Clay mineral transformations in static hydrothermal conditions within a simulated engineered barrier for nuclear waste disposal

Abstract Two cores of a compacted clay material with dominant kaolinite/smectite mixed-layer were irradiated in static hydrothermal conditions for one year and two years respectively. The experimental conditions were established in order to simulate an engineered barrier for nuclear waste disposal and a thermal gradient (from 120°C to 170°C) was applied to the cores. XRD patterns and chemical analyses of the altered samples indicate a disappearance of the high-charge smectite layers in the overall length of the cores. On the other hand, the 002 reflection shifting towards high angles with increasing temperature indicates an approximate 10% decrease in the kaolinite proportion of the mixed-layer as a function of the thermal gradient. XRD patterns of the samples after alteration also exhibit a decrease in the 001 reflection width with increasing temperature. Such a result can be related to an increase in the number of associated layers in clay particles and so, to an increase in the material aggregation. Other consequences of the alteration are the changes of the CaO clay content and the CEC with increasing temperature. Changes of the material occur at different reaction rates. The faster one is the high-charge to low-charge layer conversion, whereas the decrease in the kaolinite proportion of the mixed-layer and the change of the 001 reflection width are slower and related to thermal gradient. Except the 001 reflection width evolution, which is different in the two cores, little or no difference exists between the one-year and the two-year experiments. All the changes of the material are related to temperature, indicating that the proximity of the heat source controls the evolution rates of the clay phase. On the other hand, the present study induces qualitative reaction rates of clay transformation in static conditions, controlled by chemical diffusion related to the thermal gradient. Results are in good agreement with the nuclear storage objective, especially considering the high-charge layer conversion. In fact, the irreversible fixation of poorly hydrated cations (K + , Na + ) would be delayed, thus preserving the long term radionuclide fixation capacity of the engineered barrier. Additional experiments are now being conducted in order to estimate the real influence of the irradiations on the material alteration.