Appropriate life predictions of components are essential to ensure safety operations of nuclear power plants, especially for welds operating at high temperatures where creep crack growth can be large and dominate their lives. In this paper, for a low alloy butt weld with a given set of loadings (temperature, internal pressure and system moments), a parametric creep crack growth study has been carried out using R5 assessment procedures as follows. • The plant has been operated for 209,200 hours and will be operating further for 47,300 hours; • Initial defect depths are 1, 2, 2.3, 3, 4 and 5 mm; • Crack aspect ratios (initial defect depth: defect half length) are 0.125, 0.2, 0.23, 0.3, 0.4 and 0.5. • Postulated defects are axial (internal and external) and circumferential (internal and external) semi-elliptical surface-breaking surface defects. The study results are presented by a series of graphs which show a minimum acceptable wall thickness for each initial defect depth and aspect ratio under the assessment conditions assumed. The graphs can be used for the plant life management to reduce the decision making time on these welds and to improve the plant power generation.

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