy, TN 37379 NRC Senior Resident Inspector U.S. Nuclear Regulatory Commission Watts Bar Nuclear Plant 1260 Nuclear Plant Road Spring City, TN 37381 Douglas V. Pickett, Senior Project Manager U.S. Nuclear Regulatory Commission Mail Stop 08G9A One White Flint, North 11555 Rockville Pike Rockville, Maryland 20852-2739
ENCLOSUREI SEQUOYAH NUCLEAR PLANT (SQN) UNITS I AND 2 GENERIC REQUEST FOR RELIEF G-RR-1 TVA'S RESPONSE TO NRC'S REQUEST FOR ADDITIONAL INFORMATION NRC Question I Table 2 of Regulatory Guide 1.147, Rev. 14, identifies the Code Cases that are conditionally acceptable, i.e., for use by licensees within the limitations and conditions identified in the table.
Those conditions are identified by the staff during its review of the Code Cases and are necessary to ensure an acceptable level of quality and safety. Therefore, licensees must include those conditions in their Relief Requests. No provisions are made in Regulatory Guide 1.147 for allowing licensees to delete or alter those conditions.
Tennessee Valley Authority's (TVA)'s June 16, 2006, letter requests relief from the Code Case N-638-1 condition in Table 2 of Regulatory Guide 1.147. Specifically, the letter states in part that, min lieu of the ultrasonic examination acceptance criteria of the Construction Code in the condition for the use of Code Case N-638-1, the acceptance criteria of ASME Section XI Nonmandatory Appendix Q, as stipulated in the Conditions of RG-1.147, Rev. 14, for the use of Code Case N-504-2 will be applied for the entire structural weld overlay." Therefore, in accordance with the reasoning stated in the previous paragraph, this Relief Request must be supplemented to recognize the Licensee's compliance with the condition for N-638-1 to provide an acceptable level of quality and safety. The condition stated in the Regulatory Guide must be included in the relief request.
Question 1 (above) was superseded by an email from Brenda Mozafari (NRC) dated September 25, 2006 and later clarified by Doug Pickett (NRC) by telephone call with Rob Brown on October 11, 2006 as follows:
Identify the Ultrasonic Testing (UT) acceptance criteria that will be used for the complete full structural weld overlay and heat affected zone beneath the weld overlay. If the acceptance criteria to be used are not consistent with the respective positions stated in Regulatory Guide 1.147 for the applicable code cases, provide the technical bases for its use, and revise the scope of the submittal to be for one cycle.
Response
TVA believes that the provisions for use of the ambient temperature temper bead welding techniques, in accordance with ASME Section XI Code Case N-638-1, are applicable to the deposition of the Alloy 52/52M overlay weld filler material to the SA-508, Class 2, nozzle material. TVA also believes that for the other areas over which the filler material is deposited (i.e., the Alloy 82/182 buttering and nozzle to safe-end weld, the SA-1 82, F316L safe-end, the ER308/E308 Stainless steel safe-end to pipe weld, and the SA-376 pipe material), N-638-1 is not applicable and the strict control of the ambient temper bead weld process is not necessary to achieve a high quality weld overlay. TVA intends to use the automated gas tungsten arc weld (GTAW) process over the full length of the overlay region. In order to provide an acceptable level of quality and safety, TVA intends to comply with the RG-1.147, Table 2, special conditions for the use of N-638-1, as follows:
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TVA proposes to follow the RG-1.147, Table 2, that added the requirement for having construction type flaws in the UT qualification procedure examination standards. This is addressed as part of the Electric Power Research Institute's (EPRI) Performance Demonstration Initiative (PDI) program for weld overlays, with EPRI's PDI procedure PDI-UT-8, and as such is proposed for use with TVA's examination of pressurizer weld overlays.
TVA proposes to meet this requirement.
