Isolating radioactive waste from the biosphere
The aim of radioactive waste management is to isolate the waste from the biosphere specifically the human environment until its level of radioactivity has subsided to normal background levels. Geological disposal relies on a multi-barrier system to isolate the waste from the biosphere. The repository host rock provides a natural barrier, which is then supplemented by an Engineered Barrier System (EBS). The EBS consists of the man-made materials placed within the repository, such as waste canisters, buffer materials, backfill and seals. The EBS should have a low permeability, favourable chemistry and should safeguard against mechanical failure due to failing rocks, seismic events, etc. Research supported by the 5th Euratom Framework Programme (1998-2002) targeted the development and testing of techniques for and units of: copper canister, bentonite buffer, bentonite-based backfill and near-field rock. The work centred on the KBS-3 disposal concept created in Sweden. The full-scale disposal compartments are evaluated at a test site in the Aspo Hard Rock Laboratory. Bentonite, a fine clay, was combined with crushed rock to produce an ideal backfill material with a low hydraulic conductivity. Application of a vibrating compaction plate generated a good result, but with a higher density in the centre of the tunnel than in the ceiling regions. Furthermore, the technique of adding material at a slope rather than in horizontal layers proved beneficial. This was particularly evident in the typically problematic regions near the ceiling and walls. Equipment has been installed at the test site to monitor water uptake by the backfill. The initial data collected is encouraging, but the long-term behaviour of the backfill must be studied before the new methods can be applied at a real repository.