The basic idea of concepts for disposal of highly radioactive waste deep in rock is that the heavy, saline, stagnant formational waters are unlikely to rise to contaminate shallow groundwater. A recent concept involves placement of the waste in the lower 2 km part of up to 4 km deep holes bored in granitic rock, and relies on the sealing capacity of engineered barriers in the form of concrete and clay in the upper parts of the holes. The parts located in fracture-poor rock are sealed with dense expandable clay, while concrete is cast where pre-grouted fracture zones are intersected. The holes will converge by creep and eventually exert the seals to radial compression. Using a new rheological model based on the Kelvin model in combination with a stochastic mechanical model, the predicted radial hole convergence causes a vanishingly small increase in pressure on the seals in the first 10,000 years. In a long time perspective they will be compressed and become less permeable. Parallel conversion of the clay minerals to become less expandable will, however, reduce this potential.
Keywords: Boreholes, Clay, Concrete, Convergence, Creep, Elasticity, Radioactive waste, Rheology, Stress, Viscosity