ML041180217
| ML041180217 | |
| Person / Time | |
|---|---|
| Site: | Prairie Island  |
| Issue date: | 04/29/2004 |
| From: | Mahesh Chawla NRC/NRR/DLPM/LPD3 |
| To: | Solymossy J Nuclear Management Co |
| Chawla M, 415-8371, NRR/DLPM | |
| References | |
| TAC MB8715 | |
| Download: ML041180217 (5) | |
Text
April 29, 2004 Mr. Joseph M. Solymossy Site Vice President Prairie Island Nuclear Generating Plant Nuclear Management Company, LLC 1717 Wakonade Drive East Welch, MN 55089
SUBJECT:
SUMMARY
OF THE ROOT CAUSE ANALYSIS OF TUBE DENTS IN LOW ROW U-BENDS IN PRAIRIE ISLAND, UNIT 1, STEAM GENERATORS (TAC NO. MB8715)
Dear Mr. Solymossy:
By letters to the U.S. Nuclear Regulatory Commission (NRC) dated April 25, 2003 (ML031250066), and February 20, 2004 (ML040610107), Nuclear Management Company, LLC, submitted information pertaining to the Prairie Island, Unit 1, steam generator tube denting identified in the U-bend region of low row tubes during the 2002 refueling outage inspection.
Enclosed is the NRC staffs evaluation of the above inspection reports. As discussed in the evaluation, the NRC staff did not identify any technical issues that warranted followup action at this time. If you have any further questions, please contact me at (301) 415-8371.
Sincerely,
/RA/
Mahesh Chawla, Project Manager, Section 1 Project Directorate III Division of Licensing Project Management Office of Nuclear Reactor Regulation Docket No. 50-282
Enclosure:
As noted cc/w enclosure: See next page
ML041180217 NRR-106 OFFICE PDIII-1/PM PDIII-1/LA PDIII-1/SC NAME MChawla THarris LRaghavan DATE 04/29/04 04/28/04 04/29/04
SUMMARY
OF THE NUCLEAR REGULATORY COMMISSION (NRC) REVIEW OF ROOT CAUSE ANALYSIS OF STEAM GENERATOR TUBE DENTING PRAIRIE ISLAND, UNIT 1 DOCKET NO. 50-282 By letters to the NRC dated April 25, 2003 (ML031250066), and February 20, 2004 (ML040610107), Nuclear Management Company, LLC, (NMC) submitted information pertaining to the Prairie Island, Unit 1, steam generator tube denting identified in the U-bend region of low row tubes during the 2002 refueling outage inspection. The NRC staff requested NMC to submit a summary of the root cause analysis report during a phone call with NMC to discuss the steam generator tube inspection results during the 2002 refueling outage inspection (refer to ML030310571).
In 2001, the U-bend region of all active row 1 and 2 tubes in the Unit 1 steam generators were heat-treated to relieve the stresses in those tubes (which is intended to reduce the susceptibility to cracking). A rotating probe (with a +PointTM coil) and bobbin coil inspection of the U-bend region of the row 1 and 2 tubes was performed both before and immediately after the heat treatment. The purpose of the post-heat treatment inspection was not to re-examine the tube for degradation or denting, but to verify that the heat treatment was applied at the proper location. The heat treatment process used at Unit 1 was similar to that used at other plants.
During the 2002 refueling outage, 28 tubes were identified with dent signals greater than the licensees 2-volt dent reporting threshold. The dent signals were found near the bottom surface of tube support plate number 7 and the top surface of support plate number 6. Although only 28 tubes were identified with dents greater than 2 volts almost all tubes showed the presence of dent-like signals. No service-induced degradation was identified at these dents. Review of the
+PointTM coil data indicated that the geometry deformation is more uniform around the circumference of the tube unlike the geometry pattern for more classical denting which results from corrosion of the carbon steel tube support plate. This latter denting (classical denting) is typically more biased toward the center of the support rather than at the edges. The largest of the dent signals was approximately 6 volts. A comparison of the 2001 eddy current data (both pre- and post-heat treatment) to the 2002 data does not indicate any significant change or growth in the dent signals. Minor changes in the data are attributed to eddy current uncertainties.
The licensee reviewed other maintenance procedures (e.g., sludge lancing) to determine if they may have resulted in tube denting or result in changes to the eddy current signal response at the upper tube supports. Based on this review, the licensee concluded that no other maintenance activities could have resulted in denting or changed the eddy current signal response. A review of the U-bend heat treatment process records was also performed. No off-normal events occurred during the heat treatment process.
Based on their investigations, the licensee concluded that (1) the dent-like tube indications appear to be a result of the heat treatment process; (2) the denting has likely resulted in less
than 1 mil of inside-diameter reduction with a uniform change around the circumference of the tube (assuming the dent-like signals are truly representative of tube deformation); (3) it cannot be conclusively demonstrated that the dent-like signals are either an actual shallow deformation of the tube or an artifact of the high temperature exposure of the crevice deposits; and (4) not all of the dent-like signals are result of the heat treatment process (some of the dents were present prior to the heat treatment).
The licensee concluded that the potential geometric deformation in these dents in the U-bend region are small and would be expected to result in less severe residual stresses than classic denting of similar magnitude. Furthermore, since stress corrosion cracking has not been observed at dents in Unit 1, it is not likely that the new dents would develop stress corrosion cracks during the next cycle. The steam generators at Unit 1 are scheduled to be replaced in the fall of 2004.
Based on the material provided by the licensee, the staff did not identify any issues warranting additional follow-up.
Prairie Island Nuclear Generating Plant, Unit 1 cc:
Jonathan Rogoff, Esquire Tribal Council Vice President, Counsel & Secretary Prairie Island Indian Community Nuclear Management Company, LLC ATTN: Environmental Department 700 First Street 5636 Sturgeon Lake Road Hudson, WI 54016 Welch, MN 55089 Manager, Regulatory Affairs Nuclear Asset Manager Prairie Island Nuclear Generating Plant Xcel Energy, Inc.
Nuclear Management Company, LLC 414 Nicollet Mall, R.S. 8 1717 Wakonade Drive East Minneapolis, MN 55401 Welch, MN 55089 John Paul Cowan Manager - Environmental Protection Division Executive Vice President & Chief Nuclear Minnesota Attorney Generals Office Officer 445 Minnesota St., Suite 900 Nuclear Management Company, LLC St. Paul, MN 55101-2127 700 First Street Hudson, WI 54016 U.S. Nuclear Regulatory Commission Resident Inspector's Office Craig G. Anderson 1719 Wakonade Drive East Senior Vice President, Group Operations Welch, MN 55089-9642 Nuclear Management Company, LLC 700 First Street Regional Administrator, Region III Hudson, WI 54016 U.S. Nuclear Regulatory Commission 801 Warrenville Road Lisle, IL 60532-4351 Administrator Goodhue County Courthouse Box 408 Red Wing, MN 55066-0408 Commissioner Minnesota Department of Commerce 121 Seventh Place East Suite 200 St. Paul, MN 55101-2145 November 2003