Coordination of Actinides in Silicate Melts

Abstract

The structural environments around Th(IV) and U(VI) at concentrations ranging from 90 ppm to 7 wt.% were investigated in glasses and melts of Na di- and trisilicate compositions between 293 and 1550 K using x-ray absorption fine structure (XAFS) spectroscopy. Data for model compounds were collected at temperatures up to 2000 K in order to quantify the magnitude of anharmonic effects. Data for glasses and melts were collected and analyzed considering anharmonic (cumulant-expansion), curved-wave and multiple-scattering effects. In the glasses and melts studied, U(VI) occurs as uranyl groups [d(U(VI)-O 1.78 and 2.25(3) A] and Th(IV) occurs as a mixture of 6- and 8-coordinated Th at the 1-3 wt.% Th level, but as 8-coordinated Th at trace levels (90-1000 ppm). These two actinides do not undergo significant coordination changes during the glass-to-melt transformation. However, special care in the data analysis protocol is needed to avoid the effects of anharmonicity (due to both positional and thermal effects) which tend to artificially increase the (apparent) amount of 6-coordinated Th.