Numerical Simulation of the Interaction Between Granitic Groundwater, Engineered Clay Barrier and Iron Canister

Abstract

AbstractThe geochemical evolution of a clay engineered barrier subjected simultaneously to granitic groundwater on its external boundary and to a steady flux of iron released by canister corrosion on the other boundary is simulated for time periods up to 1000 y. A multisite ion-exchange model combined with dissolution/precipitation of accessory minerals (calcite, gypsum, quartz, goethite, pyrite, pyrrhotite, iron silicate, magnetite) is used to describe the geochemical evolution of the clay barrier. Diffusive transport of aqueous species and dissolved gases is simulated in a one dimension space. Retroaction of chemical reactions on the corrosion process of the canister is not included in the model. The temperature is assumed to be constant (25°C). Calculations are performed with the IMPACT geochemical coupled code. Two main scenarios are considered: (i) the reduction of sulfates (gypsum) is kinetically hindered, (ii) the reduction of sulfates is possible and leads to the precipitation of sulfides. Main results obtained are dealing with H2(gas) generation and control, pH transient and buffering effect of the clay barrier system. These simulations show that the hydrogen partial pressure in the clay barrier could possibly stay for long periods of time at a very low level due to interactions with mineral buffers or sinks. In the sulfate reduction scenario, a transient pH, xcursion is predicted at the vicinity of the canister. Future developpements of this work include the taking into account of a temperature gradient. Improvement of the geochemical model and reaction pathway is also necessary and will be done with the guidance of on going experimental work.