What Is the Actual Local Crystalline Structure of Uranium Dioxide, UO₂? A New Perspective for the Most Used Nuclear Fuel

Abstract

Up to now, uranium dioxide, the most used nuclear fuel, was said to have a Fm3̅ m crystalline structure from 30 to 3000 K, and its behavior was modeled under this assumption. However, recently X-ray diffraction experiments provided atomic pair-distribution functions of UO 2 , in which UO distance was shorter than the expected value for the Fm3̅ m space group. Here we show neutron diffraction results that confirm this shorter UO bond, and we also modeled the corresponding pair-distribution function showing that UO 2 has a local Pa3̅ symmetry. The existence of a local lower symmetry in UO 2 could explain some unexpected properties of UO 2 that would justify UO 2 modeling to be reassessed. It also deserves more study from an academic point of view because of its good thermoelectric properties that may originate from its particular crystalline structure.

Direct observation of pure pentavalent uranium in U2O5 thin films by high resolution photoemission spectroscopy

“…Despite decades of extensive investigations, much remains to be discovered about the peculiarity of their structural, chemical and physical properties 3–9 , or about the subtleties of the surface chemistry mechanisms governing the interactions between uranium oxides and the environment 10–13 . Progress on the latter issue is essential to make much needed advances in all aspects of treating waste from the nuclear fuel cycle.…”

Magnetic Structure of UO2 and NpO2 by First-Principle Methods

“…73 The magnetic moment of UO2 is 1.74 μB/U ion, reported by neutron diffraction measurements. 66,68,71 In our calculations, the magnetic moment of the U ion is underestimated by DFT+U and the hybrid HSE06 functional with the closest approximation obtained by the FM (111) states.…”

“…205) k-point pathway is Γ→M→R→X→Γ→R. 73 To probe the magnetic ground-state, the relative energetics of competing magnetic phases have been calculated with PBEsol+U, where the magnitude of the U influences the relative 9 energetics of the magnetic states (Fig. 3).…”

“…This is identical to observations of Pa3 ̅ crystal symmetry in UO2. 73 We therefore employed the same approach used for UO2, whereby the electronic structure of the transverse 3k AFM (U = 4.25 eV) has been calculated by the PBEsol functional (Fig. 6).…”

“…205) crystallographic distortion, where the displacement of the O 2-ions is 0.014 Å. 19,68,71,73 , which is indicative of transverse 3k AFM order. 32,73,74 Antiferroquadrupolar ordering favours Pa3 ̅ (No.…”