Measurements of the thermal conductivity of clay-sand and clay-graphite mixtures used as engineered barriers for high-level radioactive waste disposal

Abstract

Thermal conductivity measurements have been carried out on different possible constituents for engineered barriers. These consist of a mixture of selected French clay named Fo-Ca (calcic smectite) and different types of sand and/or graphite. The thermal conductivity of clay-sand mixtures depends, in addition to their bulk density and their water content, on the granulometric distribution of the sand. This conductivity increases with the median diameter of the sand. The thermal conductivity of clay-graphite mixtures is highly dependent on the type of graphite used; in some cases it can be significantly increased by the addition of a small percentage (5 mass%). Under these conditions it is not surprising to find that the increase in the thermal conductivity of ternary clay-sand-graphite mixtures depends, above all, on the graphite content. The present models (geometric model, Maxwell's model) applicable to pure compacted clays cannot be used for these types of mixtures. A specific model that takes the contact resistance between the sand particles and their average size into account has been developed and explains the experimental results for the sand mixtures.