ML032870328
| ML032870328 | |
| Person / Time | |
|---|---|
| Site: | Pilgrim |
| Issue date: | 10/02/2003 |
| From: | Riggs W Entergy Nuclear Operations |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| 2.03 115, GL-88-001 | |
| Download: ML032870328 (7) | |
Text
Entergy Nuclear Operations, Inc.
Pilgrim Station m-~ En tergy 600 Rocky Hill Road Plymouth, MA 02360 William J. Riggs Director, Nuclear Assessment October 2, 2003 U.S. NuclearjBegulaoxyCQrmmission Vtn.
ocumenwontrOl 13es-Washingfon, DC 20555
SUBJECT:
Entergy Nuclear Operations, Inc.
Pilgrim Nuclear Power Station Docket No. 50-293 License No. DPR-35 Pilgrim Relief Request (PRR)-37, Alternative Contingency Repair Plan for Generic Letter 88-01, Reactor Pressure Vessel Nozzle-to-Safe End Welds, Using ASME Code Cases N-638 and N-504-2 With Exceptions LETTER NUMBER:
2.03.115
Dear Sir or Madam:
This letter requests NRC approval of Pilgrim Relief Request (PRR) No. 37, in support of the current outage to complete an In-service Inspection contingency repair plan within the scope of Generic Letter 88-01 and BWRVIP-75. There are currently no known flaws in the nozzle-to-safe end welds being evaluated for potential weld overlay.
This relief request applies to the existing reactor pressure vessel (RPV) nozzle-to-safe end welds as an alternative to 1 OCFR50.55a(c)(3). This relief is requested under the provisions of 10CFR50.55a(a)(3)(i), in that the proposed alternative would provide an acceptable level of quality and safety. The repair plan uses a weld overlay repair method that represents an alternative to "American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code,' Section Xl Code repair.
This relief request applies to the RPV Core Spray N6A and N6B nozzles austenitic to ferritic welds.
During an inspection of the Drywell, a flaw was discovered in the capped N10 nozzle-to-end cap weld. As part of the associated investigation of potential extent of condition issues, Pilgrim has determined that it may be prudent to perform weld overlays of the Core Spray nozzle-to-safe end welds. If it is determined to be prudent to perform these overlays Pilgrim will proceed with the following repair plan.
203115
Entergy Nuclear Operations, Inc.
Pilgrim Nuclear Power Station Letter Number: 2.03.115 Page 2 This repair plan includes use of ASME Code Cases N-638 and Code Case N-504-2, but exceptions are taken to the Code Cases. NRC has authorized the use of Code Case N-638 and N-504-2 in Regulatory Guide 1.147, Rev. 13. The Staff has approved exceptions to Code Case N-504-1 (an earlier version of N-504-2) and Code Case N-638, for James A Fitzpatrick (TAC No. MB0252, dated October 26, 2000), Duane Arnold Energy Center (NRC Staff's letter dated November 19, 1999), and for the Nine Mile Point Unit 2 plant (NRC Staff's letter dated March 30, 2000). The repair plan presented in the Attachment is an alternative to the ASME Code, Section Xl 1989 Edition, pursuant to the requirements of 10CFR50.55a(c)(3), and was recently approved by the NRC for use at Fitzpatrick (TAC No. MB0252).
Based on the evaluations contained in the Attachment, Pilgrim has concluded that this alternative provides an acceptable level of quality and safety and that strict adherence to the specified requirements would result in unusual difficulty and increase dose rates without a compensating increase in the level of quality and safety. Therefore, this proposed alternative satisfies the requirements of 1 OCFR50.55a(a)(3)(i).
Review and approval of the repair plan alternative is requested to support Pilgrim restart.
If you have any questions regarding the information contained in this letter, please contact Mr. Bryan Ford (508) 830-8403.
