Thermal conductivity of MX80 powders-granules mixture and its prediction for high-level radioactive waste repository

Abstract

Bentonite granules and their mixtures with powders are considered to be a preferred sealing material for filling gaps between buffer layer and surrounding rock in high-level radioactive waste (HLW) repository. Due to continuous release of decay heat from nuclides, thermal conductivity of bentonite granules must be assessed. In this study, the effects of water content, dry density, temperature on the thermal conductivity in the directions parallel (λpar) and perpendicular (λperp) to the bedding plane of MX80 powders-granules mixture (PGM) were investigated by thermal probe technique. Test results showed that the thermal conductivity of powdered bentonite (PB) was higher than that of PGM and λpar was higher than λperp. With increasing water content, the difference rate (δPSD) in thermal conductivity between PB and PGM increased, and anisotropy coefficient α (a ratio of λpar to λperp) decreased, while the effects of dry density and temperature on δPSD and α were not obvious. The factor importance was analyzed, and results showed that the effects of water content, dry density, temperature, and particle size distribution decreased successively. A modified geometric mean (MGM) model for predicting thermal conductivity of PGM with temperature is proposed and it predicted thermal conductivity of MX80 PGM well.