Alteration kinetics of a simplified nuclear glass in an aqueous medium:: effects of solution chemistry and of protective gel properties on diminishing the alteration rate

The alteration kinetics of the French SON 68 nuclear glass simplified to its three major constituent elements (Si, B and Na) were investigated by static experiments at 90 degrees C in order to deconvolute the effects of the solution chemistry and of the protective properties of the alteration gel on the diminishing alteration rate over time. A glass dissolution experiment in static conditions showed that the initial rate r(0) was maintained even after silicon saturation of the solution. As the reaction progressed, the glass alteration rate gradually diminished over time. These results show that the driving force behind the alteration of this glass cannot be defined by the difference from saturation with respect to amorphous silica, and that reaching saturation is not a criterion for the end of alteration. The drop in the dissolution rate observed at a high degree of reaction progress is correlated with the formation of the silica gel that develops at the glass/solution interface. Confronting the experimental data with a model taking into account a diffusion boundary layer shows that the conventional tools of chemical thermodynamics are ill adapted to describing the formation and development of the silica gel layer over time. This study reveals that only a dynamic process of hydrolysis and condensation of silicon at the glass/gel interface can account for the formation of the gel layer. The glass alteration rate under silica saturation conditions would thus be highly dependent on the silicon recondensation rate in this 'dynamic percolation' concept. (C) 2000 Elsevier Science B.V. All rights reserved.