Creep-Fatigue Crack Initiation Assessments of an Instrument Guide Tube Within a Superheater Header

Abstract

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.