On the influence of temperature on the 1/2<110> screw dislocation core in uranium dioxide

Understanding the fundamental processes of plastic deformation in uranium dioxide (main nuclear fuel) is critical to predict the structural integrity of nuclear reactors under off-normal operating conditions. While the 1/2 < 110 > {001} edge dislocation has been extensively studied in recent years due to its role as rate-limiting character for plastic deformation in the primary slip systems, much less is known about the screw dislocation, despite its recently shown involvement in the composite slip process at high temperature. Here, molecular simulations are employed to investigate the evolution of the 1/2 < 110 > screw dislocation core structure in UO 2 with temperature. First, a comparison of the various interatomic models tested at 0 K addresses the stability of a zig-zag core structure predominantly spread in {001}. Then, molecular dynamics simulations confirm the stability of the screw dislocation core spread in {001} up to 1600 K, after which a transition towards more complex core structures involving the {110} and {111} planes is characterized, with direct implications on the shear-ability of the various crystallographic planes of the fluorite structure. Further analysis reveal that the transition of the screw dislocation core, that favors thermal fluctuation of the dislocation core in {111} slip planes at high temperature, is driven by the local disordering of the anionic sublattice within the dislocation core, preceding the well-known Bredig transition in UO 2 . This transition of the screw dislocation core is at the roots of the composite slip process, which was recently proposed to explain the Schmid law breakdown observed in UO 2 single crystal.