Effect of surface orientation on dissolution rates and topography of CaF

This paper reports how during dissolution differences in surface chemistry affect the evolution of topography of CaF₂ pellets with a microstructure similar to UO₂ spent nuclear fuel. 3D confocal profilometry and atomic force microscopy were used to quantify retreat rates and analyze topography changes on surfaces with different orientations as dissolution proceeds up to 468h. A NaClO₄ (0.05M) solution with pH 3.6 which was far from equilibrium relative to CaF₂ was used. Measured dissolution rates depend directly on the orientation of the exposed planes. The {111} is the most stable plane with a dissolution rate of (1.2±0.8)×10⁻⁹molm⁻²s⁻¹, and {112} the least stable plane with a dissolution rate 33 times faster that {111}. Surfaces that expose both Ca and F atoms in the same plane dissolve faster. Dissolution rates were found to be correlated to surface orientation which is characterized by a specific surface chemistry and therefore related to surface energy. It is proposed that every surface is characterized by the relative proportions of the three reference planes {111}, {100} and {110}, and by the high energy sites at their interceptions. Based on the different dissolution rates observed we propose a dissolution model to explain changes of topography during dissolution. Surfaces with slower dissolution rate, and inferred lower surface energy, tend to form while dissolution proceeds leading to an increase of roughness and surface area. This adjustment of the surface suggests that dissolution rates during early stages of dissolution are different from the later stages. The time-dependency of this dynamic system needs to be taken into consideration when predicting long-term dissolution rates.