Pore scale modeling of ion diffusion in variably saturated clays

The transport of ions in unsaturated porous media is an important fundamental process in many natural and technical settings. A realistic description of transport processes in unsaturated porous media relevant to the deep geological disposal of nuclear waste across scales is still an unresolved scientific challenge. A newly developed lattice Boltzmann based model for pore-scale transport simulations was applied to investigate ion diffusion in microscale and nanoscale pores of unsaturated clays. In traditional microscale pore-scale simulations ignoring the contribution from water films adsorbed on the surface of clay minerals due to the coarse resolution, the transport pathways related to large capillary pores depercolate under the condition of low saturation. Our improved pore-scale simulations shows that the water films covering on the surface of clay particles has non-negligible influences on ion transport as volumetric water content is less than 0.25. When water content lower than 0.1, the thin water film on clay surface controls solute diffusion. At nanoscale, electrical double layer (EDL) effects caused by the charged surface of clays can significantly enhance cation diffusion but reduce anion diffusion. Our simulations indicate that the influence from EDL on ion diffusion becomes stronger in the clay with lower water content. The present study could help improve the understanding of the diffusion mechanism of ions in variably saturated clays

Thermodynamic and Structural Modelling of Non-Stoichiometric Ln-Doped UO2 Solid Solutions,Ln = {La, Pr, Nd, Gd}

Available data on the dependence of the equilibrium chemical potential of oxygen on degrees of doping, z, and non-stoichiometry, x, y, in U1-zLnzO2+0.5(x-y) fluorite solid solutions and data on the dependence of the lattice parameter, a, on the same variables are combined within a unified structural-thermodynamic model. The thermodynamic model fits experimental isotherms of the oxygen potential under the assumptions of a non-ideal mixing of the endmembers, UO2, UO2.5, UO1.5, LnO1.5, and Ln0.5U0.5O2, and of a significant reduction in the configurational entropy arising from short-range ordering (SRO) within cation-anion distributions. The structural model further investigates the SRO in terms of constraints on admissible values of cation coordination numbers and, building on these constraints, fits the lattice parameter as a function of z, y, and x. Linking together the thermodynamic and structural models allows predicting the lattice parameter as a function of z, T and the oxygen partial pressure. The model elucidates contrasting structural and thermodynamic changes due to the doping with LaO1.5, on the one hand, and with NdO1.5 and GdO1.5, on the other hand. An increased oxidation resistance in the case of Gd and Nd is attributed to strain effects caused by the lattice contraction due to the doping and to an increased thermodynamic cost of a further contraction required by the oxidation