NOC-AE-08002323, Summary Assessment of Pressurizer Nozzle Safe End Weld Overlays: Shrinkage and Fatigue Crack Growth (RR-ENG-2-43)

Summary Assessment of Pressurizer Nozzle Safe End Weld Overlays: Shrinkage and Fatigue Crack Growth (RR-ENG-2-43)

ML081760284
Person / Time
Site:	South Texas
Issue date:	06/18/2008
From:	Jenewein B South Texas
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
NOC-AE-08002323, TAC MD1414, TAC MD1415, TAC MD1416, TAC MD1417, TAC MD1418, TAC MD1419, TAC MD1420, TAC MD1421, TAC MD1422, TAC MD1423
Download: ML081760284 (5)

Text

Nuclear Operating Company South Teas Pro/ectElectric GeneratingStation P.O Box 289 Wadsworth Texas 77483 -

June 18, 2008 NOC-AE-08002323 File No.: G25 10 CFR 50.55a U.S. Nuclear Regulatory Commission Attention: Document-Control Desk One White Flint North 11555 Rockville Pike Rockville, MD 20852-2738 South Texas Project Unit 1 Docket No. STN 50-498 Summary Assessment of Pressurizer Nozzle Safe End Weld Overlays:

Shrinkage and Fati-que Crack Growth (RR-ENG-2-43) (TAC Nos. MD1414-1423)

References:

1) Letter dated November 29, 2006, from M. J. Berg, STPNOC, to NRC S,,Document Control Desk, "Summary Assessment of Pressurizer Surge Line Weld Overlay: -Shrinkage and Fatigue Crack Growth (RR-ENG-2-43)

(TAC Nos. MD1414-1423)" (ML063450122)

2) Letter dated September 28, 2006, from David W. Rencurrel, STPNOC, to NRC Document Control Desk, "Response to Request' for Additional Information on Proposed Alternative to ASME Section XI Requirements for Application of a Weld Overlay (RR-ENG-2-43) (TAC Nos. MD1414-1423)"

(ML062850090)

Pursuant to 10 CFR 50.55a(a)(3)(i), the STP Nuclear Operatinb Company (STPNOC) requested approval to use an alternative to the requirements of the American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code),Section XI for the structural weld overlays on the South Texas Project Unit 1 and Unit 2 pressurizer spray, relief, safety, and surge nozzle safe-ends. NRC approval to perform the structural weld overlays in Unit 1 was given April 2, 2007. The surge nozzle weld overlay was performed during Unit 1 refueling outage 1RE13, and shrinkage and fatigue crack growth are addressed in Reference 1.' The pressurizer spray, relief, and safety nozzle safe-end overlays were performed during the April 2008 Unit 1 refueling outage (1 RE1 4).

STPNOC completed a stress analysis of the pressurizer spray, relief, and safety nozzle safe-ends pre-emptive weld overlays prior to restart of Unit 1. A summary of the results is attached.

,The stress analysis results support the conclusion that structural weld overlays are a suitable pre-emptive measure for anticipated flaw development.

This supplement to the request fulfills commitments made in Reference 2. Mitigative weld overlays for the pressurizer nozzle safe ends in Unit 1 and Unit 2 are complete.

There are no commitments in this submittal.

STI: 32326414 7

NOC-AE-08002323 Page 2 of 2 If there are any questions, please contact either Mr. Philip L. Walker at (361) 972-8392 or me at (361) 972-7431.

eingg/ rog rams PLW

Attachment:

Summary Assessment of Pressurizer Spray, Relief, and Safety Nozzle Safe-End Weld Overlays: Shrinkage and Fatigue Crack Growth

NOC-AE-08002323 Page 3 of 3 cc:

(paper copy) (electronic copy)

Regional Administrator, Region IV Thad Hill U. S. Nuclear Regulatory Commission Catherine Callaway 612 East Lamar Blvd., Suite 400 Jim von Suskil Arlington, Texas 76011-4125 NRG South Texas LP Mohan C. Thadani A. H. Gutterman, Esquire Senior Project Manager Morgan, Lewis & Bockius LLP U.S. Nuclear Regulatory Commission One White Flint North (MS 7 D1) Mohan) C. Thadani 11555 Rockville Pike U. S. Nuclear Regulatory Commission Rockville MD 20852 Richard A. Ratliff J. J. Nesrsta Bureau of Radiation Control R. K. Temple Texas Department of State Health Services K. M. Polio 1100 West 49th Street E. Alarcon Austin, TX 78756-3189 City Public Service C. M. Canady C. Kirksey City of Austin City of Austin Electric Utility Department 721 Barton Springs Road Jon C. Wood Austin, TX 78704 Cox Smith Matthews Senior Resident Inspector U. S. Nuclear Regulatory Commission P. 0. Box 289, Mail Code: MN1 16 Wadsworth, TX 77483

