HYDROTHERMAL INTERACTION OF WYOMING BENTONITE AND OPALINUS CLAY

Most investigations into clay-mineral stability and new mineral formation within engineered barrier system (EBS) materials for geologic repositories of nuclear waste have focused on temperatures <100 degrees C. In response to the United States Department of Energy's interest in disposing of waste packages with higher thermal loads, higher temperature (200-300 degrees C) and pressure (similar to 150 bar), long-term (6-week to 6-month), hydrothermal experiments were conducted to evaluate the interaction of Opalinus Clay (wall rock) and Wyoming bentonite (clay buffer) with synthetic Opalinus Clay groundwater. Experiments were conducted in autoclaves using a flexible gold reaction cell with water:rock ratios between 6:1 and 9:1. Run products were characterized in terms of mineralogy and geochemistry. Montmorillonite remained stable at 200 and 300 degrees C; traces of illite-smectite interstratified minerals were observed. Clay minerals in Opalinus Clay experienced significant changes at 300 degrees C, including the formation of illite, illite-smectite, and chlorite-smectite. Montmorillonite illitization within the Wyoming bentonite EBS material was likely limited by the bulk chemistry of the system (i.e. low potassium) and newly formed illite was likely limited to the Opalinus Clay fragments, nucleating on pre-existing illite in the clay rock. Zeolite minerals with compositions between analcime and wairakite formed at 300 degrees C along edges of Opalinus Clay fragments and within the bentonite matrix, but not at 200 degrees C. Aqueous fluids remained undersaturated with respect to quartz in Opalinus Clay +/- Wyoming bentonite 300 degrees C experiments, and dissolution and re-precipitation of phases such as kaolinite, calcite, and smectite likely contributed to zeolite formation. These results can be applied to understanding zeolite formation, clay-mineral phase stability, and silica saturation within EBS materials of a high-temperature repository.