ML073541072

Request for Relief No. RR-ENG-2-48 from ASME Code Case N-638-1 Requirements Regarding Start Time for 48-Hour Hold Period Before Non-Destructive Examination of Weld Overlay Repair

ML073541072
Person / Time
Site:	South Texas
Issue date:	01/10/2008
From:	Hiltz T NRC/NRR/ADRO/DORL/LPLIV
To:	Sheppard J South Texas
Thadani, M C, NRR/DORL/LPL4, 415-1476
References
Code Case N-638-1, RR-ENG-248, TAC MD5927
Download: ML073541072 (9)

Text

January 10, 2008 Mr. James J. Sheppard President and Chief Executive Officer STP Nuclear Operating Company South Texas Project Electric Generating Station P. O. Box 289 Wadsworth, TX 77483

SUBJECT:

SOUTH TEXAS PROJECT, UNIT 1 - RELIEF REQUEST NO. RR-ENG-2-48 FROM ASME CODE CASE N-638-1 REQUIREMENTS REGARDING THE START TIME FOR THE 48-HOUR HOLD PERIOD BEFORE NONDESTRUCTIVE EXAMINATION OF WELD OVERLAY REPAIR (TAC NO. MD5927)

Dear Mr. Sheppard:

The U.S. Nuclear Regulatory Commission (NRC) staff has reviewed and evaluated the information provided by STP Nuclear Operating Company (the licensee) in its letter dated June 19, 2007. The licensee requested the approval of relief request RR-ENG-2-48, for South Texas Project (STP), Unit 1, in which the licensee requested relief from certain requirements of the American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code),

Code Case N-638-1, ">Similar and Dissimilar Metal Welding Using Ambient Temperature Machine GTAW [Gas Tungsten Arc Weld] Temper Bead Technique." Code Case N-638-1 is an NRC accepted Code case that has been conditionally approved for generic use in Regulatory Guide 1.174, AInservice Inspection [ISI] Code Case Acceptability, ASME Section XI, Division 1.@

It requires that when temper bead welding is used in weld overlay of dissimilar metal butt welds, the nondestructive examinations (NDE) are required to be conducted at least 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 returned to ambient temperature. Through the RR-ENG-2-48 relief request, the licensee proposes to change the requirement to allow performance of NDE 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> after the third temper bead weld layer is completed. The overall effect of the change would be a reduction in the time needed for weld overlay installation.

Based on the information provided in the licensee=s submittal, the NRC staff concludes that the proposed modification to perform 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 the completion of third layer of temper bead welding will provide an acceptable level of quality and safety. Therefore, pursuant to Title 10 of the Code of Federal Regulations, paragraph 50.55a(a)(3)(i), the alternative proposed in the licensees request RR-ENG-2-48 is authorized for the STP, Unit 1, for the remainder of the second 10-year ISI interval, which is scheduled to end in September 2010, and only when welding is performed in accordance with relief as proposed in RR-ENG-2-48 for the safe-end welds identified in the enclosed NRC safety evaluation for STP, Unit 1.

J. J. Sheppard All other ASME Code,Section XI, requirements for which relief was not specifically requested and authorized herein by the NRC staff remain applicable, including third-party review by the Authorized Nuclear Inservice Inspector. The NRC staff=s safety evaluation is enclosed.

Sincerely,

/RA/

Thomas G. Hiltz, Chief Plant Licensing Branch IV Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket No. 50-498

Enclosure:

Safety Evaluation cc: See next page

ML073541072 NRR-028 (*)No significant change to SE input.

