Chemical diffusion in uranium dioxide – influence of defect interactions

The chemical diffusion coefficient of UO2+x was determined from electrical conductivity measurements performed during transient state, for departure from stoichiometry in the range 0<x<0.17 and for temperatures varying from 973 to 1673 K. It was found that D is a decreasing function of the departure from stoichiometry, while the oxygen diffusion coefficient reported in literature is an increasing function in the same range of departure from stoichiometry. For x<0.07, this behavior was attributed to the presence of the singly ionized (2:2:2)′ Willis defects evidenced by electrical conductivity measurements. The enthalpy of formation of these clusters amounts to ΔHf=−1.7±0.6 eV. For x⩾0.07, the decreasing function of D with x can be explained either by Willis defects α time ionized or more complex defect aggregates or via a dynamic exchange between mobile small defects and larger clusters or domains.