Fracture mechanism of fine- and coarse-grain graphite

Sumita, Junya ; Shibata, Taiju ; Tachibana, Yukio ; Kuroda, Masatoshi*

Graphite materials are used for the in-core components of High Temperature Gas-cooled Reactor (HTGR) which is a graphite-moderated and helium gas-cooled reactor. The HTGR is particularly attractive due to capability of producing high temperature helium gas, and its passive and inherent safety features. The Very High Temperature Reactor (VHTR) is one of the most promising candidates as the Generation-IV nuclear reactor systems. During operation of the HTGR, the graphite structure is subjected to various loadings such as external forces and internal stresses resulted from neutron irradiation-induced dimensional and material property changes and a thermal gradient. In order to acquire the fundamental data to evaluate the integrity of the graphite structure of HTGR by fracture mechanics, it is important to investigate the fracture toughness and strain energy release rate of graphite. In this study, the fracture toughness of fine-grain and coarse-grain graphite was measured and calculated the strain energy release rate of them. Moreover, the crack propagation in these graphite and coarse-grain graphite were observed by microscope and the difference between them was discussed from the viewpoint of the grain size.