ML20236C003
| ML20236C003 | |
| Person / Time | |
|---|---|
| Site: | Byron  |
| Issue date: | 03/17/1989 |
| From: | Richter M COMMONWEALTH EDISON CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| IEB-88-009, IEB-88-9, NUDOCS 8903210424 | |
| Download: ML20236C003 (2) | |
Text
. - -
~/7 Commonwe:lth Edison
- /
) Ons First Nationti Plaza, Chicado, Illinois O
,( v Address Reply to: Post Office Box 767 Chicago, Illinois 60690 -
s sw March 17, 1989 i
l U.S. Nuclear Regulatory Commission Attnt Document' Control Desk
, Washington, D.C.
20555
Subject:
Byron Station Unit 2 Response to NRC Bulletin 88-09 HRC Docket No. 50-455 Referoncet
-(a) NRC Dulletin No. 88-09, dated July 26, 1988 Dear Sirt Reference (a) requested that addressees establish and implement an
- inspection program to periodically confirm incore neutron monitoring system t
thimble tube integrity at Westinghouse designed nuclear power reactors.
In
- accordance with Reference (a), this' letter presents the details of the incore-thimble tube. inspection program (Action 1) and_the results of the thimble tube inspections (Action 2.a) performed during the first refueling outage for Byron
. Unit 2.
The thimble tube inspections are performed using eddy current methodology. The inspections will be performed at every refueling outage until sufficient data has been accumulated to generate a basellne and correlation for thimble tube thinning.
At that time, the Inspection frequency will be evaluated and may be adjusted as applicable.
It is expected t hat at least two inspections will be necessary to accumulate sufficient data before considering any change in inspection frequency.
To establish the' thimble tube wear acceptance criteria, a finite element structural analysis was utilized to evaluate the stresses in a worn bottom mounted instrumentation thimble tube. The wear scar geometry was assumed to be a flat bottom scar localized on one side of the thimble, which-was consistent with.the eddy current calibration standard.
It was determined that a sixty percent)(60%) maximum wall loss is acceptable in accordance with ASME Code material allowables, therefore, a wall loss criteria of 60% was adopted.
It should also be noted that this wall loss criteria (60%) will be applicable for Byron Uriit 1 and Braidwood Station Units 1 and 2 since the analysis was bounding for all four units.
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U.S. tEuclear Regulatory Commission March 17, 1989 All incore thimble tubes (58 total) for Byron Unit 2 were inspected from the seal table to the end of the thimble tubes. Three thimb.la tubes I
recorded evidence of degradation. The thimble tube at core location "D3" f
I exhibited a wall loss indication of approximately 13% at the lower tie plate.
It is believed that the wall loss is caused by vibration induced fretting wear with the lower tie plate, however, continued wear at this degraded area is not expected since all thimble tubes were slightly repositioned during the outage (due to a modification which was performed on the seal table). The thlmble tubes at core locations "H15" and "R8" exhibited a wall loss Indication of approximately 17% and 10%, respectively, between the seal table and the reactor vessel. The wall loss indications, which were typical of external defects, are believed to be preservice related and continued wear at the two degraded areas is not expected.
Based on these results, there is confidence that a thimble tube failure will not occur prior to the next refueling outage for Unit 2.
This completes the Byron Unit 2 response for Reference (a).
To the best of my knowledge and belief, the statements contained above are true and correct.
In some respect these statements are not based on my personal knowledge, but obtained information furnished by other Commonwealth Edison employees, contractor employees, and consultants.
Such information has been reviewed in accordance with company practice, and I believe it to be rollable.
Please address any questions that you or your staff may have concerning this response to this office.
Respectfully, bb- --
t1 dut M. H. Richter Nuclear bicensing Administrator
/lb 5592k cci A.B. Davis Posident Inspector - BY Subscribed and Swo to before m th s /7 day J'~,0FFICIALSEAL"_ _,_, N ere.v %,
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A 1989 l
LELIA F. MAYO U
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