OFFICE NRR/LPL4/PM NRR/LPL4/LA NRR/DCI/CNPB/BC OGC Nlo NRR/LPL4/BC NAME MThadani:sp JBurkhardt TChan(*) RUHolmes THiltz DATE 01/072008 1/7/2008 11/9/2007 1/9/2008 1/10/08 South Texas Project, Units 1 and 2 12/5/2007 cc:

Senior Resident Inspector S. M. Head, Manager, Licensing U.S. Nuclear Regulatory Commission STP Nuclear Operating Company P.O. Box 289 P.O. Box 289, Mail Code: N5014 Wadsworth, TX 77483 Wadsworth, TX 77483 C. M. Canady C. T. Bowman, General Manager, Oversight City of Austin STP Nuclear Operating Company Electric Utility Department P.O Box 289 721 Barton Springs Road Wadsworth, TX 77483 Austin, TX 78704 Ms. Marilyn Kistler J. J. Nesrsta/R. K. Temple Senior Staff Specialist, Licensing E. Alercon/Kevin Pollo STP Nuclear Operating Company CPS Energy P.O. Box 289, Mail Code 5014 P.O. Box 1771 Wadsworth, TX 77483 San Antonio, TX 78296 Environmental and Natural Resources INPO Policy Director Records Center P.O. Box 12428 700 Galleria Parkway Austin, TX 78711-3189 Atlanta, GA 30339-3064 Mr. Jon C. Wood Regional Administrator, Region IV Cox, Smith, & Matthews U.S. Nuclear Regulatory Commission 112 East Pecan, Suite 1800 611 Ryan Plaza Drive, Suite 400 San Antonio, TX 78205 Arlington, TX 76011 Director, Division of Compliance & Inspection Steve Winn/Christie Jacobs Bureau of Radiation Control Eddy Daniels/Marty Ryan Texas Department of State Health Services NRC Energy, Inc. 1100 West 49th Street 211 Carnegie Center Austin, TX 78756 Princeton, NJ 08540 Mr. Ted Enos Mr. Nate McDonald 4200 South Hulen County Judge for Matagorda County Suite 422

\1700 7th Street, Room 301 Ft. Worth, TX 76109 Bay City, TX 77414 Mr. Brian Almon A. H. Gutterman, Esq. Public Utility Commission of Texas Morgan, Lewis & Bockius P.O. Box 13326 1111 Pennsylvania Avenue, NW Austin, TX 78711-3326 Washington, DC 20004 E. D. Halpin, Site Vice President STP Nuclear Operating Company South Texas Project Electric Generating Station P.O. Box 289 Wadsworth, TX 77483

South Texas Project, Units 1 and 2 12/5/2007 Ms. Susan M. Jablonski Mr. Anthony P. Jones, Chief Boiler Inspector Office of Permitting, Remediation Texas Department of Licensing and and Registration Regulation Texas Commission on Environmental Quality Boiler Division MC-122 E.O. Thompson State Office Building P.O. Box 13087 P.O. Box 12157 Austin, TX 78711-3087 Austin, TX 78711 Mr. Ken Coates, Plant General Manager STP Nuclear Operating Company South Texas Project Electric Generating Station P.O. Box 289 Wadsworth, TX 77483

SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION APPLICATION OF WELD OVERLAYS IN PRESSURIZER NOZZLE SAFE-END WELDS STP NUCLEAR OPERATING COMPANY SOUTH TEXAS PROJECT, UNITS 1 AND 2 DOCKET NOS. 50-498 AND 50-499

1.0 INTRODUCTION

By letter dated June 19, 2007 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML071770351), STP Nuclear Operating Company (the licensee) requested U.S. Nuclear Regulatory Commission (NRC) approval to modify the 48-hour hold time requirement as defined in American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code), Code Case N-638-1, "Similar and Dissimilar Metal Welding Using Ambient Temperature Machine GTAW [Gas Tungsten Arc Weld] Temper Bead Technique" for the South Texas Project (STP), Unit 1. Code Case N-638-1 is an NRC acceptable ASME Code Case that has been conditionally approved for generic use in Regulatory Guide (RG) 1.147, Inservice Inspection Code Case Acceptability, ASME Section XI, Division 1. It requires when temper bead welding is used in weld overlay of dissimilar metal butt welds, the nondestructive examinations (NDE) are required to be conducted at least 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 returned to ambient temperature. The licensee, through RR-ENG-2-48, proposes to change this requirement to allow performance of the NDE 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> after the third temper bead weld layer is completed. The overall effect of the change would be a reduction in the time needed for weld overlay installation. The proposed alternative requested in RR-ENG-2-48 is for the remainder of the second 10-year inservice inspection (ISI) interval, which is scheduled to end in September 2010, for STP, Unit 1.

