First-principles study of effect of ideal tensile/shear strain on chemical bond length and charge density distribution of U<sub>3</sub>Si<sub>2</sub>
Authors: Wang Kun , Qiao Ying-Jie , Zhang Xiao-Hong , Wang Xiao-Dong , Zheng Ting , Bai Cheng-Ying , Zhang Yi-Ming , Du Shi-Yu ,   College of Material Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China,   Engineering Laboratory of Advanced Energy Materials, Ningbo Institute of Materials Engineering and Technology, Chinese Academy of Sciences, Ningbo 315201, China
Published: August 2022

After the Fukushima nuclear accident in 2011, U<sub>3</sub>Si<sub>2</sub> was predicted to be an important accident tolerant fuel that can replace UO<sub>2</sub>. The results of recent studies have shown that the simulation at the micro-scale of U<sub>3</sub>Si<sub>2</sub> 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 U<sub>3</sub>Si<sub>2</sub>. Therefore, employing the first principles to calculate some physicochemical data of U<sub>3</sub>Si<sub>2</sub> nuclear fuel has received extensive attention. In previous work, we predicted the ideal strength of U<sub>3</sub>Si<sub>2</sub> in several low-index crystal planes/directions by the first-principles computational tensile/shear test (FPCTT/FPCST) approach. However, the fracture behavior of U<sub>3</sub>Si<sub>2</sub> 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 U<sub>3</sub>Si<sub>2</sub> are discussed to analyze the fracture behaviors of U<sub>3</sub>Si<sub>2</sub> 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 U<sub>3</sub>Si<sub>2</sub> 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 U<sub>3</sub>Si<sub>2</sub> 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 U<sub>3</sub>Si<sub>2</sub> in the [100] crystal direction under tensile load. The toughness of U<sub>3</sub>Si<sub>2</sub> 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 U<sub>3</sub>Si<sub>2</sub> is mainly related to the elongation of the Si—Si bond. In the (100)[010] slip system, U<sub>3</sub>Si<sub>2</sub> 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 U<sub>3</sub>Si<sub>2</sub>, 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 U<sub>3</sub>Si<sub>2</sub>.