Classical molecular dynamics investigation of microstructure evolution and grain boundary diffusion in nano-polycrystalline UO₂

The High Burnup Structure (HBS) observed at pellet periphery in conventional Light Water Reactor nuclear fuels and around spots presenting high plutonium content in mixed (U, Pu) oxide fuel - MOX fuel - consists of a restructuration of the original grains into smaller ones. The process is often postulated to occur because of the accumulation of irradiation damage and the retention of fission products in the matrix. The computing power nowadays available enables for simulating larger systems at the atomic scale up to the point that nano-polycrystalline material can now be investigated by empirical potential molecular dynamics. Simulations of nano-polycrystalline UO₂ structures have been carried out at various temperatures to investigate atom mobility close to grain boundaries. The variation of Arrhénius parameters for the diffusion coefficient of oxygen, uranium and xenon as a function of the distance from a grain boundary was studied, leading to the distinction of three zones: the grain boundary layers (up to 1 nm depth) presenting enhanced diffusion, an intermediate zone (1 to roughly 2 nm depth) with intermediate diffusion values and the bulk of the grains. The following Arrhénius relations for grain boundary diffusion were derived:Oxygendiffusion: ^DO(T)=5.4×10^-4exp[(0.74eV)/(^kBT)] ^cm2/sUraniumdiffusion:^DU(T)=1.7×10 ^-6exp[(0.50eV)/(^kBT)]^cm2/sXenondiffusion: ^DXe(T)=1.4×10^-5exp[(0.50eV)/(^kBT)] ^cm2/s where k_B stands for Boltzmann's constant and T for temperature. Besides, changes in the microstructure could be "observed" over the 1 ns duration of the simulations, such as the formation of small gas clusters.