Lixiviation and percolation

The lixiviation of nuclear waste glasses is discussed in terms of percolation with the help of 3-D simulation. It is shown that the concentration of the soluble elements (Boron, Alkali) in the actual glasses is not far from the site percolation threshold for a random 3-D system. Thus, depending of small differences in composition, one may obtain percolating 'bad glasses' and non-percolating 'good glasses'. In the former, the dissolution is not congruent with a dissolution rate of boron and sodium much higher than that of silicon. In the latter, the dissolution is nearly congruent and the common rate is controlled by the dissolution of silica. Furthermore, it is shown that, even when the solution is saturated in silicic acid, the leaching process - does not stopped. The leaching front continues to progress and boron and sodium to be released in solution. This is a dynamic percolation effect due to the equilibrium dissolution-recondensation of silica. The numerical simulation show, that this effect produces a restructured silica-rich gel layer with nano-metric pores. (author)