Neutron Scattering Experiments and Electronic Structure Calculations on U₃O₇ and U₃O₈

The principal type of nuclear fuel material, which remains widely used today, is uranium dioxide (UO₂). After discharge from a nuclear reactor some sort of disposal scenario needs to be considered to manage the spent fuel in a sustainable and safe way during extensive periods of time. However, in most countries which employ(ed) commercial nuclear energy, no political decision has yet been made, leading to prolonged intermediate storage periods up to several decades. As a result, oxidation and corrosion reactions due to accidental interaction between air, water, or moisture and the spent fuel need to be considered and understood.<br/><br/>The response of conventional nuclear fuel to oxidation and corrosion is determined mainly by the physico-chemical properties of uranium. The binary uranium-oxygen system displays a very complex landscape of oxide phase fields. At temperatures below about 450 °C two single-valence compounds, uranium(IV) oxide (UO₂) and uranium(VI) oxide (UO₃), and several mixed-valence compounds (U₄O₉, U₃O₇, U₃O₈) can occur [1-4], characterized by particular associations of oxygen interstitials and vacancies. At higher temperature, long-range correlations among defects become less effective and the phase diagram becomes simpler, displaying a wide phase domain of nonstoichiometric UO_2±<i>x</i> [5]. A characteristic feature of these higher oxide compounds lies in their crystallographic modifications, which occur almost exclusively in the anion sublattice, where point defects arrange into larger clusters to form complex long-range ordered structures [6-8]. However, despite sustained efforts for almost a century, the transition and structural relations between the different compounds is yet to be fully understood.<br/><br/>New perspectives on the oxidation behavior of UO₂ were obtained by investigating the kinetic mechanisms of these materials at the nanoscale, where fundamental differences were reported compared to the macroscale observations. Recent breakthroughs were made possible by using nanoscale imaging techniques like transmission electron microscopy [9], and by evaluating atomistic models of crystal structures [8,10,11]. A key instrument has been the application of neutron scattering techniques, owing to its sensitivity towards the anion sublattice [10], and its selectivity in probing magnetic transitions [12] and the local structure [13]. In this contribution we will report our current understanding of the uranium oxide systems U₃O₇ and U₃O₈ in ground state, based on latest neutron scattering data and ab-initio calculations.<br/><br/>[1] Grenthe I.<i>, et al.</i>, "Uranium", in: Morss L.R., Edelstein N.M., F. J. (Eds.) The Chemistry of the Actinide and Transactinide Elements, Springer, Dordrecht, 2008.<br/>[2] K.O. Kvashnina<i>, et al.</i>, Phys. Rev. Lett., 111 (2013) 253002.<br/>[3] D.A. Andersson<i>, et al.</i>, Inorg. Chem., 52 (2013) 2769.<br/>[4] S.D. Conradson<i>, et al.</i>, Phys. Rev. B, 96 (2017) 125114.<br/>[5] P.Y. Chevalier<i>, et al.</i>, J. Nucl. Mater., 303 (2002) 1.<br/>[6] B.T.M. Willis, J. Chem. Soc., Faraday Trans. II, 83 (1987) 1073.<br/>[7] L. Desgranges<i>, et al.</i>, Inorg. Chem., 48 (2009) 7585.<br/>[8] G. Leinders<i>, et al.</i>, Inorg. Chem., 55 (2016) 9923.<br/>[9] G. Leinders<i>, et al.</i>, Inorg. Chem., 55 (2016) 3915.<br/>[10] G. Leinders<i>, et al.</i>, Inorg. Chem., 60 (2021) 10550.<br/>[11] A. Soulié<i>, et al.</i>, Inorg. Chem., 58 (2019) 12678.<br/>[12] A. Miskowiec<i>, et al.</i>, Phys. Rev. B, 103 (2021) 205101.<br/>[13] L. Desgranges<i>, et al.</i>, Inorg. Chem., 56 (2017) 321.