Skorski, and three co-authors, of the Pacific Northwest National Laboratory, reported in 2011 that the 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 (coating) 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.
The authors' modeling has focused on using the AFCI glass system in a carbonate, sulfate, and phosphate rich environment. They evaluated the minerals predicted to form to determine the likelihood of the formation of a protective layer on the surface of the glass. They have also modeled individual ions in solutions vs. pH and the addition of aluminum and silicon. These results allow the authors to understand the pH and ion concentration dependence of mineral formation. They have determined that iron minerals are likely to form a complete incrustation layer and the authors plan to look more closely at Vivianite [Fe3(PO4)2•8(H2O)] and Siderite [FeCO3] in the next stage of the project.
The authors concluded that using mineral formation to stabilize the dissolution of glass offers an opportunity to increase the waste loading of the glass, especially if the waste stream is tailored to form the desired minerals. This increase comes from the requirement of the specific ions from the waste stream to create the protective minerals
on the glass surface. Through these investigations the authors have examined various mineral phases that may form and the feasibility of these phases as passivation barriers. Once the authors attempted to calculate the additional waste loading they realized the equilibrium of the mineral may be changed by the addition of other ions in the waste stream. While the actual increase is not known at this time it is likely that the waste loading can be increased to help increase the formation of protective minerals.
Future Work and Conclusions
The authors' modeling research has shown the potential of encrusting species in iron systems. Both the carbonate and phosphate systems have shown to have a high likelihood of forming an encrusting layer and the sulfate system has a moderate chance. The authors plan to investigate the iron system through the close of FY11. This investigation will attempt to recreate some of these phases on the surface of the glass through glass dissolution and solution spiking methods. They will use a rapidly dissolving glass with a high concentration of iron pared with a saturated carbonate or phosphate solution. This glass will be placed in a static dissolution test and will be evaluated using SEM and XRD to look for the alteration regions as well as any minerals that may have formed on the surface of the glass.
The authors also plan to continue their geochemical modeling of glass systems to optimize mineral phases that have a high likelihood of reducing the dissolution rate of the glass. As part of this modeling the authors will examine what elements may interfere with the precipitation of the desired phases. This will be an iterative process
that will also take melting characteristics into account. This glass will be melted to compare the melt properties and limited corrosion testing will be performed.
The AFCI glass matrix will be tested in a static testing environment to evaluate the validity of the presented models. Any mineral formation will be examined as well as the general dissolution rate of the glass. If the dissolution rate is found to be different from what was entered into the model it will be adjusted and re-run accordingly.
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