ML063610092

Request for Relief Number 06-GO-001 Alloy 600 Pressurizer Weld Overlays - Submittal of Committed Information

ML063610092
Person / Time
Site:	Catawba
Issue date:	12/19/2006
From:	Morris J Duke Energy Carolinas, Duke Power Co
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
06-GO-001
Download: ML063610092 (17)

Text

jAA Duke

-IEnergy, JAMES R. MORRIS Vice President Catawba Nuclear Station 4800 Concord Rd. / CNO1VP York, SC 29745-9635 803 831 4251 803 831 3221 fax December 19, 2006 U.S. Nuclear Regulatory Commission Attention:

Document Control Desk Washington, DC 20555-0001

Subject:

Duke Power Company LLC d/b/a Duke Energy Carolinas, LLC (Duke)

Catawba Nuclear Station, Unit 1 Docket Number 50-413 Request for Relief Number 06-GO-001 Alloy 600 Pressurizer Weld Overlays - Submittal of Committed Information On July 27,

2006, as modified by letters dated September 11, 2006, September 27,

2006, and November 16f 2006, Duke submitted the subject request for relief regarding a proposed alternative approach to support application of full structural weld overlays on various pressurizer nozzle-to-safe end welds.

The September 27, 2006 letter contained the following two commitments:

1.

The following information will be submitted to the NRC within fourteen days of completion of the final UT on each unit included in this relief request.

Also included in the results will be a discussion of any repairs to the overlay material and/or base metal and the reason for the repair.

A listing of flaw indications detected The disposition of all 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 www. duke-energy. com

U.S. Nuclear Regulatory Commission Page 2 December 19, 2006

2.

Prior to entry into Mode 4 from the Catawba Unit 1 outage in the Fall of 2006, a summary of the results of the stress analyses demonstrating that the preemptive full structural weld overlay will not hinder the components from performing their design function will be submitted to the NRC.

The information required by Item 2 above is included in the attachment.

The attached information demonstrates that the preemptive full structural weld overlays satisfy the applicable requirements of ASME Section XI, Code Case N-504-2, and Appendix Q, and thus constitute long-term, Code acceptable mitigation against pressurized water stress corrosion cracking in the subject welds.

The information required by Item 1 above was previously transmitted to the NRC via letter dated December 14, 2006.

If you have any questions concerning this information, please contact L.J. Rudy at (803) 831-3084.

Very truly yours, James R. Morris LJR/s Attachment

U.S. Nuclear Regulatory Commission Page 3 December 19, 2006 xc (with attachment):

W.D. Travers, Administrator, Region II U.S. Nuclear Regulatory Commission Atlanta Federal Center 61 Forsyth St.,

SW, Suite 23T85

Atlanta, GA 30303-8931 J.F. Stang, Jr.,

NRC Senior Project Manager U.S. Nuclear Regulatory Commission 11555 Rockville Pike Mail Stop 0-8 H4A Rockville, MD 20852-2738 A.T. Sabisch, NRC Senior Resident Inspector Catawba Nuclear Station

Attachment Summary of Design and Analyses of Preemptive Weld Overlays for Pressurizer Nozzle Locations Containing Alloy 600 Materials

Structural Integrity Associates, Inc.

3315 Almaden Expressway Suite 24 San Jose, CA 95118-1557 Phone:

408-978-8200 Fax: 408-978-8964

%w'A.stmudintom mtaylor@structint.com December 15, 2006 SIR-06-480, Rev. 0 MJT-06-023 Mr. Mitch Hatley Duke Energy Corporation McGuire Nuclear Station, Mail Code MG05SE 13225 Hagers Ferry Road Huntersville, NC 28078

Subject:

Summary of Weld Overlay Design and Analysis Calculations for Pressurizer Surge, Spray and Safety/Relief Nozzle-to-Safe End Welds at Catawba Nuclear Station, Unit 1

Reference:

Duke Energy Corporation, McGuire Nuclear Station Unit 2 and Catawba Nuclear Station Unit 1, Request for Alternative 06-GO-001, Revision 1, Sept. 27, 2006

Dear Mr. Hatley:

The following attachment is transmitted in support of Duke's response to commitments in the above-referenced request for alternative: : A summary of the results of stress and fracture mechanics analyses demonstrating that the preemptive full structural weld overlays satisfy the applicable requirements of ASME Section XI, Code Case N-504-2, and Appendix Q, and thus constitute long-term, Code acceptable mitigation against PWSCC in the subject welds.

