The mechanical and thermal properties of (Th,U)Si compounds: A systematic density functional theory research

Abstract Thorium-based materials have been utilized as candidate in Generation-IV nuclear reactors. The thermo-mechanical properties of traditional thorium-based nuclear fuels may not satisfy the operating requirements of the nuclear reactor. For example, the thermal conductivity of ThO2 decreases to 3 Wm−1K−1 when the temperature reaches 1200 K, which may not be compatible with the nuclear reactors. ThC is found to be unstable at zero pressure while a phase transition will happen under high pressure. Hence it is necessary to explore the potential (Th, U)Si ternary nuclear fuel. In this study, not only the mechanical and thermal properties of the Th-Si compounds were calculated, but also the structures and thermo-mechanical properties of (Th, U)Si ternary compounds constructed by replacing Th atoms with U atoms in the binary compounds, are predicted through first principles. The Th-Si and (Th, U)Si compounds are both ductile with high Young's modulus (130GPa). The configuration of Th3Si2 has the strongest mechanical isotropy in binary and ternary compounds, while the mechanical isotropy of Th4Si4U2 decreases by substituting Th atoms in Th3Si2(Th6Si4) with U atoms. Additionally, the mechanical isotropy of Th2Si4U2 is elevated dramatically by replacing Th atoms in ThSi(Th4Si4) with U atoms. Lastly, Th3Si2 and its derived ternary compound Th5Si4U were selected to explore the effect of atomic substitution on thermal performance. Specifically, U-substituted Th5Si4U displays isotropy on lattice thermal conductivity which is similar to Th3Si2(Th6Si4), but it will reduce the electron thermal conductivity after replacing Th with U atom.