Simulation of geochemical interactions between bentonite, concrete and host rocks, and their influence on radionuclide migration

In the Canadian concept for a deep geological repository for used nuclear fuel waste, Highly-compacted Bentonite (HB) and Low-Heat High-Performance Concrete (LHHPC) are potential engineered barrier materials. Mineralogy and pore water compositions are different for bentonite, LHHPC and potential host rocks, such as limestone and granite. Consequently, chemical alterations are expected at interfaces between these materials, which could potentially lead to modifications of the transport properties of the barrier system. Reactive transport simulations have been performed with the MIN3P-THCm to investigate long-term chemical interactions driven by diffusion-dominated transport across the interfaces. Two scenarios were simulated to investigate interfacial reactions and their effects on solution composition, mineralogy, porosity, and diffusive transport in the barrier system: 1) HB/LHHPC/host rock; 2) HB/host rock. In the first scenario, simulation results show that due to the relatively high pH of pore water in LHHPC (pH = 9.7), substantial alterations occur at the interfaces of bentonite/LHHPC. However, after 100,000 years the alteration reactions remain restricted to a distance of 2 cm from the interfaces. The alteration mechanism depends mainly on the mineralogy of the host rock and chemical composition of the pore water. In the case of granitic host rock, Calcium Silicate Hydrate (CSH) phases present in LHHPC are predicted to transform into tobermorite, phillipsite and saponite near the interface within 1000 years. The simulations indicate that clogging occurs after about 3500 years in the concrete adjacent to bentonite due to the precipitation of tobermorite, sepiolite, saponite, phillipsite and calcite. In the case of limestone host rock, saponite and sepiolite are the dominant minerals formed in the LHHPC near the interface within a time frame of 1000 years. In this case, clogging is predicted after about 1500 years in the limestone adjacent to the LHHPC, mainly due to the precipitation of saponite, sepiolite, calcite and phillipsite. In comparison, the alterations of the interfaces in scenarios without LHHPC are not significant. Simulations of I-129 transport across the interfaces suggest that radionuclide migration can be significantly retarded if interface alteration leads to clogging.