The tie bar component of the advanced gas-cooled reactor (AGR) is manufactured from the nickel-based superalloy Nimonic PE16. During service the component undergoes microstructural and associated mechanical property changes attributable to irradiation-induced and thermally induced processes. The extent of these changes varies with local temperature/flux variations along the tie-bar length. Most importantly, a region of relatively poor stress rupture properties is created towards the bottom of exposed bars, where the irradiation temperature is in the range 350 to 400°C.

In this paper, the microstructure and microchemistry of two highly irradiated bars are reported and the particular features associated with the stress rupture “trough” are identified.

A mechanism is proposed for the intergranular failure observed within this region.