The development of a suitable technique for isolating unreprocessed nuclear reactor wastes from the biosphere has led to the Swedish multibarrier concept KBS 2 with two engineered components, a thick-walled copper canister and a clay body which confines the canister. The clay consists of well fitting blocks of highly compacted Na bentonite made by "isostatic" compression of bentonite powder. They are not water saturated when placed in the deposition holes but take up water from the surrounding rock, swell and ultimately form a tight contact with the rock and the canisters. When the bentonite is in physical equilibrium with the surroundings it forms a medium with a number of valuable properties, such as self-healing and ion exchange capacities. The healing means that initial joints between blocks and voids, or local passages in the clay cased by slight rock or canister displacements, will be sealed by the swelling potential of the clay