New techniques make disposal of highly radioactive waste in very deep holes safe

A presently much discussed concept for disposal of highly radioactive waste (HLW) implies disposal in the lower part of very deep boreholes, called VDH here. The waste packages will be located in very salt, stagnant groundwater in rock that is much less permeable than shallow rock. A disadvantage is that some of the techniques for installation of waste and clay seals have not yet been demonstrated and that retrieval of damaged or stuck canisters is deemed difficult. The concept requires precise adaption of canister and seal positions to the rock structure, which will be known by site investigations including pilot borings for a relatively low cost at a very early stage. The uppermost parts of the 4 km deep deposition holes may need to be supported by casings and installation of supercontainers with HLW and clay seals must be made in clay mud for stability reasons. Intersected fracture zones must be stabilized by grouting and the deposition holes plugged here by concrete cast on site. The grout and concrete are novel and have talc as superplasticizer for reaching long-term strength and chemical stability. Compared with more shallow disposal the VDH concept is competitive with respect to cost and construction time, as well as to long-term safety, since seismic and tectonic impact are less detrimental and future glaciations will cause much less disturbance. The most important value of VDH is that the groundwater that can possibly become contaminated by failing engineered barriers will stay at more than 2000 m depth and that there is no mechanism that can bring it up to the biosphere