Roles of clay and concrete in isolating high-level radioactive waste in very long holes

Groundwater flow transports possibly released radionuclides from underground repositories to the biosphere. It can also make construction difficult as is obvious from examining technical solutions for disposal of high-level radioactive waste (HLW) in long subhorizontally bored holes (KBS-3H) and in very deep boreholes (VDH). The presence of intersected, water-bearing fracture zones requires concrete for sealing these parts of the holes while the rest contains canisters surrounded and separated by dense, expandable clay. Casting of the concrete should be preceded by grouting of the fractured rock using cementitious materials composed so that mutual physical and chemical interaction do not degrade either of them. For the sake of rock stability the horizontal holes have to be located at very moderate depth, 400-500 m, where the rock has a high average hydraulic conductivity, while the slimmer, steep holes reaching down to 4 km are kept stable by using clay mud in the construction phase and dense clay for long term performance. The rock at this depth is much less permeable than higher up and the groundwater sufficiently salt to be maintained there, causing only local thermally induced circulation of possibly contaminated water. The KBS-3H concept involves practical difficulties and risks in the installation of the clay seals and waste canisters, for which the risk of shearing by slip of frequently intersected steep fractures is a major threat after closure of the repository. The VDH concept relies on effective sealing of the upper part of the deep holes and puts less demand on the seals in the lower, waste-bearing part, for which the buoyancy conditions of the groundwater make it a major barrier to upward migration of possibly released radionuclides.Godkänd; 2013; 20130612 (mohhat