AN ASSESSMENT OF BRANCH TECHNICAL POSITION 5-3 TO DETERMINE UNIRRADIATED RTndt FOR SA-508 CL. 2 FORGINGS

摘要

10 CFR 50.61 and 10 CFR 50, Appendix G require the evaluation of the unirradiated nil ductility transition reference temperature (RT_(NDT)) determined in accordance with ASME Section Ⅲ, paragraph NB-2331. For extended operation, components outside of the traditional beltline may require evaluation of RT_(NDT) per NB-2331. Many of these components do not have sufficient test data to determine RT_(NDT) in accordance with NB-2331 due to different test requirements during fabrication. The US NRC has provided methods in Branch Technical Position (BTP) 5-3 of NUREG 0800 to estimate a bounding RT_(NDT) for these components using available test data. This paper compares Branch Technical Position 5-3, paragraph 1.1(4) estimates of RT_(NDT) using 10℉ Charpy Ⅴ-notch data to the ASME B&PV Section Ⅲ, Paragraph NB-2331 values of RT_(NDT), for a sample of 35 SA-508 CI. 2 reactor vessel shell forgings. The BTP 5-3, Paragraph 1.1(4) estimates underestimate NB-2331 values by an average of 13.8℉ and by as much as 40℉. This non-conservatism is attributed to the use of 10℉ Charpy V-notch data for the BTP 5-3, paragraph 1.1(4) estimate and the difficulty of using only Charpy V-notch data to estimate a parameter based on Charpy V-notch and dropweight data. If BTP 5-3, paragraph 1.1(4) estimates for RT_(NDT) are to be used in 10 CFR 50.61 and 10 CFR 50 Appendix G calculations, the bias and measurement uncertainty of BTP 5-3, paragraph 1.1(4) should be accounted for, such that the estimate of RT_(NDT) bounds the NB-2331 RT_(NDT) with 2σ confidence, as shown in Regulatory Guide 1.99, Revision 2.