In order to extend the boiler lives at Advanced Gas-Cooled Reactor (AGR) nuclear power stations in the UK, new temperature measuring instrumentation to monitor reactor gas temperature has been proposed to install on the bore of an intact boiler tube to provide additional boiler operating data to support the station lifetime extension. This paper details a creep-fatigue crack initiation assessment of the proposed installation of an instrument guide tube within the superheater header using the latest R5 high temperature assessment procedures based on detailed finite element thermal transient stress analysis values for a bounding start-up and shutdown cycle. The fatigue damage at welds has been calculated based on both weld and parent material properties. The new approach for assessing weldments has been used in this paper. This new approach involves splitting the existing Fatigue Strength Reduction Factor (FSRF) into a Weldment Endurance Reduction (WER), which accounts for reduced fatigue endurance due to weld imperfections, and a Weldment Strain Enhacement Factor (WSEF), which accounts for material mismatch and local geometry. The creep assessments of the weld material locations have been carried out on both parent and weld material properties including the welding residual stress. The total creep-fatigue damage is then obtained as the sum of fatigue damage, Df, and creep damage, Dc.

This paper provides an overview of developments that have been undertaken in recent years in order to extend the Crack Opening Area (COA) solutions for Leak-Before-Break (LBB) within the R6 Flaw Evaluation Procedures. For high temperature operations, the effect of creep strains on COA has been studied using finite element (FE) models of a centre-cracked plate with combined primary and secondary stresses. Primary creep, as well as secondary creep, has been included in the studies. The ultimate aim of this work is to further develop and validate by FE analysis, simplified expressions for the prediction of COA under realistic loading conditions for high temperature operations. For COA evaluations in general, work has been undertaken by FE modelling of the following aspects: • Straight-fronted cracks under combined loading. • Converging/diverging cracks whereby the length on one surface is different to that on the other surface (i.e. cases where there is a through-wall variation in crack length). • Weld strength mis-match effects. • Component features effects.