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

The alteration kinetics of the French SON 68 nuclear glass simplified to it s three major constituent elements (Si, B and Na) were investigated by stat ic experiments at 90 degrees C in order to deconvolute the effects of the s olution 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 g lass alteration rate gradually diminished over time. These results show tha t the driving force behind the alteration of this glass cannot be defined b y 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 i nto account a diffusion boundary layer shows that the conventional tools of chemical thermodynamics are ill adapted to describing the formation and de velopment 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 altera tion 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.