On the Coordination of Actinides and Fission Products in Silicate Glasses

The local structure around Th, U, Ce and Nd in leached silicate glasses was examined using XAFS spectroscopy at their L3 edges and also at the K edge of Fe, Co, Ni, Zr and Mo. Pellets of inactive borosilicate glasses with a simplified or a complex composition were leached statically at 90 deg C, at pH buffered to 0 or 6 for 28 days (surface/volume, S/V, ratios of 0.1 cm-1). These glasses are compared to another SON68 sample (denoted 'SP1' in this paper) that was statically leached for 12 years under similar conditions, except for a higher S/V of 12 cm-1 and a higher unconstrained pH of 9.6. The speciation of Fe, Co, Ni, Zr and Mo in the simple and the complex unleached are similar. In the statically leached glasses, the speciation of these transition metals is mostly identical to in the unleached glasses, except in the gels formed at the surface of the glasses leached at low pH, where large speciation differences are observed. Surface precipitates, especially for Fe (as ferrihydrite), Mo (possibly sidwillite) and Th (as ThO2) were detected. Finally, the drying of the gels considerably affects the metal speciation by enhancing metal polymerization.