Long-term laboratory investigation on backfill

Crushed salt is the preferred backfill material for a technical barrier in a final repository of radioactive waste in salt formations because it will have the same properties as the host rock after full compaction. However, it must be compacted by the convergence of the host rock to stabilize the underground openings and to reduce sufficiently the permeability in the EDZ and the backfill. High temperatures close to the waste containers favour the compaction process of pure crushed salt quite well. At lower temperatures, addition of bentonite supports the compaction process and favourably reduces the permeability. On basis of our laboratory test results we recommend a mixture of 85% crushed salt and 15% Ca-bentonite as a backfill barrier component in the lower temperature range.