The necessity to predict the durability of high level nuclear waste (HLW) glasses on extended time scales has led to various thermodynamic & kinetic approaches. Advances in the measurement of medium-range order in glasses has led to the understanding that the molecular structure of a glass, controls glass durability by establishing the distribution of ion exchange sites, hydrolysis sites & access of water to those sites. During the early stages of glass dissolution, a "gel" layer resembling a membrane forms through which ions exchange between glass & leachant. The hydrated gel layer exhibits acid/base properties, manifested as the pH dependence of the thickness & nature of the gel layer. This layer ages into clay or zeolite minerals by Ostwald ripening. Thermodynamic & structural approaches to the prediction of glass durability are compared vs Ostwald ripening.
