Hierarchical Modeling of HLW Glass-Gel-Solution Systems for Stage 3 Glass Degradation
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Hierarchical Modeling of HLW Glass-Gel-Solution Systems for Stage 3 Glass Degradation Carol M. Jantzen1 and Charles L. Crawford1 1 Environmental & Chemical Process Technology Savannah River National Laboratory Aiken, SC 29898, U.S.A. ABSTRACT The necessity to a priori predict the durability of high level nuclear waste (HLW) glasses on extended time scales has led to a variety of modeling approaches based primarily on solution (leachate) concentrations. The glass composition and structure control the leachate and the gel compositions which in turn control what reaction products form: the leached layer is a hydrogel and reacts with the solution (leachate) to form secondary phases some of which cause accelerated glass dissolution which is undesirable. Glasses with molar excess alkali that is not bound to glass forming (Al,Fe,B)O4 structural groups in the glass resume accelerated leaching. The hydrogels of the glasses that resume accelerated leaching at long times contain excess alkali and the leachates contain excess strong base, [SB]ex. The [SB]ex further accelerates aluminosilicate gel aging into analcime with time. Glasses with no excess molar structural alkali do not resume accelerated leaching: the glass generates weak acids, [WA], in the leachate favoring hydrogel aging into clays. These data indicate that the gel layer transforms to secondary phases in situ in response to interactions with the chemistry of a continuously evolving leachate. INTRODUCTION Current theories of glass dissolution suggest that all glasses typically undergo an initial rapid rate of dissolution denoted as the “forward rate” (Figure 1-Stage I). However, as the contact time between the glass and the leachant lengthens some glasses come to “steady state” equilibrium and corrode at a “steady state” or “residual rate (Figure 1-Stage II) rate while other glasses undergo a disequilibrium reaction with the leachant solution that causes a sudden change in the solution pH or the silica activity in solution [1]. The “resumption to an accelerated rate” (Figure 1-Stage III) after achieving “steady state” or “residual” dissolution is undesirable as it causes the glass to return to a rapid dissolution characteristic of initial dissolution and confounds long term durability modeling of nuclear waste glasses.
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Figure 1. Generally accepted glass dissolution mechanisms.[2] The initial rate of dissolution is often referred to as Stage I but encompasses Mechanisms I, II, and III (see Figure 1 labelling) because multiple mechanisms are operative including regimes that are inter-diffusion controlled, hydrolysis controlled, and diffusion or affinity controlled which leads to a dissolution rate drop.[3] In the region of diffusion/affinity control, which starts in Stage I and continues into Stage II, the concentration of the glass components increase in solution, the solution approaches saturation with respect to some rate-limiting alteration phase(s) but is not at final or steady state saturation. In other words, the dissolution rate slows down as the chemi
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