  Oxygen self-diffusion is key to understanding stoichiometry and defect
structures in oxide nuclear fuels. Experimentally, low activation-barrier
oxygen migration was found in ThO$_2$, a candidate nuclear fuel, possibly due
to short-circuit diffusion mechanisms. Here, we perform extensive molecular
dynamics simulations to show that various types of extended defects can enhance
oxygen self-diffusion with a much-reduced activation barrier in ThO$_2$. In
this work, we consider extended defects including 1D (dislocation), 2D (grain
boundary), and 3D (void) defects. Due to the distinct characteristics of each
type of extended defect, the modulation of oxygen diffusion varies. These
results provide a quantitative description of oxygen transport, which is
significantly enhanced within a close distance (nanometer scale) from the
extended defects. Among all these defects, grain boundary, particularly the
$\Sigma 3$ twin boundary with a low formation energy, exhibits the strongest
effect on increasing oxygen transport.
