Microstructural response of the fuel phase in U-7Mo dispersion fuel irradiated at different powers

Abstract Irradiation conditions such as fission power, fission rates, and temperature are important parameters for nuclear fuels because they influence the microstructural behavior and ultimate irradiation performance. Two U-7Mo/Al-3.5 wt.% Si dispersion fuel plates were irradiated in the Advanced Test Reactor edge-on to the core in the RERTR-9B experiment at different powers to study the effects of power on fuel phase swelling behavior. The results of non-destructive measurements have been reported in a previous paper [1] , and the destructive examination results are being reported here. In particular, the microstructural evolution of the U-Mo fuel phase in irradiated fuel plates has been investigated by cutting transverse cross-sections, using generated scanning electron microscopy micrographs, and image processing methods to assess the changes in microstructural features, such as fission gas pore growth and non-recrystallized grain fraction. In this study, the focus was to examine the evolution of fission gas pores from both the constrained and unconstrained regions of U-Mo fuels irradiated at different powers. Based on an estimated linear gradient, the rate of change of fission gas pore size with locally calculated fission densities is more prominent in the higher power specimen. As such, the average porosity and pore area is approximately 11% and 2 × larger in the fuel irradiated at higher power than that of the lower power specimen. The growth of fission gas pores can accelerate the pore interconnectivity and thus fission gas outside the fuel kernel (FGO), which are two precursors for swelling in U-Mo fuels. The influence of increased power, internal plate stresses, and fission-induced creep on fuel kernel deformation and lateral mass transfer is also highlighted.