Oxidation kinetics of SPS-densified U3Si2 fuels—Microstructure impact

Abstract

U3Si2 is a potential candidate for accident tolerant fuels because of its high uranium density and excellent thermal conductivity in comparison to UO2. However, U3Si2 suffers from oxidation, steam corrosion, and subsequent disintegration/pulverization. The detailed investigation of kinetics that incorporates fundamental treatment of oxidation of U3Si2 is scarcely reported, and the oxidation mechanisms have not been fully elucidated. In this paper, the oxidation behavior of microcrystalline (mc-) and nanocrystalline (nc-) U3Si2 have been systematically investigated using a thermogravimetric analysis (TGA) apparatus through a series of isothermal and non-isothermal kinetic studies. The isothermal kinetic study with a model-fitting approach indicates oxidation activation energy of 85 kJ/mol for dense mc-U3Si2 and 96.4 kJ/mol for nc-U3Si2 pellets, while the isoconversional approach leads to an activation energy in the range of 70–85 kJ/mol for mc-U3Si2 and 75–86 kJ/mol for nc-U3Si2 with three most common model-free methods, including Kissinger–Akahira–Sunose, Flynn–Wall–Ozawa, and Friedman methods. The derivation of oxidation activation energies using both isothermal and isoconversional methods highlights the approach to evaluate the oxidation resistance of nuclear materials using TGA quantitatively and makes it possible to compare among various nuclear fuels.