Formation and growth of irradiation-induced defect structures in ceria

Radiation damage effects are of primary concern for materials used in nuclear energy production. In this study, emphasis was given to the processes of formation and growth of radiation-induced defect structures in oxide fuels. Due to the natural complexity of oxide fuels, which consist of both a metal sublattice and an oxygen sublattice, radiation effects are much more complex in oxides than in metals. As a result, there are many radiation effects that are still not well understood despite numerous research efforts engaged in the past. This study was aimed to help clarify some of these effects, such as the evolution process of dislocation structures during irradiation and how it is affected by various irradiation conditions. In order to develop an understanding of the radiation damage process in the common fluorite-type ceramic oxide fuel, ceria (CeO2) was selected as a surrogate material of UO2 for this study. According to previous studies, ceramic materials with a fluorite crystal structure possess high radiation tolerance. Using CeO2 single crystals allowed for the observation of the intrinsic behavior of defects while excludes the effects of grain boundaries. To reveal the basic mechanisms responsible for the evolution of microstructure induced by irradiations, a group of coordinated experiments were designed by incorporating multiple techniques consisting of ion irradiation, in situ transmission electron microscopy (TEM) and ex situ TEM observation. Radiation damage in the materials was induced by irradiating them with krypton and xenon ions from an accelerator. Irradiation experiments were conducted at three temperature regimes: room temperature, 600°C and 800°C, in order to inspect the temperature dependence of atomic defect transportation. Ion energies were carefully chosen for low and high energy irradiations in order to produce a deposited ion peak within the specimen at low energy and a uniform distribution of defects at high energy. In situ TEM analysis was used in order to take advantage of real-time recording of defect nucleation and growth under gas ion irradiation, and ex situ TEM analysis was used to characterize the radiation-induced features at high image resolution along with complementary elemental analysis techniques such as X-ray energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS). In addition to the experimental investigation, a rate theory model, as a part of the multi-scale simulation approach, was employed to study the growth behaviors of dislocation loops. The computational results were found to be consistent with the experimental observations