Radiation Damage on Multiple Length Scales in Uranium Dioxide

Radiation damage in UO2 has been well studied but there exists little correlation between point defect accumulation, lattice structure changes and microstructure. This is partly because irradiated nuclear fuel is highly radioactive and its defect chemistry is extremely complicated resulting from fission of the material and consequent fission products being embedded in the fuel matrix [Olander1976]. To adequately study the evolution of defects from point defects through to microstructure features, the resulting defects have to be intentionally simplified for characterization. Ion accelerators have the unique capability of creating simple microstructure features using specific ions, without the added complication of fission and neutron activation from nuclear reactors. As an example, H+ ions have been used to create (only) a distribution of dislocations that were studied using various techniques. The ability to tune the energy or type of the ion to achieve desirable implantation depth and ideally simple microstructure renders it a lucrative instrument for this type of analysis.;X-ray diffraction (XRD) studies and transmission electron microscopy (TEM) have been utilized to study extended structure changes and microstructure evolution. Ion beam irradiations create displacements and displacement networks, voids, surface fracturing, gas bubbles and several other microstructure changes to model nuclear reactor damage [Noris1972]. Using an ion accelerator, it has been possible to isolate these radiation induced defects and study their subsequent evolution with increasing dose. Insofar, since all of the phenomena caused by radiation damage originate from point defects, the elucidation of radiation effects on the atomic scale is crucial. This is rendered complicated due to aperiodic irradiation defects. This lack of periodicity renders standard approaches, such as TEM and XRD ineffective, as these methods probe average structure over tens of Angstroms. Therefore, techniques that are sensitive to short range order are required to understand the defect detail on atomic scale. X-ray absorption fine structure spectroscopy (XAFS) measures the population-weighted local structure and chemical speciation of the examined elements making it perhaps the most incisive method for determining the local range order in irradiated materials. In this study, Extended X-ray Absorption Fine Structure (EXAFS) measurements have shed significant insight into the local chemistry evolution from irradiation damage in UO2. With the aforementioned defect characterization techniques, this project has developed three significant scientific conclusions:;1. Irradiation of UO2 creates changes with some similarity to oxidation, but the increase in lattice parameter (as compared to oxidation) indicates differences in microstructure.;2. TEM invisible defect clusters are predominantly responsible for lattice structure changes in UO2, confirmed using EXAFS and Cluster Dynamics simulation.;3. The combination of EXAFS, Cluster Dynamics, CrystalMaker Analysis, MD and Raman measurements support the existence of hyper and hypo-stoichiometric regions in the material after irradiations.;4. The lattice parameter, the microstructure visible to TEM and the lattice structure invisible to TEM all scale with DPA.