Modeling of the vacancy diffusion and dislocation motion induced mesoscale-macroscale creep deformations of dense polycrystalline UO2 under irradiation and high temperature conditions

As a competitive nuclear fuel, UO₂ is widely applicable in various types of nuclear reactors, including both the current and next-generation reactors. Under extreme irradiation conditions, irradiation creep and thermal creep deformations occur within the fuel, significantly influencing the thermo-mechanical behavior evolution of fuel elements and the service safety of nuclear reactors. To predict the macroscale creep deformations and reveal the underlying mechanism for polycrystalline nuclear fuels, the three-dimensional governing equations are established to describe the multi-field coupling behaviors of vacancy diffusion, dislocation motions and the associated mechanical deformations. The corresponding numerical algorithms and codes for the multi-field coupling calculations are developed to simulate the tensile creep tests for polycrystalline UO₂ under different conditions. The good agreement between the predicted macroscale creep rates and various experimental data validates the effectiveness of the developed models and algorithms. The influences of applied stress, temperature, fission rate and grain size on the multi-scale creep behaviors and dominant mechanisms are analyzed. The research results indicate that: (1) the differing stress-related vacancy equilibrium concentrations at various grain-boundary regions lead to the vacancy flux from the tensile boundaries to other boundaries, resulting in the macroscale elongation along the uniaxial creep tension direction; (2) the irradiation-enhanced diffusion coefficient and grain-boundary vacancy equilibrium concentration are responsible for the increased diffusional creep contributions at the temperatures lower than 1200 K, while the fuel fission rates have a minor impact on dislocation creep contributions within the intermediate temperature range from 1000 K to 1400 K; (3) within the low-temperature range below 1000 K, the macroscopic creep deformations of polycrystalline UO₂ are dominantly attributed to irradiation-induced diffusional creep; at temperatures above 1000 K, the creep mechanism becomes more complex and varies with stress, fission rate and grain size; (4) the fuel fission rate affects the transition temperature from irradiation creep to thermal creep; meanwhile, the grain size influences the activation temperature and the stress required to initiate dislocation annihilation-induced creep. This study offers an effective approach to predict the macroscale creep deformations and elucidate the underlying mechanism for nuclear fuels.