RG-1.147, Table 2, added the requirement to use the Ultrasonic Acceptance Standards in NB-5330 and the associated subsections N-5331, "Fabrication," and NB-5332, "Preservice Examination." For the pressurizer nozzle preemptive weld overlays (PWOLs), the design "code-of-record" TVA proposes to use is the 2001 Edition through the 2003 Addenda (2001A03) of ASME Section III. Thus, for fabrication of the weld overlay region, Table 2 of RG-1.147 requires the acceptance cdteria of Subsection NB-5331 to be applied to the PWOL UT volumetric examinations. However, the use of the NB-5331 criteria is impractical under the overlay processes because it is extremely limiting on the acceptance for the overlay weld. The impracticality with the use of the NB-5331 criteria results from the fact that the PDI-UT-8 examination process is highly sensitive. The PDI UT type examination detects very small insignificant indications within the weld filler which cannot be readily characterized. Under the requirements of NB-5331 (b), these types of indications would be classified as lack of fusion. Classifications in this manner would render the indications rejectable and would precipitate numerous unnecessary repairs on the overlay without adding quality to the weld's integrity. In lieu of the criteria of NB-5331, TVA proposes to use the acceptance criteria of Appendix Q, paragraph Q-4100(c), in keeping with the guidelines of RG-1.147 for use of N-504-2.
TVA's intent with this proposal is to apply the criteria of paragraph Q-4100(c) over the full thickness of the overlay weld material, and to indications that are judged, by the examiner, to be within the heat affected zone (HAZ) under the overlay material. The maximum expected depth of the HAZ into the existing nozzle material, based upon TVA experience with the use of the automatic GTAW, is approximately 1/8 inch. The criteria can be properly used for the full length of the overlay material where N-638-1 applies and where accurate and useable UT data can be obtained. Regions where there are practical limitations on the collection of usable UT data can best be defined as those volumes which would fall outside of the required examination volume as defined in ASME Section Xl, Appendix Q, Figure Q-4100-1. TVA intends to perform a surface examination of the tapering ends of the overlay.
For the volumes defined by Appendix Q, Figure Q-4100-1, TVA proposes to apply the acceptance criteria of Q-4100(c). The use of the acceptance criteria in this manner is also supported by the fact that the primary reason for the application of the overlays is the greater potential for primary water stress corrosion cracking in the Alloy 600 dissimilar metal (DM) nozzle-to-safe end welds. The overlay ends are not in close proximity to the DM welds. Thus, if flaws in the Alloy 600 weld propagate, they would arrest at the interface with the ferritic base material or the Alloy 52/52M weld filler metal and would be contained in the volume of material that is subject to the preservice examination requirements (e.g., within the examination volume defined by Figure Q-4300-1). For the TVA nozzle configurations, the UT examination procedures uses at least a refracted longitudinal beam transducer with the angle of the ultrasonic beam transmitted at an angle of either 60 degrees, or 70 degrees, and a straight beam transducer to investigate the weld overlay material and HAZ volumes.
This technique maximizes the amount of usable volumetric data that WVA can obtain on the integrity of the overlay region. However, fully usable UT volumetric examination data is practical to obtain axially along the length of the overlay until the transducer approaches the tapering ends of the overlay region. At the ends of the overlay, the weld filler materials taper to blend with the existing curvature at the nozzle end and the pipe end geometries. This Page 2 of 5
specific area limitation can extend approximately 0.2 inches to 0.5 inches (or more) back from the actual end of the deposited weld material, depending upon the geometry encountered at the nozzle and pipe ends as the transducer is moved around the circumference of the overlay volume. As the UT transducer approaches the ends of the overlay weld material, the filler material thickness decreases and the transducer's leading edge contacts the curvature of the existing nozzle and pipe. This causes the transducer to rotate the sound beam direction away from the weld volume of interest. In addition, the tapering overlay filler material outside of the volume defined by Figure Q-4100-1 (i.e., A-B-C-D) is not ground and polished to the same finish as the areas over the welds because of the need to minimize the potential for damage to the base metal materials at the nozzle and pipe ends. These physical limitations impede the transducer's ability to provide usable data at the ends of the overlay material. Performance of additional UT data collection on the taper ends also would be conducted in very high radiation fields. Recent experience from volumetric examination of the DM welds on the WBN pressurizer, during the ongoing Unit 1, Cycle 7 outage, indicated that examination personnel would be working in contact radiation fields of 450-500 mrem/hour. The increased stay-time in these high radiation areas, to prepare the tapered end surfaces and perform the UT examinations, is not justified by improved quality or the amount of usable data.
