Solution and Migration of Impurity Ions in UO2, U3O8 and Y2O3
Atomic scale computer simulation techniques based on the Born description of forces between ions, have been used to predict the solution and migration behaviour of impurities in ceramic materials.
Static defect simulation techniques based on the Mott-Littleton approximation are shown to be particularly effective models for the study of the radiologically important fission products iodine, ruthenium and caesium. The accommodation mechanisms of these species are predicted in both UO2±x and U3O8-z, as a function of stoichiometry. In U3O8-z many of the fission products occupy large complex traps, which can exist in several different configurations. Migration often relies on a complex mechanism whereby the fission product moves through a number of different defect cluster configurations. Essentially two types of mechanisms have been found. In one, migration of the fission product is controlled by slow uranium self-diffusion. In the other, fission product migration is mediated by oxygen vacancies. In the latter case, it is the fission product association with a trap site or its migration across a trap site which is the rate determining step.
As static defect simulations can only predict migration enthalpies but not absolute diffusion coefficients or pre-exponential terms, molecular dynamics techniques are also used to study the self-diffusion of intrinsic defects in UO2. Unfortunately this technique is only effective for fast diffusing species with activation energies which are smaller than approximately 1 eV. As such only oxygen migration could be considered.
Finally the effects of defect-defect interactions and defect clustering are studied in Y2O3. In particular, the dependence of solution behaviour upon defect ion radius has been considered. A large reduction of the solution energy can be observed as impurities form clusters with intrinsic defects and with each other via co-doping mechanisms.