Global interim storage of HLW has reached a level that requires large extension of the storage capacity, which puts pressure on regulatory authorities and national parliaments for finding and applying ways of safe disposal of such waste. An important option is to use very dense natural expandable clay for isolating spent nuclear fuel in boreholes where it will be exposed to high temperature for hundreds to a few thousand years. The clay must be placeable and homogeneous and be able to sustain significant shear strain and temperatures up to 150oC without leaking or losing its ductile behaviour and self-healing potential. In this document the long-term function of such seals, which have the form of dense smectite blocks and soft smectite mud surrounding the containers/canisters will be described with respect to the impact of degrading physical/chemical mechanisms. Focus is on clay barriers for isolating spent nuclear fuel in up to 3-4 km deep boreholes but aspects are also provided on disposal in mined repositories at a few hundred m depth below the ground surface.

In either case the dense clay surrounding the waste containers will expand and enclose them, and consolidate the surrounding mud, which successively becomes denser, while the dense clay seal softens until its swelling pressure and the pressure of the mud is the same. 

The clay seals in deep boreholes used for disposal of spent nuclear fuel consist of a central core of dense expandable clay in perforated tubes (“supercontainers”) submerged in clay mud according to a concept termed VDH. In the lower parts of 3-4 km deep boreholes these tubes, made of copper, Navy Bronze, titanium or steel, host canisters lined with highly compacted expandable clay. In the upper parts of the holes the same type of supercontainers with no waste but with dense smectite clay blocks make up a primary barrier to possibly released radionuclides. A second barrier is the heaviness of the strongly saline groundwater at depth, which prevents such water to reach high up to the biosphere. The role of the mud is to save the supercontainers from touching the borehole walls when being placed, and to seal voids in the borehole walls with clay. The dense clay and soft clay mud will interact physically and ultimately become a homogeneous silicified clay body. Creep strain in the rock causes the deposition holes to con, which increases the radial pressure on the clay seals and thereby eliminates flow and diffusive migration of possibly contaminated porewater from the deployment part to move to the ground surface.