ML080990790
| ML080990790 | |
| Person / Time | |
|---|---|
| Site: | Byron  |
| Issue date: | 04/14/2008 |
| From: | David M NRC/NRR/ADRO/DORL/LPLIII-2 |
| To: | Pardee C Exelon Generation Co |
| david marshall NRR/DORL 415-1547 | |
| References | |
| TAC MD6203 | |
| Download: ML080990790 (7) | |
Text
April 14, 2008 Mr. Charles G. Pardee Chief Nuclear Officer and Senior Vice President Exelon Generation Company, LLC 4300 Winfield Road Warrenville, IL 60555
SUBJECT:
BYRON STATION, UNIT 2 - REVIEW OF SPRING 2007 STEAM GENERATOR TUBE INSERVICE INSPECTION REPORTS (TAC NO. MD6203)
Dear Mr. Pardee:
By two letters dated July 31, 2007, as supplemented by letter dated February 20, 2008, you submitted reports related to the steam generator tube inservice inspections for the spring 2007 refueling outage at the Byron Station (Byron), Unit 2, in accordance with the plants technical specifications (TSs).
The Nuclear Regulatory Commission (NRC) staff has completed its review of these reports and concludes that you have provided the information required by the Byron TSs, and that no additional follow-up is required at this time. A copy of the NRC staff evaluation is enclosed.
Sincerely,
/RA/
Marshall J. David, Senior Project Manager Plant Licensing Branch III-2 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket No. STN 50-455
Enclosure:
Evaluation of Steam Generator Tube Inservice Inspection Reports cc w/encl: See next page
- SE date NRR-106 OFFICE LPL3-2/PM LPL3-2/LA DCI/CSGB/BC LPL3-2/BC NAME MDavid:sp THarris for EWhitt AHiser*
RGibbs DATE 04/14/08 04/11/08 03/24/08 04/14/08
Byron Station, Unit 2 cc:
Corporate Distribution Exelon Generation Company, LLC Via e-mail Byron Distribution Exelon Generation Company, LLC Via e-mail Illinois Emergency Management Agency Division of Nuclear Safety Via e-mail Mr. Dwain W. Alexander, Project Manager Westinghouse Electric Corporation Via e-mail Howard A. Learner Environmental Law and Policy Center of the Midwest Via e-mail Byron Resident Inspector U. S. Nuclear Regulatory Commission Via e-mail Attorney General Springfield, IL 62701 Via e-mail Ms. Lorraine Creek RR 1, Box 182 Manteno, IL 60950 Chairman, Ogle County Board P.O. Box 357 Oregon, IL 61061 Mrs. Phillip B. Johnson 1907 Stratford Lane Rockford, IL 61107 Chairman Will County Board of Supervisors Will County Board Courthouse Joliet, IL 60434 Mr. Barry Quigley 3512 Louisiana Rockford, IL 61108
ENCLOSURE OFFICE OF NUCLEAR REACTOR REGULATION EVALUATION OF STEAM GENERATOR TUBE INSERVICE INSPECTION REPORTS FOR SPRING 2007 REFUELING OUTAGE BYRON STATION, UNIT 2 DOCKET NO. 50-455 By letters dated July 31, 2007 (Agencywide Documents Access and Management System (ADAMS) Accession Nos. ML072120471 and ML072120520), as supplemented by letter dated February 20, 2008 (ADAMS Accession No. ML080510729), Exelon Generation Company, LLC (the licensee) submitted information pertaining to the steam generator (SG) tube inservice inspection (ISI) performed at Byron Station (Byron), Unit 2, during the spring 2007 refueling outage (RFO) 13.
Byron 2 has four Westinghouse Model D5 SGs, which are designated 2A through 2D. All four SGs were inspected during RFO 13. Each SG has 4,570 thermally-treated Alloy 600 tubes with an outside diameter of 0.750 inches and a nominal wall thickness of 0.043 inches. The tubes are hydraulically expanded for the full-depth of the tubesheet at each end. The tubes are supported by Type 405 stainless steel support plates with quatrefoil-shaped holes. The U-bend region of the tubes installed in Rows 1 through 9 was thermally-treated after bending in order to reduce stress.
