ML062620234

Relief Request 06-ON-004 Request for Additional Information

ML062620234
Person / Time
Site:	Oconee
Issue date:	09/11/2006
From:	Brandi Hamilton Duke Energy Corp
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
06-ON-004
Download: ML062620234 (8)

Text

N BRUCE H HAMILTON Dukeg Vice President kEnergy Oconee Nuclear Station Duke Energy Corporation ONO0 VP / 7800 Rochester Highway Seneca, SC 29672 864 885 3487 864 885 4208 fax bhhamilton@duke-energy.com September 11, 2006 U. S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001

SUBJECT:

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

Oconee Nuclear Station, Units 1, 2, &3 Docket Numbers 50-269, -270, -287 Relief Request 06-ON-004 Request for Additional Information On August 24, 2006 Duke submitted Relief Request 06-ON-004 pursuant to 10 CFR 50.55a(a)(3)(i), requesting NRC approval to use alternatives to the American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code), Section Xl inservice inspection (ISI) requirements for the Oconee Nuclear Station, Unit 1, 2, &3. This proposed alternative approach is to support application of full structural weld overlays on various pressurizer nozzle-to-safe end welds and will provide an acceptable level of quality and safety.

On August 29, 2006 NRC Staff electronically requested additional information regarding several issues contained within the relief request. The Duke response is attached.

If you have any questions or require additional information, please contact Randy Todd at (864)-885-3418.

Sincerely, Bruce H. Hamilton Enclosure A[)-7 www.duke-ee'iy corn

ENCLOSURE OCONEE NUCLEAR STATION RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION REQUEST FOR RELIEF No. 06-ON-004 Pressurizer Alloy 600 Weld Overlays (TAC Nos. MD2887,2888,2889)

1. In your submittal dated August 24, 2006, you state that a preemptive full structural weld overlay is proposed for the components listed under Section 1.0. Please indicate what types of nondestructive examinations (NDE) will be performed prior to the full structural weld overlay installation. If pre-welding NDE is not to be performed, please confirm that in all cases, a full structural overlay will be installed and discuss your justification for not performing the NDE prior to welding.

Response

NDE will not be performed prior to weld overlay. Each overlay applied will be designed to be a full structural overlay. Surface examination and repair required by Code Case N-504-2 and ASME Section XI Nonmandatory Appendix Q will be performed prior to weld overlay. Part of the reason for applying the overlay is that the rigorous NDE required by Section XI Appendix VIII for detection of PWSCC cannot be applied to some of these locations because the geometry is not suitable.

2. Please discuss your repair strategy as a result of NDE. That is, if a flaw is detected in the weld by the NDE prior to weld overlay, confirm that a full-structural weld overlay is applied, and confirm that the weld overlay thickness calculation is based on the worst case flaw.

Response

NDE will not be performed prior to weld overlay. The design basis flaw for the purpose of structural sizing of the overlay is assumed to be 360 degrees and 100% through the original wall thickness of the dissimilar metal weld (DMW). For the crack growth analysis the initial flaw size is assumed to be 360 degrees and 75% through the original wall thickness. The 75%

through-wall assumption is selected based upon the PDI qualified inspection of the overlay at the conclusion of the weld overlay process, which includes the outer 25% of the original weld.

If flaws are detected in the post-overlay inspection, they will be evaluated in accordance with the requirements of Code Case N-504-2 and Appendix Q.

3. On page 7 of your transmittal you indicate that items 4 - 6 will be completed after startup. It is the staffs expectation that the requirements of N-504-2 (g)(2) and (g)(3) be completed prior to startup. Please discuss why relief is not necessary for items 4 - 6 prior to placing the plant into service and why, if not completed, the components will perform its intended function after deposition of the overlays.

Response

The analyses and evaluations described in items 4 through 6 will be completed prior to entry into mode 4.

