ML17191A889
| ML17191A889 | |
| Person / Time | |
|---|---|
| Site: | Dresden  |
| Issue date: | 09/23/1998 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML17191A888 | List: |
| References | |
| NUDOCS 9809250002 | |
| Download: ML17191A889 (2) | |
Text
UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 2056Ml001 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION DRESDEN NUCLEAR POWER STATION, UNIT 3 CORE SHROUDEXAMINATION RESULTS AND THE FLAW EVALUATION COMMONWEAL TH EDISON COMPANY DOCKET NO. S0-249
1.0 INTRODUCTION
. By letter dated May 23, 1997, the Commonwealth Edison Conipany {the licensee} submitted, for NRC review, its core shroud examination results and the associated flaw evaluation for the detected flaws for Dresden 3 plant. The ultrasonic {Un examination of the core Shroud was conducted in 1997. About 72.S% of the total axial welds were examined in that effort, and no reportable indications were found. However, during the core plate support ring segment weld examination, the licensee identified circumferential cracking in the H2, H3, and HS welds, and two composite flaws in the vicinity of the segment welds of the support ring. The two composite.
flaws are of a "T" shape, with their circumferential segments in the HS circumferential weld and their axial segments in the vicinity of the V23 and V2S ring segment {axial) welds below the H5 weld. The segment close to V23 has an axial length of 2.8 inches and a maximum depth of O.SO inch; the segment close to V2S has an axial length of 1.2 inches and a maximum depth of 0.25 inch. The licensee intended to demonstrate through an analytical flaw evaluation that the unit could be operated without repairs for four fuel cycles.
Report CMED-060298 was referred to in the submittal for the evaluation of the two composite flaws. Hence, the review of CMED-060298 is also included in the staffs safety evaluation {SE).*
2.0 EVALUATION 2.1 Licensee For circumferential flaws and the circumferential segments of the two composite flaws, the licensee cited its installed comprehensive shroud repair as a means to structurally replace circumferential welds H1 through H7. For the axial segments of the two composite flaws, the licensee used {1} the limit load analysis of Section XI of the ASME Code and {2} the maximum primary stress criterion of Section Ill of the ASME Code, to calculate the allowable flaw depth under the normal/upset and emergency/faulted loading conditions. The licensee used a conservatively assumed crack geometry: 4 inches long {axially through the thickness} and 0.5 inch deep {radially}. The licensee then estimated the amount of crack growth at the end of four fuel cycles to be 3.S inches using a crack growth rate of Sx10*5 inch per hour. The predicted final crack depth at the end of four additional fuel cycles is, therefore, 4.0 inches {O.S inch + 3.S inches}.
Using the limit load analysis and the maximum primary stress criterion, the licensee determined
- that the most limiting allowable flaw depth was from applying the maximum primary stress criterion at the emergency/faulted condition. This allowable flaw depth is 8.827 inches. Since the predicted final crack depth at the end of four additional fuel cycles (4.0 inches) is far less than the allowable flaw depth {8.827 inches of the radial dimension of 10.68S inches for the support ring), the licensee concluded that continued operation for Dresden 3 without repair is justified.
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2 2.2 NBC Staff 2.2.1 Circumferential Flaws The licensee's core shroud repair was designed to structurally replace the circumferential welds from the H1 weid to the H7 weld. This repair was approved by the staff on December 6, 1995, in a letter from John F. Stang (USNRC) to D. L. Farrar (licensee). Hence, the licensee is not required to perform the flaw evaluation on a circumferential flaw, or the circumferential segment of a composite flaw, that was detected in the reinspection of the core shroud.
2.2.2 Axial Flaws The licensee's flaw evaluation for the axial segments of the composite flaws was documented in CMED-060298, which included a limit load analysis in accordance with Section XI of the ASME Code and an analysis based on the maximum primary stress criterion.of Section Ill. The maximum primary stress criterion is from NG'-3000 of Section Ill for core support structures.
The staff considered the licensee's use of the limit load analysis appropriate because the limit load analysis was selected according to the guidelines (based on fluence levels) established in the report, GENE-523-113-0894, Revision 1, "BWR Core Shroud Inspection and Evaluation Guidelines," dated March 1995. This report was approved by the NRC on June 16, 1995.
However, GENE-523-113-0894, Revision 1, does not contain detailed procedures for the limit load analysis of axial flaws. This information was documented in CMED-060298. The staff evaluated the limit load analysis for axial flaws in CMED-060298 and found the licensee's safety factors, 2.8 for the normal and upset conditions and 1.4 for the emergency and faulted
_condition, were incorrect. The safety factors of 2.8 for the normal and upset conditions and 1.4 for the emergency and faulted condition specified in IWB-3640 and Appendix C of Section XI of the ASME Code, were originally derived for circumferential flaws on pipes. For axial flaws on pipes, one should use safety factors of 3.0 for the normal and upset conditions and 1.5 for the emergency and faulted condition, as specified in Appendix C. Based on the underlying theory of Appendix C, the staff determined that the safety factors for axial flaws on pipes (3.0 for the normal and upset conditions and 1.5 for the emergency and faulted condition) are applicable to flaws on components with geometries other than pipes (such as the core plate support ring).
- The difference in safety factors (2.8 and 1.4 v.s. 3.0 and 1.5) is not significant to the results from the limit load analysis and does not affect the limiting allowable flaw depth of 8.827 inches determined by the licensee based on the primary stress criterion of Section Ill. Hence, the licensee's results and conclusions are acceptable. The staff also confirmed that the remaining portion of the flaw evaluation methodology for axial flaws, including the use of the bounding crack growth rate of 5x10-5 inches per hour, is appropriate.
3.0 CONCLUSION
S The staff has reviewed the licensee's submittal and except for an insignificant error in the safety factors, has determined that the flaw evaluation is performed in accordance with the procedures and criteria in the ASME Code and the assumed crack growth rate is adequate for this application. Since the predicted final flaw depth at the end of the four fuel cycles (4.0 inches) is far less than the allowable flaw depth (8.827 inches), the staff concludes that continued operation for Dresden 3 without repair is acceptable for four fuel cycles.
Author: Simon Sheng*, EMCB/DE Date:
September 23, 1998