Computer Simulations of Hydration, Alteration, and Release from Waste Glasses Using A Cell Model
- PDF / 269,983 Bytes
- 8 Pages / 612 x 792 pts (letter) Page_size
- 30 Downloads / 238 Views
II1.11.1
COMPUTER SIMULATIONS OF HYDRATION, ALTERATION, AND RELEASE FROM WASTE GLASSES USING A CELL MODEL Fernando C. Pérez-Cárdenas, Hao Gan, and Ian L. Pegg Vitreous State Laboratory, The Catholic University of America, Washington, DC 20064
ABSTRACT A probabilistic cell model to study the hydration, alteration, and elemental release into solution from nuclear waste glasses is presented. This computational model considers the glass alteration as the product of hydration and multiple phase transformations. The model reveals that the elemental redistribution occurring as a consequence of these transformations can produce transport barriers that would affect the glass alteration and release into a surrounding aqueous solution. Importantly, the model also exhibits an apparent saturation concentration in solution, which has a kinetic origin and is lower than the true thermodynamic value. We also highlight the interplay among the different mechanisms that give rise to the glass corrosion process.
INTRODUCTION An essential aspect of nuclear and hazardous waste immobilization by vitrification is the chemical durability of the glass product. A wide variety of tests have been designed to assess the durability of waste glasses under repository conditions. These methods determine either the release of chemical species from the glass into a surrounding aqueous solution during the corrosion process (e.g., PCT, MCC-1, TCLP, soxhlet, etc.) or the depth of alteration in a solid glass sample exposed to hydrating conditions, as is the case in the Vapor Hydration Test (VHT) [1]. The interpretation of the results from these tests is sometimes complicated by the fact that the various mechanisms involved in the glass alteration and leaching process are complex and highly interrelated. The study of simple models that are able to reproduce some of the more prominent features that are experimentally observed can help to elucidate the governing processes and the interactions between them. This paper presents a computational cell model that is used to simulate the hydration, alteration, and release from waste glasses. It is an extension of a previous study of the alteration of high-sodium waste glasses under VHT conditions [2]. In the present work, we have extended the scope to include the exchange of chemical species between the glass matrix and a surrounding aqueous solution. The model considers that several phase transformations can take place during the glass alteration, and that this process and the release of elements into solution are strongly coupled. Our simulations show that the redistribution of mobile species within the solid matrix, as the alteration front advances toward the center of the glass sample, plays a critical role in the nature of the alteration and release evolution curves. Rather than attempting to predict the outcome of a particular experimental situation, we present simulation results that illustrate possible underlying microscopic mechanisms that give rise to general trends observed in the alteration and releas
Data Loading...
