Publisher Summary Eliminating the risk of transportation of radionuclides from radioactive waste stored underground to the biosphere is the goal set by all countries that make use of nuclear power. Different countries plan geologic disposal in rocks, salt and clay sediments. The design principle is that highly radioactive waste, such as burnt-out fuel and products from processed waste, is enclosed in containers—canisters—isolated by smectitic (smectite-rich) clay. These canisters are placed in vertical boreholes or long horizontal bored or blasted tunnels at a depth of 300–800 meters. Low- and medium-level waste has shorter lifetimes and can be stored at smaller depth with less effective isolation. Selection of smectitic clays with suitable bulk density to embed the canisters is based on the valuable properties of clay buffer: (1) very low hydraulic conductivity, (2) very low anion diffusion capacity and fairly low transport capacity of positively charged radionuclides, (3) a high swelling potential for self-sealing of gaps and openings in the buffer and its contacts with the rock and canisters, (4) favorable rheological properties, such as sufficient bearing capacity to minimize settlement of the heavy canisters and sufficient softness to avoid transfer of high tectonically induced shear stresses to the canisters, (5) sufficient thermal conductivity to transfer heat caused by the radioactive decay to the rock without being too hot, (6) colloid filtering capacity, and (7) capacity to filter microbes.