Geochemical Behaviour and Stability of the Czech B75 Bentonite during interaction with Water at 90°C

The B75 bentonite is a potential material to be used as engineered barrier in a high-level radioactive waste (HLRW) disposal concept in Czech Republic. The B75 bentonite is natural non-activated Ca-Mg bentonite from the Rokle deposit. The bentonite barrier in HLRW repository will be exposed to heat generated from the radioactive decay of the waste and to hydration by groundwater coming from the surrounding rocks In this context, batch experiments were performed in which B75 bentonite was exposed to distilled water and natural granitic water (pH 7.65, calcium-sodium, bicarbonate-sulphate type of water) at 90°C. The bentonite material consisted of montmorillonite, illite, kaolinite, quartz, calcite/Mg-calcite, siderite, anatase and Fe-dolomite/ankerite. XRD data and solute chemistry analysis show no evidence of significant changes in mineralogical and chemical composition of bentonite interacted with water for 6 months at 90°C. The most prominent changes are related to cation-exchange and carbonate buffer reactions. No wewly formed mineral phases could be detected. The chemical changes are related to the Fe2O3, FeO and CO2 content. It can be concluded that used Czech B75 bentonite is stable during its 6 month interaction with water and that different initial water composition has no influence on its stability.