In final disposal, compressed bentonite is used as the buffer material, which is installed in blocks in the deposition hole to surround the canister. Bentonite is clay that becomes plastic, swelling and slippery when wet. In the repository, the use of bentonite is based on its ability to swell when it makes contact with water.
The buffer fills the entire empty space between rock and the canister and protects the canister, for example, from minor rock movements. It also limits and slows down the release of radionuclides if a single canister were to get broken for some reason.