Optimal mixing scheme for graphite–bentonite mixtures used as buffer materials in high-level waste repositories

The buffer material in deep geological repositories for disposing high-level radioactive nuclear waste must have sufficient thermal conductivity to dissipate the decay heat. The graphite is a potential addictive to improve the thermal conductivity of bentonite buffer material, thus the thermal-hydro-mechanical performance was experimentally investigated. A series of tests including the free swelling index, grain size distribution, swelling pressure, saturated hydraulic conductivity and thermal conductivity measurements were performed on compacted graphite–bentonite mixtures (GB) at various graphite ratios (5%, 10%, 15%, 20%, 30%, 40%) and meshes (50, 100, 200, and 325 mesh), indicating that adding graphite could significantly influence the properties of GB, depending on the water content and dry density as well. It was also found that the optimal graphite ratio was approximately 15–20% by weight, and that the proper graphite should have a grain size ranging from 100 to 200 mesh. Meanwhile, the pore size distribution (PSD) curves of compacted GB showed that graphite particles graded as too large or too small were prone to forming macropores by point-edge mode contact with the generally smaller bentonite particles or agglomerates. Moreover, as graphite is hydrophobic, water can easily pass through the surface of the graphite sheets, even when the bentonite expands after hydration.