ML20205T376
| ML20205T376 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 11/04/1988 |
| From: | Cartwright W VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.) |
| To: | NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM) |
| References | |
| 88-515A, IEB-88-009, IEB-88-9, NUDOCS 8811140231 | |
| Download: ML20205T376 (5) | |
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.s VIROIN1A E1.aCTRIC AMD POWER COMPANY R cuwoxn.Vamoxx A 9 0 2 61 November 4, 1988 United States Nuclear Regulatory Commission Serial No.
88-515A Attention: Document Control Desk NAPS /JHL Washington, D. C. 20555 Docket Nos.
50 338 50 339 License Nos. NPF-4 NPF-7
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Gentlemen;
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l VIRGINIA ELECTRIC AND POWER CONPANY
!EElli ANNA POWER STATION UNITS 1 AND 2 RESPONSE TO NRC BULLETIN NO. 88-09 THINBLE TUBE THINNING IN WESTINGHOUSE REACTORS j
NRC Bulletin No. 88-09, dated July 26, 1988, requested licensees to establish an inspection program to monitor thinning of incore neutron monitoring system thimble tubes.
The Bulletin specifically requested that the inspection program include the establishment of thimble tube wear acceptante criterion, an appropriate inspection frequency, and an inspection methodology that is capable of detecting wear of the thimble tubes.
North Anna Power Station has implemented a thimble tube inspection program as 4
i described in NRC Bulletin No. 88 09.
A description of the North Anna inspection program is provided in the attachment.
The implementation of the thimble tube inspection program will ensure continued compliance with 10CFR50, Appendix A. General Design Criteria 14, "Reactor Coolant Pressure i
Boundary".
The information provided in this response is true and accurate to the best of my knowledge and belief.
If you have any questions or require further information, please contact us immediately.
i Very truly yours, b.L W 2 g
W. R. Cartwright i
i Vice President - Nuclear Attachment b
GG11140231 881104 PDR ADOCK 05000338 i j Q
cc:
U. S. Nuclear Regulatory Comission Region !!
101 Marietta Street, N. W.
Suite 2900 Atlanta, Georgia 30323 Mr. J. L. Caldwell NRC Senior Resident Inspector North Anna Power Station l
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COMMONWEALTH OF VIRGINIA COUNTY OF HENRICO
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The foregoing document was acknowledged t'efore nie, in and for the County and Commonwealth aforesaid, today by W. R. Cartwright who is Vice President Nuclear, of Virginia Electric and Power Company.
He is duly authorized to execute and file 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 YTdayof EtnNo
, 19 38__.
My Comission expires:
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26
, 19 90 kti Y IhdL Notary Public r
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s 8IIAC)tiENT NORTH ANNA POWER STATION UNIT NOS. 1 AND 2 RESPONSE 10 NRC BULLETIN NO. 88 09 THINBLE TUBE THINNING IN WESTINGHOUSE REACTORS North Anna Power Station has implemented an inspection program to monitor thimble tube integrity.
The program was recommended by Westinghouse Electric Corporation and implemented during the 1987 refueling outages for both Units 1 and 2.
The program consisted of eddy current inspection and the application of acceptance criteria for repair if thinning was detected.
Future inspections of thimble tubes will be performed every refueling outage.
This l
inspection frequency is based on the thimble material properties after manufacturing (cold working) ard that after a 50% wall loss the thimble will i
no longer satisfy the ASME Boiler and Pressure Vessel Code requirements, Section NB3133 for tubing subjected to external pressure.
This program is adequate to detect degradation from thimble tube wear.
The Westinghouse recommended acceptance criterion is based on ASME Boiler and Pressure Vessel Code, Section NB3133.
After a 50% wall loss the thimble will no longer meet the requirements for tubing subjected to external pressure.
In addition, the acceptance criterion was based on the fact that the worst case i
thimbles had experienced approximately 50% tube wall thinning over five effective full power years.
A wear rate of approximately 10% tube wall thinning per effective full power year was then conservatively assumed.
The i
basis for this assumption is that the eddy current inspection results are l
based on the wear scar extending one fourth of the way around the thimble and that the wear rate is linear with time.
Therefore, the acceptance criterion requires tube repair when eddy current testing detects greater than 40% tube i
wall thinnirg, and requires tube replacement or isolation for greater than 50%
l tube wall thinning.
(future thimble tube inspection results will be evaluated l
to determine if the acceptance criteria and inspection frequency need to be
[
revised.)
North Anna management decided to conservatively reduce the Westinghouse accep-l tance criterion for tube repair to reflect 18 month operating cycles.
Tube t
repair is required when there is between 35 and 49% tube wall thinning and tube replacement or isolation is required when there is greater than 50% tube i
wall thinning.
The basis for the revision in the acceptance criterion is the frequency that refueling outages are performed on North Anna Units 1 and 2.
j North Anna Units I and 2 are on a 18 month refueling frequency.
With a 10%
wear rate per effective full power year, it can be assumed that a 15% wear rate would occur between refueling outages.
Based on this assumption, the i
acceptance criterion for tube repair was revised to 35% tube wall thinning, so the requirements of ASME Section NB3133 are satisfied.
When tube wall thin-ning is detected, it is evaluated by engineering, and reviewed and approved by the Station Nuclear Safety and Operating Committee (SNSOC).
When tube repair or isolation is required, SNSOC review and approval of the repair procedure is also required.
Tube repair consists of retracting the thimble tube approxi-mately two to three inches out of the core to allow placing new metal in the area of the wearing surface.
Tube isolation consists of first retracting the
a tube from the core approximately two inches and then closing the isolation valve at the seal table to remove it from service.
Inspection results from the Unit 11987 refueling outage indicated six thimble tubes had wall thinning greater than 35%.
These thimble tubes were evaluated and, as a result, retracted out of the core approximately two to three inches to allow placing new metal in the area of the wearing surface.
Inspection results from the Unit 2 1987 refueling outage indicated that eight thimble tubes had wall thinning greater than 35%.
These thimbles were evaluated and, as a result, retracttd out of the core approximately three inches to allow placing new metal in the area of the wearing surface.
Of these eight thimbles requiring repair, three thimble tubes had wall thinning such that thimble tube isolation and removal from service was required.
The revised (conservatively reduced from the Westinghouse recommendations)
I acceptance criterion and the requirements for performing engineering evaluations for thimble tube wall thinning have been implemented by approved station procedures and will be used during future inspections.
The implementation of the thimble tube inspection program will ensure continued compliance with 10CFR50, Appendix A, General Design Criteria 14 "Reactor Coolant Pressure Boundary".
The results of thimble tube inspections will be documented and maintained within their records retention schedule.
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