The licensee provided the scope, extent, methods, and results of its SG tube inspections in the documents noted above. In addition, the licensee described corrective actions (i.e., tube plugging or repair) taken in response to the inspection findings.
Based on the review of the reports submitted, the Nuclear Regulatory Commission (NRC) staff has the following observations and comments:
At the end of RFO 13, Byron 2 was in its second sequential 90 effective full-power month (EFPM) ISI period. The SGs had operated for 72 EFPM in this period.
Visual inspections of the secondary side of all 4 SGs were performed. Erosion of the primary moisture separator components was identified in all 4 SGs. The erosion in the affected areas was not projected to penetrate through wall or produce loose parts over the next two operating cycles, when the next inspection of the separators is currently planned.
One tube (in row 33, column 65) was plugged and stabilized along with two other adjacent tubes that contained possible loose part signals with no detectable flaws (row 33, column 64 and row 34, column 65). These signals were at the 8th tube support plate. This foreign object is adjacent to two tubes that were plugged during RFO 2 and RFO 7 due to what was believed at the time to be pitting degradation. Because the indications in these two tubes were attributed to pitting, they were not stabilized. As a result of the RFO 13 findings, eight additional tubes were preventatively plugged and stabilized to completely surround these two non-stabilized, plugged tubes. The indications in these two tubes are now attributed to the foreign object. Removal of a nearby inspection port and inspection in the area of the foreign object is currently planned for the next refueling outage when the activities can be planned and scheduled.
The licensee concluded that outside diameter initiated stress corrosion cracking in the U-bend region is not a potential damage mechanism until stress corrosion cracking occurs at other regions of the SG (e.g., hot-leg expansion transition and tube support plate region).
Approximately 30 percent of the tubes were inspected with a +Point' probe from 3 inches above to 17 inches below the top of the hot-leg tubesheet.
The licensee uses visual +Point' probe and bobbin probe inspections to identify loose parts and loose part wear in the flow distribution baffle region. For the last several outages, visual inspections of the 2nd baffle plate have been performed in one SG on a rotating basis. For the SGs that are not visually inspected, a +Point' probe is used to inspect for loose parts and loose part wear.
After reviewing the licensees February 20, 2008, supplemental letter, the NRC staff conducted a phone call with the licensee on March 17, 2008, to clarify several issues. During this call, the licensee clarified the following:
There was one mid-cycle outage in which limited SG tube inspections were performed. This outage occurred in May/June 2002, and the SGs had operated for approximately 151 EFPM.
There were no other mid-cycle SG tube inspections although the plant was shut down approximately 2 weeks prior to the scheduled start of RFO 6 and the plant entered the refueling outage early.
There was an administrative error in the table summarizing the inspections in the 90 EFPM sequential period. The starting point for the 90 EFPM period is RFO 9, as indicated in the table on the next page (prepared by the NRC staff).
The sampling strategy employed at Byron 2 involves sampling without replacement. As a result, the percentage of tubes inspected for each outage in the 90 EFPM period represents a different population of tubes (such that the sampling requirements in the technical specifications (TSs) are met). Once 100 percent of the tubes (or portions of tubes) are inspected during a sequential period, subsequent sampling in that period would involve tubes (or portions of tubes) that were already inspected.
Extended Byron 2 Cumulative Operating Duration RFO Cumulative EFPM Cumulative EFPM per period Comments 1
14.304 First ISI 2
28.248 13.944 Start of 120 EFPM period 3
41.808 27.504 4
56.088 41.784 5
70.824 56.52 6
86.604 72.3 7
103.548 89.244 8
120.456 106.152 End of 120 EFPM 9
137.112 122.808 1st outage of 90 EFPM period 10 153.876 19.572 11 171.420 37.116 12 188.856 54.552 13 206.292 71.988 Based on its review of the information provided, the NRC staff concludes that the licensee provided the information required by their TSs. In addition, the NRC staff concludes that there are no technical issues that warrant follow-up action at this time because the inspections appear to be consistent with the objective of detecting potential tube degradation and the inspection results appear to be consistent with industry operating experience at similarly designed and operated units.
Principal Contributor: E. Wong Date: April 14, 2008