The chemical state of complex uranium oxides

Uranium oxides are fascinating materials not only owing to their technological significance in nuclear fuel applications [1] but also with respect to their dynamical properties [2], valence orbital configuration [3] and elementary excitations [4]. Understanding of the electronic, magnetic and crystal structural properties of uranium dioxide (UO2) was advanced significantly [5,6] after establishing a Mott-Hubbard insulator behavior in UO2 [see e.g. Ref. 7]. Uranium dioxide with a fluorite type crystal structure is thermodynamically stable, but upon oxidation generates a series of the mixed-valence U oxides U4O9/U3O7 and U3O8 where the stochiometry depends on the temperature and oxygen partial pressure [8]. These processes are typical for the nuclear fuel cycle. The main component of the nuclear fuel rods UO2 is generally considered to be the safest chemical form for disposal purposes because of its low water solubility and stability over a wide range of environmental conditions. Uranium dioxide in contact with air can be quickly oxidized to U3O8 even at very low temperature (~200°C). The formation of U3O8 from UO2 results furthermore in volume increase which can potentially damage the first confinement barrier (aka fuel cladding) in case of direct storage of fuel elements. The phase of U3O8 occurs as end product not only with UO2 as starting material but also when other nuclear fuel sources like U dicarbides and U hydrides are used [9,10]. To avoid the risk related to the release of radionuclides to the environment during the disposal of nuclear waste [1] , U3O8 is often recycled to spent nuclear oxide fuel [11]. Uranium oxides are not only used in power plants but also, e.g., catalysis applications. The use of U3O8 was encouraged in U3O8/SiO2 catalysts which can efficiently destroy a range of hydrocarbons and chlorine-containing pollutants [12], because of the believed availability of two distinct oxidation states U(VI) and U(IV) and easy conversion between them [13]. In all applications a direct observation of the U chemical state is of paramount importance.