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. The

ISI code of record for STP, Unit 1 second 10-year ISI interval, which started in September 2000 and is scheduled to be complete in September 2010, is the 1998 Edition of Section XI of the ASME Code.

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 licensee sets forth in RR-ENG-2-48 its proposed alternative that provides an acceptable level of quality and safety. The requirement of 10 CFR 50.55a(a)(3)(i) provides the regulatory basis to evaluate the proposed modification.

3.0 TECHNICAL EVALUATION

3.1 Requirements for Which Relief is Requested Paragraph 50.55a(b) of 10 CFR approves the use of the Code cases listed in NRC RG 1.147, Revision 14. NRC RG 1.147, Revision 14, lists ASME Code Case N-638-1 as a conditionally acceptable Section XI Code case. Code Case N-638-1 states, in part, that the final weld surface shall be examined using NDE methods when the completed weld has been at ambient temperature for at least 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br />.

3.2 Licensees Proposed Alternative The licensee, through RR-ENG-2-48, proposes to start the 48-hour hold period before performance of NDE immediately after completion of the third temper bead layer.

3.3 Components for Which Relief is Requested The licensee will perform proactive repairs for five (5) ASME Class 1 pressurizer spray line nozzle safe-end welds at STP, Unit 1 (identifying numbers 6"RC1003BB1, 6"RC1004NSS, 6"RC1009NSS, 6"RC1012NSS, and 6"RC1015NSS). Each has an Alloy 182/82 weld connecting the low alloy steel nozzle to an austenitic stainless steel safe end, and a stainless steel weld connecting the safe end to the stainless steel piping.

3.4 Staff Evaluation Code Case N-638-1 requires that when temper bead welding is used, surface and ultrasonic examinations shall be performed when the completed weld has been at ambient temperature for at least 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br />. This delay was provided to allow sufficient time for hydrogen cracking to occur (if it is to occur) in the heat affected zone (HAZ) of ferritic materials prior to performing examinations, to ensure detection of hydrogen cracking by NDE. However, based on research and industry experience, the Electric Power Research Institute (EPRI) has provided a technical basis for starting the 48-hour hold after completion of the third temper bead weld layer rather than waiting 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> after the weld overlay cools to ambient temperature.

EPRI found that weld layers beyond the third temper bead layer are not designed to provide tempering to the ferritic HAZ during ambient temperature temper bead welding. EPRI has documented its technical basis in Technical Update report 1013558, "Repair and Replacement Applications Center: Temperbead Welding Applications 48-Hour Hold Requirements for Ambient Temperature Temperbead Welding" (ADAMS Accession No. ML070670060).

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 temper bead welded component should be very tolerant of the moisture ..." EPRI also notes that over 20 weld overlays and 100 repairs have been performed using temper bead 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 ISIs.

An ASME Technical Basis Paper (ADAMS Accession No. ML070790679) which supports 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 nonwater-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 staff finds that starting the 48-hour hold time after completion of the third temper bead weld layer is acceptable. The staff finds that the licensee has provided sufficient technical justification to show that hydrogen cracking in the weld overlay would not likely occur under the proposed alternative. A staff review of operational experience and research in this area has found no technical objection to the proposed alternative. Therefore, 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 to perform NDE because any delayed hydrogen cracking, were it to occur, would be 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 temper bead weld layer.

4.0 CONCLUSION

The staff has reviewed the licensees submittal and determined that the proposed modification to perform 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 temper bead weld layer will provide an acceptable level of quality and safety.

Therefore, pursuant to 10 CFR 50.55a(a)(3)(i), the staff authorizes the use of RR-ENG-2-48 for the remainder of the second 10-year ISI interval at STP, Unit 1 which is scheduled to end in September 2010, and only when welding is performed as described in RR-ENG-2-48 for those safe-end welds identified in Section 3.3 above, at the STP, Unit 1.

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: J. Collins Date: January 10, 2008