The high-temperature creep behavior transition in stoichiometric nanocrystalline (Pu_x,U_1−x)O₂ mixed oxides

The study of high-temperature creep properties of (Pu,U)O₂ mixed oxides (MOX) ceramics is an increasingly important and attractive issue in the field of nuclear fuels. Using the molecular dynamics method, we study the creep behavior and creep mechanism transition of nanocrystalline (NC) MOX affected by temperature, stress, grain size, and Pu contents. It is found that the creep of NC MOX is dominated by diffusional creep and larger stress can activate the grain boundary sliding. At the Bredig transition temperature of MOX, its anionic sublattice pre-melts, which increases the creep rate due to intensified ions diffusion. Thus, the increase of temperature and stress leads to a transition in the creep mechanism from Coble creep with grain boundary sliding to Nabarro–Herring creep. Based on the method of analogy, we develop a theoretic model of the cationic vacancy concentration rate for high-temperature diffusional creep of NC MOX. This model can well predict the cationic vacancy concentration in the secondary creep stage. The findings of this study could not only give us a deep understanding of the creep behaviors of MOX ceramics at higher temperatures, but provide valuable insights in the variations of cationic vacancy concentration during steady-state creep stage of NC MOX.