Experimental Study and Monte Carlo Modeling of Calcium Borosilicate Glasses Leaching
- PDF / 100,211 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 40 Downloads / 191 Views
0985-NN06-02
Experimental Study and Monte Carlo Modeling of Calcium Borosilicate Glasses Leaching Mehdi Arab1, Celine Cailleteau1, Frederic Angeli1, and Francois Devreux2 1 CEA/DTCD/SECM/Laboratoire d'etudes du Comportement a Long Terme, CEA Centre Valrho, BP 17171, Bagnols-sur-ceze, 30207, France 2 Laboratoire de Physique de la Matiere Condensee, CNRS & Ecole Polytechnique, Palaiseau Cedex, 91128, France
ABSTRACT During aqueous alteration of glass an alteration layer appears on the glass surface. The properties of this alteration layer are of great importance for understanding and predicting the long-term behavior of high-level radioactive waste glasses. Numerical modeling can be very useful for understanding the impact of the glass composition on its aqueous reactivity and longterm properties but it is quite difficult to model these complex glasses. In order to identify the effect of the calcium content on glass alteration, seven oxide glass compositions (57SiO2 17B2O3 (22−x)Na2O xCaO 4ZrO2; 0 < x < 11) were investigated and a Monte Carlo model was developed to describe their leaching behavior. The specimens were altered at constant temperature (T = 90°C) at a glass-surface-area-to-solution-volume (SA/V) ratio of 15 cm-1 in a buffered solution (pH 9.2). Under these conditions all the variations observed in the leaching behavior are attributable to composition effects. Increasing the calcium content in the glass appears to be responsible for a sharp drop in the final leached boron fraction. In parallel with this experimental work, a Monte Carlo model was developed to investigate the effect of calcium content on the leaching behavior especially on the initial stage of alteration. Monte Carlo simulations performed with this model are in good agreement with the experimental results. The dependence of the alteration rate on the calcium content can be described by a quadratic function: fitting the simulated points gives a minimum alteration rate at about 7.7 mol% calcium. This value is consistent with the figure of 8.2 mol% obtained from the experimental work. The model was also used to investigate the role of calcium in the glass structure and it pointed out that calcium act preferentially as a network modifier rather than a charge compensator in this kind of glasses. INTRODUCTION Monte Carlo modeling methods can be very useful for simulating glass alteration [1]. A model based only on hydrolysis and recondensation mechanisms was proposed by Devreux [2-4] in which the glass is constructed on a lattice of tetrahedral sites (cristobalite). The networkforming atoms (Si, Al, B) are first randomly distributed over the lattice points in proportions corresponding to the simulated glass composition. Sodium atoms are then distributed in the glass; they may be charge compensators (near aluminum atoms and tetracoordinated boron atoms) or network modifiers creating nonbridging bonds between two silicon atoms.
The element reactivity is specified by dissolution probabilities that depend on the type and coordination number of eac
Data Loading...
