ML12130A261
| ML12130A261 | |
| Person / Time | |
|---|---|
| Site: | Millstone  |
| Issue date: | 05/01/2012 |
| From: | Scace S Dominion Nuclear Connecticut |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| 12-244, TAC ME7352 | |
| Download: ML12130A261 (6) | |
Text
Dominion Nuclear Connecticut, Inc.
Millstone Power Station MAY 1 -.2012 jioDominion Rope Ferry Road Waterford, CT 06385 U. S. Nuclear Regulatory Commission Serial No.12-244 Attention: Document Control Desk NSSL/WDC RO Washington, DC 20555 Docket No. 50-336 License No. DPR-65 DOMINION NUCLEAR CONNECTICUT, INC.
MILLSTONE POWER STATION UNIT 2 RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION REGARDING STEAM GENERATOR TUBE INSERVICE INSPECTION REPORT FOR END OF CYCLE 20 (TAC NO. ME7352)
By letter dated October 7, 2011, Dominion Nuclear Connecticut, Inc. (DNC) submitted information summarizing the results of the end of cycle 20 steam generator tube inspections at Millstone Power Station Unit 2 (MPS2). In a letter dated March 29, 2012, the NRC transmitted a request for additional information (RAI) to DNC related to the inspections. DNC agreed to respond to the RAI by May 17, 2012.
Attachment I provides DNC's response to the NRC's RAI.
If you have any questions regarding this submittal, please contact Wanda Craft at (804) 273-4687.
Sincerely,
/St~ h nýE.Scac~e --
Site Vice President - Millstone
Attachment:
- 1. Response to Request for Additional Information Regarding Steam Generator Tube Inservice Inspection Report for End of Cycle 20 Commitments made in this letter:
- 1. None
Serial No.12-244 Docket No. 50-336 Page 2 of 2 cc: U.S. Nuclear Regulatory Commission Region I 475 Allendale Road King of Prussia, PA 19406-1415 J. S. Kim Project Manager - Millstone Power Station U.S. Nuclear Regulatory Commission One White Flint North 11555 Rockville Pike Mail Stop 08-C2A Rockville, MD 20852-2738 NRC Senior Resident Inspector Millstone Power Station
Serial No.12-244 Docket No. 50-336 ATTACHMENT 1 RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION REGARDING STEAM GENERATOR TUBE INSERVICE INSPECTION REPORT FOR END OF CYCLE 20 DOMINION NUCLEAR CONNECTICUT, INC.
MILLSTONE POWER STATION UNIT 2
Serial No.12-244 Docket No. 50-336 RAI Response for EOC 20 SG Tube Inspection Report Attachment 1, Page 1 of 3 By letter dated October 7, 2011, Dominion Nuclear Connecticut, Inc. (DNC) submitted information summarizing the results of the end of cycle 20 steam generator tube inspections at Millstone Power Station Unit 2 (MPS2). In a letter dated March 29, 2012, the NRC transmitted a request for additional information (RAI) to DNC related to the inspections. DNC agreed to respond to the RAI by May 17, 2012. This attachment provides DNC's response to the NRC's RAI.
RAI 1
There were a number of indications reported in Table 3 of the October 7, 2011, submittal that appearto be in the free span region and are classified as foreign object wear. Please discuss the basis for concluding that the indications were the result of foreign object wear (e.g., visual inspection confirming a wear scar).
DNC Response There are three tubes listed in Table 3 identified as being in steam generator (SG) 1 with recorded eddy current signals indicative of foreign object wear with a foreign object present. The foreign object was confirmed visually and identified in the October 7, 2011 report as weld slag, but could not be removed. These three tubes (R119 C66, R120 C67, and R121 C66) were stabilized and plugged.
Seven of the twelve tubes listed in Table 3 identified as being in SG 2 with recorded eddy current signals indicative of foreign object wear with a foreign object present during previous outages, were confirmed visually, and the objects were removed. The degradation in these tubes was not severe so these seven tubes were left in service and the wear scars are evaluated each examination with essentially no changes detected. These seven tube locations are: R28 C5, R29 C4, R59 C10, R123 C46, R124 C45, R125 C48, and R126 C49.
There are five remaining tubes listed in Table 3 identified as being in SG 2 with recorded eddy current signals indicative of foreign object wear without a foreign object present. Visual examinations were performed at these tube locations. No foreign object was found visually at any of these locations; however, the most likely cause of the wear scars was determined to be a foreign object based on size, shape, and location. These five tube locations are: R44 C5, R98 C143, R118 C41, R119 C42, and R128 C 107. Wear scars at three of these five tube locations (R98 C143, R118 C41, and R119 C42) were first detected during a previous outage. These wear scars were evaluated during this examination with essentially no changes detected.
