Atomistically-informed cross-slip and slip-system transition modeling in uranium dioxide

Understanding the mechanical behavior of uranium dioxide (UO 2 ) at high-temperature is essential for assessing the structural integrity of nuclear fuel under reactor operating conditions. The pronounced plastic anisotropy observed in UO 2 single crystals is believed to originate from the complex behavior of 1/2 < 110 > screw dislocations and their ability to cross-slip between the various slip modes of the fluorite structure, as initially suggested by Sawbridge and Sykes more than fifty years ago. However, direct evidence supporting this hypothesis, either from atomic-scale simulations or transmission electron microscopy, has so far been lacking, leaving the microscopic origin of cross-slip and its possible contribution to the plastic anisotropy unresolved. In this work, we combine molecular dynamics, theoretical modeling and discrete dislocation dynamics simulations to investigate the mobility of the 1/2 < 110 > screw dislocation in UO 2 as a function of temperature, stress and orientation. Atomistic simulations reveal that above ∼ 1500 K, the 1/2 < 110 > screw dislocation progressively transitions from planar glide in {001} or {110} slip systems to frequent cross-slip in 1/2 < 110 > {111}, ultimately leading to a full slip-system transition near 1700 K. This behavior correlates with a temperature-induced reorganization of the dislocation core, from a configuration electrostatically-confined within {001} to a {111}-spreading state as the oxygen sublattice disorders. An atomistically-informed stochastic model of cross-slip and slip-system transition is then formulated and implemented in the discrete dislocation dynamics simulation, successfully reproducing atomistic simulations, experimental slip traces and the plastic anisotropy trends observed in UO 2 .