  We report the effects of radiation damage on helium diffusion in zircon using
data from molecular dynamics simulations. We observe an increase in activation
energy for helium diffusion as a result of radiation damage and increasing
structural disorder. The activation energy in a heavily damaged region is
smaller than in a completely amorphous system which is correlated with
remaining order in the cation sublattices of the damaged structure not present
in the fully amorphized system. The reduction of activation energy is related
to the disappearance of fast diffusion pathways that present in the crystal.
Consistent with the change in activation energy, we observe the accumulation of
helium atoms in the damaged structure and discuss the implications of this
effect for the formation of helium bubbles and zircon's performance as an
encapsulation material for nuclear waste.
