Structure and motion of tritium and helium in the breeding-blanket-relevant liquid lithium-lead eutectic alloy: ab-initio molecular dynamics simulations.

The lead-rich liquid lithium-lead eutectic alloy, Li17Pb83, is one of the candidates to be used in breeding blanket modules of future fusion reactors where tritium breeding is essential to provide the necessary fuel for fusion. It has several properties that favour its use, such as the breeding capacity of Li, the neutron multiplication capacity of Pb, the ease of circulation for off-site tritium recovery, and the capability to, at least partly, refrigerate the system. It is therefore important to understand the properties of the tritium generated in the breeding reactions, and its effects on the hosting liquid. The same applies to helium nuclei that are generated in a 1:1 ratio to tritium in the breeding reactions. We have performed first principles molecular dynamics simulations to study the structural changes observed in liquid Li 17 Pb 83 when tritium or helium is added. In one set of simulations we have made calculations for several amounts of tritium, with molar concentrations ranging from 0.20 to 0.03, without any He atoms. In the other set of simulations we have included helium atoms with molar concentration 0.11, and no tritium. Tritium atoms are found to bind preferentially with Li, modifying substantially the Li-Li correlation functions. We also observe the presence of long-lasting di-tritium molecules when tritium concentration is not too low, which also tend to bind to Li atoms. The velocity autocorrelation functions of tritium, Li and Pb are evaluated, and analysed in order to obtain the corresponding vibrational properties of the different species. Helium atoms tend to aggregate together forming a cluster whose characteristics are reported, together with the correlation functions of He atoms with Li and Pb. The motion of He atoms within this cluster is found to be sub-diffusive, while an estimate of He mobility outside the aggregate is also given.