Sincerely, Attachments:
Pilgrim Relief Request No. 37 (5 pages) cc:
Mr. Travis Tate, Project Manager Office of Nuclear Reactor Regulation Mail Stop: 0-8B-1 U. S. Nuclear Regulatory Commission 1 White Flint North 11555 Rockville Pike Rockville, MD 20852 U.S. NRC, Region 1 475 Allendale Road King of Prussia, PA 19406 Senior Resident Inspector Pilgrim Nuclear Power Station 2
ATTACHMENTTO ENTERGY LETTER 2.03.115 Proposed Alternative for the Repair of Reactor Pressure Vessel Nozzle-to-Safe End welds Pilgrim Relief Request (PRR-37)
A.
COMPONENT IDENTIFICATION A full structural weld overlay repair is proposed for the weldment associated with the austenitic to ferritic-N6A and N6B RPV nozzle-to-safe end welds.
These welds fall within the scope of GL 88-01 and BWRVIP-75.
The weld overlay material for the proposed repair is as follows:
For machine (GTAW) welding, the weld material is ASME Section II, Part C, SFA 5-14 Filler Wire ER NiCrFe-7 UNS N06052 F-No. 43 known commercially as Alloy 52.
For manual (SMAW) welding, the weld material is ASME Section II, Part C, SFA 5-11 Weld Electrode E NiCrFe-7 UNS W86152 known commercially as Alloy 152.
B EXAMINATION AND REPAIR REQUIREMENTS Weld overlays will be designed consistent with the requirements of NUREG-0313, Revision 2 (which was implemented by Generic Letter 88-01), ASME Code Cases N-504-2, N-638, and ASME,Section XI, Paragraph IWB-3640.
Welder Qualification and Welding Procedures All welders and welding procedures will be qualified in accordance with ASME Section IX and any special requirements from Section Xl or applicable code cases. A manual shielded metal arc weld (SMAW) procedure will be qualified to facilitate localized repairs and to provide a seal weld, prior to depositing the overlay. This procedure will make use of UNS W86152 SMAW electrodes consistent with the requirements of ASME Section IX. The repair activities will be performed by qualified personnel from Welding Services Incorporated (WSI) and shall be in accordance with WSI's Nuclear Repair (NR)
Certificate of Authorization. The repairs will be performed in accordance with WSI Welding Procedure Specification for welding Alloy 52/152.
Welding Wire Material A consumable welding wire highly resistant to intergranular stress corrosion cracking (IGSCC) will be used for the overlay material. This material, designated UNS N06052 is a nickel-based weld filler material, commonly referred to as Alloy 52, and will be applied using the gas tungsten arc welding (GTAW) process. Alloy 52 is identified as F-No. 43 Grouping for Ni-Cr-Fe, classification UNS N06052 Filler Metal. Alloy 52 contains about 30% chromium that imparts excellent corrosion resistance to this material. Alloy 152 welding wire may be used for seal welding activities.
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Weld Overlay Design The weld overlays will extend around the full circumference of the nozzle weldment location in accordance with NUREG-0313, Revision 2, Code Case N-504-2 and Generic Letter 88-01 and BWRVIP-75. The specific thickness and length will be computed according to guidance provided in ASME Section Xl, Code Case N-504-2 and ASME Section Xl, Paragraph IWB-3640,1989 Edition. The overlay will completely cover the indication location and the Inconel 182 weld deposit butter with the highly corrosion resistant Alloy 52 material. In order to accomplish this objective, it may be necessary to weld on the low alloy steel (LAS) nozzle material. A temper bead welding approach will be used for this purpose according to provisions of ASME Code Case N-638. This code case provides for machine gas tungsten-arc welding (GTAW) temper bead weld repairs to P No. 3 nozzle materials (SA 508 Cl. 2) at ambient temperature. The temper bead approach was selected because temper bead welding supplants the requirement for post weld heat treatment (PWHT) of heat-affected zones in welded LAS material.
Examination Requirements The repair, PSI and ISI examination of the weld overlay repairs will be performed in accordance with the ISI Program and Plan along with of NUREG-0313, Revision 2, Generic Letter 88-01, and approved plant procedures as specified by the ISI Repair/Replacement Program.
