ML20237D139
| ML20237D139 | |
| Person / Time | |
|---|---|
| Site: | Mcguire, Catawba, McGuire, 05000000 |
| Issue date: | 12/14/1987 |
| From: | Tucker H DUKE POWER CO. |
| To: | NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM) |
| References | |
| NUDOCS 8712230069 | |
| Download: ML20237D139 (3) | |
Text
s DmfE POWER GOMPANY P.O. DOX 33180 CHARIXYTTE, N.C. 28242 HALH.TUCKEH Tenernoxe vice eneminewr (704) 0.73-4tk31 l
strotaan emonwmm December 14, 1987 U.S. Nuclear Regulatory Commission
, Document Control Desk Washington, D.C.
20555 Subj ec t: McGuire Nuclear Station Docket Nos. 50-369, -370 Catawoa Nuclear Station Docket Nos. 50-413, -414 Gentlemen:
On November 20, 1987, a telephone conference call was held between Duke staff and NRC staff concerning Rod Cluster Control Assembly (RCCA) axial repositioning at McGuire and Catawba. This letter is in followup to that discussion.
Background
Several domestic and foreign Westinghouse Plants have observed cladding wear indications on Rod Cluster Control Assemblies (RCCAs).
The problem was first identified at Point Beach, Unit 2 (Wisconsin Electric Power) during the EOC-9 refueling outage.
The following observations were made as a result of hot cell examinations performed on the most severely damaged RCCA:
Axially continuous wear scars caused by sliding contact between the RCCA rodlets and the upper internals guide cards and continuous guide region during stepping and scramming events.
- Localized fretting wear scars caused by flow induced vibratory contact between the RCCA rodlets and the upper internals guide cards when the RCCAs are operated in the fully withdrawn, parked position.
- Axial hairline cracks at the rodlet tips caused by a combination of absorber swelling induced clad stress and radiation induced effects on cladding material properties.
The most probable effect of radiation on the cladding is radiation enhanced segregation of impurity elements to the grain bound-aries.
No operational problems have been reported for wear or hairline axial cladding cracking. The cladding degradation caused by fretting with the guide cards during RCCA operation at the fully withdrawn position was determined to be the life limiting effect.
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t Document Control-Desk December 14, 1987 Page 2 Duke has performed several video inspections of RCCAs (MNS-2, EOC-1 'and CNS-1, EOC-1,2) and has identified' similar wear scars; ' Although the wear was easily
. identified, the amount of wear in terms of depth of penetration could not be quantified.
Based on the observations made at Point Beach and 'at other 14 X 14 and 15 X 15 plants, Westinghouse estimated the service life of 17 X 17 RCCAs to be 10 years and recommended axial repositioning after.10 years to extend,the service life to 15 years.
Recently obtained 17 X 17 RCCA examination data has indicated a much higher wear rate ' than that previously observed in the 14 X 14 and 15 X 15 RCCA designs.
Multiple indications of wear were observed on the 17 X 17 hybrid B4C RCCAs. at Tihange -3 in Belgium after 2 annual cycles of operation (the maximum
~
wear depth was 57% of the cladding wall thickness).
All RCCA's were well within allowable operating limits for B4C type RCCA's, with no indication of any claiding penetration. Data is also available on foreign non-B4C RCCA's.
An Edf plant in France experienced 22 thru wall penetrations on 15 of their 17 X 17 AgInCd RCCAs
'after 4. cycles of operation (about 4 1/2 years) with no reported operational problems.
j Future Plans at MNS and CNS A summary of Duke's strategy to minimize the concerns associated with 17 X 17 RCCA cladding wear due to vibratory contact with the upper internals guide cards is as follows:
(1)
Reposition the operating elevation of the fully withdrawn RCCAs at MNS-1, MNS-2, CNS-1, and CNS-22 The result of this change is that the interfacing surfaces between the RCCA rodlets and the upper internals guide cards is shifted such that unaffected cladding surfaces are positioned at the guide cards.
Increasing the operating elevation of the RCCAs is preferred over j
decreasing the elevations due to the resulting minimal impact on core reac-tivity and shutdown margin.
Pre-operational testing at McGuire and Westing-I house drawing reviews have indicated that the operating elevation of fully I
withdrawn RCCAs can be increased by at least 2 Steps (from 228 to 230 steps l
withdrawn).
(2) Duke intends to implement axial repositioning at MNS-1, MNS-2, and CNS-1 at the earliest possible convenience following the completion of the necessary safety evaluations and plant changes.
Axial repositioning at CNS-2 is planned for the E0C-2 refueling outage in 1988.
Duke plans to perform eddy current / profilometry inspections on the McGuire 17 X 17 RCCAs at Unit I during the EOC-5 refueling (November 1988) and at Unit 2 during the EOC-4 refueling (June 1988).
(3) Duke will evaluate the need for RCCA inspections at Catawba based on the results of the McGuire RCCA examinations.
7. A...
Document Control Desk
[
December 14, 1987 Page 3-Summary With respect to technical specification 3/4.1.3.6, Duke understands that RCCAs are limited to the - insertion limits of the specification.
Rod withdrawals to 231 steps may be accomplished within the existing specifications.
Further, small perturbations of rod: overlap are acceptable.
Finally, Duke understands that.a license amendment would be necessary to adjust the existing limits downward. Duke will be pursuing this latter change in the future.
' Duke is providing this information in order to facilitate NRC understanding of this operational situation.
If the NRC has any further questions in this area, please contact Duke through the normal licensing channels.
L L
Very truly yours, t
f
), f,/
f Hal B. Tucker RIG /316/jge xc:
Dr. J. Nelson Grace Regional Administrator U.S. Nuclear Regulatory Commission l
Region II l
l 101 Marietta Street, NW, Suite 2900 l
f Atlanta, Georgia 30323 Mr. Darl Hood, Project Manager i
Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Washington, D.C.
20555 Mr. W.T. Ordere NRC Resident Inspector 1
McGuire Nuclear Station I
Mr. P.K. Van Doorn NRC Resident Inspector Catawba Nuclear Station j
.I