Thermochemistry of nuclear waste glasses: application to weathering studies

Leaching experiments under static conditions have been conducted in water at 90°C with a borosilicate and an albite glass to check whether the kinetics r of the reaction are determined by the affinity A of dissolution through the law r=r0[1−exp(−A/RT)]. These experiments have been conducted with the goal of approaching saturation conditions thanks to the fast kinetics ensured by a high ratio between the surface area of glass and the volume of the solution (80 and 343 cm−1 for the borosilicate and an albite glass, respectively). The leaching rates decrease by several orders of magnitude over month-long experiments. The ionic activities as calculated from the concentrations of species in the solution are used to define the chemical affinity A=RT ln(K/Q) where Q is the ionic activity product, K the reaction equilibrium constant, R the gas constant and T the temperature. The constant K has been calculated from the Gibbs-free energy of formation of glasses determined in a another paper. Because the [1−exp(−A/RT)] term does not drop below 0.75, the general rate equation does not account for the marked decrease of the reaction rate. Although the affinity of dissolution can be considered to be a rate controlling factor, the decrease of leaching rate is likely due to the formation of a gel at the glass–solution interface that constitutes a diffusion barrier limiting the transfer of reacting aqueous species to the glass.