A mechanistic model for understanding nuclear waste glass dissolution

Abstract

Abstract Alteration of the French R7T7 borosilicate glass in aqueous media may be described by a combination of equations governing surface reaction and diffusion mechanisms, with an empirical law of partial silicon retention in the alteration film. The dissolution process is controlled by the silicic acid activity at the interface between the gel and the pristine glass; the alteration film acts as a silicon diffusion barrier. The LIXIVER computer code describes the chronological evolution of the dissolution rates, the silicon concentrations at the reaction interface, and the concentrations of glass matrix components in solution between 0 and 300°C for various glass and solution compositions under static or dynamic leaching conditions. LIXIVER is not designed to model the long-term glass and solution behavior, but to investigate the effects of leaching parameters on the dissolution kinetics within a laboratory time frame (several months). This article discusses the results of modeling experiments, the principles behind the model, and simulations intended to qualify LIXIVER by specifying its scope of applications. An order of magnitude for the apparent silicon diffusion coefficient in the alteration film is proposed at 90°C for a glass-surface-area-to-solution-volume ( SA/ V) ratio of 0.5 cm −1.