Obtaining solubility constants to describe the incongruent dissolution of SON68 waste glass by an equilibrium ideal solid solution model approach

One of the waste forms from the Belgian programme for nuclear energy is vitrified high level waste as SON68. When exposed to distilled or synthetic interstitial clay water, SON68 dissolves incongruently meaning that some elements (Si, Al, Ca) are less soluble than others (B, Li, Na). The objective of this study is to describe the composition of the leachant in contact with inactive SON68 glass by assuming congruent glass dissolution and formation of a secondary solid phase represented as an ideal solid solution in equilibrium with the leachant. Experimental SON68-glass dissolution data in distilled water were available at three temperatures. The solubility of the different end members in the solid solution is optimized using data at each temperature. The temperature dependence of the end members was described by Van 't Hoff equation. In this way, one model describe the composition of the leachant and the secondary phase during glass dissolution at different temperatures. The model, calibrated for the distilled water system could successfully describe the composition of the leachant in dissolution experiments in a synthetic clay water system. This study shows that geochemical model approach allows a macroscopic description of the incongruent dissolution for different temperatures or reaction progresses.