BENTONITE GAP FILLING FOR HIGH-LEVEL WASTE DISPOSAL

The safe disposal of high-level radioactive waste is based on the multi-barrier concept in which bentonite (clay) will be used as the construction material for one of the barriers. The bentonite will surround the waste package containing spent nuclear fuel in the disposal hole in the deep geological repository. Bentonite, because of its unique characteristics, will be used as the damping, sealing, filling and construction material within the disposal system. The principal geotechnical challenge will be to limit the penetration of water into the container from the surrounding rock and thus to extend the lifetime of the container as well as to restrict the transportation of water and gases from the disposal location. In addition, the sorption capacity of bentonite is important in terms of the capture of certain radionuclides which might escape from the container as a result of natural corrosion processes. The main requirements of the bentonite with regard to geotechnical properties consist of very low hydraulic conductivity (max 10(-12) m/s), high swelling ability (sigma(SW)> 1MPa), rheological stability, high plasticity, and good thermal conductivity. The construction of the disposal hole will exert an influence on the geotechnical properties of the buffer material. According to the vertical disposal system (Sweden, Finland, etc.) the disposal well will be drilled into the rock massif with a diameter which will allow for the installation of bentonite blocks around the container. However, certain tolerances and a degree of clearance will be required due to the practical requirements of bentonite block and container installation. The objective of this study therefore was to investigate both the methods and materials which might be used for the filling of the gap between the rock wall of the disposal well and the bentonite blocks. The experimental work was split into three parts, the first of which involved the selection of Czech bentonite as the basic material; this was followed by extensive laboratory tests which aimed to determine the relevant geotechnical parameters. The second part consisted of the selection and testing of methods for the compaction of the bentonite, and the final part concerned the selection of methods for the placing of the compacted bentonite into the gap.