4. On page 5 of your submittal, you indicate that Code Case N-638-3 was recently approved by ASME, increasing the limit for temper bead welding over ferritic material. In your Table 3 list of alternatives you cite N-638-3 and the associated white paper as the basis for your alternative to exceed 100 sq.in. The staff has not approved this Code Case in Reg. Guide 1.147, Rev 14., and as such, the staff may take considerable time to evaluate your alternative.

Another approach may be to provide a basis that is consistent with other alternatives that the staff has approved. Part of your discussion should include similarities between your plant and those listed in your precedents section of the submittal and why the resultant overlay will not prevent the component from performing its design function.

Response

The white paper referenced in our submittal was part of an ASME Code action that sought (and succeeded) in relaxing an arbitrary limitation that was included in N-638 to restrict the use of the ambient temperature bead welding to a surface area of less than 100 square inches. Later revisions to this Code Case (N-638-2 and N-638-3), approved by ASME Code in 2005 and 2006 respectively, have extended the 100 in2 limit to 500 in .

The white paper addressed three potential technical concerns that the 100 in2 limitation may have been intended to prevent: residual stresses, tempering of the weld HAZ, and the possible (but unlikely) development of delayed hydrogen cracking in the underlying, ferritic base material. The applicability of the white paper to the ONS surge nozzle weld overlay design is addressed individually for each of these potential concerns below:

(a) Residual Stresses - Using as an example a BWR Feedwater Nozzle, the white paper cites EPRI sponsored analyses [1] of an overlay that just equaled 100 in2 coverage over the ferritic2 steel base metal. Two axisymmetric finite element models were created, one with the 100 in weld overlay and the other with the weld overlay extended on the nozzle side until it blended into the nozzle taper surface (approximately 126 in2). Figure 1 shows the post overlay residual stress on the nozzle inside surface for both models. It is seen that the extended overlay configuration did not significantly alter the residual stress results, and if anything, made the axial stresses even more compressive.

The Feedwater nozzle configuration modeled in [1] was roughly similar to the ONS surge nozzles; however it is not necessary to rely on this similarity, since nozzle specific residual stress analyses are being conducted as part of the Duke PWOL project. The resulting post-overlay inside surface residual stress distributions for the ONS surge nozzles are shown in Figure 2. It is seen from this figure that the ONS surge nozzle weld overlay design, with its

-120 in2 coverage over the ferritic steel base metal, creates favorable compressive residual stresses on the inside surface of the nozzle.

(b) HAZ Tempering - The white paper cites past programs which have demonstrated that temper bead welding using automatic GTAW provides adequate tempering of the HAZ in P-1 and P-3 materials and does not degrade strength or fracture toughness for temper bead weld overlays. Reference [2] presents results of a bimetallic weld overlay mockup of a 12 inch diameter, SA-508 Class 2 low alloy steel nozzle. The overlay applied to this nozzle covered

-119 in2 of the low alloy steel nozzle (approximately the same as the ONS surge nozzle overlay). Microstructure and microhardness measurements were performed on the HAZ of this overlay, as well as mechanical property tests (Charpy and Tensile) of a groove weld in the same nozzle with similar coverage area. The mechanical property results verified that the weld overlay repair did not degrade the strength or toughness of the low alloy steel HAZ.

Microstructure and microhardness results demonstrated adequate tempering of the material, such that Hydrogen embrittlement would not be expected. This demonstration was conducted on a weld overlay geometry with essentially identical low alloy steel (LAS) coverage as the ONS surge nozzle overlay.

(c) Delayed Hydrogen Cracking - Inspections of the above described mockup, as well as extensive inspections of temperbead weld overlays in mockups and in the field, have been performed, of overlays with LAS coverages ranging from less than 10 in2 up to and including 325 in2. These have shown that hydrogen induced cracking has not been a problem with repairs produced by the automatic GTAW temper bead process. The process is by its nature a low hydrogen process, and diffusion of hydrogen is very rapid for low alloy steels.

