FLiBe is a molten fluoride salt of lithium and beryllium, used in 📝fusion blanket designs as coolant and tritium breeder in a single fluid; the name is a coinage spelled from its chemical constituents — fluorine, lithium, beryllium.
FLiBe is not a fusion invention: the salt was developed in the 1950s and 1960s for the molten-salt fission reactor program at Oak Ridge National Laboratory, whose Molten Salt Reactor Experiment ran on fluoride salts of the same family. Fusion adopted it because one liquid happens to carry the blanket's whole job description. The 📝lithium in the salt breeds 📝tritium when fusion neutrons strike it; the 📝beryllium acts as a 📝neutron multiplier, supplying the neutron surplus breeding requires; and the salt itself absorbs the neutrons' energy as heat and carries it to the power cycle. Its low vapor pressure at high temperature and its chemical stability distinguish it from water or liquid-metal coolants, at the cost of a high melting point that keeps the whole loop above roughly 460 degrees Celsius.
A liquid 📝blanket of FLiBe can surround a fusion core with few structures for neutrons to damage, which is why the salt anchors several liquid-immersion blanket concepts.