Predictive Modeling of Diffusional Processes Within the Hydrated Layer of Vitrified Nuclear Waste Forms
Upon contact between a nuclear waste glass and an aqueous solution, a structurally altered, highly hydrated layer forms. The distribution of the glass constituents within this layer has been investigated through development of mathematical models representing the fundamental mechanisms that describe the formation of those gel layers. These mechanisms include: migration due to electrochemical potential gradients; structural alteration reactions resulting in the formation of a transition zone, a structurally unique region between the gel layer and bulk glass; alteration product formation through precipitation; dissolution of the layer; and generation of an electric double layer at the solution-gel layer boundary. Special attention has been focused on two of these processes: the influence of the electric double layer on species migration which is shown to be unimportant when compared with dissolution, and the growth kinetics of the layer. In determining the growth kinetics, emphasis has been placed on modeling potential rate controlling processes in the transition zone. In the absence of dissolution and solution feedback effects, layer growth is proportional to time if processes in the transition zone control growth, and proportional to SQRT.(t) when diffusion through the gel layer is rate controlling. The glass constituents are modeled by four elements that are representative of the major categories of experimentally determined behavior. These elements are: silicon, sodium, and two generic waste products which are largely insoluble in solution and therefore, tend to accumulate in the gel layer. A distinction is made as to whether these elements are bound to the glass, released from the glass network and mobile, or part of an immobile precipitate. Parametric studies of the influence of the different fundamental processes have been conducted through the use of a computer code named GELOH, which represents the numerical implementation of the system of coupled, partial differential equations describing the model. These numerical investigations have shown the ability of the proposed model to reproduce a wide range of experimentally observed behaviors. On the basis of the information developed in this work, an interpretation of the aqueous interactions between actual borosilicate waste glasses and water are presented.
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign