Lattice thermal expansion of Pu1−yAmyO2−xplutonium–americium mixed oxides

Plutonium oxide melt structure and covalency

Advances in nuclear power reactors include the use of mixed oxide fuel, containing uranium and plutonium oxides. The high-temperature behaviour and structure of PuO2-x above 1,800 K remain largely unexplored, and these conditions must be considered for reactor design and planning for the mitigation of severe accidents. Here, we measure the atomic structure of PuO2-x through the melting transition up to 3,000 ± 50 K using X-ray scattering of aerodynamically levitated and laser-beam-heated samples, with O/Pu ranging from 1.57 to 1.76. Liquid structural models consistent with the X-ray data are developed using machine-learned interatomic potentials and density functional theory. Molten PuO1.76 contains some degree of covalent Pu-O bonding, signalled by the degeneracy of Pu 5f and O 2p orbitals. The liquid is isomorphous with molten CeO1.75, demonstrating the latter as a non-radioactive, non-toxic, structural surrogate when differences in the oxidation potentials of Pu and Ce are accounted for. These characterizations provide essential constraints for modelling pertinent to reactor safety design.

Electron correlation and relativistic effects on the electronic properties of a plutonium and americium mixed oxide (PuAmO4): from single-particle approximation to dynamical mean-field theory

First-principles calculations were performed on a plutonium and americium mixed oxide (PuAmO4), aiming at revealing the effects of electron correlation, Pu/Am 5f-conduction electrons' hybridization, and relativity on its electronic properties. The many-body calculation suggests that the spin-orbit-coupling (SOC)-splitting of j = 5/2 and j = 7/2 manifolds are both in the weakly and moderately correlated states, respectively, implying that the jj coupling scheme is more appropriate for Pu/Am 5f electrons. The density of states, 5f occupation numbers, and Green's functions all suggest that both Pu and Am 5f electrons exhibit the coexistence of the localized and delocalized states. The admixture of 5fn atomic configurations, Pu/Am 5f-conduction electrons' hybridization, and dual characteristics of 5f electrons yield average occupation numbers of 5f electrons n5f = 4.78 and 5.86 for Pu and Am ions, respectively. Within the DFT+DMFT calculation, the weighted-summation-derived occupation numbers in terms of 5f4/5f5/5f6 and 5f5/5f6 configurations for Pu and Am 5f electrons, respectively, are in reasonable agreement with those of other DFT-based calculations.

Breaking the hard-sphere model with fluorite and antifluorite solid solutions

Using the hard-sphere model with the existing tabulated values of ionic radii to calculate the lattice parameters of minerals does not always match experimental data. An adaptation of this crystallographic model is proposed by considering the cations and anions as hard and soft close-packed spheres, respectively. We demonstrate the relevance of this "hybrid model" by combining Pauling's first rule with experimental unit-cell parameters of fluorite and antifluorite-structured systems to revise the ionic radii of their constitutive species.

New sample stage for characterizing radioactive materials by X-ray powder diffraction: application on five actinide dioxides ThO2, UO2, NpO2, PuO2 and AmO2

A new sample stage for characterizing radioactive materials by X-ray powder diffraction was developed at the ATALANTE facility (CEA Marcoule, France) using a conventional (non-nuclearized) Bruker D8 goniometer mounted in Bragg–Brentano geometry. The setup consists of a removable, fully hermetic sample stage, with a 200 µm-thick beryllium window, that can be plugged onto a glove-box, allowing the sample to be introduced in an hermetic medium that also encapsulates the glove-box atmosphere throughout the analysis process. The whole setup is thus hermetically unplugged from the glove-box and positioned on the centre of the goniometer. No preliminary decontamination and/or decontainment of the sample is necessary. The device was developed to avoid an expensive and time-consuming nuclearization of the diffractometer while also keeping it easily accessible for maintenance. Ultimately, keeping the diffractometer out of a glove-box also limits the volume of the final nuclear wastes, and thus the removable sample stage is the only `active' part. X-ray diffraction results of two NIST standards LaB6 and α-Al2O3 as well as five actinide dioxides ThO2, UO2, NpO2, PuO2 and AmO2 are presented to show the efficiency of the setup.

Plutonium and Americium Aluminate Perovskites

Both AmAlO₃ and PuAlO₃ perovskites have been synthesized and characterized using powder X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared spectroscopy (FT-IR), and ²⁷Al magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR). AmAlO₃ perovskite showed a rhombohedral configuration (space group R3̅c) in agreement with previous studies. The effect of americium α-decay on this material has been followed by XRD and ²⁷Al MAS NMR analyses. In a first step, a progressive increase in the level of disorder in the crystalline phase was detected, associated with a significant crystallographic swelling of the material. In a second step, the crystalline AmAlO₃ perovskite was progressively converted into amorphous AmAlO₃, with a total amorphization occurring after 8 months and 2 × 10¹⁸ α-decays/g. For the first time, PuAlO₃ perovskite was synthesized with an orthorhombic configuration (space group Imma), showing an interesting parallel to CeAlO₃ and PrAlO₃ lanthanide analogues. High-temperature XRD was performed and showed a Imma → R3̅c phase transition occurring between 473 and 573 K. The thermal behavior of R3̅c PuAlO₃ was followed from 573 to 1273 K, and extrapolation of the data suggests that cubic plutonium perovskite should become stable at around 1850 K (R3̅c → Pm3̅m transition).

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.