Unusual Difficulty In Meetinq Specified Requirements A repair method with a more corrosion resistant material would require draining the reactor vessel. In addition, preheat and post weld heat treatment are required for welding on nozzle material by ASME Section III, Subparagraph NB4622.7. These requirements are highly impractical without draining the reactor vessel, and may distort the P3 components involved. If the vessel is drained, the radiation dose rates in the nozzle areas would increase significantly, resulting in increased personnel exposure.
Therefore, consistent with ALARA practices and prudent outage scheduling and utilization of outage personnel, there is no vessel drain down planned for the overlay.
The weld overlay will be completed with water on the inside surface of the nozzle. This approach (i.e., no vessel drain down) shortens outage duration and reduces radiation exposure to personnel. For the same reasons if a flaw were to be identified in the existing overlayed weld material this flaw would not be removed.
The altemative, as described below, provides an acceptable level of quality and safety while neither draining the reactor vessel nor applying preheat and post weld heat treatments. Therefore, the alternative alleviates the impracticality of following certain code requirements for this repair activity.
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C.
ALTERNATIVE TO REPAIR REQUIREMENTS The repair will utilize ASME Code Case N-504-2, Altemative Rules for Repair of Class 1, 2, and 3 Austenitic Stainless Steel Piping," and Code Case N-638, Similar and Dissimilar Metal Welding Using Ambient Temperature Machine GTAW Temper Bead Technique,' with the following exceptions and clarifications.
Clarification of Code Case N-504-2 for ApplicabilitV to Nickel-Based Austenitic Steel Code Case N-504-2 was prepared specifically for austenitic stainless steel material. An alternate application to nickel-based austenitic materials (i.e., Alloy 52) is requested due to the specific configuration of the nickel-based austenitic weldment.
Exception from Code Case N-504-2 Paragraph (b)
Code Case N-504-2 paragraph (b) requires that the reinforcement weld metal shall be low carbon (0.035 % maximum) austenitic stainless steel. In this application, a nickel-based filler is required and Alloy 52 has been selected in place of low carbon austenitic stainless steel.
Exception from Code Case N-504-2 Paractraph (e)
Code Case N-504-2 paragraph (e) requires as-deposited delta ferrite measurements of at least 7.5 for the weld reinforcement. These measurements have no meaning for nickel-based materials and will not be performed for this overlay.
Exception from Code Case N-504-2 Paragraph (h)
Code Case N-504-2 paragraph (h) requires a system hydrostatic test of completed repairs if the repaired flaw penetrated the original pressure boundary or if there is any observed indication of the flaw penetrating the pressure boundary during repairs. A system leak test of completed repairs will be used in lieu of a hydrostatic test.
Use of Code Case N-638 Applicability Code Case N-638 shall be applied to the nozzle material.
Exception from Code Case N-638 Section 1.0 paragraph (a)
Code Case N-638 Section 1.0 paragraph (a) states that the maximum area of an individual weld based on the finished surface shall be 100 square inches, and the depth of the weld shall not be greater than one-half of the ferritic base metal thickness. Pilgrim proposes to use a circumferential overlay of approximately 150 square inches.
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D.
BASIS FOR THE ALTERNATIVE Clarification of Code Case N-504-2 for Applicability to Nickel-Based Austenitic Steel The weldment being addressed is austenitic material having a mechanical behavior similar to austenitic stainless steel. Accordingly, this alternative provides an acceptable level of quality and safety. Therefore, Code Case N-504-2 should be interpreted to apply equally to both materials.
Exception from Code Case N-504-2 Para-graph (b)
A consumable welding wire highly resistant to IGSCC was selected for the overlay material. This material, designated UNS N06052 is a nickel-based alloy weld filler material, commonly referred to as Alloy 52, and will be applied using the gas tungsten arc welding (GTAW) process. Alloy 52 contains about 30% chromium that imparts excellent stress corrosion cracking resistance to this material. By comparison, Alloy 82, is identified as an IGSCC resistant material in NUREG 0313 Revision 2 and contains about 18 to 22% chromium while Alloy 182 has a nominal chromium composition of 13 to 17%.