Safety and quality of the overlay are maintained by the performance of two liquid dye penetrant (LPT) surface examinations, which compensate for the lack of meaningful UT data at the tapered ends of the overlay. Initially, a LPT examination (on the existing nozzle and pipe surface area prepared to accept the overlay and for an additional area extending a minimum of 2 inches in length past the expected overlay area) is to be performed prior to the application of the overlay. Any unacceptable indications found during the initial LPT surface examination are evaluated for the need to repair the flaw. Such repairs may include activities that range from minor surface polishing and blending of the indication into the surrounding area, to full-depth metal removal of the flaw and welded repair in accordance with the site's ASME Section XI Repair and Replacement Program "code-of-record." In addition, following the application of the overlay and after the final grinding and polishing of the overlay surface to facilitate post-overlay examinations, a LPT surface examination is performed on the overlay surface and for an additional area extending 2 inches past the end of the overlay. These surface examinations provide added assurance of the quality of the overlay process. For these two surface examinations, TVA proposes to apply the surface examination acceptance criteria of paragraph Q-4100(b), in accordance with RG-1.147 condition to use the requirements of Appendix Q. Note also that the probability of inducing cracks in the HAZ with the GTAW overlay application process is greatly reduced in the areas where weld material is deposited over the stainless steel welds, safe-ends, and piping components.
For the post-overlay preservice volumetric UT examinations, the use of the ASME Section III NB-5330, Subsection NB-5332, criteria results in the use of same basic acceptance criteria per Table 2 of RG-1.147 and the use of Appendix Q for preservice examination requirements (i.e., the criteria from IWB-3000). TVA is therefore meeting these acceptance criteria. In addition, the procedure for recording and evaluation, of any indications encountered during the post-overlay UT examination, is performed in accordance with the requirements of the PDI-UT-8, Revision F. Plots of the suspected flaw indications in a cross sectional schematic of the overlay weld are also made, where appropriate, to aid in the identification of the origins of any such indications. This data is also used in future inservice examinations of the PWOLs which follow the requirements of ASME Section XI, Appendix 0, Article Q-4000.
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In support of these proposed modifications to the N-638-1 requirements from Table 2 of RG-1.147, and in keeping with recent industry precedents, TVA will provide the results of their post-overlay UT examinations to the NRC within 14 days of completion of the last oveday examination.
NRC Question 2 On page A4-2 of Attachment 4 to your submittal, you state that it is not practical to perform the full UT of the 1.5T band. Your basis is that the existing nozzle configurations shown in the TVA drawings contained in Attachment I prevent you from completing the full UT. Please provide additional information to support your conclusion. The information provided should discuss the achievable amount of area that will be successfully examined for each pre-emptive weld overlay design configuration. Secondly, the UT examination should be performed on the maximum extent achievable.
Response
Please note that Attachment I in G-RR-1 is a general sketch of the pressurizer nozzle overlay areas and does not depict the actual configuration of each of the six individual pressurizer nozzle areas. In addition, the final configurations of the overlay areas vary as the actual welds are deposited in the oveday process. These variations in the final overlay configuration also influence the amount of total coverage obtained during the required examinations.
As discussed in response to question 1, the PWOL area where the ambient temper bead welding requirements of N-638-1 applies is the deposited weld filler material over the SA-508, Class 2 nozzle material. Thus, the extension of the examination area, for a band around the overlaid area for at least a distance of 1.5 times the thickness (1.5t) of the component, only applies at the nozzle end of the overlay weld.
