  Fusion energy stands out as a promising alternative for a future decarbonised
energy system. To be sustainable, future fusion nuclear reactors will have to
produce their own tritium. In the so-called breeding blanket of a reactor, the
neutron bombardment of lithium will produce the desired tritium, but also
helium, which can trigger nucleation mechanisms owing to the very low
solubility of helium in liquid metals. An understanding of the underlying
microscopic processes is important for improving the efficiency, sustainability
and reliability of the fusion energy conversion process. A spontaneous creation
of helium drops or bubbles in the liquid metal used as breeding material in
some designs may be a serious issue for the performance of the breeding
blankets. This phenomenon has yet to be fully studied and understood. This work
aims to provide some insight on the behavior of lithium and helium mixtures at
experimentally corresponding operating conditions (843 K and pressures between
0.1 and 7 GPa). We report a microscopic study of the thermodynamic, structural
and dynamical properties of lithium-helium mixtures, as a first step to the
simulation of the environment in a nuclear fusion power plant. We introduce a
microscopic model devised to describe the formation of helium drops in the
thermodynamic range considered. A transition from a miscible homogeneous
mixture to a phase-separated one, in which helium drops are nucleated, is
observed as the pressure is increased above 0.175 GPa. The diffusion
coefficient of lithium (2 {\AA} 2 /ps) is in excellent agreement with reference
experimental data, whereas the diffusion coefficient of helium is in the range
of 1 {\AA} 2 /ps and tends to decrease as pressure increases. The radii of
helium drops have been found to be between 1 and 2 {\AA}.