Austin, TX Centennial, CO Charlotte, NC Stonlngton, CT Silver Spring, MD Sunrise, FL Unlontown, OH Whittier, CA Toronto, Canada 512-533-9191 303-792-0077 704-597-5554 860-536-3982 301-445-8200 954-572-2902 330-899-9753 562-944-8210 905-829-9817

Mr. Mitch Hatley, Duke Corporation SIR-06-480, Rev. O/MJT-06-023 December 15, 2006 Page 2 of 2 If you have any questions or comments regarding this summary, please contact one of the undersigned.

Prepared by:

Verified by:

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12/15/06 Date 12/15/06 Date Moses Taylor, P.E.

Associate Richard A. Mattson, P.E.

Senior Associate Approved by:

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12/15/06 Date Moses Taylor, P.E.

Associate nnn Attachments cc: D. Llewellyn M. Ruis V

Structural Integrity Associates, Inc.

Summary of Design and Analyses of Preemptive Weld Overlays for Pressurizer Nozzle Locations Containing Alloy 600 Materials Attachment to SIR-06-480, Rev. 0 fMJT-06-023 I of 11 Structural Integrity Associates, Inc.

1.0 Introduction Duke has applied full structural weld overlays (WOLs) on dissimilar metal welds (DMWs) of four 6" pressurizer safety/relief nozzles, one 4" pressurizer spray line nozzle, and one 14" pressurizer surge line nozzle at the Catawba Nuclear Station, Unit 1. The purpose of these overlays is to eliminate dependence on the primary water stress corrosion cracking (PWSCC) susceptible Alloy 82/182 welds as pressure boundary welds and to mitigate any potential future PWSCC in these welds. The overlays were installed using a PWSCC resistant weld filler material, Alloy 52M [1].

The requirements for design of weld overlay repairs are defined in ASME Code Case N-504-2

[2], supplemented for this application by the Relief Request [3]. Weld overlay repairs are considered to be acceptable long-term repairs for PWSCC susceptible weldments if they meet a conservative set of design assumptions which qualify them as "full structural" weld overlays.

The design basis flaw assumption for full structural weld overlays is a circumferentially oriented flaw that extends 3600 around the component, completely through the original component wall.

A combination of internal pressure, deadweight and seismic stresses is applied to the overlaid nozzles containing this assumed design basis flaw, and they must meet the requirements of ASME Code,Section XI, IWB-3641 [4].

ASME Section III stress and fatigue usage evaluations are also performed that supplement existing piping, safe end, and nozzle stress reports, to demonstrate that the overlaid components continue to meet ASME Code,Section III. The original construction Code for the pressurizer was ASME Section III, 1971 Edition through Winter 1972 Addenda. However, as allowed by ASME Section XI, Code Editions and Addenda later than the original construction Code may be used. ASME Section III, 2001 Edition with Addenda through 2003 [5] was used for these analyses.

In addition to providing structural reinforcement to the PWSCC susceptible locations with a resistant material, weld overlays have also been shown to produce beneficial residual stresses that mitigate PWSCC in the underlying DMWs. The weld overlay approach has been used to repair stress corrosion cracking in U.S. nuclear plants on hundreds of welds, and there have been no reports of subsequent crack extension after application of weld overlays. Thus, the compressive stresses caused by the weld overlay have been effective in mitigating new crack initiation and/or growth of existing cracks. In addition, the weld residual stresses from the overlays act as compressive mean stresses in fatigue crack growth assessments.

Finally, evaluations are performed, based on as-built measurements taken after the overlays are applied, to demonstrate that the overlays meet their design basis requirements, and that they will not have an adverse effect on the balance of the piping systems. These include comparison of overlay dimensions to design dimensions, evaluations of shrinkage stresses and added weight effects on the piping systems.

