  Uranium is considered as a very important nuclear energy material because of
the huge amount of energy released. As the main products of spontaneous decay
of uranium, helium is difficult to react with uranium for its chemical
inertness. Therefore, bubbles will be formed inside uranium, which could
greatly reduce the performance of uranium or cause the 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 at extreme conditions. Here, we explored the
structural stability of 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: U4He with space group Fmmm and U6He with space group
P-1. Both are metallic and adopt layered structures. Electron localization
function calculation combined with charge density difference analysis indicate
that there are covalent bonds between U and U atoms in both Fmmm-U4He and
P-1-U6He. Compared with the elastic modulus of ${\alpha}$-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-U4He and P-1-U6He at high-temperature. It is found
that Fmmm-U4He and P-1-U6He undergo one-dimensional superionic phase
transitions at 150 GPa. Our study revealed exotic structure of U-He compounds
beyond the form of bubble under high-pressure and high-temperature, that might
be relevant to the performance and safety issue of nuclear materials at extreme
conditions.