Attachment NOC-AE-08002323 Page 1 of 2 South Texas Project Unit 1 Summary Assessment of Pressurizer Spray, Relief, and Safety Nozzle Safe-End Weld Overlays:,

Shrinkage and Fatigue Crack Growth Shrinkage Assessment:

Calculated Limits on design stress due to shrinkage were determined prior to completing the weld overlay.

The limits were calculated using WESTDYN 7.1, a program acceptable for use in piping analysis, with an assumed pipe temperature of 70°F. A displacement of 0.25-inch was applied to the pressurizer safe end as cold spring loading for the spray nozzle. This displacement is assumed as the main loading input to represent shrinkage due to the nozzle weld overlay.

The resulting bending stress for the spray nozzle as determined by computer calculations is 13,089 psi. The code allowable stress for this piping at 70°F is 2Sm = 32,000 psi.

Consequently, the piping qualifies under code stress requirements where shrinkage does not exceed 0.25-inch.

Actual Actual shrinkage measured at 90° intervals around the circumference of the spray nozzle weld overlay is 0.203 inch. This shrinkage results in an applied stress of 10,628 psi compared to the Code allowable of 32,000 psi.

The maximum shrinkage measured at 900 increments around the circumference of the three Safety nozzles and the Relief nozzle following the overlay is 0.263-inch on safety nozzle N3.

The highest applied bending stress experienced due to shrinkage is 30,589 psi on safety nozzle N4B, compared to the code allowable stress 2Sm = 40,000 psi for this piping at 70°F.

Therefore, the three Safety nozzles and the Relief Valve Inlet piping meet the allowable code stresses.

Pressurizer Maximum Applied Code Nozzle Shrinkage Stress Allowable Spray Nozzle N2 0.203 inch 10,628 psi* 32,000 psi Safety Nozzle N3 0.263 inch 21,190 psi 40,000 psi Relief Nozzle N4A 0.181 inch 16,653 psi 40,000 psi Safety Nozzle N4B 0.219 inch 30,593 psi 40,000 psi Safety Nozzle N4C 0.248 inch 21,559 psi 40,000 psi

• This stress value was determined as a ratio of the actual shrinkage to the assumed shrinkage (0.25-inch).

Attachment NOC-AE-08002323 Page 2 of 2 In addition, the calculated stress in the unreinforced fabricated tee in the Relief piping determined by applying a Stress Intensification Factor (SIF) is 23,057 psi. The code allowable stress for this piping at 70°F is 2Sm = 28,200 psi. Therefore, the code stress limitation is not exceeded.

The design pressure and normal operating pressure are 2,485 psig and 2,235 psig, respectively. With the additional stress provided by the structural weld overlay, the total stresses remain significantly below the code allowable stress. Because the piping is designed for significantly more shrinkage and still satisfy code design requirements, the actual shrinkage experienced results in even greater safety margin.

Fatigue crack growth assessment:

WCAP-16611-P, "South Texas Units 1 and 2 Pressurizer Safety/Relief, Spray, and Surge Nozzles Structural Weld Overlay Qualification," August 2006, addresses the issue of fatigue crack growth. If an assumed flaw in the piping extends through 75% of the pipe thickness, more than 19 years can be expected to pass before the flaw grows through the base metal. A table below provides the specific nozzle and an expected service life relative to the ratio of assumed initial flaw depth to the original wall thickness.

Pressurizer Safe End' Expected Service Life Spray Nozzle N2 19 Years Safety Nozzle N3 40 Years Relief Nozzle N4A 40 Years Safety Nozzle N4B 40 Years Safety Nozzle N4C 40 Years Consequently, such a flaw would not grow through-wall before the next inservice inspection.

There is additional assurance in that post-weld overlay inspection did not identify any flaws in the inspectable volume of the base metal for the pressurizer spray, relief, and safety nozzle safe-end weld overlays.