The chemical diffusion coefficient of UO2+x was determined from electrical conductivity measurements performed during transient state, for departures from stoichiometry of x = 0 to 0.17 and at 973 to 1673K. It was found that the chemical diffusion coefficient was a decreasing function of the departure from stoichiometry, while the reported O diffusion coefficient was an increasing function over 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 revealed by electrical conductivity measurements. The enthalpy of formation of these clusters amounted to -1.7eV. For x above 0.07, the decreasing function of the chemical diffusion coefficient with x could be explained either by Willis defects or more complex defect aggregates or via a dynamic exchange between mobile small defects and larger clusters or domains.

Chemical Diffusion in Uranium Dioxide – Influence of Defect Interactions. P.Ruello, G.Chirlesan, G.Petot-Ervas, C.Petot, L.Desgranges: Journal of Nuclear Materials, 2004, 325[2-3], 202-9