Nonetheless, the post weld soaks specified in the Code are intended as post hydrogen bake outs permitting NDE after the repair has returned to ambient temperature. N-638, since it does not impose the post weld bake, requires a 48-hour hold time prior to NDE, to verify that the unlikely event of hydrogen induced cold cracking has not occurred. The Duke weld overlay procedure will conform to the 48-hour hold time requirement prior to performing NDE.

Furthermore, the metallurgical aspects discussed above are independent of the surface area of the repair but related to parameters of the qualified welding procedure.

Finally, it is important to note that the above theoretical arguments have been proven in practice by extensive field experience with temperbead weld overlays, with LAS coverage ranging from less than 10 in2 up to and including 325 in2 . Table 1 below provides a partial list of such applications. It is seen from this table that the original DMW weld overlay was applied over 20 years ago, and WOLs with LAS coverage in the 100 in2 range have been in service for 5 to 15 years. Several overlays have been applied with LAS coverage significantly greater than the 100 in2 . Relief requests for these large overlays have been previously approved.

These overlays have been examined with PDI qualified techniques, in some cases multiple times, and none have shown any signs of new cracking or growth of existing cracks. One such specific incidence [Constellation Energy Relief Request (ML060240110) for Calvert Cliffs] was referenced in Duke's original submittal.

5. Please verify that NDE will be performed after 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> from the time the welded component has achieved ambient temperature per Code Case N-638-1.

Response

No request for alternative to this requirement is intended. NDE will be performed after 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> from the time the welded component has achieved ambient temperature as required by Code Case N-638-1.

6. Please discuss if the NDE requirement under 4.0(b) of N-638-1, ultrasonic examination of the 1.5T band on either side of the overlay, will be achieved. If this area requirement cannot be met, please discuss the achievable amount of area that will be successfully examined for each preemptive weld overlay weld design configuration you wish to apply. Secondly, clarify whether the ultrasonic test examination will be performed on the maximum extent achievable.

Response

(a) Code Case N-638-1 addresses the use of the temper bead welding technique including those welds made in deep cavities in ferritic material. In the case of weld overlays to be applied at ONS, this technique will be used to apply a non-ferritic overlay to the P1 ferritic nozzle base material adjacent to the dissimilar metal weld (DMW). The PDI qualified

ultrasonic examination procedure is designed and qualified to examine the entire volume of the overlay weld as well as the region of the P1 material containing the weld heat affected zone (HAZ) and a volume of unaffected base material beyond the HAZ. In addition to verifying the soundness of the weld, a purpose of these examinations is to assure that delayed cracking that may be caused by hydrogen introduced during the temper bead welding process is not present. In the unlikely event that this type of cracking does occur, it would be initiated on the surface on which the welding is actually performed or in the HAZ immediately adjacent to the weld. The most appropriate technique to detect surface cracking is the surface examination technique that Duke will perform on the weld overlay and the adjacent base material in a band at least 1.5 times the thickness of the base material on either side of the overlay. The maximum achievable inspection volume is 100% of the volume susceptible to weld induced flaws.

(b) While it would be possible to extend the examination volume to a larger extent on either side of the weld overlay, it would not be possible with current technology to ultrasonically inspect 100% of the volume within 1.5 times the thickness of the base material because of geometric considerations. Inspection of an increased volume would result in increased dose to inspection personnel without a compensating increase in safety or quality because there is no plausible mechanism for formation of new flaws or propagation of existing flaws in the region. The overlay volume is small relative to the volume of the underlying pipe and does not present the same concerns as those related to welds in deep cavities contemplated by the requirements of Code Case N-638-1. Therefore, the examinations tailored for overlay inspection and required by Code Case N-504-2 and Appendix Q as modified in the request for relief provide full assurance that the weld and adjoining base material are fully capable of performing their intended function. Approval by ASME in 2005 and 2006 of Code Cases N-638-2 and N-638-3 respectively recognizes that inspection of the larger volume is not necessary to assure quality and safety c) The NRC has previously granted relief on this specific issue for temper bead welding for use at other plants for the reasons mentioned above. Specifically, San Onofre Nuclear Generating Station Unit 2 in the Spring of 2006, Millstone Power Station, Unit No. 3 in January 2006, and Three Mile Island Unit 1 in Fall 2003 have received approval to use inspection methods essentially identical to those proposed by Duke.

