Cs diffusion mechanisms in UO₂ investigated by SIMS, TEM, and atomistic simulations

Experimental investigations and atomistic simulations are combined to study the cesium diffusion processes at high temperature in UO₂. After ¹³³Cs implantation in UO₂ samples, diffusion coefficients are determined using the depth profile evolution after annealing as measured by secondary ion mass spectrometry. An activation energy of 1.8 ± 0.2 eV is subsequently deduced in the 1300-1600 °C temperature range. Experimental results are compared to nudged elastic band simulations performed for different atomic paths including several types of uranium vacancy defects. Activation energies ranging from 0.49 up to 2.34 eV are derived, showing the influence of the defect (both in terms of type and concentration) on the Cs diffusion process. Finally, molecular dynamics simulations are performed, allowing the identification of preferential Cs trajectories that corroborate experimental observations.