Theoretical study of the structural and thermodynamic properties of U-He compounds under high pressure

Uranium is considered as a very important nuclear energy material because of the huge amount of energy it releases. As the main product of the spontaneous decay of uranium, it is difficult for helium to react with uranium because of its chemical inertness. Therefore, bubbles will be formed inside uranium, which could greatly reduce the performance of uranium or cause safety problems. Additionally, nuclear materials are usually operated in an environment of high-temperature and high-pressure, so it is necessary to figure out the exact state of helium inside uranium under extreme conditions. Here, we explored the structural stability of the U-He system under high pressure and high temperature by using density functional theory calculations. Two metastable phases are found between 50 and 400 GPa: U 4 He with space group Fmmm and U 6 He with space group P 1&cmb.macr;. Both are metallic and adopt layered structures. Electron localization function calculation combined with charge density difference analysis indicates that there are covalent bonds between U and U atoms in both Fmmm -U 4 He and P 1&cmb.macr;-U 6 He. Regarding the elastic modulus of α-U, the addition of helium has certain influence on the mechanical properties of uranium. Besides, first-principles molecular dynamics simulations were carried out to study the dynamical behavior of Fmmm -U 4 He and P 1&cmb.macr;-U 6 He at high-temperature. It was found that Fmmm -U 4 He and P 1&cmb.macr;-U 6 He undergo one-dimensional superionic phase transitions at 150 GPa. Our study revealed the exotic structure of U-He compounds beyond the formation of bubbles under high-pressure and high-temperature, which might be relevant to the performance and safety issues of nuclear materials under extreme conditions. Besides the helium bubbles in uranium at ambient conditions, helium could exist as He 2 dimer and atomic configuration in Fmmm -U 4 He and P 1&cmb.a.line;-U 6 He under high pressure. Both Fmmm -U 4 He and P 1&cmb.a.line;-U 6 He exhibited one-dimensional superionic phase transitions.