Uranium surroundings in borosilicate glass from neutron and x-ray diffraction and RMC modelling.

Neutron and high-energy x-ray diffraction measurements have been performed on multi-component 55SiO(2)·10B(2)O(3)·25Na(2)O·5BaO·ZrO(2) borosilicate host glass loaded with 30 wt% UO(3). Both the traditional Fourier transformation technique and the reverse Monte Carlo simulation of the experimental data have been applied to get structural information. It was established that the basic network structure consists of tetrahedral SiO(4) units and of mixed tetrahedral BO(4) and trigonal BO(3) units, similar to the corresponding host glass. Slight changes have been observed in the oxygen surroundings of the Na and Zr modifier cations; both the Na-O and Zr-O distances decrease and a more compact short-range structure has been obtained compared to the host glass. For the U-O correlations two distinct peaks were resolved at 1.84 and 2.24 Å, and for higher distances intermediate-range correlations were observed. Significant correlations have been revealed between U and the network former Si and B atoms. Uranium ions take part in the network forming, which may be the reason for the observed good glassy stability and hydrolytic properties.