It is well known that within a few hours after startup of a nuclear reactor, the temperature gradient within a fuel element causes migra-tion of voids/bubbles radially inwards to form a central hole. To understand the atomic processes that control this migration of voids, we performed molecular dynamics (MD) simulations on single crystal UO_2 with voids of diameter 2.2 nm. An external temperature gradient was applied across the simulation cell. At the end of the simulation run, it was observed that the voids had moved towards the hot end of the simulation cell. The void migration velocity obtained from the simulations was compared with the available phenomenological equa-tions for void migration due to different transport mechanisms. Surface diffusion of the slowest moving specie, i.e. uranium, was found to be the dominant mechanism for void migration. The contribution from lattice diffusion and the thermal stress gradient to the void migra-tion was analyzed and found to be negligible. By extrapolation, a crossover from the surface-diffusion-controlled mechanism to the lat-tice-diffusion-controlled mechanism was found to occur for voids with sizes in the urn range.
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