Deformation and fracture of irradiated polygranular pile grade A reactor core graphite

Highlights: → Mechanical properties of PGA nuclear graphite specimens were tested. → Load-displacement characteristics were consistent with quasi-brittle behaviour. → Micro-cracks observed in non-linear part of load-displacement curve pre-peak load. → Micro-testing showed surface tilts consistent with twinning. → Irradiated specimens failed at 20% lower load for 15% weight loss material. - Abstract: Pile grade A (PGA) graphite is used as a moderator in UK gas cooled nuclear reactors. This is a polygranular, aggregate material with quasi-brittle behaviour. When exposed to the service environment the material is subject to radiolytic oxidation that results in mass loss and an attendant increase in porosity. In the present work both unirradiated and irradiated small specimens of PGA graphite have been subjected to diametral compression. A novel trench-probe loading method is also described that allows micro-scale specimens prepared by focused ion beam milling to be fractured in a focused ion beam work station. This allows the fracture characteristics of selected regions of the graphite microstructure to be interrogated. The load-displacement and fracture characteristics of both the unirradiated and irradiated PGA graphite are compared and shown to be consistent with quasi-brittle behaviour. In addition, surface features consistent with elastically induced twins are observed associated with filler particles of the graphite. The results are discussed with respect to the quasi-brittle behaviour of this polygranular graphite.