Practical limitations are encountered during the performance of volumetric examinations of the completed weld overlay regions. UT scanning of the transition areas in the tapered and blended ends of the overlays encounters physical limitations that impede the collection of qualified and usable data of the thinning overlay weld material volume. The use of the surface examination data in these small overlaid regions becomes more reliable as an indicator of the quality of the overlay and the effects on the existing base materials. It should also be noted that the generic industry approved PDI/UT procedures for qualification of equipment and personnel to perform volumetric examinations of weld ovedays of similar and dissimilar metal welds apply only to examination volumes generally depicted in Appendix Q, Figures Q-4100-1 and Q-4300-1 as required by Table 2 of RG-1.147 in conjunction with N-504-2. Performance of the UT examination beyond the Appendix 0 defined areas will not be properly qualified through the approved industry qualification process (i.e., with the use of PDI-UT-8). While UT data can be collected in the blend area ends using the same equipment, the usefulness of the data collected is limited by the physical configuration and the qualification certification processes.
Extension of the volumetric examination areas beyond the overlay regions for a dimension equal to 1.5t of the component past the end of the temper bead weld overlay area, or for a maximum distance of up to 5 inches, results in the collection of data in areas of the nozzle and piping materials where indications may already exist. These indications may have already been accepted based on the original fabrication standards of the nozzle and piping components. In addition, LPT surface examinations are performed on overlay polished surfaces and in a band at the ends of the overlay extending 2 inches past the end of the overlay. The potential for the Page 4 of 5
deposition of the overlay material to induce new flaws in any volume beyond this 2-inch region is insignificant. Thus, the imposition of the extra 1.5t bandwidth is unneeded and has the potential to result in unnecessary repairs of the nozzle and pipe base metal areas.
Volumetric examination of the overlay weld material and the HAZ cannot be properly performed from the curved surfaces of the nozzle assembly that extends toward the pressurizer vessel wall, or from the extended pipe surfaces at the end (near the stainless steel process piping).
The PDI-UT-8 procedures and equipment uses refracted longitudinal waves transmitted through the materials in the examination region. Examination of the overlay volume and the associated HAZ cannot be properly performed by bouncing the UT sound waves back through the extended regions of the nozzle and pipe wall materials. No usable data can be collected on the overlay region by extending the UT examination over the extra volume as required by application of the 1.5t provision of N-638-1.
TVA intends to perform the UT examination on the final overlay weld volume, over the length, as described in TVA's response to Question 1. The volume covered by this examination includes the overlay HAZ volume that is approximately 1/8 inch deep into the original nozzle, weld, and pipe materials. The surface (LPT) examination, following the completion of the overlay, covers the area that extends 2 inches beyond the end of the overlay region.
In accordance with the requirements with the use of N-504-2 and Appendix Q, TVA proposes to perform the ASME Section XI preservice UT examination of the overlay regions. The preservice examinations of the original DM weld and the similar metal weld areas includes the volume of the outer 25 percent of thickness of the original welds and an additional 0.5 inch band of adjacent base metal. Inclusion of the added N-638-1 bandwidth of 1.5t volume and for 25 percent of the outer wall thickness over the whole length of the overlay unnecessarily increases the examination volumes. This added volume increases the examination time and radiation exposure to inspection personnel with no added benefit to the quality of the overlays.
Illustrations of the two basic volumes proposed to be covered by TVA's examinations are shown in the attached sketches. Sketch No. 1 illustrates the examination volume for the fabrication and construction type examination required with the application of the two code cases. Sketch No. 2 illustrates the examination volume for the preservice examination to be performed in accordance with the requirements for the use of N-504-2 and Appendix Q.
As stated G-RR-1, Attachment 4, the areas most prone for the introduction of cracks during the overlay process will be in the heat affected zones (HAZ) between the overlay weld material and the existing surfaces on the SA-508 nozzle, the Alloy 82/182 buttering and nozzle to safe-end weld, the SA-182, F316L safe-end piece, the ER308/E308 Stainless steel safe-end to pipe weld, and the SA-376 pipe material. These areas will be thoroughly covered by the combination of the two surface examinations and the construction/fabrication and preservice UT examinations performed to meet the Appendix Q requirements. TVA is proposing the use of the PWOLs as a mitigation activity. The designs of the weld overlays are such that the presence of a flaw, within the existing nozzles, welds, and pipe materials, and which grows to 100 percent through-wall, will not encroach upon the required full structural integrity of the overlaid areas. The combination of the surface and volumetric examination methodologies and the application of the acceptance criteria provide an acceptable level of quality and safety with the proposed PWOL modifications.
