Kr and Xe irradiations in lanthanum (La) doped ceria: Study at the high dose regime

In order to understand cavity and bubble formation and growth in oxide nuclear fuel materials, ion beam irradiation experiments were conducted with two common fission gas species: Kr and Xe. Ceria (CeO2) was selected as a surrogate material for uranium dioxide (UO2) due to its many similar properties to UO2. Ion beam energies were chosen such that both cavities and gas bubbles structures were induced by ion irradiations. The ion irradiation experiments were carried out at 600 °C, at which temperature, cavity/gas bubble structures are believed to be immobile in this material. Lanthanum (La) was chosen as a dopant in CeO2 to investigate the effect of impurities. The presence of La in the CeO2 lattice also introduces a predictable initial concentration of oxygen vacancies, similar to the introduction of oxygen vacancies by the existence of Pu3+ in MOX fuel [1]. The influence of two La concentrations, 5% and 25%, were examined.The study focused on the high dose regime where cavity/gas bubble structures were clearly identifiable with their sizes and number densities readily measurable. Cavity/gas bubble coarsening by coalescence was identified with TEM (Transmission Electron Microscopy) characterizations of as-irradiated La doped CeO2 specimens. The results revealed that lanthanum trapping has significant influence on the cavity/bubble growth in the material lattice by comparing the cavity/gas bubble size distributions between 5% La doped ceria and 25% La doped ceria. Lattice and kinetic Monte Carlo calculations described in a previous work have provided insights to the interpretations of the experimental results [2].Solid state Xe precipitates were observed in low energy Xe implantation in 5% La doped ceria to a very high fluence of 1 × 1017 ions/cm2 at 600 °C. The solid state Xe precipitate structures are represented by faceted morphology. Very similar observations of solid state/near solid state Xe bubbles were made by Nogita et al. in the outer region of UO2 pellet irradiated to a pellet average burnup of 49 GWd/t [3].

► Xe and Kr bubble formation and growth were studied in La doped CeO2 by ion irradiation.
► Evolution of cavity/gas bubble has two stages: an initial nucleation stage and a coarsening stage.
► La dopant was shown to play an important role in influencing the cavity/gas bubble growth behaviors.
► Solid state Xe precipitates were observed in irradiated 5% La doped CeO2 at very high dose.