Structural Investigation of Uranium–Neptunium Mixed Oxides Using XRD, XANES, and ¹⁷O MAS NMR

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 U_1–<i>x</i>Np_<i>x</i>O₂ (<i>x</i> = 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 L_III and Np L_III 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

Structural Properties and Charge Distribution of the Sodium Uranium, Neptunium, and Plutonium Ternary Oxides: A Combined X‑ray Diffraction and XANES Study

The charge distributions in α-Na₂UO₄, Na₃NpO₄, α-Na₂NpO₄, Na₄NpO₅, Na₅NpO₆, Na₂PuO₃, Na₄PuO₅, and Na₅PuO₆ are investigated in this work using X-ray absorption near-edge structure (XANES) spectroscopy at the U-L₃, Np-L₃, and Pu-L₃ edges. In addition, a Rietveld refinement of monoclinic Na₂PuO₃, in space group <i>C</i>2/<i>c</i>, is reported for the first time, and the existence of the isostructural Na₂NpO₃ phase is revealed. In contrast to measurements in solution, the number of published XANES data for neptunium and plutonium solid phases with a valence state higher than IV is very limited. The present results cover a wide range of oxidation states, namely, IV to VII, and can serve as reference for future investigations. The sodium actinide series show a variety of local coordination geometries, and correlations between the shape of the XANES spectra and the local structural environments are discussed herein

Optimization of Uranium-Doped Americium Oxide Synthesis for Space Application

Americium 241 is a potential alternative to plutonium 238 as an energy source for missions into deep space or to the dark side of planetary bodies. In order to use the ²⁴¹Am isotope for radioisotope thermoelectric generator or radioisotope heating unit (RHU) production, americium materials need to be developed. This study focuses on the stabilization of a cubic americium oxide phase using uranium as the dopant. After optimization of the material preparation, (Am_0.80U_0.12Np_0.06Pu_0.02)­O_1.8 has been successfully synthesized to prepare a 2.96 g pellet containing 2.13 g of ²⁴¹Am for fabrication of a small scale RHU prototype. Compared to the use of pure americium oxide, the use of uranium-doped americium oxide leads to a number of improvements from a material properties and safety point of view, such as good behavior under sintering conditions or under alpha self-irradiation. The mixed oxide is a good host for neptunium (i.e., the ²⁴¹Am daughter element), and it has improved safety against radioactive material dispersion in the case of accidental conditions

A New Look at the Structural Properties of Trisodium Uranate Na₃UO₄

The crystal structure of trisodium uranate, which forms following the interaction between sodium and hyperstoichiometric urania, has been solved for the first time using powder X-ray and neutron diffraction, X-ray absorption near-edge structure spectroscopy, and solid-state ²³Na multiquantum magic angle spinning nuclear magnetic resonance. The compound, isostructural with Na₃BiO₄, has monoclinic symmetry, in space group <i>P</i>2/<i>c</i>. Moreover, it has been shown that this structure can accommodate some cationic disorder, with up to 16(2)% sodium on the uranium site, corresponding to the composition α-Na₃(U_1–<i>x</i>,Na_<i>x</i>)­O₄ (0 < <i>x</i> < 0.18). The α phase adopts a mixed valence state with the presence of U­(V) and U­(VI). The two polymorphs of this compound described in the literature, <i>m</i>- and β-Na₃(U_1–<i>x</i>,Na_<i>x</i>)­O₄, have also been investigated, and their relationship to the α phase has been established. The completely disordered low-temperature cubic phase corresponds to a metastable phase. The semiordered high-temperature β phase is cubic, in space group <i>Fd</i>3̅<i>m</i>