ML20247F695
| ML20247F695 | |
| Person / Time | |
|---|---|
| Site: | Mcguire |
| Issue date: | 09/07/1989 |
| From: | Tucker H DUKE POWER CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| IEB-88-009, IEB-88-9, NUDOCS 8909180243 | |
| Download: ML20247F695 (3) | |
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Dune POWER GOMPANY
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P.O. DOX 33180 CHARIDTTE, N.C. 28242 IIAL H. TUGIGR niernonz va0E PMrtertDENT (704)373 4531 moos man ruopmmon September 7, 1989 Document Control Desk U. S. Nuclear Regulatory Commission Washington, D. C.
20555
Subject:
McGuire Nuclear Station, Unit 2 Docket No. 50-370 i
NRC Bulletin 88-09 Thimbic Tube Thinning in Westinghouse Reactors Gentlemen:
NRC Bulletin 88-09 required that Duke Power Company establish and implement an inspection program to periodically confirm incore neutron monitoring system thimble tube integrity at McGuire Nuclear Station. Please, find attached my response for McGuire Nuclear station Unit 2. Thimble wear does not appear t1 j
be a problem at McGuire Unit 2 at the present time.
I declare under penalty of perjury that the statements set forth herein are true and correct to the best of my knowledge.
Very truly yours, as k. f v
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Hal B. Tucker JGT/4/M288-09 xc:
Mr. S. D. Ebneter Regional Administrator, Region II U. S. Nuclear Regulatory Commission 101 Marietta Street, NW, Suite 2900 Atlanta, Georgia 30323 i
I Mr. P, T Var. Doorn i
NRC Resident Inspector McGuire Nuclear Station
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7 8909180243 890907 PDR ADOCK 05000370
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NJg' BULLETIN 88-09 RESPONSE McGUIRE 2 END OF CYCLE 5 INCORE GUIDE THIMBLE EXAMINATIONS The McGuire Unit 2 End-of-Cycle 5 incore thimble measurements were performed to meet the requirements of NRC Bulletin 88-09.
McGuire 2 is a 4-loop Westinghouse PWR with 58 incore guide thimbles. Each thimble is manufactured from cold worked stainless steel 316 with a 0.201 inch internal diameter and a 0.049 inch wall thickness. Data collection, analysis, and acceptance criterion generation for the McGuire 2 End-of-Cycle 5 tests were performed by Westinghouse.
The McGuire 2 Er.d-of-Cycle 5 exams utilized an eddy current measurement
' technique. The probe was a differential probe operating in absolute mode of sufficient diameter to ensure a'high fill factor for the test.
Calibrations were performed using a standard containing five 90 degree one-inch tapered flaws of 10%, 20%, 40%, 60%, and 80% maximum through-wall depth. This technique has been used by Westinghouse and other vendors for this test at sites both in the U.S.
and abroad with good success. All the calibration flaws were easily detectable.
However, higher than expected noise levels during testing made defects between 10% and 15% depth difficult to distinguish. Data collection and analysis were performed using a MIZ-18 system and DDA-4 analysis software. The McGuire 2 thimbles showed very little degradation, with only two indicating any detectable wall loss. The maximum detected wall loss was 33% on the thimble at core location M-07.
This wear was located at the top of the lower core plate, consistent with indications seen at other plants. The other wear indication was 25% wall loss also located at the top of the lower core plate on the thimble at core location F-07.
Other 4-loop plants have shown greater wall loss after fewer cycles of operation, and in general have shown wear on a greater number of thimbles. These results indicate McGuire 2 does not have a serious thimble wear problem. Therefore, no further action is planned for this outage.
A wall loss acceptance criterion was developed by Westinghouse using a finite element analysis model of the McGuire 2 guide thimbles.
This finite element analysis calculated an allowable wall loss of 60% of the nominal wall thickness before action must be taken to preclude thimble failure. The flaw geometry used in the model was a 90 degree circumference flat bottom scar of one inch in length. This geometry is consistent with the eddy current test standard, and should be conservative with respect to the existing flaw since the detected flaws were less than one inch in length. Also, using the 90 degreo circumference should provide conservative results since any greater actual scar circumference will result in the eddy current current test overstating the depth of penetration, which would lead to higher than actual stress intensities being calculated by the finite element analysis. More detailed calculations of allowable wall loss which eliminate many of these conservatism are being performed as part of the Westinghouse Owners Group task on flux thimble wear.
Duke Power is participating in this task.
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1 The limited plant data tilable at this time indicate that a linear wear rate prediction is conservat for predicting future thimble conditions. Knowing the maximum wall loss of 33., the number of fuel cycles to date (5), and an eddy current uncertainty of 10%, the approximate wear rate per cycle is:
'JR = (33% + 10%)/5 - 8.6%/ cycle This wear rate can then be used to predict a minimum number of fuel cycles until the 60% criterien is met:
N = [60% - (33% + 10%)] / 8.6% / cycle - 1.98 fuel cycles Accordingly, the next inspection is currently planned for the end of cycle 6 l
outage. At that time any thimbles exceeding the applicable wear criterion will have the appropriate action taken on them to eliminate the risk of leakage. This action may include repositioning, capping, or replacement of the thimble, depending on the amount of wear found and the wear rate. Future inspection frequency will be based on the applicable wear rate information resulting from the Westinghouse Owners Group task and from inspections performed at McGuire.
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