Attachment to SIR-06-480, Rev. 0 /MJT-06-023 2 of 11 SA

2.0 Analysis Summary and Results 2.1 Weld Overlay Structural Sizing Calculations Detailed sizing calculations for weld overlay thickness were performed using the "Codes and Standards" module of the pc-CRACK computer program [6], which incorporates ASME Code,Section XI, IWB-3640 evaluation methodology. Loads and stress combinations were provided by Duke. Both normal operating/upset (Level A/B) and emergency/faulted (Level C/D) load combinations were considered in this evaluation, and the design was based on the more limiting results. The resulting minimum required overlay thicknesses are summarized in Table 2-1.

Because of weld metal dilution concerns over the low alloy steel nozzle, a dilution weld layer is specified, in addition to the thickness required for structural reinforcement, to allow for the possibility that the minimum required chromium content for PWSCC resistance (24%) may not be achieved in the first layer [7].

The weld overlay length must consider: (1) length required for structural reinforcement, (2) length required for access for preservice and inservice examinations of the overlaid weld, and (3) residual stress improvement. In accordance with ASME Code Case N-504-2, the minimum weld overlay length required for structural reinforcement was established by evaluating the axial-radial shear stress due to transfer of primary axial loads from the pipe into the overlay and back into the nozzle, on either side of the weld(s) being overlaid. Axial weld overlay lengths were established such that this stress is less than the ASME Section III limit for pure shear stress. The resulting minimum length requirements are summarized in Table 2-1.

The overlay length and profile must also be such that the required post-WOL examination volume can be inspected using Performance Demonstration Initiative (PDI) qualified nondestructive examination (NDE) techniques. This requirement can cause required overlay lengths to be longer than the minimums for structural reinforcement. A typical weld overlay design for the Catawba Unit 1 pressurizer nozzles is illustrated in Figure 2-1. Because of the relatively short lengths of the original safe-ends, it was necessary to extend the overlay over both the DMW and the adjacent stainless steel (SS) welds, to ensure sufficient overlay length for inspectability and residual stress improvement. The designs were reviewed by qualified NDE personnel to ensure that they meet inspectability requirements for both welds, and the overlays were designed to satisfy full structural requirements for both the DMWs and the SS welds.

Table 2-1: Weld Overlay Structural Thickness and Length Re uirements Safety/Relief Spray Nozzle Surge Nozzle Nozzle Minimum*

Nozzle Side 0.397 0.292 0.427 Thickness (in.)

Safe End Side 0.365 0.242 0.469 Minimum**

Nozzle Side 0.932 0.590 1.175 Length (in.)

Safe End Side 1.503 0.917 1.683

- Weld dilution layer (0.08") must be added

• • - Additional length requirements apply for inspectability nent to SIR-06-480, Rev. 0 /MJT-06-023 3 of II Structural Integrity Associates, Inc.

Attachr

2.2 Section III Stress Analyses Stress intensities for the weld overlaid Safety/Relief, Spray and Surge nozzles were determined from finite element analyses for the various specified load combinations and transients using the ANSYS software package [8]. Linearized stresses were evaluated at a total of nine stress locations - three paths as shown in Figure 2-2, each evaluated at the intrados, extrados and cheek locations for 3-dimensional models. (3-dimensional models were used for the safety/relief and spray nozzles because of the adjacent elbows. The surge nozzle was deemed to be adequately modeled by a 2-dimensional, axisymmetric model.) The stress intensities at these locations were evaluated in accordance with ASME Code,Section III, Subarticles NB-3200 and NB-3600 [5],

and compared to applicable Code limits. A summary of the stress and fatigue usage comparisons for the most limiting locations is provided in Table 2-2. The stresses and fatigue usage in the weld overlaid nozzles are within the applicable Code limits. Figure 2-2 illustrates a typical stress model and stress paths evaluated. In general, the limiting location for the Section III stress analyses was found to be the section of the original pipe at the end of the overlay (Path 1 in Figure 2-2).

