Selenium redox speciation and coordination in high-burnup UO_2 fuel: Consequences for the release of ~(79)Se in a deep underground repository

摘要

The chemical state of Se in high-burnup UO_2 spent nuclear fuel (SNF) was investigated by micro X-ray absorption near-edge structure (l-XANES) spectroscopy. The data were first evaluated in terms of linear combination fits of reference compounds. In a second step, the XANES data were fitted to theoretical (ab initio) spectra via geometrical optimization of atomic clusters around a central Se atom, using the FDMNES and FitIt software packages. Best fits were obtained assuming substitution of Se in occupied or vacant oxygen sites within the UO_2 lattice, with 20–25% local expansion. Based on these results and chemical arguments we argue that the long-lived fission product ~(79)Se may be stabilized to sparingly soluble Se(-II) in SNF. This would explain the failure to detect dissolved Se in SNF leaching experiments.