ML20246L826
| ML20246L826 | |
| Person / Time | |
|---|---|
| Site: | Catawba  |
| Issue date: | 03/16/1989 |
| From: | Tucker H DUKE POWER CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| IEB-88-009, IEB-88-9, NUDOCS 8903240285 | |
| Download: ML20246L826 (3) | |
Text
--
4 e;
DUKE POWER GOMPANY P.O. Box 33180 CHARLOTTE, N.O. 28242
- HAL B. TUCKER ret.zenone-vios emessoswr (704) ay34tu)g NUCLBAB PRODUCTION March 16, 1989
. Document Control Desk U. S. Nuclear Regulatory Commission Washington', D. C.
20555 t
Subject:
Catawba Nuclear Station, Unit 1 Docket Nos. 50-413 NRC Bulletin No. 88-09 Thimble Tube Thinning in Westinghouse Reactors Gentlemen:
NRC Bulletin 88-09 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.
Please find attached my.
response for Catawba Nuclear Station, Unit 1.
Thimble' wear does not appear to be
'a problem at Catawba Unit 1 at the present time. This report was scheduled for:
submittal to the NRC'on January'2, 1989. However, transmittal of the report was delayed due to an administrative oversight.
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,
/4
.f Q Hal B. Tucker JGT09.D4/lcs-Attachment i
xc:
Mr. S. D. Ebneter Regional Administrator, Region II U. S. Nuclear Regulatory Commission 101 Marietta St., NW, Suite 2900 Atlanta, Georgia 30323 l
Mr. P. K. VanDoorn NRC Resident Inspector McGuire Nuclear Station ho
'f
/
b a
o!.
RESPONSE TO NRC BULLETIN 88-09 CATAWBA UNIT 1 END OF CYCLE 3 INCORE GUIDE THIMBLE TUBE EXAMINATION The thimble examination requested by NRC Bulletin 88-09, " Thimble Tube Thinning in Westinghouse Reactors," was performed at Catawba Nuclear Station Unit 1 on December 1 and 2, 1988 during the end of cycle 3 refueling outage.
Catawba 1 is a 4-loop Westinghouse PWR with 58 incore guide thimble tubes. The thimble tube design used at Catawba 1 is identical to those at McGuire Nuclear Station Unit 1.
Each tube has a 0.201 inch internal diameter, a 0.049 inch wall thickness, and is manufactured of 316 stainless steel. Westinghouse conducted all phases of the Catawba examination, including data collection, data analysis, and generati.on of a thimble wall loss acceptance criterion.
The Unit i end of cycle 3 examination technique was identical to that used at McGuire Unit 1.
Results of the McGuire Unit 1 inspection were submitted to the NRC per H. B. Tucker's December 22, 1988 letter to the Document Control Desk. An absolute type eddy current probe with a high fill factor was used to determine volumetric reduction in the thimble tube wall relative to nominal conditions.
The volumetric information suppli.ed by the probe is translated to a depth of penetration in the MIZ-18 using depth versus volume relationships derived from known defects machined into a calibration standard. The calibration standard used for the Catawba Unit 1 end of cycle 3 examination contained 5 defects of 90 degrees circumferential extent.
Each defect was a one-inch long tapered scar varying in depth from 0% to either 10%, 20%, 40%, 60%, or 80% of nominal wall thickness over the length of the defect. This calibration standard should provide conservative calculations of wear depth, since any wear scar of greater than 90 degrees circumferential extent will generate higher volumetric loss for an equivalent wear depth than the 90 degrees calibration scar, causing the eddy current system to overstate the depth of penetration. Limited data available at this time indicate wear occurs over a greater than 90 degrees circumference.
For this test, an eddy current uncertainty of 10% is used. Additionally, the minimum detectable flaw was determined to be 10% of wall thickness.
The eddy current test results showed one thimble with detectable wear. The thimble at core location M-07 has an indication of 55% through-wall wear at a location approximately 31 inches below the top surface of the lower core plate.
A 55% maximum wall loss indication after 3 cycles is consistent with results from other 4-loop, 12-foot core plants. The unique nature of the Catawba Unit 1 data is in the number of thimbles showing detectable wear. Most 4-loop, 12-foot plants which show wear have indications on a number of thimbles.
Catawba Unit 1 appears more like McGuire 1 in that very few thimbles are involved.
- Overall, thimble wear does not appear to be a serious problem at Catawba Unit 1, which is similar to the conclusion reached for McGuire Unit 1.
This similarity was expected, since the thimble design is identical and both units are equipped with j
the lower internals anti-vibration sleeves.
The acceptance criterion for thimble wall thinning calculated for Catawba Unit 1
{
was 60% through-wall wear. A finite element model of the Catawba Unit 1 thimbles was generated by Westinghouse and used to calculate peak stress intensities in the thimble wall resulting from a 60% through-wall defect. The defect used in the analysis was a flat bottom scar of 90 degrees circumferential extent, which is similar to the machined defects on the eddy current calibration standard. The calculated peak stress intensity at 60% through-wall wear was significantly less than that allowed for the thimble material.
Further calculation of acceptable wall loss and determination of wear scar geometry is being performed by the Westinghouse Owners Group.
Duke Power is participating in this task.
l o
The application of the 60% through-wall wear criterion to the Catawba Unit 1 I
thimble at core location M-07 has led to the decision to isolate the thimble above the seal table by installing a cap on the tube. This decision was made after applying the 10% eddy current measurement uncertainty to the 55% measured defect. This led to a possible 65% wall loss, which is greater than the 60%
criterion. No further action beyond capping the M-07 thimble is planned for this outage. Predictions of future wear on the remaining thimbles is difficult, since the linear wear rate model is not necessarily applicable when the thimbles show zero wear at this point. Accordingly, the next examination is planned for end of cycle 5 at Catawba Unit 1, after two more cycles of operation. The results from the end of cycle 5 test and the Owners Group work will be used to quantify a wear rate.
Future actions will be determined after the end of cycle 5 exam based on these results.
l ii 1
l l
l
_______.a