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

Up to now, uranium dioxide, the most used nuclear fuel, was said to have a Fm 3̅ 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 Fm 3̅ 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 Pa 3̅ 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.