The Swedish KBS concepts for final deposition of canisters with nuclear wastes from reactors, imply that the canisters be placed in deposition holes which are bored from the floors of tunnels at about 500 m depth in Swedish crystalline bedrock. The canisters are isolated from the rock by a ″buffer mass″ consisting of pure bentonite in the case of unreprocessed wastes, and the deposition plant is finally sealed by backfilling shafts and tunnels with bentonite/sand mixtures. The bentonite, which surrounds the canisters, is applied in the form of blocks of highly compacted bentonite. The blocks are produced by isostatic compaction of commercial bentonite powder under pressures in the range of 50-100 MPa, which yields block densities of 2. 1-2. 2 t/m**3 when using Wyoming Na bentonite MX-80 powder. The compacted bodies are cylindrical and they can be manufactured by sawing, drilling etc. so that cylindrical discs, rods, annuli, and practically any shape can be obtained. The particularly attractive properties of the highly compacted Na bentonite are reported