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Attachment (Enclosure 1)
TVA Sketch I and Sketch 2
ILLUSTRATION SKETCH NO.]
I N-504-2.
APPENDIX 0. FICURE Q-4100-1 AND PDI-UT-8 I
OVERLAY EXAMINATIO4 VOLUME A-B-C-D ALLOY 600 82/182 WELD OVERLAY-ALLOY 52/52U ID-E b
z N
A NOZZLE SA-508.
CLASS 2 BUTTERING ALLOY 600 82/1152 STAINLESS STEEL WELD-ER308/E308 PEAT AFFECTED ZONE (HAZ) g0,l25S-J STAINLESS STEEL PIPE SA-376 APPLICABLE VOLUMETRIC EXAMINATION VOLUME '.
FOR USE OF APPENDIX Q ACCEPTANCE CRITERIA IO/13l00 a *I1l AM
.Z\\34..l.l-I..?OWI
- V.,IleSV~f.. ftl
- 3b.-lr-..--b Leg i~me.eg K\\
ILLUSTRATION SKETCH NO.2 POST-OVERLAY SURFACE EXAMINATION AREA ALLOY 600 82/182 WELD BUTTERING ALLOY 600 82/182 PRESERVICE AND INSERVICE VOLUMSETRIC EXAMINATION VOLUMES FOR USE OF N-504-2. APPENDIX 0, FIGURE 0-4300-1. PDI-UT-8 AND TABLE IWB-3514-2 ACCEPTANCE CRITERIA EXAMINATION VOLUME a-b-c-d --
STAINLESS STEEL PIPE SA-376 9/f/2O 4 114 POO w:\\oqo-4, I -ro.\\vra ov~r ieyo~t mill won a.:
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let roo.dw
ENCLOSURE2 LIST OF COMMITMENTS SQN COMMITMENTS SCHEDULE TVA will provide the results of the post-overlay This data will be provided within 14 days ultrasonic (UT) examinations of the preemptive after completion of the last UT examination weld overlays (PWOLs) on the associated unit of the PWOLs, as appropriate for the pressurizer spray, relief, safety, and surge line applicable unit's outage.
nozzles to the NRC (6 locations, each unit).
This data will include:
0 a list of indications detected 1 8
the disposition of all detected indications using the acceptance standards in accordance with ASME Section Xl, Appendix Q, (i.e., Tables IWB-3514-2 or IWB-3514-3, as appropriate); and, if possible,
[ the type and nature of the indications2 Also included in this data will be a discussion of any needed repairs to the overlay material and/or the base metal or existing welds, and the reason for the repairs.
Subsequent inservice examination of the structural weld overlays on pressurizer welds will be in accordance with ASME Section XI, Appendix Q, Q-4300. Where PWOLs have been installed over dissimilar metal welds that have been examined with PDI qualified procedures and have no recordable indications, subsequent inservice examinations will be in accordance with Appendix Q-4300(b) through (f) and Q-4310.
The recording criteria of the UT examination procedure to be used for the post-overlay examinations (i.e.,
TVA's N-UT-66 procedure, which uses Revision F of the EPRI PDI-UT-8 procedure ) requires that all indications, regardless of amplitude, be investigated to the extent necessary to provide accurate characterization, Identity, and location. Additionally, PDI-UT-8 requires that all indications, regardless of amplitude, that cannot be clearly attributed to the geometry of the overlay configuration be considered as flaw indications 2 Ultrasonic examination procedure PDI-UT-8, Revision F also requires that all suspected flaw Indications are to be plotted on a cross sectional drawing of the overlay weld and that the plots should accurately Identify the specific origin of the reflector.
TVA confirmed by telecon that the area beyond point A toward the nozzle area on the drawing (sketch #1),
as discussed with NRC on October 19, 2006, will be polished per Westinghouse/PCI. This area will be UT examined to the extent practical keeping in mind that the UT procedure is limited as the volume narrows.