Neptunium characterization in uranium dioxide fuel: Combining a XAFS and a thermodynamic approach

The effect of Np incorporation in a UO₂ matrix on redox state of U and Np cations has been investigated by X-ray absorption spectroscopy (XAS) on three samples: (U_0.9Np_0.1)O₂, (U_0.9Np_0.1)O_2 + x and pure NpO₂, the latter of which was used as a reference for Np(IV) oxidation state. XANES at the Uranium LIII edge and XANES and EXAFS at the Neptunium LIII edge show that Uranium only is oxidized to higher (V) and (VI) oxidation states in the hyperstoichiometric (U_0.9Np_0.1)O_2 + x sample in comparison to the stoichiometric (U_0.9Np_0.1)O₂ where U appears at the (IV) oxidation state. This contrasts to Neptunium cation that always remains at the (IV) oxidation state whatever the oxygen stoichiometry.Thermodynamics calculations were performed to complete the experimental study and lead to the same conclusion. Separate UO₂–NpO₂ phases and homogeneous solid solution were considered. The latter case shows that the energy of mixing is insignificant in this system. The combined experimental and theoretical approaches demonstrate that any excess of oxygen in the system is carried by Uranium.