First-principles study of effect of ideal tensile/shear strain on chemical bond length and charge density distribution of U₃Si₂

Abstract

After the Fukushima nuclear accident in 2011, U₃Si₂ was predicted to be an important accident tolerant fuel that can replace UO₂. The results of recent studies have shown that the simulation at the micro-scale of U₃Si₂ serving as a candidate for accident tolerant fuel is not deep enough. It is not sufficient to build fuel databases and models at a macro-scale to effectively predict some properties of U₃Si₂. Therefore, employing the first principles to calculate some physicochemical data of U₃Si₂ nuclear fuel has received extensive attention. In previous work, we predicted the ideal strength of U₃Si₂ in several low-index crystal planes/directions by the first-principles computational tensile/shear test (FPCTT/FPCST) approach. However, the fracture behavior of U₃Si₂ has not been explained much. Therefore, in this work, the effects of ideal tensile/shear strain on the chemical bond length and charge density distribution of U₃Si₂ are discussed to analyze the fracture behaviors of U₃Si₂ in these low-index crystal planes/directions. The effect of strain is achieved by using the incremental simulation elements in the specified crystal plane/direction. The crystal structures of U₃Si₂ under different strains are optimized by using the first principles based on density functional theory. The variation ranges of chemical bond length and the charge density distributions of U₃Si₂ under different ultimate strains are summarized and calculated respectively. The results show that the elongation of the U—U bond is the main contributor to the tensile deformation of U₃Si₂ in the [100] crystal direction under tensile load. The toughness of U₃Si₂ in the [001] crystal direction is mainly due to the elongation of the U—Si bond and U—U bond. However, the tensile deformation produced in the [110] crystal direction of U₃Si₂ is mainly related to the elongation of the Si—Si bond. In the (100)[010] slip system, U₃Si₂ has great deformation and the crystal breaks when the Si—Si bond length reaches a limit of 3.038 Å. For the (001)[100], (110)[<inline-formula><tex-math id="M1">\begin{document}$ \bar 1 $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20221210_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20221210_M1.png"/></alternatives></inline-formula>10] and (001)[110] slip systems of U₃Si₂, the crystal is broken under small shear deformation, and the change of its bond length is not obvious, reflecting that the sudden decrease of the strain energy or stress in these several slip systems may be related to the strain-induced structural phase transition of U₃Si₂.