Thermodynamics and kinetics of intrinsic point defects in plutonium dioxides

Abstract

Thermodynamic and kinetic properties of intrinsic point defects in plutonium dioxides (PuO2), including their formation, migration, and effects on oxygen (O) self-diffusion, are systematically studied using atomic simulations in this work. Coulomb interactions among charged defects and ions are found play a key factor in determining the energetics and structures of defects in PuO2, where point defects are energetically prefer to binding together forming defect pairs and less relaxation volumes are introduced by these bound point defects. Further calculations of defect migration properties reveal different migration mechanisms of O and Pu point defects, where O point defects prefer to migrate along [100] direction with an energy barrier of 0.31 eV by the vacancy mechanism, while Pu point defects prefer to migrate along [100] direction with an energy barrier of 2.57 eV by the interstitial mechanism. This confirms the cation sublattice is much more stable than the anion sublattice in the fluorite structure PuO2. Given the dominance of O defects in PuO2, their effects on O self-diffusion are investigated. Both vacancies and interstitials could significantly enhance O self-diffusivity. Under the Meyer-Neldel rule, activation energy and pre-exponential factors of O diffusion show a dependence of point defect concentrations. Specifically, internal stresses introduced by defects are found responsible for the different dependency of the activation energy on the vacancy and interstitial concentrations respectively. Finally, an empirical equation is derived to connect the point defect concentration, i.e. O/Pu ratio, and activation energy of O self-diffusion in PuO2.