Thermochemistry of nuclear waste glasses: application to weathering studies

Leaching experiments under static conditions have been conducted in water a t 90 degreesC 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 = r(0)[1 - exp(-A/RT)]. These experiments have been condu cted 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 albit e glass, respectively). The leaching rates decrease by several orders of ma gnitude over month-long experiments. The ionic activities as calculated fro m the concentrations of species in the solution are used to define the chem ical affinity A = RT ln(K/Q) where Q is the ionic activity product, K the r eaction 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 do es 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 interfa ce that constitutes a diffusion barrier limiting the transfer of reacting a queous species to the glass. (C) 2001 Elsevier Science B.V. All rights rese rved.