ML20245D840

Responds to NRC Bulletin 88-009, Thimble Tube Thinning in Westinghouse Reactors, Including Test Method Utilized for Plant Insp,Which Involved Absolute Type Eddy Current Probe W/Data Gathering & Reduction Performed on MIZ-18 Computer

ML20245D840
Person / Time
Site:	Catawba
Issue date:	04/18/1989
From:	Tucker H DUKE POWER CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
IEB-88-009, IEB-88-9, NUDOCS 8905010189
Download: ML20245D840 (3)

Text

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' April 18,'1989 U. S.l Nuclear Regulatory Commission Washington,'D.C. 20555 Document Control Desk

Subject:

Catawba Nuclear Station, Unit 2 Docket No. 50-413 NRC Bulletin No. 88-09 Thimble Tube Thinning in Westinghouse Reactors Gentlemen:

NRC Bulletin No. 88-09, Thimble Tube Thinning in Westinghouse Reactors, requested that Duke Power Company establish and implement an inspection program to periodically confirm incore neutron monitoring system thimble tube integrity at Catawba Nuclear Station. The Catawba Unit 2 incore instrument thimbles were examined from March 12 to March 19, 1989 during the end of Cycle 2 refueling outage.

The test method utilized for the Catawba Unit 2 inspection involved an absolute type eddy current probe with data gathering and reduction performed on a M1Z-18 computer system. All testing was performed by Echoram (a Westinghouse-subsidiary). The minimum detectable flaw was approximately 10%, and the measurement. uncertainty was 10%. System calibrations were performed every 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> using a standard tube containing machined, tapered defects of 10%, 20%,

.40%, 60%, and 80% maximum wall loss. The defects are each 1.5 inches in axial length,- and 90 degree in circumferential extent. This method is identical to those already used at McGuire Unit 1 and Catawba Unit 1 during previous thimble tube inspections. Twenty-two out of the fifty-eight incore instrument guide thimbles showed measurable wear. Three thimbles presented measurable wear at more than one location. A maximue wall loss of 48%, was measured at location G-05. The amount and distribution of thimble wear observed from this test is consistent with expectations derived from information supplied by similar plants.

A map of the Catawba Unit 2 core showing the thimble locations and the amount of wear observed at each location is attached.

As a result of this inspection, eleven thimbles will be repositioned prior to cycle 3 startup, which is currently scheduled for May 16, 1989. Thimbles to be repositioned are at core locations D-12, D-10, G-09, B-08, J-08, M-07, J-01, G-05, L-05, K-06, and C-07. The thimble at C-07 is being repositioned as a precautionary measure, as the eddy current probe did not insert far enough during the test to fully inspect the thimble. The other ten thimbles were selected based on a combination criterion of a 60% allowable wall thickness predicted loss at the end of cycle 3 and a linear wear rate assumption. The 60% allowable wall loss was determined by a finite element analysis of the Catawba Unit 2 incore j i

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'U. S. Nuclear Regulatory Commission April 18, 1989 Page Two thimbles. The scar model used in the finite element analysis was a flat bottom scar of 90 degrees circumferential extent and 1.5 inches axial length. This model should provide conservative stress intensities relative to the actual wear scar. The peak stress intensity (calculated at 60% through wall wear and a system pressure of 2500 psi) was 19.87 ksi. This peak stress intensity compares favorably to the allowable intensity of 25.7 ksi from the ASME Boiler and Pressure Vessel Code. The linear wear rate model is currently considered by Westinghouse to be conservative with respect to the actual wear rate at higher wear values.

The cutoff point for selecting thimbles to be repositioned was determined to be 30% measured through wall wear at the end of Cycle 2 refueling outage.

Repositioning thimbles with measured wear at or above 30% though wall assures that Catawba Unit 2 will be able to operate for the entire duration of cycle 3 without any thimbles exceeding the 60% through wall-wear limit. The most limiting thimble for cycle 3 is at core location A-09, which has 29% measured wear (see Attachment 1). Using a 10% uncertainty in the wear value and a linear wear model, the allowable operating time until the 60% through wall limit is reached is given by:

Allowable Operating Time (days) = [60% - (29% + 10%)) / WR The wear rate (WR) is calculated by:

WR = (29% + 10%) / 907 days = 0.0430% / day Therefore, the Allowable Operating Time is 488 days.

The 907 day figure in the wear rate calculation is the total number of days the unit has operated at or above mode 4 since the installation of the thimbles.

Cycle 3 is designed to extend for approximately 320 effective full power days of operating. The thimbles will be reinspected at the end of cycle 3 refueling outage to determine any further action. The Unit 2 end of cycle 3 refueling outage is currently scheduled to start on June 3, 1990.

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,

[ km 11al B. Tucker JGT11.D4/lcs Attachment xc: Mr. S. D. Ebneter Regional Administrator, RII U.S. Nuclear Regulatory Commission 101 Marietta St., NW., Suite 2900 l Atlanta, Georgia 30323 l

l Mr. W. T. Orders 1

NRC Resident inspector Catawba Nuclear Station

ATTACHMENT 1

. . . , CATAUBA UNIT 2, EOC 2 INCORE FLUX THIMBLE EDDY CURRENT RESULTS l

R P N M L K J H G F E D C B A 240" 37% NDD 1 NDD NDD NDD 2 NDD NDD NDD NDD 3 245" NDD 27% NDD 4 259" 262" 226" 251" 43% 48% 17% 23% 5 261" 262" 174' 250" 26% NDD 38% 28% 25% 6 263" 180" 30% NDD 21% NDD 7 190" 177" 175" NDD NDD NDD 35% 23% NDD NDD 38% 8 262" 175" NDD 43% NDD 29% 9 169" NDD NDD 42% 10 NDD NDD NDD NDD NDD 11 J 320" NDD NDD 31% 12 218" )

22% NDD NDD NDD 13 232" 27% NDD _

NDD NDD 14 l 238" 25% NDD 15 KKX -Distance from top of thimble in inches YYY -Throughwall wear in percent (NDD indicates No Detectable Defect)

NOTE: The following thimbles had more than one defect:

THIMBLE WEAR LOCATION A09 28% 239" G09 30% 179" K06 31% 228"