Table 2-2: Limiting Stress Results for Weld Overlaid Nozzles Nozzle Load Combination Type Calculated Allowable Eqn.10: Primary + Secondary (P +Q) (ksi)*

67.58**

47.88 Safety/

Level A/B Eqn.12/13: Simplified Elastic-Plastic Anal 42.25**

47.88 Relief (P +Q) (ksi)

Fatigue Cumulative Usage Factor 0.051 1.000 Eqn.10: Primary + Secondary (P +Q) (ksi)*

149.09**

48.53 Spray Level A/B Eqn.12/13: Simplified Elastic-Plastic Anal 45.72**

48.53 (P +Q) (ksi)

Fatigue Cumulative Usage Factor 0.986 1.000 Surge Level A/B Eqn. 10: Primary + Secondary (P +Q) (ksi)*

48.28 48.34 Fatigue Cumulative Usage Factor 0.994 1.000 Primary stress acceptance criteria are met via the sizing calculations discussed in Section 2.1.

• • - Elastic analysis exceeds the allowable value of 3Sm, however, criteria for simplified elastic-plastic analysis and thermal ratchet are met.

2.3 Residual Stress and Section XI Crack Growth Analyses Weld residual stresses for the Catawba Unit 1 pressurizer nozzle weld overlays were determined by detailed elastic-plastic finite element analyses. The analysis approach has been previously documented to provide predictions of weld residual stresses that are in reasonable agreement with experimental measurements [7]. Two-dimensional, axisymmetric finite element models were developed for each of the nozzles. Modeling of weld nuggets used in the analysis to lump the combined effects of several weld beads is illustrated in Figure 2-3. The models simulated an inside surface (ID) repair at the DMW location with a depth of approximately 50% of the original wall thickness. This assumption is considered to conservatively bound any weld repairs that may have been performed during plant construction from the standpoint of producing tensile residual stresses on the ID of the weld. The models also simulated the SS pipe to safe-end weld.

Attachment to SIR-06-480, Rev. 0 /MJT-06-023 4 of 11 Structural Integrity Associates, Inc.

The residual stress analysis approach consists of a thermal pass to determine the temperature response of the model to each individual lumped weld nugget as it is added in sequence, followed by an elastic-plastic stress pass to calculate the residual stress due to the temperature cycling from the application of each nugget. Since residual stress is a function of welding history, the stress passes for each nugget are performed sequentially, over the residual stress fields induced from all previously applied weld nuggets. The resulting residual stresses were evaluated on the inside surface of the original welds and safe-end components, as well as on several paths through the DMW and SS welds (Figures 2-4 and 2-5).

The residual stress calculations were then utilized, along with stresses due to applied loadings and thermal transients, to demonstrate that assumed cracks that could be missed by inspections will not exceed the overlay design basis during the ASME Section XI inservice inspection interval due to fatigue or PWSCC. In the fatigue crack growth analyses, 25% of the original 40 year design quantity of each applied transient was assumed to be applied in the 10 year interval.

Initial flaw sizes for the crack growth assessments were assumed consistent with the post-overlay UT inspections performed. Fatigue crack growth results are summarized in Table 2-3 for initial flaw sizes of 25%, 50% and 75% of the original pipe wall thickness. In all cases, the maximum crack depth at the end of the ten-year inspection interval is less than the weld overlay design basis flaw (the original wall thickness plus dilution layer for the DMW or just the original wall thickness for the SS welds, since no dilution layer was specified). Since the exam volume for the PDI qualified post-overlay UT inspections includes the weld overlay plus the outer 25% of the original wall thickness, a 75% through wall crack is the largest flaw that could escape detection by this examination.

For crack growth due to PWSCC, the total sustained stress intensity factor during normal plant operation was determined as a function of assumed crack depth, considering internal pressure stresses, residual stresses, steady state thermal stresses, and stresses due to sustained piping loads (including deadweight). Zero PWSCC growth is predicted for assumed crack depths at which the combined stress intensity factor due to sustained steady state operating conditions is less than zero. For all nozzles, considering the worst case paths in the DMWs, the sustained stress intensity factors remained negative for crack depths up to and beyond 75% of the original wall thickness. Therefore, no crack propagation due to PWSCC is predicted in the overlaid nozzles.

