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

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 ( lambda par ) and perpendicular ( lambda perp ) 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 lambda par was higher than lambda perp . With increasing water content, the difference rate ( delta PSD ) in thermal conductivity between PB and PGM increased, and anisotropy coefficient alpha (a ratio of lambda par to lambda perp ) decreased, while the effects of dry density and temperature on delta PSD and alpha 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.