Obtaining Solubility Constants to Describe the Incongruent Dissolution of SON68 Waste Glass by an Equilibrium Ideal Soli
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Obtaining Solubility Constants to Describe the Incongruent Dissolution of SON68 Waste Glass by an Equilibrium Ideal Solid Solution Model Approach Diederik Jacques and Karel Lemmens Waste and Disposal Department, SCK•CEN, Boeretang 200, B-2400 Mol, Belgium ABSTRACT One of the waste forms resulting from the Belgian programme for nuclear energy is the R7T7 high level waste glass, simulated by the SON68 glass. When exposed to distilled water or synthetic interstitial clay water, SON68 glass dissolves incongruently, releasing some elements (B, Li, Na) faster to the solution than others (Si, Al, Ca). The objective of this study is to describe the composition of the leachate in contact with inactive SON68 glass by assuming congruent glass dissolution followed by the precipitation of a secondary solid phase represented as an ideal solid solution in equilibrium with the leachate. Experimental SON68 dissolution data in distilled water was available at three temperatures. The solubility of the different end members in the solid solution is optimized using the available data for each temperature. The temperature dependence of the different end members was then obtained by Van 't Hoff equation. The calibrated model can describe the composition of the leachate and the altered glass phase during glass dissolution at different temperatures. The model, calibrated for the distilled water system, could successfully describe the composition of the leachate during glass dissolution experiments in a synthetic clay water system.
INTRODUCTION One of the waste forms resulting from the Belgian program for nuclear energy is vitrified high level waste such as the R7T7 glass, simulated by the SON68 glass. An important aspect of the safety analysis of waste disposal in a Boom Clay layer is the stability of the glass in the clay environment. Understanding the incongruent release of glass constituents and the possible formation of secondary gel or crystalline phases is indispensable. The objective of the study is to describe the composition of the aqueous solution generated when inactive SON68 waste glass is dissolving using a geochemical equilibrium model based on ideal solid solutions (similar to [1] and [2]). In this way, the incongruent release of some major elements (Si, Al, Ca) is described. The model approach starts from very simplifying ideas: (i) congruent dissolution of the glass, and (ii) precipitation of a single ideal solid-solution phase that represents the altered glass phase, gel phase, and/or secondary phases. Using the experimental data, the solubility constants of the end members in the ideal solid solution and their temperature dependence are fitted for SON68 glass in distilled water. These fitted values are then extrapolated to describe the composition of the leachate when SON68 glass is dissolving in synthetic interstitial clay water. An alternative approach is to use a mechanical mixture of pure phases in equilibrium with the leachate instead of an ideal solid solution. Sometimes, however, a sequence of different minerals i
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