Kinematics and Thermodynamics of Non-Stoichiometric Oxidation Phase Transitions in Spent Fuel

Abstract

ABSTRACTAt low temperatures (<200°C), spent fuel from power reactors oxidizes from its UO2 lattice to a U4O9 lattice but with an oxygen-to-uranium (O/U) ratio of ∼2.4. Also, the weight gain time response has a plateau as the O/U approaches 2.4. Part of this response results from a geometrical dependency as a U4O9 oxidation front propagates into grain volumes of UO2. It may also be indicative of a metastable, non-stoichiometric U4O9 phase whose existence may inhibit the transition kinetics to the next expected phase of U3O8. To gain a mechanistic understanding and to plan future oxidation tests, lattice kinematic and thermodynamic models are developed for lattice deformations and energetics of lattice phase changes (UO2 → U4O9 → U3O7 → U3O8) that include zeroth order influences on oxidation kinetics due to interstitial oxygen atoms and vacancies plus interstitial and substitutional actinides and fission decay products in spent fuel.