The effect of cluster configuration on vacancy mediated uranium migration is investigated for stoichiometric UO2 and hyper-stoichiometric UO2+x. The minimum enthalpy pathway involves reconfiguration of the most stable cluster to a metastable configuration, in stoichiometric and hyper-stoichiometric systems. Additionally, a much larger number of alternative metastable configurations were identified for UO2+x, compared to UO2. This would lead to a larger contribution to the Arrhenius pre-exponential term with hyper-stoichiometry. Consequently, the often made assumption that re-orientation of the ground state cluster offers access to the migration activation energy cannot be assumed to be valid. Crown Copyright (C) 2014 Published by Elsevier B.V. All rights reserved.