Investigation of the deformation and fracture of a nuclear-grade graphite using in situ high-temperature computed x-ray micro-tomography

Isotropic and near-isotropic graphites are used as moderating material and major core components of operating nuclear fission reactors, such as the UK Advanced Gas-cooled Reactors (AGRs). As such materials are subjected to various stresses and loads in service, the deformation and fracture behaviour of nuclear graphite at operating temperatures is of critical importance to the integrity of the core. Due to experimental limitations, most mechanical tests performed to date to evaluate such properties in graphites have carried out at ambient temperatures, but these experiments can never fully describe the behaviour of graphite at realistic temperatures, i.e ., ~650°C for AGRs and ~1000°C outlet temperatures for Gen IV reactors. For reliable evaluations of the reactor core integrity, in situ at-temperature characterisation of nuclear graphites must be undertaken. In the current work, we present the first quantitative three-dimensional, under-load characterisation of the evolution of damage in these graphites by performing fracture toughness tests at temperatures up to 1000°C with simultaneous real time imaging using computed x-ray micro-tomography. In this work, a unique in situ ultrahigh temperature synchrotron radiation x-ray tomography facility that permits in situ investigations of damage evolution under load at temperatures up to 2000°C in vacuo or in a selective environment, was adopted. The experiments were undertaken at the hard x-ray micro-tomography beamline (BL 8.3.2) at the Advanced Light Source (Lawrence Berkeley National Laboratory). The material investigated was an unirradiated near-isotropic (1:1.1) moulded Gilsocarbon graphite extracted from stock AGR reactor core bricks. The filler coke particles, produced by calcination of Gilsonite, had a diameter of around 500 µm. The size of the specimens investigated in this study was similar to those irradiated surveillance specimens trepanned from the core bricks. Flexural strength tests (plain beam specimens under three-point bending) and fracture tests (beam specimen with sharp deep notch; radius ≤ 50 µm) were performed at three temperatures: room temperature (RT), 650°C and 1000°C.  A full x-ray tomography scan was performed at each step as the specimen was incrementally loaded. The tomography scans were then reconstructed in 3D to feed into LaVision software for full 3D strain and deformation measurements. With respect to strength properties, the nominal flexural strength measured at high temperature was found to be higher than at room temperature, with localised strain concentrations forming preferentially around the filler particles. For the fracture tests, the crack-tip opening displacement and the strain distribution ahead of the crack tip were analysed. Results indicated that crack-initiation fracture toughness K Ic values varied with temperature, and that the filler particles in the graphite structure served to deflect the fracture path and as such provided a means of extrinsic toughening to promote crack-growth toughness, as reflected in the rising R-curve behaviour of the material.  The relationships between the measured mechanical properties and the complex multiscale structure of the graphite will be discussed in detail.