FLiBe

FLiBe is a molten salt made from a mixture of lithium fluoride (LiF) and beryllium fluoride (BeF₂). It is both a nuclear reactor coolant and solvent for fertile or fissile material. It served both purposes in the Molten-Salt Reactor Experiment (MSRE).

The 2:1 mixture forms a stoichiometric compound, Li₂BeF₄, which has a melting point of 459 °C, a boiling point of 1430 °C, and a density of 1.94 g/cm³. Its volumetric heat capacity is 4540 kJ/m³K, which is similar to that of water, more than four times that of sodium, and more than 200 times that of helium at typical reactor conditions. Its appearance is white to transparent, with crystalline grains in a solid state, morphing into a completely clear liquid upon melting. However, soluble fluorides such as UF₄ and NiF₂, can dramatically change the color salt in both solid and liquid state. This made spectrophotometry a viable analysis tool, and it was employed extensively during the MSRE Operations

The eutectic mixture is slightly greater than 50% BeF₂ and has a melting point of 360 °C. This mixture was never used in practice due to the overwhelming increase in viscosity caused by the BeF₂ addition in the eutectic mixture. BeF₂, which behaves as a glass, is only fluid in salt mixtures containing enough molar percent of Lewis base. Lewis bases, such as the alkali fluorides, will donate fluoride ions to the beryllium, breaking the glassy bonds which increase viscosity. In FLiBe, beryllium fluoride is able to sequester two fluoride ions from two lithium fluorides in a liquid state, converting it into the tetrafluorberyllate ion BeF₄⁻².

The chemistry of FLiBe, and other fluoride salts, is unique due to the high temperatures at which the reactions occur, the ionic nature of the salt, and the reversibility of many of the reactions. At the most basic level, FLiBe melts and complexes itself through

${\displaystyle 2LiF_{(s)}+BeF_{2(s)}\longrightarrow 2Li_{(l)}^{+}+BeF_{4(l)}^{-2}}$.

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