Compressive loading unloading behavior of nuclear graphite grades of different forming method and raw cokes

Nuclear graphite is used for core structural components and neutron moderators in high temperature gas-cooled reactors. As graphite is a brittle material fail at relatively low strains (e.g., ∼0.5% in tension and ∼2% in compression), cracking of these components can occur throughout the life of the reactor under the influence of thermal and mechanical stresses. While a lot of studies have been performed on the fracture of graphite, most studies have been concerned on crack initiation and propagation, with little concerns on the damage processes that lead to the very first stage of crack initiation. In this study, the graphite damage processes before the main crack formation were investigated based on the microstructure change during load relaxation. For this, 4-1/3 notched flexure strength test specimens made of nuclear graphite grades IG-110, NBG-18 and PCEA of different forming methods (isotropic molding, vibrational molding and extrusion, respectively) and ingredients (coke, binder) were subjected to 10 cyclic compressive loading-unloading, and the changes in the microstructure of notch-tip areas were examined by X-ray tomography