What Is the Actual Local Crystalline Structure of Uranium Dioxide, UO 2 ? A New Perspective for the Most Used Nuclear Fuel

Up to now, uranium dioxide, the most used nuclear fuel, was said to have a Fm3̅m 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 , in which UO distance was shorter than the expected value for the Fm3̅m 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 has a local Pa3̅ symmetry. The existence of a local lower symmetry in UO could explain some unexpected properties of UO that would justify UO 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.