ML18153C676

Suppls 881007 Response to NRC Bulletin 88-009,consisting of Results of Eddy Current Insps of Thimble Tube Thinning. Insps Indicated No Detectable Degradation of Thimble Tubes

ML18153C676
Person / Time
Site:	Surry
Issue date:	07/08/1991
From:	Stewart W VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.)
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
88-515C, IEB-88-009, IEB-88-9, NUDOCS 9107220042
Download: ML18153C676 (5)

Text

e e VIRGINIA ELECTRIC AND POWER COMPANY RICHMOND, VIRGINIA 23261 July 8, 1991 United States Nuclear Regulatory Cpmmission Serial No. 88-515C Attention: Document Control Desk *NURPC Washington, D. C. 20555 Docket No. 50-280 50-281 License No. DPR-32 DPR-37 Gentlemen:

VIRGINIA ELECTRIC AND POWER COMPANY SURRY POWER STATION UNITS 1 AND 2 NRC BULLETIN 88-09: THIMBLE TUBE THINNING UNIT 2 INSPECTION RESULTS AND PROGRAM In our response (Serial No.88-515) to NRC Bulletin 88-09 dated October 7, 1988, Virginia Electric and Power Company submitted an alternate schedule for completion of the required thimble tube inspections consistent with the provisions of paragraph 2 of the Bulletin.

The thimble tube design at Surry is a double walled asymmetric configuratiqn in which the inner wall forms the pressure boundary. Eddy current inspections were performed during the Surry Power Station Unit 2 1991 refueling outage. These inspections indicated no detectable degradation of the thimble tubes.

As committed to in our letter (Serial No. 88-5158 dated January 18, 1991 ), which reported Unit 1 inspection results, the details of our future inspection program for both Unit 1 and Unit 2 are being provided in Attachment 1. Should you have any questions or require further information, please contact us ..

Very truly yours, I.

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W. L. Stewart Senior Vice President - Nuclear.

Attachment cc: U. S. Nuclear Regulatory Commission Region II 101 Marietta Street, N. W.

Suite 2900 Atlanta, Georgia 30323 Mr. M. W. Branch NRC Senior Resident Inspector Surry Power Station

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COMMONWEALTH OF VIRGINIA )

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COUN1Y OF HENRICO )

The foregoing -document was acknowledged before me, in and for the County and Commonwealth aforesaid, today by W. L. Stewart who is* Senior Vice President - Nuclear, of Virginia Electric and* Power Company. He is duly authorized to execute and rile the foregoing document in behalf of that Company, and the statements in the document are true to the best of his knowledge and belief.

Acknowledged before me this 8 rt' day of , 19!11_.

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My Commission Expires: , 191Y_.

I~ Notary Public (SEAL)

ATTACHMENT 1 NRC BULLETIN 88-09 THIMBLE TUBE INSPECTION PROGRAM SURRY POWER STATION

• NRC BULLETIN 88-09 THIMBLE TUBE INSPECTION PROGRAM SURRY POWER STATION Surry *units 1 and 2 were originally built with a single-wall thimble tube design. - These -.

tubes were replaced with a double-walled asymmetric configuratioh. These replacements took place three fuel cycles ago. An inner tube (0.212 inch I. D. with 0.0428 inch nominal wall) is used for passage of the neutron flux detector, with three thermocouples (0.040 inch diameter) permanently attached to the exterior of the inner tube. Enclosing this tube/thermocouple assembly is a second outer tube. This outer tube protects the thermocouples, however it is the inner tube which serves as the reactor coolant pressure boundary. Any external source of degradation to the inner

. tube would first require. penetration of the outer tube. Thus, detection of degradation of the outer tube does not imply degradation of the pressure boundary. Monitoring the outer tube does provide early detection of the potential for degradation of pressure boundary integrity of the inner tube as required by NRC Bulletin 88-09.

Virginia Electric and Power Company has addressed the requirement to establish an inspection program to monitor thimble tube performance for Surry Power Station as indicated in the responses to Bulletin Action Item 1 below:

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the establishment, with technical justification, of an appropriate thimble tube wear acceptance criterion ... " *

• _Response:

The inner tube can lose 60% of its wall thickness without exceeding ASME Code material allowables under design conditions. This thimble tube wear limit was determined using finite element structural analysis under the assumption that the outer tube would be completely worn through.

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the establishment, with technical ju-stification, of an appropriate inspection frequency ... "

Response

Based on the expected wear rates of the double-wall thimble tubes with respect to the single-wall thimble tubes and the data presently available relative to the inspection of double-wall thimble tubes, Surry will inspect the Unit 2 thimble tubes at the end of every other operational cycle. Based on the limited outer tube wear observed on a Unit 1 thimble tube, Surry Unit 1 thimble tubes will be inspected during the next refueling outage. If the next inspection of Unit 1 does not show any degradation of the inner tube and no additional degradation of any of the outer tubes, then Unit 1 thimble tube inspections will be conducted on an every other operational cycle basis.

This inspection frequency is subject to change based on accumulation of data or evidence of wear on an inner tube.

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"the establishment of an inspection methodology that is capable of adequately detecting wear of the thimble tubes ... "

Response

This requirement is met by use of conventional bobbin coil eddy current inspection techniques applied inside the inner tube. Using this approach; a limited scope investigation was performed by Westinghouse to assess the adequacy of this inspection technique in the assessment of wear damage in double-wall bottom mounted instrumentation tubing. The results of the Westinghouse investigation show that the presence *of thermocouple wires in the annulus and the non-concentric configuration of the thimble tubes limit the detection of wear scar damage to approximately 40% of the outer wall thickness. Wear scars that penetrate to the outside surface of th(:} inner tube can be detected. The impact of the thermocouple wires. and. the non-concentric configuration will have less effect on detecting damage to the inner tube. This inspection methodology is adequate to detect inner tube wall degradation before the inner tube wall is reduced to a thickness below ASME Code allowable limits .

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