Thermal properties of GMZ bentonite pellet mixtures subjected to different temperatures for high-level radioactive waste repository

Bentonite pellets or their mixtures with powdered bentonite are potential buffer/backfill materials for the deep geological disposal of high-level radioactive waste (HLW) and are used to backfill the joints between neighbouring bentonite blocks or the construction gaps between bentonite blocks in the buffer layer and surrounding rock. Knowledge of the thermal properties of buffer/backfill materials is essential for the thermal evaluation and design of HLW repositories. The thermal properties were measured on a mixture of GMZ07 bentonite pellets and powdered bentonite with 70% pellet content over a wide range of water contents, dry densities, and temperatures. The test results revealed that the thermal conductivity of the specimen compacted with pure bentonite powder was higher than that of the mixture. This was due to the different pore-size distributions that influenced the conductive heat transfer in the specimens. The difference in thermal conductivity between the two types of specimens made by different methods decreased with increasing dry density, which was mainly attributed to the preparation of bentonite pellets using a compaction–crushing method. In addition, the destruction of bentonite pellets increased gradually with increasing dry density. The thermal conductivity, thermal diffusivity, and volumetric heat capacity of the mixture increased with increasing temperature. For a given dry density, the size of the inter-aggregate pores and aggregates of the mixture was larger than that of compacted pure GMZ07 bentonite, resulting in a decrease in the extent or quality of connections between neighbouring aggregates and a reduction in the thermal conductivity.