ML080380037
| ML080380037 | |
| Person / Time | |
|---|---|
| Site: | Millstone  |
| Issue date: | 03/28/2008 |
| From: | John Hughey NRC/NRR/ADRO/DORL/LPLI-2 |
| To: | Christian D Virginia Electric & Power Co (VEPCO) |
| Sanders, Carleen, NRR/DORL 415-1603 | |
| References | |
| TAC MD3450 | |
| Download: ML080380037 (5) | |
Text
March 28, 2008 Mr. David A. Christian President and Chief Nuclear Officer Virginia Electric and Power Company Innsbrook Technical Center 5000 Dominion Boulevard Glen Allen, VA 23060-6711
SUBJECT:
MILLSTONE POWER STATION, UNIT NO. 3 - NUCLEAR REGULATORY COMMISSION STAFF REVIEW OF STEAM GENERATOR TUBE INSPECTION REPORTS FOR THE 2005 REFUELING OUTAGE (TAC NO. MD3450)
Dear Mr. Christian:
By letters dated October 26, 2005 (Agencywide Documents Access and Management System accession number ML053070547), and October 2, 2006 (ML062860044), Dominion Nuclear Connecticut, Inc. (DNC) submitted information concerning the 2005 steam generator (SG) tube inspections at Millstone Power Station, Unit No. 3 (MPS3), which were performed during the cycle 10 refueling outage. DNC submitted additional information regarding the SG tube inspections by letter dated December 18, 2007 (ML073620218).
Enclosed is the Nuclear Regulatory Commission staffs evaluation of DNCs submittals. Based on its review, the NRC staff concludes that the licensee has provided the information required by MPS3s technical specifications and no additional follow-up to these reports is required at this time. This closes out the TAC number MD3450.
If you have any questions please contact Carleen Sanders at 301-415-1603.
Sincerely,
/ra/
John D. Hughey, Project Manager Plant Licensing Branch 1-2 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket No. 50-423
Enclosure:
Safety Evaluation cc w/encl: See next page
ML080380037 OFFICE LPL1-2/GE LPL1-2/PM LPL1-2/LA CSGB/BC LPL1-2/BC NAME CSanders JHughey ABaxter AHiser HChernoff DATE 02/25/2008 02/27/2008 2-13-2008 3/28/2008 3/28/08
REVIEW OF THE 2005 STEAM GENERATOR TUBE INSERVICE INSPECTION REPORTS MILLSTONE POWER STATION UNIT 3 DOCKET NUMBER 50-423 By letters dated October 26, 2005 (Agencywide Documents Access and Management System accession number ML053070547), and October 2, 2006 (ML062860044), Dominion Nuclear Connecticut, Inc. (DNC or the licensee) submitted information concerning the 2005 steam generator (SG) tube inspections at Millstone Power Station, Unit No. 3 (MPS3), which were performed during the cycle 10 refueling outage (3R10). DNC submitted additional information regarding the SG tube inspections by letter dated December 18, 2007 (ML073620218).
MPS3 has four Westinghouse Model F SGs, each of which contains 5,626 U-bend thermally treated Alloy 600 tubes. Each tube has a nominal outside diameter of 0.688 inches and a nominal wall thickness of 0.040 inches. During SG fabrication, the tubes were hydraulically expanded at both ends, over the full depth of the 21.23-inch thick tubesheet. The tubesheet was drilled on a square pitch with 0.98-inch spacing. There are 59 rows and 122 columns in each SG. The radius of the row 1 U-bends is 2.20 inches. The U-bends in rows 1 through 10 were stress relieved after forming. Eight Type 405 stainless steel support plates, which have broached quatrefoil holes, support the vertical section of the tubes, and six anti-vibration bars support the U-bend section of the tubes.
At the time of this inspection, the SGs had accumulated 13.9 effective full-power years (EFPY) of operation.
The licensee provided the scope, extent, methods, and results of their SG tube inspections in the documents referenced above.
After review of the information provided by the licensee, the NRC staff has the following comments/observations:
The licensee reported that J-Tubes 1, 15, 16, and 30 in SG C showed signs of erosion at the interface between the nozzle weld and the header interface, on the inside diameter of the header. Ultrasonic testing and weld repair (overlay) was performed on these nozzles during the outage. These same J-Tubes were also repaired in SG B during 3R10, and in SG D during MPS3s cycle 9 refueling outage. SG A underwent J-Tube repairs during the next refueling outage, MPS3s cycle 11 refueling outage.
During visual inspections of the lower deck in the secondary side of the SG, the licensee observed some leakage at the riser barrel/downcomer slip joint. In addition, some minor roughness or pitting was observed on the primary moisture separators in the location where the feedwater over sprays. No significant loss of material in the primary moisture separators was observed, and the licensee reported the conditions were similar to those observed during previous inspections.
In the four SGs at MPS3, there are 8,783 overexpansions (regions of the tubes in the tubesheet that were expanded into anomalies in the tubesheet). Of these 8,783 overexpansions, 3,260 are on the hot-leg sides of the SGs. Of these 3,260 ENCLOSURE
overexpansions, 1,054 were inspected during the 2005 outage, all of which were in SGs A and C. This sample included all overexpansions with bobbin voltages greater than 28 volts and a sample of the overexpansions with voltages between 18 and 28 volts.
These inspections were focused on the upper region of the tubesheet since these were considered (by the licensee) to be the greatest risk to tube integrity. No cracks were observed at any of the overexpansions or anywhere else in the SGs. The licensee indicated that they would have expanded the inspection (more tubes and the depth within the tubesheet), as required.
