Tunnel backfill and plug

After installing the canister and buffer material, the deposition tunnels are filled with a granule mixture, consisting of bentonite pellets crushed to different sizes. The advantage of clay is its low hydraulic conductivity and long-term chemical and mechanical stability.

The tunnels will be filled in stages throughout the final disposal operations. The backfill material prevents the tunnels and shafts from becoming groundwater flow channels and maintains the mechanical stability of the tunnels. The tunnel backfill material also holds the bentonite buffer surrounding the canister in place. In addition, backfilling the tunnels prevents access to the facilities, which means that the final disposal area does not need to be guarded after the end of the operational phase of the disposal facility.

The deposition tunnel must remain watertight under all conditions. Therefore, a wedge-shaped massive steel-reinforced concrete plug is constructed at the end of the deposition tunnel filled with bentonite clay. The concrete plug is approximately 6 m long and 4–6 m in diameter. Each plug contains approximately 160 m3 of concrete and approximately 20 t of reinforcements. The construction of the concrete plug has already been tested in the Full-Scale In-Situ System Test.