State of knowledge and challenges of U-Si compounds for use in light water reactor accident tolerant fuel designs

Research in uranium silicide binary allows has been renewed in recent years for use in light water reactor accident tolerant fuel concepts due to the high thermal conductivity and hypothesized oxidation tolerance of these compounds. The enhanced thermal conductivity would decrease the radial fuel temperature profile, thus decreasing the thermal stresses observed in the system. However, research on binary U-Si compounds has historically focused on high fissile density compounds U₃Si and U₃Si₂ in support of the RERTR campaign. The properties and microstructures in these studies had a large degree of variability, which required a complete reassessment if these fuels were to be considered for further irradiation testing. Experiments at LANL have emphasized fabrication of nearly stoichiometric, low oxygen impurity content specimens of U₃Si, U₃Si₂, USi, and U₃Si₅ for thermophysical properties and oxidation testing to aid modeling and simulation communities with accurate data to predict how these novel fuels will behave in-pile. However, many unknowns still remain in the system. Only a handful of the U-Si compounds have had any irradiation testing, which have shown some degree of amorphization and irregular pore shapes making the swelling behavior difficult to predict. Fission product silicides have also had limited characterization in the literature with regards to thermodynamics and physical properties. Likewise, the impact of phase transformations and phase stability near USi_2-x as a function of temperature has largely been overlooked in the literature. This paper will look at the to-date properties of U-Si compounds as a function of temperature to the respective melt point of the phase in relation to the reference LWR fuel UO₂. The impact of the high temperature phase transformation in U₃Si₅ will also be discussed in relation to operating conditions in an LWR. Finally, an overview of remaining challenges that exist in the U-Si system will be presented.