ML20195B942
| ML20195B942 | |
| Person / Time | |
|---|---|
| Site: | Point Beach  |
| Issue date: | 10/26/1988 |
| From: | Fay C WISCONSIN ELECTRIC POWER CO. |
| To: | NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM) |
| References | |
| CON-NRC-88-100 IEB-88-009, IEB-88-9, VPNPD-88-521, NUDOCS 8811020264 | |
| Download: ML20195B942 (6) | |
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{{#Wiki_filter:. _ - _ _ _ _ _ _ - _ _ _ _ _ _ _ _ _ _ _ _ - _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _. Wisconsin Electnc acoumr 231 W MICHIGAN.P o. Box 2046.VILWAUKEE,WI 53201 H14) 2212345 VPNPD-88-521, NRC-88-100 October 26, 1988 U. S. NUCLEAR REGULATORY COMMISSION Document Control Desk Mail Station P1-137 Washington, D. C. 20555 Gentlement DOCKETS 50-266 AND 50-301 NRC BULLETIN 88-09 THIMBLE TUBE THINNING IN WESTINGHOUSE REACTORS POINT BEACH NUCLEAR PLANT, UNITS 1 AND 2 NRC Bulletin 88-09 dated July 26, 1988 requires all licenscos to establish and impicment an inspection program to periodically confirm the integrity of the thimble tubes for the incore neutron monitoring system. Wisconsin Electric has established such an inspection program for the thimble tubos at our Point Beach Nuclear Plant, Units 1 and 2. Attached is a report of the incore neutron flux thimble tube inspections conducted during the Spring 1988 Point Beach Unit 1 Refueling Outage. Wo believe this report is responsive to the information requested in Bulletin 88-09. Wo shall provide a similar report on the Point Beach Unit 2 thimble tube inspection. This inspection is presently scheduled for completion in early November 1988. Our report will be forwarded to you upon completion of the Fall 1988 Point Beach Unit 2 Refueling Outage. I very truly yours, hi/ C. W. [y 8811020264 001026 Vice President PDR ADOCK 05000 o Nuclear Power 0 Attachment Copics to NRC Regional Administrator, Pegion III NRC Resident Inspector subscribed and sworn to before me this R f it.l.a of October, 1988. $ btrv. nt..]t l t I l Notary Public, State of Wisconsin My Commission expires < ;7 On
j O o IEB 88-09 j T8tIMBLE TUBE THINNING PBNP UNIT 1
1.0 INTRODUCTION
This report is a summary of the inspection performed on the neutron flux mapping thimble tubes during the Spring 1988 Unit 1 Refueling outage. Included in this summary is a description of the work performed, inspection results, and plans for future inspections and maintenance actions. A technical justification for taking a thimble tube out of service and/or repositioning it is provided. Detailed data from the inspections may ba found in the Cramer & Lindell inspection report, "Record of BaE*line Eddy Current Inspection of the Replacement Incore Thimbles, Unit 1," dated April 29, 1988. This report is on file at PBNP. 2.0 HISTORY OF THIMBLE TU) T The original incore thimble tubes were replaced in both units in 1985 by the currently installed thimble tubes. It was necessary to replace these tubes due to internal blockages. No leaking incore thinble tubes were ever discovered during the first 14 years of operation. The replacement tubes are made of stainless steel type 316 with a nominal outer diameter of.313" and a nominal inside diameter of.210". The original tubes measured .300" 0.D. and a.200" I.D. The additional size was used to prevent blockages. 3.0 SCOPE OF INSPECTION Eddy urrent examination of all the Unit I neutron flux mapping thimble tubes was initiated during the 1988 spring refueling outage. The intent of the inspection was to obtain data on tube condition after three years of service and detect any tube damage or any abnormal conditiens. The I inspection was conducted by Cramer & Lindell Engineers, Inc. The inspection was performed in accordance with Cramer & Lindell procedure SS-017, "Hulti-frequency Eddy current Inspection of.300" to.315" 0.D. Stainless Steel Incore Detector Thimble Tubes." Data was recorded on magnetic tape and strip charts for future use, j l 4.0 RESULTS OF INSPECTION The overall incore thimble tube condition in Unit 1 after three years of service is fair. The tubes are of high quality and are free from defects and inconsistencies. Most tubes recorded no damage, although some abnormal conditions were recorded at the lower core plate in the form of distorted signals or damage signals. Six tubes recorded damage at the lower core plate. Five of these tubes recorded minor damage ranging from 4 to 32 percent wall loss. Another tube, 1-G6, had an indicated wall loss estimated conservatively at 44 percent. See Figure 1 for axial location of damage on the thimble tubes. Figure 2 provides the core location where damage was noted.
2 5,0 CAPPING AND REPOSITIONING CRITERIA Based on a conservative calculation of the collapse strength for a tube, it was determined that an incore thimble tube could collapse at design pressure after it has worn 83% through wall. This calculati'n assumes uniform wear around the circumference of the tube with no credit taken for any reinforcement around the worn area. For a typical tube, fretting wear will be localized resulting in a large volume of sound tube material surrounding the worn area. Consequently, prior to collapse, the wall loss would have to be in excess of 83% of nominal wall thickness. To ensure that tube failure does not occur between inspections, tubes shall be capped and taken out of service, if a cumulative wall loss in excess of 60% is predicted in the subsequent operating cycles before the next inspection. The 60% capping limit was established as follows: FACTOR % WALL LOSS Haximum allowable wall loss 83% I Error in eddy current inspection -10% Uncertainty in wall loss geometry -10% Capping Limit 63% To prolong the life of thimble tubes experiencing wear, the tube may be repositioned to move the worn area away from the lower core plate. This will move the degraded portion of the tube into an area where fretting is not occurring and place intact tube material in the area where fretting is occurring. By doing this, the thimble tube life can be extended without undue risk of failure. The repositioning of tubes will be evaluated on a case-by-case basis. The previously discussed plugging limit will be adhered to in all cases. I 6.0 INSPECTION FREQUENCY 1 The frequency of inspection is based on the maximum wall loss noted in a region of active fretting and the projected wear which would occur based on l a known wear rate. After 3 years of operation, the highest damage signal discovered on the Unit 1 thimble tubes was determined to be 40% through wall. This wear occurred over a 3-year period, resulting in an average wear rate of l 14%/ year. I t i i
. The maximum inspection frequency for thimble tubes will be determined as follows: max meas WR F = Inspection frequency - (years) WL,,, Maximum allowable wall loss (%) = WL,,,, = Wall loss measured (%) WR = Wear rate (%/ year) The following is an example of an inspection frequency calculation: WL,,x = 60% (capping limit) WL,,,, = 25% (measured wall loss) i WR = 15%/ year (wear rate) %~ % = 2.3 years F= 15%/ year 3ased on the calculation above, the thimble tube would be inspected in 2 years. 7.0 INSPELTION TECHNIQUE The thimbla tubes were inspected using eddy current. The inspection was conducted in iccordance with section V of the ASME Boiler and Pressure Vessel Code, 1980 Edition / Winter 1981 Addenda, I
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