Attachment to SIR-06-480, Rev. 0 /MJT-06-023 5 of 11 Structural Integrity Associates, Inc.

Table 2-3:

Table2-3:

Limiting Fatigue Crack Growth Results for Weld Overlaid Nozzles DMW Safety/Relief Spray Nozzle Surge Nozzle Nozzles Initial Flaw Size Flaw Size (in.)

Flaw Size (in.)

Flaw Size (in.)

(% of Orig. Thick.)

Initial Final Initial Final Initial Final Circumferential Flaws 25%

0.2975 0.2986 0.2188 0.2196 0.377 0.487 50%

0.5950 0.5961 0.4375 0.4375 0.754 0.873 75%

0.8925 0.8946 0.6563 0.6563 1.131 1.183 Axial Flaws 25%

0.2975 0.2975 0.2188 0.2191 0.377 0.378 50%

0.5950 0.5950 0.4375 0.4376 0.754 0.755 75%

0.8925 0.8927 0.6563 0.6574 1.131 1.132 Original thickness 1.27 0.955 1.588

+ dilution layer*

SS Welds Safety/Relief Spray Nozzle Surge Nozzle Nozzles Initial Flaw Size Flaw Size (in.)

Flaw Size (in.)

Flaw Size (in.)

(% of Orig. Thick.)

Initial Final Initial Final Initial Final Circumferential Flaws 25%

0.1950 0.2038 0.1475 0.2230 0.360 0.434 50%

0.3900 0.4078 0.2951 0.3770 0.721 0.843 75%

0.5850 0.6105 0.4426 0.5139 1.081 1.404 Axial Flaws 25%

0.1950 0.1992 0.1475 0.2140 0.360 0.452 50%

0.3900 0.4375 0.2951 0.4783 0.721 0.883 75%

0.5850 0.6528 0.4426 0.5728 1.081 1.224 Original thickness*

0.78 0.590 1.441

• - Allowable crack depth at end of ten year inspection interval.

V Structural Integrity Associates, Inc.

Attachment to SIR-06-480, Rev. 0 /MJT-06-023 6 ofll

Figure 2-1:

Illustration of Typical Weld Overlay Design for Catawba Unit 1 Pressurizer Nozzles Attachment to SIR-06-480, Rev. 0 /MJT-06-023 7 of I S

AN EXTRA)OS LOCATION Safety/Relief Nozzle Figure 2-2:

Typical Finite Element Model for Section III Stress Evaluation Showing Stress Paths 2'

/Ir 6

= 2Lay"r6 Layor3 Figure 2-3:

Typical Finite Element Model for Residual Stress Analysis Showing Nuggets used for Welding Simulations Attachment to SIR-06-480, Rev. 0 /MJT-06-023 8 of II Structural Integrity Asociates, Inc.

Figure 2-4:

80

-70

-60

-50 40

-30

-20 is 10i Finite Element Model for Residual Stress Analysis Showing Paths used in Crack Growth Evaluations Post-Repair McGulrelCatawba Pressurizer Spray Nozzle -

Post-ButtWeld ID Surface Axial Stress

• Post-WOL1

-- Post-WOL4-70F

-)(4-Post-WOL4-650F 2

1.8 1.6 1.4 1.2 1

0.8 0.6 0.4 0.2 0

-0.2

-0.4

-0.6

-0.8

-1

-1.2

-1.4

-1.6

-1.8

-2 Distance from ID Weld Repair Centeoline (in)

V Structural Integrity Associates, Inc.

