The copper canister is isolated from the surrounding rock with buffer material. A number of blocks of tightly compressed bentonite are installed as buffer material between the canisters and the rock in the final disposal hole.
Bentonite is a naturally occurring type of clay that expands manifold when coming into contact with water and that, on the other hand, conducts almost no water at all. Expanding bentonite fills the space surrounding the final disposal canisters. Therefore, the bentonite prevents water from coming anywhere near the copper canisters. In the event of a possible canister leak, it also stops the radioactive substances from coming into contact with the rock.
The bentonite barrier surrounding the canisters also protects it against mechanical wear, i.e., possible movements of the rock. Bentonite behaves somewhat like modelling clay: it buckles when necessary, but can also recover its shape because of its elasticity. Additionally, it fills up and seals quickly any cracks that can possibly occur while the rock moves.