Interactions of corrosion products and bentonite: An extended multicomponent reactive transport model

Radioactive waste disposal in deep geological repositories envisages various engineered barriers such as carbon–steel canisters and compacted bentonite. Canister corrosion and the chemical interactions of corrosion products with bentonite are key reactions for the long term performance of a repository. Samper, Lu, and Montenegro (Physics and Chemistry of the Earth 2008; 33S: S306-S316) reported numerical models to simulate canister corrosion and the interactions of corrosion products with bentonite for the near field of a repository in granite. Here we present an updated version of their reactive transport model which accounts for: (1) Three types of sorption sites in the bentonite; (2) Kinetically-controlled canister corrosion, (3) Kinetically-controlled magnetite precipitation; and (4) The competition effect of Ni2+ for sorbing sites. Accounting for kinetically-controlled canister corrosion leads to a significant reduction in the corrosion rate. Uncertainties in the surface complexation reactions play a minor role in the time evolution of the computed pH in the bentonite and the granite. Computed iron concentrations, however, are very sensitive to changes in the surface complexation reactions. The apparent distribution coefficient of Fe computed with the three-site model is 10 times larger than that obtained with a single-site model. The concentration of dissolved Fe computed with kinetic magnetite precipitation is smaller than that obtained with magnetite precipitation at local equilibrium. The largest difference in the concentration of dissolved Fe occurs after 3 × 104 years. The competition of Ni2+ for sorption sites affects significantly the chemical evolution of the bentonite porewater. The sorption of Ni2+ on bentonite releases protons and therefore the pH in the bentonite is smaller than that computed without Ni2+ transport. The sorption of Ni2+ leads to a decrease of the concentration of sorbed Fe and an increase of the concentration of dissolved Fe in the bentonite porewater for t < 105 years.