Multiscale Modeling of Nucleation and Growth of Defect clusters in UO2

Microstructural evolution of UO2 due to irradiation damage at elevated temperatures affects both mechanical and thermal properties of UO2. Atomistic modeling plays an important role in understanding the dynamics underlying the accumulation of irradiation damage.;A multi-scale study has been carried out to understand nucleation and growth of clusters in irradiated UO2. The size cluster distribution and cluster compositions were the focus of investigations, specially the nature of the off-stoichiometric clusters formed.;A novel Cluster Dynamics (CD) model that describes the nucleation and evolution of defect clusters in multi-component systems has been developed. The model has been used to predict clustering of vacancies and interstitials into voids and dislocation loops, respectively, in irradiated UO2. The model reproduces well a range of experimental data on nucleation and growth behavior and its temperature dependence. A very important feature of this model is its ability to predict the off-stoichiometry of defect clusters, allowing, in turn, for the tracking of off-stoichiometry of the matrix. The effect of migration energy of point defects on the concentration and average size of voids has been studied. Also, the effect of irradiation conditions such as irradiation temperature, irradiation dose rate and irradiation dose on clusters concentration and composition has been investigated.;Preliminary results from this model show that Frenkel defects, as opposed to Schottky defects, dominate the nucleation process in irradiated UO 2. Vacancy clusters tend to grow mainly by absorbing oxygen vacancies and the migration energy of uranium vacancies is the rate limiting energy in nucleation and growth of voids. The results also show that, in a stoichiometric UO2 under irradiation, vacancy clusters (voids) tend to have both hypo- and hyper- stoichiometric composition with a higher fraction of hyper-stoichiometric composition clusters. A hyper-stoichiometric cluster composition indicates that the matrix would become oxygen rich even if the initial state is perfectly stoichiometric.