Reactive transport model of interactions of corrosion products and bentonite

Carbon-steel and compacted bentonite have been proposed as candidate materials for overpack and buffer, respectively, in deep geological repositories for nuclear waste disposal. Here, we present 1-D and 2-D axisymmetric water flow and multicomponent reactive solute transport models to simulate canister corrosion, the interactions of corrosion products with bentonite and the long-term hydrochemical evolution of porewater composition in the near field of a repository in fractured granite. Proton surface complexation is highly effective in buffering pH in bentonite porewater which increases due to canister corrosion. Other mechanisms such as calcite dissolution/precipitation, Fe exchange and dissolution/precipitation of Fe minerals are much less effective in buffering pH. Magnetite is the main corrosion product. Most of the Fe diffuses from canister into bentonite where it precipitates. Siderite precipitation is two orders of magnitude smaller than magnetite precipitation due to the limited availability of dissolved carbonates. The amounts of dissolved, sorbed and exchanged iron are in turn two orders of magnitude smaller than precipitated siderite. Bentonite porosity decreases due to magnetite precipitation. The maximum decrease which occurs at the canister–bentonite interface is 0.1 and is insufficient to clog bentonite pores. Apparent distribution coefficient, Kd, of corrosion products derived from computed dissolved and exchanged Fe concentrations increases strongly with time, indicating that the use of a constant Kd for corrosion products is largely unrealistic.