On the reduced damage tolerance of fine-grained nuclear graphite at elevated temperatures using in situ 4D tomographic imaging

Fine-grained nuclear graphite is one of the key structural materials for several Generation IV high-temperature nuclear fission reactor designs; surprisingly though, its deformation and fracture behaviour at elevated temperatures remain somewhat obscure. In light of this, the current study focused on investigating the flexural strength and fracture toughness of two fine-grained graphite (SNG623 and T220) using real-time X-ray micro-tomography imaging at room temperature, 750 °C and 1100 °C. Specifically, nonlinear-elastic fracture mechanics-based JR(Δa) R-curves at these temperatures were presented with evolution of damage and failure micro-mechanisms, local strain distributions and J-integral fracture analysis, purveying notable findings. Compared to the coarser-grained Gilsocarbon nuclear graphites used in the current advanced gas-cooled reactors, these modern fine-grained graphites display deficient fracture resistance in the form of far less stable crack growth prior to catastrophic fracture and reduced failure strain at 1100 °C. Moreover, their elevation in strength and toughness at high temperatures is remarkably lower to that of Gilsocarbon graphite. Based on in situ high-temperature Raman spectroscopy mapping, we believe that one of the major causes of this behaviour can be attributed to the smaller magnitude of 'frozen-in' residual stress relaxed at elevated temperatures compared with Gilsocarbon graphite.