Nucleation of helium in liquid lithium

Abstract

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}.