Application of the GRAAL model to leaching experiments with SON68 nuclear glass in initially pure water

Abstract

Based on a review of the current state of knowledge concerning the aqueous alteration of SON68 nuclear glass we have proposed a mechanistic model, GRAAL (Glass Reactivity with Allowance for the Alteration Layer) [P. Frugier, S. Gin, Y. Minet, T. Chave, B. Bonin, N. Godon, J.E. Lartigue, P. Jollivet, A. Ayral, L. De Windt, G. Santarini, J. Nucl. Mater. 380 (2008) 8]. This article describes how the GRAAL model hypotheses are solved using a calculation code coupling chemistry and transport. The geochemical solution of this model combines three major phenomena: chemical equilibria in solution, water and ion transport by convection or diffusion, and element diffusion through the passivating reactive interphase. The model results are compared with experimental data for SON68 glass leached in initially pure water both in a closed system and in renewed media. The comparison shows the model very satisfactorily accounts for variations in the pH and the element concentrations in solution as a function of time, the glass surface area in contact with solution, and the solution renewal rate. This success is due to the fact that the diffusion of elements through the alteration gel is taken into account in the model. This mechanism cannot be disregarded under most experimental conditions — if only to predict the solution pH — and must therefore be an integral part of the geochemical model.