Investigation of swelling behaviors of U-10Zr metallic fuel in the low temperature regime via a cavitational void swelling model

The uranium zirconium (U-Zr) metallic nuclear fuel has long been considered as a promising candidate fuel type for fast breeder reactors. One of the key performance issues of the U-10Zr metallic fuel is its rather strong swelling, which would lead to significant fuel cladding mechanical interaction (FCMI) at relatively high fuel burn-ups. It is therefore imperative to understand the fuel swelling behaviors and the underlying physics mechanisms. Recently published experimental results of U-10Zr fuel swelling behaviors in a low temperature regime (400-600K) showed perceivable swelling of the in-pile irradiated fuel. However, the underlying physics mechanism behind this swelling is not clear. In this work, we performed in-depth analyses on the microstructural features of the irradiated fuel with metallography and conducted a finite-element based simulation on a computational platform, COMSOL, to determine the hydrostatic stress in the irradiated fuel. The spherical objects in the metallography result have been identified as cavities with relatively low fission gas pressure instead of equilibrium gas bubbles. A rate theory code based on the cavitational void swelling model has been developed and the fuel swelling behaviors of U-10Zr fuel in the low temperature regime have been assessed. The simulation results suggest a much lower migration energy for vacancy diffusion in the metallic fuel compared to what is currently used, and a new set of key parameters for the rate theory model were determined.