Effect of soft-mould pressing method on anisotropy of the graphitic matrix spheres: Dry-bag isostatic vs. Quasi-isostatic

The anisotropy of graphitic matrix in fuel elements for high-temperature gas-cooled reactors (HTR) has an important effect on the distribution of thermal stress and irradiation stress, which should be strictly controlled. In this study, the graphitic matrix spheres were prepared through quasi-isostatic pressing and dry-bag isostatic pressing. The anisotropy was compared from the elastic modulus, crushing load, coefficient of thermal expansion (CTE) and thermal conductivity in different directions. The causes of anisotropy were investigated from the aspects of raw material microstructure, charging, pressing, as well as heat treatment. The graphite microstructure and the preferred orientation of graphite particles in the production process were found as the main reasons for the anisotropy of thermal and mechanical properties. The dry-bag isostatic pressing method can effectively reduce anisotropy and increase the production capacity compared with the quasi-isostatic method, which is consistent with the development trends of high quality and low cost fuel elements.