Reactive transport modeling of the interaction between a high-pH plume and a fractured marl: the case of Wellenberg

Abstract

In the context of the proposed low- and intermediate-level radioactive waste repository at Wellenberg (Switzerland), calculations simulating the interaction between hyperalkaline solutions and a fractured marl, at 25 °C, have been performed. The aim of these calculations is to evaluate the possible effects of mineral dissolution and precipitation on porosity and permeability changes in such a fractured marl, and their impact on repository performance. Solute transport and chemical reaction are considered in both a high-permeability zone (fracture), where advection is important, and the wall rock, where diffusion is the dominant transport mechanism. The mineral reactions are promoted by the interaction between hyperalkaline solutions derived from the degradation of cement (a major component of the engineered barrier system in the repository) and the host rock. Both diffusive/dispersive and advective solute transport are taken into account in the calculations. Mineral reactions are described by kinetic rate laws. The fluid flow system under consideration is a two-dimensional porous medium (marl, 1% porosity), with a high-permeability zone simulating a fracture (10% porosity) crossing the domain. The dimensions of the domain are 6 m per 1 m, and the fracture width is 10 cm. The fluid flow field is updated during the course of the simulations. Permeabilities are updated according to Kozeny's equation. The composition of the solutions entering the domain is derived from modeling studies of the degradation of cement under the conditions at the proposed underground repository at Wellenberg. Two different cases have been considered in the calculations. These 2 cases are representative of 2 different stages in the process of degradation of cement (pH 13.5 and pH 12.5). In both cases, the flow velocity in the fracture diminishes with time, due to a decrease in porosity. This decrease in porosity is caused by the precipitation of calcite (replacement of dolomite by calcite) and other secondary minerals (brucite, sepiolite, analcime, natrolite, tobermorite). However, the decrease in porosity and flow velocity is much more pronounced in the lower pH case. The extent of the zone of mineral alteration along the fracture is also much more limited in the lower pH case. The reduction of porosity in the fractures would be highly beneficial for repository performance, since it would mean that the solutions coming from the repository and potentially carrying radionuclides in solution would have to flow through low-conductive rock before they would be able to get to higher-conductive features. The biggest uncertainty in the reaction rates used in the calculations arises from the surface areas of the primary minerals. Additional calculations making use of smaller surface areas have also been performed. The results show that the smaller surface areas (and therefore smaller reaction rates) result in a smaller reactivity of the system and smaller porosity changes.