Advances in researches on buffer/backfilling materials—bentonite pellets and pellet mixtures

The bentonite pellet is considered as an alternative buffer/backfilling material to fill technological voids and empty space in high-level radioactive waste (HLW) repository. The previous studies on the bentonite pellets are carefully reviewed and summarized, including their manufacturing methods, emplacement techniques, thermal conductivity, hydraulic behavior, structural change and mechanical behavior. Correspondingly, the research subjects worth further investigation are put forward. The results in the literatures indicate that the pellets can be manufactured and emplaced using several techniques, which together with size gradation and packing protocol can influence the packing dry density and homogeneity. For the pellet mixtures, the thermal conductivity is mainly governed by dry density, water content and temperature, and the hydro-mechanical behavior is related to size gradation, dry density and temperature. Upon liquid or suction controlled hydration, the initial loose-structured pellet mixtures will gradually transfer to the cemented state and finally present a homogeneous appearance at saturation. However, much longer duration is required before getting a completely homogeneous state. Considering the complexity of the operation conditions in a HLW repository, the improvements on emplacement techniques of the pellets and the investigations on the hydro-mechanical behavior and structural change under the coupled thermo-hydro-chemo-mechanical conditions should be further conducted.