  A cluster dynamics (CD) model has been developed to investigate the
nucleation and growth of point defect clusters, i.e., interstitial prismatic
loops and nanoscale and sub-nanoscale voids, in ThO2 during irradiation by
energetic particles. The model considers cluster off-stoichiometry due to the
asymmetry of point defect generation on the O and Th sublattices under
irradiation, as well as the point defect diffusivities and the defect binding
energies to clusters. The energies were established using detailed molecular
dynamics simulations considering the statistical variability of cluster
configuration. A high-order adaptive time-integration has been used to solve
the model. The predicted loop density and their average size is in good
agreement with reported experimental observations for proton irradiated ThO2 at
600oC. The model did not predict void evolution due to the sluggish kinetics of
cation vacancies, explaining the absence of voids in proton irradiated ThO2
(and other oxides) at relatively low temperatures.
