An analytical model for the Amoeba effect in UO2 fuel pellets

Significant temperature gradients are present across the fuel particles when a nuclear reactor is producing power. If local temperatures are sufficiently high, nuclear fuel kernels can migrate up the thermal gradient. During this process, the barrier retaining fission product is progressively damaged, and the damage can lead to complete failure of the coating system. This phenomenon is called the Amoeba effect. In this model, the Amoeba effect is analyzed in terms of an interactive transport phenomenon between the solid-state diffusion of oxygen ions in a UO2 kernel and the flow of CO gas molecules through the pyrocarbon buffer layer surrounding the kernel. For mathematical simplicity, a cylindrically-shaped kernel is assumed. The results show that not only a concentration gradient in oxygen ions but also a temperature gradient with a negative heat of transport for oxygen ions can cause the Amoeba effect in UO2 fuel particles.