The NRC staff notes that crack-like indications were observed in three distinct locations within the tubesheet at another plant in 2004. This information was summarized in NRC Information Notice 2005-09, Indications in Thermally Treated Alloy 600 Steam Generator Tubes and Tube-To-Tubesheet Welds, dated April 7, 2005. The steam generator tubes at this plant were fabricated from thermally treated Alloy 600. The locations of these crack-like indications included overexpansions, the tack expansion region, and the tube-to-tubesheet weld. Based on inspections performed in the tubesheet region in units with thermally treated Alloy 600 tubes, it is not clear to the NRC staff that overexpansions are more susceptible to cracking than the tack expansion region or the tube-to-tubesheet weld. As discussed in NRC Generic Letter 2004-01, Requirements for Steam Generator Tube Inspections, it is the NRC staff position that all tube locations susceptible to cracking should be inspected with techniques capable of finding cracks regardless of the safety significance unless an amendment has been approved to eliminate the need for such inspections. Given the operating experience noted above, the NRC staff would expect that future inspections would inspect the tack expansion region and the tube-to-tubesheet weld unless it was concluded that these areas were not susceptible to cracking. The NRC staff's position is that that these areas should be inspected with probes capable of finding cracks at these locations or an amendment obtained to exclude these areas from inspection. The NRC staff understands that the licensee for Millstone 3 will be requesting an amendment to alter the repair criteria for flaws detected within the tubesheet region (e.g., the tack expansion) for their Fall 2008 outage.
The licensee indicated that difficulty was experienced traversing the tight radius U-bend tubes. This difficulty is believed to be due to the bobbin probe design and not related to a change in tube geometry. Because of this condition, MPS3 will perform future inspections of the tight radius U-bend tubes (rows 1 and 2) with a rotating coil.
The licensee noted 32 single volumetric indications in the 2005 Annual Report. Although they did not provide the depth of each indication, they indicated that the depths of these indications ranged from 9 to 35 percent through wall.
The licensee focused their inspections of dents and dings on the hot-leg. They indicate that with all other variables being equal (e.g., material susceptibility, chemical environment, stress levels), the temperature dependency of cracking makes it much more likely to develop in the highest temperature portions of the tube first. The NRC staff agrees that with all other factors being equal, the lower the temperature the less likely cracking will occur. However, it is often difficult to demonstrate that all of the other factors are equal. This is supported by operating experience where certain regions of the tube (at cooler temperatures) may crack before similar but hotter locations. These considerations are important in developing an inspection strategy.
The licensee indicated that they categorized the results of their inspections in SG A as Category C-1. Because the licensee found one defective tube in SG A, the inspection should have been classified as Category C-2. Given the inspections performed during the 2005 outage, a change in classification from C-1 to C-2 has no impact.
In summary, the scope of inspections at Millstone 3 appears to be consistent with the objective of detecting the forms of degradation to which the tube is potentially susceptible. As noted above, the NRC staff would expect that future inspections would inspect the tack expansion region and the tube-to-tubesheet weld unless it was concluded that these areas were not susceptible to cracking. In addition, the results of the inspections appear to be consistent with industry operating experience with thermally treated Alloy 600 steam generator tube material.
Principal Contributor: A. Johnson Date: February 1, 2008
Millstone Power Station, Unit No. 3 cc:
Lillilan M. Cuoco, Esquire Senior Counsel Dominion Resources Services, Inc.
Building 475, 5th Floor Rope Ferry Road Waterford, CT 06385 Edward L. Wilds, Jr., Ph.D.
Director, Division of Radiation Department of Environmental Protection 79 Elm Street Hartford, CT 06106-5127 Regional Administrator, Region I U.S. Nuclear Regulatory Commission 475 Allendale Road King of Prussia, PA 19406 First Selectmen Town of Waterford 15 Rope Ferry Road Waterford, CT 06385 Mr. J. W. "Bill" Sheehan Co-Chair NEAC 19 Laurel Crest Drive Waterford, CT 06385 Mr. Evan W. Woollacott Co-Chair Nuclear Energy Advisory Council 128 Terry's Plain Road Simsbury, CT 06070 Senior Resident Inspector Millstone Power Station c/o U.S. Nuclear Regulatory Commission P. O. Box 513 Niantic, CT 06357 Ms. Nancy Burton 147 Cross Highway Redding Ridge, CT 00870 Mr. Joseph Roy, Director of Operations Massachusetts Municipal Wholesale Electric Company Moody Street P.O. Box 426 Ludlow, MA 01056 Mr. J. Alan Price Site Vice President Dominion Nuclear Connecticut, Inc.
Building 475, 5th Floor Rope Ferry Road Waterford, CT 06385 Mr. Chris Funderburk Director, Nuclear Licensing and Operations Support Dominion Resources Services, Inc.
5000 Dominion Boulevard Glen Allen, VA 23060-6711 Mr. William D. Bartron Licensing Supervisor Dominion Nuclear Connecticut, Inc.
Building 475, 5th Floor Rope Ferry Road Waterford, CT 06385 Mr. David A. Sommers Dominion Resources Services, Inc.
5000 Dominion Boulevard Glen Allen, VA 23060-6711 Mr. David A. Christian President and Chief Nuclear Officer Virginia Electric and Power Company Innsbrook Technical Center 5000 Dominion Boulevard Glen Allen, VA 23060-6711