After the canisters and the buffer material have been installed, the final disposal tunnels are filled up with clay blocks and pellets. The advantages with clay are its low water conductivity and its long-term chemical and mechanical stability.
Precompressed Friedland clay blocks and bentonite pellets are Posiva’s selected backfill material for the final disposal tunnels. An alternative to this are blocks of crushed bentonite that contain bentonite as more than 30% of their volume.
Tunnels are filled up in phases for the entire operational period of the final disposal facility. At the end of the final disposal operations, the technical facilities, the access tunnel and the shafts will also be backfilled.
From the point of view of long-term safety, it is essential to fill up and seal up the final disposal facilities in such a manner that the conditions inside the bedrock will be restored as faithfully as possible to the natural conditions preceding the excavation of the facilities.
The filling material prevents the tunnels and shafts from turning into flow routes for groundwater and maintains the mechanical stability of the tunnels. The filling material in tunnels also keeps the bentonite barrier surrounding the canisters from moving. Furthermore, filling up the tunnels prevents anyone from entering the facilities, which means that no particular guarding is needed for the final disposition facility area after the operation phase is over.