Effect of air volume fraction on the thermal conductivity of compacted bentonite materials

The thermal conductivity of compacted bentonite is one of the key parameters in the design of engineered barrier system in the high-level radioactive waste (HLW) repositories where this material is proposed for use as buffer. Due to the extreme low thermal conductivity of air, it appears important to better understand the role of air volume fraction phi(g) (V-a/V) while assessing the repository performance. In this study, the thermal conductivity lambda of compacted bentonite was analyzed in the full range of air volume fraction phi(g). The analyses made implied that the thermal behaviour of compacted bentonite could be roughly divided into three zones according to the air volume fraction phi(g): at the low phi(g), the effect of air volume fraction was negligible, as the soil was nearly saturated; at the medium phi(g), the thermal behaviour could be significantly weakened with the presence of air. This was attributed to the fact that the channels for air were gradually connected with the increase of air volume fraction; at the high phi(g), the increase of air volume fraction had insignificant effect on the thermal behaviour. This is because the channels for air were fully connected in the soil specimen, and the increase of air volume fraction could not significantly create "extra" channel for air in that case. A new thermal conductivity model was thus proposed, allowing the thermal conductivity in the full range of air volume fraction to be calculated. The good agreement between the calculated and measured thermal conductivity values for six types of bentonite collected from existing literatures showed the good performance of the proposed method as well as the relevance of the identified mechanism.