Engineering Glass Passivation Layers -Model Results

Abstract

SUMMARYThe immobilization of radioactive waste into glass waste forms is a baseline process of nuclear waste management not only in the United States, but worldwide. The rate of radionuclide release from these glasses is a critical measure of the quality of the waste form. Over long-term tests and using extrapolations of ancient analogues, it has been shown that well designed glasses exhibit a dissolution rate that quickly decreases to a slow residual rate for the lifetime of the glass. The mechanistic cause of this decreased corrosion rate is a subject of debate, with one of the major theories suggesting that the decrease is caused by the formation of corrosion products in such a manner as to present a diffusion barrier on the surface of the glass. Although there is much evidence of this type of mechanism, there has been no attempt to engineer the effect to maximize the passivating qualities of the corrosion products.This study represents the first attempt to engineer the creation of passivating phases on the surface of glasses. Our approach utilizes interactions between the dissolving glass and elements from the disposal environment to create impermeable capping layers. By drawing from other corrosion studies in areas where passivation layers have been successfully engineered to protect the bulk material, we present here a report on mineral phases that are likely have a morphological tendency to encrust the surface of the glass. Our modeling has focused on using the AFCI glass system in a carbonate, sulfate, and phosphate rich environment. We evaluate the minerals predicted to form to determine the likelihood of the formation of a protective layer on the surface of the glass. We have also modeled individual ions in solutions vs. pH and the addition of aluminum and silicon. These results allow us to understand the pH and ion concentration dependence of mineral formation. We have determined that iron minerals are likely to form a complete incrustation layer and we plan to look more closely at Vivianite [Fe 3 (PO 4 ) 2 •8(H 2 O)] and Siderite [FeCO 3 ] in the next stage of the project.

Exploration and Modeling of Structural changes in Waste Glass Under Corrosion

“…Uncertainties remain regarding kinetic limitations of petalite formation; 62 however, similar minerals were endorsed in analogous modeling efforts. 63 Reaction titration modeling for NDS predicted Si and Na concentrations at 24,840 and 20,780 ppm, respectively, for M/V ratios of 75 g/ 500 mL (Figure 46). Si and Na concentrations continually increase up to 80 g NDS added.…”

“…Kim and Kirkpatrick observed the sorption of aqueous boron onto silica gel through the formation of both strong and weak surface connections, with a preferential attachment of IV B. 63 For the gels formed on ISG, IV B also preferentially attached due to the increase in normalized area for short equilibration times. As the equilibration time increased, however, III B sorption was also observed.…”