Performance of strongly compressed MX-80 pellets under repository-like conditions

Maturation of clay prepared from strongly compressed MX-80 pellets takes a long time and the swelling pressure and hydraulic conductivity remain high for several months after water saturation. The present study was made with hydration under heated conditions for testing the hypothesis that thermally induced stresses will cause disintegration of grains and speed up the homogenization, leading to normal swelling pressure and hydraulic conductivity. This was confirmed by the experiments. In the vicinity of spent fuel canisters with a high thermal output, this process will be partially counteracted by cementation of the clay particles caused by precipitation of dissolved silica/aluminium in the hydration phase. The latter effect is not significant at temperatures of 110 and 125 °C but can be substantial at 150 °C.