Grain-scale study of the grain boundary effect on UO2 fuel oxidation and fission gas release under reactor conditions

Abstract

In a nuclear reactor, when the fuel cladding fails, the water/steam (H2O) will enter the fuel rod and react with the fuel producing UO2+x and hydrogen (H2). This paper investigates the oxidation of Uranium Dioxide (UO2) and its effect on fuel behaviors under reactor conditions. A grain-scale reactive transport model is applied to simulate the chemical reactions between UO2 and H2O. Coupled thermal conduction, mass transfer and chemical reactions are considered in the model. The complex physicochemical processes are simulated on the microstructures derived from real images of UO2. The effect of grain boundary and grain size on the temperature drop across the fuel is first studied and the relationship between the grain boundary density and temperature drop is revealed. Simulations of oxidation are then conducted with different grain boundaries and sizes. The impact of oxidation on temperature drop and fuel effective thermal conductivity is investigated. Simulation results demonstrate that fuel oxidation results in higher temperature drops by lowering the effective fuel thermal conductivity. Studies on the effect of grain boundary diffusivity on oxidation are also carried out. Our results show that higher grain boundary diffusivity can lead to faster oxidation in fuel. Oxidation effect on the fission gas diffusion in the fuel is also studied. It is found that oxidation enhances the fission gas (Xenon) diffusion in the fuel. In a finer-grained fuel, the concentration of fission gas can increase up to 40% due to fuel oxidation. The results indicate that accounting for the effects of grain boundary density and diffusivity is imperative for accurately predicting fuel oxidation and fission gas release for nuclear fuel at microscale. The presented model provides a numerical tool to quantitatively analyze the effects of fuel microstructure on fuel behaviors under reactor conditions.