Ion-Exchange Interdiffusion Model with Potential Application to Long-Term Nuclear Waste Glass Performance

Ion exchange and interdiffusion are critical processes in glass applications. In the field of aqueous glass corrosion, it is difficult to conclusively deconvolute the process of ion exchange from other processes, principally dissolution of the glass matrix, due to the formation of alteration layers, Therefore, we have developed a method to isolate alkali diffusion that involves contacting glass coupons with a solution of (LiCl)-Li-6 dissolved in functionally inert dimethyl sulfoxide. We employ the method at temperatures ranging from 25 to 150 degrees C with various glass compositions. Glass compositions include simulant nuclear waste glasses, such as SON68 and the ISG, glasses in which the nature of the alkali element was varied, and glasses that contained more than one alkali element. An interdiffusion model based on Fick's second law was developed and applied to all experiments to extract diffusion coefficients. The model expands established models of interdiffusion to the case where multiple types of alkali sites are present in the glass. Activation energies for alkali ion diffusion were calculated. The interdiffusion model derived from laboratory experiments is expected to be useful for modeling glass corrosion in a geological repository when silicon concentrations are high.