Water saturation and flow in a surface disposal facility: Numerical study for the Dessel repository

Water saturation and flow in the near field was studied previously without interactions between cementitious components and the earth cover. Existing numerical studies were extended for (i) including vapor flow, (ii) including flow through voids and fracture networks, (iii) full-scale analysis with cementitous components and earth cover including also heterogeneities as gravel layers and voids, (iv) transient flow conditions (resaturation), and (v) a framework for conceptualization of the effect of concrete degradation on water flow. A key element of the study was the application of the concept of a rough fracture wall which allows for numerical simulations of water flow in cracked or fractured porous media under unsaturated conditions. Depending on the hydric conditions (relative humidity or matric head), water flows through the whole fracture, mainly through irregularities at the fracture surface (pits) or as a film on the surface. In addition, the approach allows quantifying water redistribution, water fluxes and saturation degree in fractured porous media or even in a (fractured) monolith – void system. For a range of parameter values, it was shown that fractures are mainly unsaturated, but water flux was mainly occurring through the fractures. However, for a few specific conditions, the reverse was found. Resaturation, saturation degree distribution and water flux distribution were numerically assessed for an intact system. At equilibrium, the cementitous materials are fully saturated due to their high suction potential. However, it would take more than 1000 years to saturate the whole facility under intact conditions. Using the dual permeability approach and a conceptualization of increasing physical and chemical degradation of the cementitious materials, the most important characteristics of resaturation of cracked concrete was demonstrated. For most cases (except extremely low infiltration rates), the fractures equilibrate quickly after imposing a boundary flux at the top. The matrix domain resaturation is slower and depends, beside on the water flux, on the hydraulic conductivity of the matrix. When the difference between the conductivity of the matrix and the imposed boundary flux is not sufficiently large, a complex resaturation pattern was found. Otherwise, a typical pattern in four stages (from top to bottom) was identified: (i) matrix domain close to the boundary is completely saturated with a net flux towards the fracture, (ii) matrix and fracture are in equilibrium with each other, (iii) zone with water transfer from fracture to matrix, (iv) fracture in equilibrium with boundary flux whereas matrix is still at each initial state.