Present understanding of R7T7 glass alteration kinetics and their impact on long-term behavior modeling

Glass alteration is a complex phenomenon, the kinetics of which result from the convolution of numerous mechanisms, most of which depend not only on the intrinsic glass properties (composition, structure, surface condition, etc.) but also on the environment (leaching solution volume, flow rate and composition, temperature, pH, surrounding materials, etc.). Considerable progress has been made during the last two decades toward understanding these mechanisms and their interrelations, largely through studies undertaken to predict the long-term behavior of nuclear waste containment glasses. By the mid-1990s, most of these basic mechanisms were thought to be well known, and their interdependence relatively well described by models based essentially on the concepts of chemical affinity. For nuclear waste glasses, if allowance is made for the diffusion and sorption of silica in the gel layer, this kind of model seems satisfactory to account for a single set of data. However, for different data sets, the solubility parameter (C* or K, depending on the model) must be fitted to the leaching conditions and environment. This point clearly demonstrates that this parameter, the value of which depends on the way in which it was reached, is not a thermodynamic property of the material: C* is clearly not an intrinsic glass solubility limit. A major research effort was therefore undertaken in France to identify the true causes of the significant drop in the alteration rate observed under `saturation conditionsʹ, and to assess the extent to which the available long-term behavior models had to be revised. The main results of this work on SON68 (`R7T7ʹ) glass are reviewed, and their impact on glass modeling is discussed.