Sodium saturated bentonite clay compacted to a high density is a very efficient isolation for preventing radiotoxic nuclides from deeply buried canisters with radioactive wastes from reaching the biosphere. The main function of the bentonite, which is applied in the form of blocks between the rock and the canisters in large boreholes, is to provide a practically impervious barrier. The bentonite blocks take up water and swell so that they completely fill the space between rock and canisters. The swelling potential, which is permanent, makes the bentonite self-sealing. This means that rock joints, which may be opened, are sealed by extruding bentonite.