Exception from Code Case N-504-2 Paracraph (e)
The composition of nickel-based Alloy 52 is such that delta ferrite is not formed during welding. Ferrite measurement requirements were developed for welds of 300 series stainless steel that require delta ferrite to develop corrosion resistance. Weld of Alloy 52 and Alloy 152 are 100% austenitic and contain no delta ferrite due to the high nickel composition (approximately 60% Ni and low iron content).
Exception from Code Case N-504-2 Paraciraph (h)
In lieu of the hydrostatic pressure test requirements defined in Code Case N-504-2, the required pressure test shall be performed in accordance with the Third Interval ISI Program and Plan and Code Case N-41 -2 with the exception that the volumetric examination performed shall be an ultrasonic examination of the weld overlay.
Use of Code Case N-638 Applicability Code Case N-638 was developed for temper bead applications to similar and dissimilar metals. It permits the use of machine gas tungsten arc welding (GTAW) at ambient temperature without the use of preheat or PWHT on Class 1, 2, and 3 components.
Temper bead welding methodology is not new. Numerous applications over the past decade have demonstrated the acceptability of temper bead technology in nuclear environments. Temper bead welding achieves heat affected zone (HAZ) tempering and grain refinement without subsequent post weld heat treatment (PWHT). Excellent HAZ toughness and ductility are produced.
Exception from Code Case N-638 Section 1. paragraph (a)
The 100 square inch restriction is based on groove weld repairs. The overlay repair on the Core Spray nozzles will cover up to approximately 150 square inches of the pipe considering circumference and length.
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On the potential core spray nozzle repair, blending the repair into the nozzle transition would produce a covered low alloy steel area of approximately 150 square inches. If the 100 square inch criterion were strictly adhered to, the repair could produce a notch near the nozzle transition, which could produce an undesirable stress concentration. Blending the repair into the nozzle is the preferred option. The increase in the weld overlay area is consistent with industry practice and poses no safety concern. The material properties are not degraded by this increased area. Shrinkage stresses are considered beneficial in that they tend to create compressive stresses that retard crack growth rates.
E.
CONCLUSION Weld overlays involve the application of weld metal circumferentially over and in the vicinity of a flawed weld to restore ASME Section Xl margins as required by ASME Code Case N-504-2. Weld overlays have been used in the nuclear industry as an acceptable method to repair flawed welds. The use of overlay filler material that provides excellent resistance to IGSCC provides an effective barrier to crack extension.
The design of the overlay for RPV nozzle weldment uses methods that are standard in the industry for size determination of pipe-to-pipe overlays. There are no new or different approaches used in these overlay designs that would be considered first of a kind or inconsistent with previous approaches. The overlay is designed as a full structural overlay in accordance with the recommendation of NUREG-0313, Revision 2, which was forwarded by Generic Letter 88-01 and by Code Case N-504-2 and by ASME Section Xl Paragraph IWB-3640.
Temper bead techniques, as defined by Code Case N-638, will produce a tough corrosion resistant overlay deposit that meets or exceeds all code requirements for the weld overlay.
Pilgrim concludes that the contingency repair plan is justified and presents an acceptable level of quality and safety to satisfy the requirements of 10CFR50.55a(a)(3)(i). Similar proposed alternatives to the requirements of 10CFR5O.55a(c)(3) have been previously approved by the NRC for James A Fitzpatrick (TAC No. MB0252, dated October 26, 2000), Duane Arnold Energy Center (NRC Staff's letter dated November 19, 1999), and Nine Mile Point Unit 2 plant (NRC Staff's letter dated March 30, 2000).
F.
DURATION OF THE PROPOSED ALTERNATIVE The proposed alternative applies to the repairs of RPV nozzle-to-safe end welds for the scheduled outage and for the remaining service life of these welds. Re-inspection will be per BWRVIP-75 Guidelines.
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