Fission gas transport and its interactions with irradiation-induced defects in lanthanum doped ceria

Combined experimental and modeling efforts have been extremely productive in understanding irradiation-induced displacement damage in metal and metal alloy systems. In order to help understand the fundamental mechanisms of irradiation-induced defect formation and evolution in nuclear fuel, similar combined modeling and experimental efforts have been carried out. Ceria (CeO2) was selected as a surrogate material for Uranium Dioxide (UO2) due to its many similar properties. Lanthanum (La) was chosen as a dopant in CeO2 to investigate the effect of impurities in a controlled manner. The presence of La in the CeO2 lattice introduces a predictable initial concentration of oxygen vacancies, making it possible to characterize hypo-stoichiometric effects in CeO2. The influence of two La concentrations, 5% and 25%, were examined. Radiation damage was induced using low energy ion implantations and high energy ion irradiation experiments, where the ion beam energy was selected for high displacement damage levels and/or high levels of implanted Xe or Kr. A combination of in situ TEM (Transmission Electron Microscopy), ex situ TEM experiments and X-ray related techniques were used to study the evolution of defect clusters and the influence of two common fission products, Xe and Kr. The irradiations were performed on thin film, single crystal materials so that the material composition and crystallinity could be directly controlled. The irradiation damage caused the formation of complex microstructures with dislocation loops, voids or bubbles, and dislocation networks at higher doses. The Burgers vectors of the dislocation loops were determined and the loops were found to be mainly [111] type Burgers vector pure edge loops. They have been tentatively identified as interstitial type. La, as an impurity, has revealed a strong defect trapping effect. Various sets of quantitative experimental results were obtained to characterize the dose and temperature effects of irradiation. These results also help to benchmark simulation codes being developed with a kinetic Monte Carlo model. These experimental results include size and size distributions of dislocation loops, voids and gas bubble structures created by irradiation. More importantly, this systematic experimental work has provided key insights into the understanding of the mechanisms of defect evolution in the materials investigated. A model including both defect production and annihilation mechanisms has been proposed to explain the observed defect kinetics in the lower dose regime. A coalescence driven model has been proposed for void/bubble growth in the higher dose regime. Experimental results also revealed that lanthanum trapping has significant influence on the void/bubble growth in the CeO2 lattice. Lattice and kinetic Monte Carlo calculations have provided key insights to the interpretations of experimental results