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

摘要

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 U3S1 and U_3Si_2 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 U3S1, U_3Si_2, US_1, and U_3Si_5 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 UO_2 The impact of the high temperature phase transformation in U_3Si_5 will also be discussed in relation to operating conditions in an LWR Finally, an overview of remaining challenges that exist in the U-S_1 system will be presented.