Irradiation-induced internal stresses in polycrystalline α-uranium: a mesoscale mechanical approach

Several advanced nuclear reactor designs incorporate metallic fuel; however, the initial stages of metallic fuel microstructure evolution have not been studied since the 1960s. The development of new fuels and fuel performance models can be greatly accelerated by understanding the physical mechanisms driving the early evolution of fuel microstructure during irradiation. In this work, a mesoscale model based on irradiation-induced growth strains of single crystal alpha-uranium is developed to understand the initial irradiation-induced deformation of polycrystalline alpha-uranium. Three-dimensional simulations on single crystal and polycrystalline alpha-uranium are performed to study irradiation-induced distortion and the development of internal stresses within the material. The elastic strain energy density, von Mises stress and hydrostatic stress are investigated to understand how plasticity and fracture may develop in the material. It is found that at extremely low burnups (e.g., 0.0005 atomic %, or 4.7 MWd/MTU), tensile internal stresses on the order of hundreds of megapascals develop on the grain boundaries and triple points, making either plastic deformation or intergranular cracking likely, depending on the strength of the grain boundaries. In addition, it is found that the application of external stress, even 100 MPa of hydrostatic compression, will not suppress the development of the tensile internal stresses, indicating that plastic flow or fracture will occur during irradiation regardless of other stresses that may arise in service.