Clay is capable of swelling and remaining watertight.
As a backfill material in the waste emplacement tunnels, the role of clay material is to keep infiltrating water away from the disposal containers and retard the transport of any released radionuclides. Bentonite clay fulfils both of these requirements. It can take up large amounts of water, which causes it to swell. It also has the ability to bind radionuclides on the long term and to retard their migration. For these reasons, the voids surrounding the waste containers are filled with swelling bentonite.
Cracks in clayey soil. When clay dries out, it shrinks and forms cracks. When it rains, the water is absorbed, the clay swells and the cracks close. (Image: Nagra)