Serial No.12-244 Docket No. 50-336 RAI Response for EOC 20 SG Tube Inspection Report Attachment 1, Page 2 of 3
RAI 2
Please discuss the results of the secondary side inspections and channel head inspections.
DNC Response Prior to the installation of eddy current probe manipulators, the hot and cold leg channel heads staywell welds and divider plate welds were visually examined in both SG 1 and SG 2. The examination revealed no evidence of divider plate or staywell weld degradation, and no foreign objects were identified.
During the MPS2 refueling outage in April 2011 (2R20), visual examinations were performed on all previously installed plugs. No indications of plug degradation, leakage, or misplacement were identified.
A visual examination of steam drum components was performed in SG 1. In the areas examined, sludge accumulation was light, with a harder underlying crystalline coating of sludge noted. Arch bars, J-tabs and fan bars appeared to be in good condition. Sludge deposits on the primary and secondary moisture separators were light and tightly adhering. Very little deposit was removed from these surfaces when rubbed with a gloved hand. The primary separators examined were in good condition with no evidence of material degradation. The curved arm assemblies within the primary separators were inspected and found to be in good condition. The edges of the steam outlet to the assemblies were sharp, indicating no noticeable flow assisted corrosion.
In short, no evidence of material degradation was identified during the examination of steam drum components.
Post-lancing visual examinations in SG 1 identified no loose sludge in the annulus at the top of tubesheet. The no-tube lane and staywell regions were clear as well. The blowdown flow holes in the tubesheet showed no evidence of flow induced erosion.
The first support lattice, shroud, and shroud support components examined were in good condition. Jacking studs showed no indication of movement between the shell and shroud. A three inch screw was located near the cold leg periphery at tubes R121 C126 and R120 C127, as discussed in the inspection report. After several attempts it was determined that the screw dimensions prevented retrieval. The surrounding tubes were stabilized and plugged.
There was a light dusting of sludge noted in the annulus of SG 2 and random small pieces of debris were noted at the cold leg 90 degree handhole, which is the location of one of the water lance suction feet. Many of the tube crevices were noted as encircled with a small amount of orange deposit. This was noted during the last outage as well.
The no-tube lane and staywell region were clean and the blowdown flow holes showed no evidence of erosion. The first support lattice, shroud, and shroud support
Serial No.12-244 Docket No. 50-336 RAI Response for EOC 20 SG Tube Inspection Report Attachment 1, Page 3 of 3 components examined were in good condition with no evidence of material degradation.
Jacking studs showed no indication of movement.
RAI 3
There were a number of newly reported dents and dings on both the hot-leg and cold-leg of both SGs. Please discuss the nature of these new dents and dings and what may have caused them.
DNC Response In 2005, Millstone lowered the threshold amplitude for reporting dents and dings from 3 volts to 2 volts. For SG 1, the 2R20 inspection was the third inspection since changing the reporting threshold and for SG 2, it was the second inspection.
It is not uncommon to have tubes that were just below the reporting threshold during previous outages to record a signal amplitude above the reporting threshold value during a subsequent outage.
Locations of dents or dings reported during 2R20 were reviewed in previous eddy current examination history to determine if the signal existed in the previous data or if it is new. If during the review of historical data the dent or ding was found to be present but below the reporting threshold, the 2R20 signal response was compared to the previous signal response to determine if there was any significant change. If there was no significant change in the dent or ding signal response, then the indication is designated with the code 'HR' for History Review. If the signal is not found in the previous data, the signal is designated as 'NEW'.
SG 1 had nine newly reported dings, four newly reported dents and one 'NEW' ding.
SG 2 had 10 newly reported dings and one newly reported dent.
The majority of the newly reported dents and dings were not significantly above the reporting threshold of 2 volts. The average voltage of the newly reported dents and dings was 2.76 volts. Of the 24 newly reported dents or dings in SG 1 and SG 2, none showed significant change from previous outages. These previously existing indications could have originated during fabrication or could have been caused by impact from a foreign object. Definitive causes are indeterminate.
A new ding was reported in SG 1 R138 C67 at the top of tubesheet cold leg + 11.51" elevation. The indication had a voltage of 3.67 volts which is a relatively small ding signal. The tube is on the periphery and had apparently been impacted by a foreign object. The ding location was examined with the rotating +PointTM coil and no degradation was detected.