ML15218A266
| ML15218A266 | |
| Person / Time | |
|---|---|
| Site: | Oconee  |
| Issue date: | 05/03/1996 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML15218A265 | List: |
| References | |
| NUDOCS 9605150365 | |
| Download: ML15218A266 (2) | |
Text
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UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-001 SAFETY-EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION UPPER HEAD-TO-TUBESHEET WELD FLAW EVALUATION DUKE POWER COMPANY OCONEE NUCLEAR STATION UNIT 2 DOCKET NO. 50-270
1.0 INTRODUCTION
On May 3, 1996, the Duke Power Company (the licensee) submitted a report for NRC approval in accordance with Paragraph IWB-3610 of Section XI of the American Society of Mechanical Engineers (ASME) Code. This report contains the licensee's evaluation of flaw indications in the upper head-to tubesheet weld in Steam Generator A. The flaw was discovered during ultrasonic (UT) examination during the 1996 refueling outage at the Oconee Nuclear Station, Unit 2 (Oconee 2).
The examinations were performed in accordance with the requirements of ASME Code,Section XI, 1989 Edition. The report indicates that the UT indication from the upper head-to-tubesheet weld exceeds the allowable flaw size specified in IWB-3500 of Section XI of the ASME Code and requires flaw evaluation using IWB-3610 and Appendix A of the ASME Code.
2.0 DISCUSSION In accordance with the requirements of ASME Section XI, 1989 Edition, the licensee provided the results of the 1996 refueling outage UT examinations performed on the upper head-to-tubesheet weld. The submittal also provided the evaluation of the single flaw that exceeds the ASME acceptance criteria.
This flaw was characterized as a subsurface flaw with length of 56 inches and depth of 0.8 inch, and is located 3.8 inch from the outer surface and 3.9 inch from the inner surface. The licensee did not perform an assessment of the accuracy of the UT measurements. However, in this case, the flaw evaluation indicates the flaw is relatively insensitive to flaw size because it is a subsurface flaw located close to the midplane of the weld. Hence, in this case, flaw size accuracy is not significant.
Methods and acceptance criteria that are acceptable to the staff for evaluating flaw indications exceeding the allowable flaw size in IWB-3500 are described in IWB-3610 and Appendix A of Section XI of the ASME Code for this subsurface crack.
In the licensee's fatigue analysis, the crack growth rate curves used were from air environment curves in Figure A4300-1 of Appendix A. The licensee used 420 heatup/cooldown cycles to bound the flaw growth from the normal and upset transients.
ENCLOSURE 9605150365-9605063 PDR ADOCK 05000270 P
-2 The residual stress distribution model was based on work of Ferrill, et. al.
[1].
For the-upper head-to-tubesheet weld.configuration this results in a residual stress at the center of the wall of a small or negative value.
Hence, the licensee used no residual stress in the fatigue evaluation.
Although this may be a nonconservative assumption, the margins in the flaw evaluation are extremely large and the licensee's assumption is not significant.
In determining the fracture toughness of the weld, the licensee used a reference temperature of 600F, in accordance with the criteria in Table 3-1 of the B&W Owners Group report [2].
The weld was fabricated using a submerged arc process by Babcock & Wilcox using Mn Mo Ni filler wire with Linde 80 type flux. This weld is similar to the welds in reactor pressure vessels that have been greviously reviewed and accepted by the staff. A reference temperature of 60 F is acceptable.
The licensee's evaluation indicates that the limiting condition for the evaluation is the heatup cycle. The margin in the flaw evaluation of the upper head-to-tubesheet weld exceeds the ASME Code IWB-3610 criteria by a factor of 5.13 for the heatup cycle. The licensee performed an evaluation assuming the flaw was twice the depth dimension. The margin of this flaw evaluation would exceed the ASME Code IWB-3610 criteria by a factor of 3.38 for the heatup cycle. These evaluations indicate that flaws significantly larger than the flaw discovered during UT examination would meet the ASME Code IWB-3610 criteria.
4.0 CONCLUSION
S
- 1) The licensee's evaluation indicates that the flaw in the upper head-to tubesheet weld satisfies the criteria from Paragraph IWB-3612 of Section XI of the ASME Code.
- 2) Based on Conclusion (1), the weld is acceptable for service for the remainder of the plant's operating license. However, the area containing this flaw shall be reeaxamined during the next three inspection periods as required by IWB-2420 of the ASME Code.
5.0 REFERENCES
[1] Ferrill, D.A., Juhl, P.B., and Miller, D.R., "Measurement of Residual Stresses in a Heavy Weldment," Welding Journal, WRC Supplement, Vol. 45, No.
11, November 1966.
[2] BAW-10046A, Rev. 2, "Methods of Compliance With Fracture Toughness and Operational Requirements of 10 CFR 50, Appendix G," B&W Owners Group Materials Committee Topical Report, June 1986.
Principal Contributor: B. Elliot Dated: May 3, 1996