Wednesday, June 20, 2007 - 10:20 AM
The Summit (Boise Centre on the Grove)

Molecular Dynamics study of diffusional creep in nanocrystalline UO2

Tapan Desai, Paul Millett, and Dieter Wolf. Idaho National Laboratory, Idaho Falls, ID

We have performed Molecular Dynamics (MD) simulations to study creep in nanocrystalline UO2 at temperatures well above the oxygen sub-lattice melting. In the absence of any external loading, we found that the uranium ions diffuse only via the grain boundaries (GB) whereas the oxygen ions show lattice as well as GB diffusion. When these microstructures are subjected to constant-stress loading at levels low enough to avoid microcracking and dislocation nucleation from the GBs, our simulations reveal that in the absence of grain growth UO2 deforms via GB diffusion creep (also known as Coble creep). The creep activation energy agrees well with the zero-stress diffusional activation energy of the slowest moving species, i.e. the uranium ions. Thus the rate-limiting mechanism for the Coble creep is the GB diffusion of uranium ions.