COORDINATION OF ACTINIDES IN SILICATE MELTS

The structural environments around Th(IV) and U(VI) at concentrations ranging from 90 ppm to 7 wt.% were investigated in glasses and melts o f Na di-and trisilicate compositions between 293 and 1550 K using x-ra y absorption fine structure (XAFS) spectroscopy. Data for model compou nds were collected at temperatures up to 2000 K in order to quantify t he magnitude of anharmonic effects. Data for glasses and melts were co llected and analyzed considering anharmonic (cumulant-expansion), curv ed-wave and multiple-scattering effects;In the glasses and melts studi ed, U(VI) occurs as uranyl groups [d(U(VI)-O approximate to 1.78 and 2 .25(3) Angstrom] and Th(IV) occurs as a mixture of 6- and 8-coordinate d 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 coordin ation changes during the glass-to-melt transformation. However, specia l 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.