Attachment to SIR-06-480, Rev. 0 /MJT-06-023 9 ofll

McGulrelCatawba Pressurizer Spray Nozzle ID Surface Hoop Stress

-50.00

--60.00

-70.00 2

1.8 1.6 1.4 1.2 1

0.8 0.6 0.4 0.2 0

-0.2

-0.4

-0.6

-0.8

-1

-1.2

-1.4

-1.6

-1.8

-2 Distance from ID Weld Repair Centerdlne (in)

Figure 2-5:

Spray Nozzle Residual Stress Results along Inside Surface of Original Butt Weld and Safe-End - Plant-Specific to McGuire/Catawba 3.0 Conclusions The design of the Catawba Unit 1 weld overlays was performed taking guidance from the requirements of ASME Code Case N-504-2 [2], amended in accordance with the Relief Request

[3]. The weld overlays are demonstrated to be long-term mitigation of PWSCC in these welds based on the following:

" In accordance with ASME Code Case N-504-2, structural design of the overlays was performed to meet the requirements of ASME Section XI, IWB-3640 based on an assumed flaw 100% through and 3600 around the original welds. The resulting full structural overlays thus restore the original safety margins of the nozzles, with no credit taken for the underlying, PWSCC-susceptible material.

• The weld metal used for the overlay is Alloy 52M, which has been shown to be resistant to PWSCC [1], thus providing a PWSCC resistant barrier. Therefore, no PWSCC crack growth is expected into the overlay.

" Because of the short safe-end lengths in the original nozzle designs, the overlays were extended to cover the adjacent stainless steel pipe to safe-end welds. Although not susceptible to PWSCC, covering them with the overlays was necessary to ensure inspectability and effective residual stress improvement of the DMWs. The overlays were also designed as full structural over the stainless steel welds, thereby providing additional structural margin.

Attachment to SIR-06-480. Rev. 0 /MJT-06-023 10 of 11 Strutural Integrity Asociates, Inc.

P

" No credit was taken in the overlay designs for the first overlay layer, which could have been diluted by the base metal during the welding process.

" Application of the weld overlays was shown to not impact the conclusions of the existing nozzle Stress Reports. Following application of the overlay, all ASME Code,Section III stress and fatigue criteria are met.

" Nozzle specific residual stress analyses were performed, after first simulating severe ID weld repairs in the nozzle-to-safe end welds, prior to applying the weld overlays. The post weld overlay residual stresses were shown to result in beneficial compressive stresses on the inside surface of the components, and well into the thickness of the original DMWs, assuring that future PWSCC initiation or crack growth into the overlay is highly unlikely.

• Fracture mechanics analyses were performed to determine the amount of future crack growth which would be predicted in the nozzles, assuming that cracks exist that are equal to or greater than the thresholds of the NDE techniques used on the nozzles. Both fatigue and PWSCC crack growth were considered, and found to be acceptable.

Based on the above observations and the fact that similar nozzle-to-safe end weld overlays have been applied to other plants since 1986 with no subsequent problems identified, it is concluded that the Catawba Nuclear Station Unit 1 pressurizer surge, safety/relief and spray nozzle dissimilar metal welds have received long term mitigation against PWSCC.

4.0 References

1. "Materials Reliability Program (MRP): Resistance to Primary Water Stress Corrosion Cracking of Alloys 690, 52, and 152 in Pressurized Water Reactors (MRP-111)," EPRI, Palo Alto, CA: 2004, 1009801

2. ASME Code, Code Case N-504-2, "Alternative Rules for Repair of Classes 1, 2, and 3 Austenitic Stainless Steel Piping,Section XI, Division 1."

3. Request No. 06-GO-00 1, Duke Corporation, McGuire Nuclear Station Unit 2 and Catawba Nuclear Station Unit 1, Request for Alternative 06-GO-001.

4. ASME Boiler and Pressure Vessel Code,Section XI, 1998 Edition (with Addenda up to 2000).

5. ASME Boiler and Pressure Vessel Code,Section III, 2001 Edition through 2003 Addenda.

6. pc-CRACK for Windows, Version 3.1-98348, Structural Integrity Associates, 1998.

7. Materials Reliability Program Report MRP-169, "Technical Basis for Preemptive Weld Overlays for Alloy 82/182 Butt Welds in PWRs," August 2005.

8. ANSYS/Mechanical, Release 8.1 (w/Service Pack 1), ANSYS Inc., June 2004.

A Attachment to $1R-06-480, Rev. 0 /MJT-06-023 11 of 11StuurlIertyAscasIc