ML20196B242
| ML20196B242 | |
| Person / Time | |
|---|---|
| Site: | Point Beach  |
| Issue date: | 12/02/1988 |
| From: | Fay C WISCONSIN ELECTRIC POWER CO. |
| To: | NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM) |
| References | |
| CON-NRC-88-121 IEB-88-009, IEB-88-9, VPNPD-88-585, NUDOCS 8812060221 | |
| Download: ML20196B242 (7) | |
Text
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Wisconsin Electnc men court,r 231 W. MICHIGAN.P o BOX 2046, MILWAUKEE.WI 53201 H14) 2212M5 VPNPD-88-585 NRC-88-121 December 2, 1988 U.
S. NUCLEAR REGULATORY COMMISSION Document Control Desk Mail Station Pl-137 Washington, D. C.
20555 Gentlement DOCKET 50-301 NRC BULLETIN 88-09 THIMBLE TUBE THINNING IN WESTINGHOUSE _ REACTORS POINT BEACH NUCLEAR PLANT, UNIT 2 NRC Bulletin 88-09 dated July 26, 1988, requires all licensees to establish and implement an inspection program to confirm periodically the integrity of the thimble tubes for the incore neutron monitoring system.
Wisconsin Electric has established such an inspection program for the thimble tubes at our Point Beach buclear Plant, Units 1 and 2.
Atta:hed is the report of the incore neutron flux thimbic tube inspections conducted during the Fall 1988 Point Beach Unit 2 Refueling Outage.
Our letter dated October 26, 1988, provided the results of the Unit 1 program.
The attached report contains the information requested in Bulletin 88-09 and completes our response.
Very truly yours, l$O C. W. Fa 50 Vice President 8"@
Haclear Power n
{
Attachmer.t y
Copies to NRC Regional Administrator, Region III gg NRC Resident Inspector Mg Subterib(d and sworn to before me this A
- day of December Sq l CR h m M, 1988.
fpl
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Bea I
Notary Public, State o'f Wisconsin
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My Commission expires S-2 7 - WJ.
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THIMBLE TUBE THINNING IN WESTINGHOUSE REACTORS Attachment Page 1 NRC BULLETIN 88-09 THIMBLE TUBE THINNING PBNP UNIT 2
1.0 INTRODUCTION
This report is a summary of.he inspection performed on the neutron flux mapping thimble tubes during the October -
November 1988 Unit 2 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 tube repositioning is provided.
Detailed data from the inspection will be available in the forthcoming Cramer & Lindell inspection report.
This information was compiled from the Crimc0 & Lindell Engineers, Inc., "Damaged Pubo/ Condition Roport" dated November 7, 1988.
This report is on file at PBNP.
2.0 HISTORY OF THIMBLE TUBES The original incore thimble tubes were replaced in both units in 1985 by the currently installed thimble tubes.
It was necessary to replace these tubos due to internal blockages.
No leaking incore thimble tubes were discovered during the first fourteen years of operation.
The replacement tubos are made of stainicss steel typo 316 with a nominal outer diameter of 0.313" and a nominal inside diamoter of 0.210".
The original tubos measured 0.300" O.D.
and a 0.200" I.D.
The additional size was used to preclude internal blockages.
3.0 SCOPE OF INSPECTION Eddy current examination of all the Unit 2 neutron flux mapping thimble tubes was initiated after roccipt of NRC Bulletin 88-09.
The intent of the inspection was to obtain data on tubo condition after three years of service and detect any tube damage or any abnormal conditions.
The inspection was conducted by Cramcr & Lindell Engineers, Inc.
The inspection was performed in accordance with Cramer &
Lindell Procedure SS-028, "Multi-Frequency Eddy Current Inspection of 0.300" to 0.315" O.D.
Stainless Steel Incore Detcetor Thimble Tubes."
Data were recorded on magnetic tape and strip charts for future use.
o l
THIMBLE TUB'd THINNING IN WESTINGHOUSE REACTORS Attachment Page 2 4.0 RESULTS OF INSPECTION I
The overall incore thimble tube condition in Unit 2 after three years of service is fair.
The tubes are of high quality and are free from defects and inconsistencies.
Most s
tubes recorded no damage, although some abnormal conditions (dtstorted signals or damage signals) were recorded at the lowar core plate. the diffuser plate, the core support forging, and the tie plates.
Twelve tubes recorded damage at various axial positions in the core.
The highest wall losses were discovered in four tubes at the lower core plate.
Using a conservative method of analysis, the damage on these four tubes ranged from 29 to 39% wall loss.
The tube in core position J-12 recorded the largest amount of i
damage with 39% wall loss at the lower core plate, 25% wall j
loss at the core support forging, and 21% wall loss at the l
diffuser plate.
Axial and radial distributions of damage are shown in Figures 1 and 2.
l 5.0 CAPPING AND REPOSITIONING CRITCRIA i
Based on a conservative calculation of the collapse strength for a tube, it was determined tha*. an incore thimbic tube i
will collapse at design pressure when it has less than 17%
I of its original wall thickness.
This calculation assumes uniform wear around the circumforence of the tube, with no credit taken for any reinforcemet.* around the worn area.
i For a typical tube, fretting wear will be localized, I
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 j
wall thickness.
To ensure that tube failure does not occur 1stween inspections, tubes will be capped and taken out of service i
if a cumulative wall loss in excess of 60% is predicted in the subsequent operating cycles before the next scheduled inspection.
The 60% capping limit was established as follows:
~
FACTOR
% WALL LOSS Maximum allowable wall loss 83%
l Error in eddy current inspection
-10%
l Uncertainty in wall loss geometry
-10%
l Capping Limit 63%
r s-,-
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l
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THIMBLE TUBE TIIINNING IN WESTINGHOUSE ?EACTORS Attachuent Page 3 c
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 rink of failore.
The repositioning of tubes will be evaluated on a case-by-case basis.
The previously discussed capping Jimit will be adhered to in i
all cases.
6.0 INSPECTION FREQUENCY 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 a known wear rate.
After 3 years of operation, the highest damage signal l
discovered on the Uait 2 thimble tubes was determined to be i
39% through wall.
This wear occurred over a 3-year period, resulting in an average wear rate of 13%/ year.
The maximum inspection frequency for thimble tubes will be determined as follows:
Emax E meas F=
gg F = Inspection frequency - (years)
WL,,x
= Maximum allowable wall loso (%) (capping limit)
WL,,,, = Wall loss measured (%)
l WR
= Wear rate (%/ year)
I The following is an example of an inspection frequency calculation:
WL
= 60% (capping limit) l max W7,,,,, = 25% (measured wall loss)
= 15%/ year (wear rate) 2.3 years F
n ear Based on the calculation above, the thimble tube would be inspected in 2 years.
i l
TliIMBLE TUBE THINNING IN WESTINGHOUSE REACTORS Attachment Page 4 7.0 7.NSPECTION TECHNIQUE The thimble tubes were inspected using eddy current.
The inspection w4s conducted in acccrdance with section V of the ASME Boiler and Pressure Vessel Code, 1980 Edition / Winter 1981 Addenda.
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