Molecular dynamics studies of displacement cascades in the uranium dioxide matrix

In the context of studies on long-time storage of irradiated spent fuel, molecular dynamics simulations have been carried out in order to understand the physical phenomena, on the atomic scale, linked to modifications and damage that the uranium dioxide structure undergoes during a-decay irradiation in repository conditions. Simulations of atomic displacement cascades over an energy range from 1 to 20 keV for the initial primary knock-on atom (PKA) do not show any amorphization of the structure in agreement with what has been found experimentally, and there is very little correlation between the initial orientation of the PKA and the cascade morphology. The number of Frenkel pairs, as a function of the initial energy of the PKA, exhibits a power-law behaviour with an exponent of 0.9 which is contrary to the theoretical linear Norgett-Robinson-Torrens law. Finally, for both species the vacancies have a tendency to aggregate and cluster near the core of the cascade while interstitial atoms are preferentially located at the periphery of the branches corresponding to subcascades.