Grain growth and pore coarsening in dense nano-crystalline UO_2+x fuel pellets

Dense nano-sized UO_2+x pellets are synthesized by spark plasma sintering with controlled stoichiometries (UO_2.03 and UO_2.11) and grain sizes (~100 nm), and subsequently isothermally annealed to study their effects on grain growth kinetics and microstructure stability. The grain growth kinetics is determined and analyzed focusing on the interaction between grain boundary migration, pore growth, and coalescence. Grains grow much bigger in nano-sized UO_2.11 than UO_2.03 upon thermal annealing, consistent with the fact that hyper-stoichiometric UO_2+x is beneficial for sintering due to enhanced U ion diffusion from excessive O ion interstitials. The activation energies of the grain growth for UO_2.03 and UO_2.11 are determined as ~1.0 and ~2.0 eV, respectively. As compared with the micrometer-sized UO₂ in which volumetric diffusion dominates the grain coarsening with an activation energy of ~3.0 eV, the enhanced grain growth kinetics in nano-sized UO_2+x suggests that grain boundary diffusion controls grain growth. The higher activation energy of more hyper-stoichiometric nano-sized UO_2.11 may be attributed to the excessive O interstitials pinning grain boundary migration.