Abstract
Uranium–neptunium mixed dioxides are considered as fuels and targets for the transmutation of the minor actinides in fast neutron reactors. Hereafter, a local and atomic scale structural analysis was performed on a series of U1–xNpxO2 (x = 0.01; 0.05; 0.20; 0.50; 0.75; 0.85) synthesized by the sol–gel external gelation method, for which longer range structural analysis indicates that the process yields solid solutions. The oxidation state of IV for uranium and neptunium cations was confirmed using U LIII and Np LIII edge X-ray absorption near edge structure (XANES). The atomic scale structure was probed with 17O magic angle spinning nuclear magnetic resonance (MAS NMR) for the anion. Structural distortions due to the substitution of U by the smaller Np cation were detected by 17O MAS NMR.
Highlights
A local and atomic scale structural analysis was performed on a series of U1−xNpxO2 (x = 0.01; 0.05; 0.20; 0.50; 0.75; 0.85) synthesized by the sol−gel external gelation method, for which longer range structural analysis indicates that the process yields solid solutions
The oxidation state of IV for uranium and neptunium cations was confirmed using U LIII and Np LIII edge X-ray absorption near edge structure (XANES)
Such Np solid solutions have been investigated by XRD at high temperatures to develop the understanding of their thermodynamic properties in the hyperstoichiometric region of the system.[3−5] Further, as these materials possess unusual magnetic ordering at low temperatures significant investigative work have been done.[6−9] Neptunium-237 Mössbauer spectroscopy was performed in a series of U1−xNpxO2 (x = 0.15, 0.25, 0.5, 0.75, 1) compounds and demonstrated that Np remains IV valent oxidation state for all compositions.[7]
Summary
Understanding the incorporation mechanism of minor actinides (MA), that is, Np, Am, Cm, in uranium dioxides and mixed U− Pu oxides (MOX) as solid solutions is essential starting point for the detailed understanding of these materials, which will become relevant, as they are considered as fuels and targets for the transmutation of the minor actinides in fast neutron reactors.[1,2] This is an innovative approach to dramatically reduce the long-term radiotoxic impact of these materials and is considered in two modes: the homogeneous MA recycling scenario which relies on the introduction of a few percent of MA in all of the fuel pins of the reactor and, the heterogeneous MA recycling concept which considers deployment of the MA in a UO2 matrix in dedicated fuel rods and assemblies. High resolution 17O MAS NMR was used for the first time in such solid solutions to probe the local environment around the oxygen
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