Evolution of steel-bentonite interactions under hydrothermal gradients: Mineralogical and geochemical implications for nuclear waste disposal

Carbon steel and bentonite are chosen as engineered barriers for high-level radioactive waste containment in deep geological repositories, which requires an evaluation of their long-term interactions to ensure durability. Two laboratory experiments were conducted under hydrothermal gradients with steel-Ca-Mg-Na-bentonite and Na-bentonite unsaturated columns over 25 and 27 months, respectively. The experiments measured changes in the porewater distribution, the specific surface area, the cation exchange capacity of the bentonites, and the oxidation state of Fe. These were determined in different sections along the columns. Mineralogical analyses were performed using X-ray diffraction and scanning electron microscopy. The results obtained showed water saturation at the conclusion of the experimental time. The specific surface area of bentonites decreased near the steel interface, while the cation exchange capacity exhibited an overall increase. The corrosion rate decreased over time, with bentonite showing enriched Fe content within the first 3.5 mm from the steel interface. New Fe-bearing minerals, including ferri-chlorite, magnetite, hematite, maghemite, and siderite, were identified as reaction products at the steel-bentonite interface. Aqueous species are redistributed in the bentonite's porewater, increasing with time the concentrations of Na + , Ca 2+ , Mg 2+ and SO 4 2− and decreasing those of Fe, due to the imposed hydraulic gradient and the precipitation reactions. This study compares the evolution of these bentonite properties with a previous study at short experimental times of 1 and 6 months of reaction. These findings are relevant to deep geological disposal conditions, where bentonite influences the formation and nature of iron clay minerals over time.