7. With respect to hydrostatic testing, please verify if Code Case N-416-2 is listed within your current ISI Program Plan.

Response

Code Case N-416-2 is not listed within the current ISI Program Plan. The requirements of Code Case N-416-2 were incorporated into Section XI and are implemented in ASME Boiler and Pressure Vessel Code Section XI 1998 Edition through 2000 Addenda applicable to the fourth inspection interval for each of the units for which relief is requested.

Residual Axial Stress Along Inside Wall 5

Length of 100 ino2 Weld Overlay lo 0

-5 j-10 4 -15

-20 100 in*2

-25

-- Extended

-- An .

-7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 Axial Distance (Y)From Weld Centerline (in)

Residual Hoop Stress Along Inside Wall 0 .

Length of 100 W eld OverlayI inrA2 N4 1

-10 \

F 0

-25

-30 -* 10O irA2

-- Extended

-350

-7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 Axial Distance (Y) From Weld Centerline (in)

Figure 1 Residual Stress Distribution from White Paper (see Text)

r-e Pos-P.r Oconee Pressurizer Surge Nozzle --- Post-WOL ID Surface Axial Stress Pot-WCL4-70F

• -P.n-WOL4-650F Safe End (SS) ,WSCC Susceptible Nozzle (LAS)

-70.00

-60.00

- 50.00

- 20.00

--

• 10,00

=so

-70.00

-40.00

-- 40.00

-- 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.ý6 -1.8

-2 Distance from ID Weld Repair Centerine (in) f-0-Po,,-Repir Oconee Pressurizer Surge Nozzle- -'Pot-WOL40FPtWL1l IDSurface Hoop Stress -Post-WOL4-.50F Safe End (SS) 'PWSCC Suseptible Nozzle (LAS)

-70.00 ýegion

-60.00

-50.00

-40.00 -.-- e'

-30.00 10.00 000n 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 IDWeld Repair Centerline (in)

Figure 2 Calculated Residual Stress Distribution for Oconee (see Text)

Table 1 - Dissimilar Metal Weld Overlay Experience Date Date Plant Component Nozzle Diameter Approx. LAS P tm e(in) Coverage (in2)

April 2006 Davis Besse Hot leg drain nozzle 4 16 February SONGS Unit 2 PZR spray nozzle 8 50 2006 safety/relief nozzles 6 28 November Kuosheng Unit 2 Recirculation outlet 22 250 2005 nozzle April 2004 Susquehanna Unit I Recirc. inlet nozzle 12 100 Recirc. outlet nozzle 28 325 November 200: TMI Unit I Surge line nozzle 11.5 75 October 2003 Pilgrim Core spray nozzle 10 50 CRD return nozzle 5 20 October 2002 Peach Bottom Units 2 Core spray nozzle 10 50

&3 Recirc. outlet nozzle 28 325 CRD return nozzle 5 20 October 2002 Oyster Creek Recirc. outlet nozzle 26 285 December Duane Arnold Recirc. inlet nozzle 12 100 1999 June 1999 Perry Feedwater nozzle 12 100 June 1998 Nine Mile Point Unit 2 Feedwater nozzle 12 100 March 1996 Brunswick Units I &2 Feedwater nozzle 12 100 February Hatch Unit I Recirc. inlet nozzle 12 100 1996 January 1991 River Bend Feedwater nozzle 12 100 March 1986 Vermont Yankee Core spray nozzle 10 50 References (Question 4):

1. "Justification for the Removal of the 100 Square Inch Limitation for Ambient Temperature Temper Bead Welding on P-3 Material", EPRI-NP- 1011898, February 2005.

2. "Inconel Weld-Overlay Repair for Low-Alloy Steel Nozzle to Safe-End Joint", EPRI NP-7085-D, January 1991.