FLiBe

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FLiBe is a molten salt made from lithium fluoride (LiF) and beryllium fluoride (BeF2) in a 2:1 ratio. This mixture forms Li2BeF4 and melts at about 459°C, boils around 1,430°C, and has a density of roughly 1.94 g/cm3. It holds a high volumetric heat capacity comparable to water and a specific heat about 60% that of water. In solid form it is white to transparent crystals and becomes a clear liquid when melted.

FLiBe can act as both a nuclear reactor coolant and a solvent for fertile or fissile materials. It was used this way in the Molten-Salt Reactor Experiment (MSRE) at Oak Ridge National Laboratory.

There is a slightly BeF2-rich eutectic that would melt at about 360°C, but it was not used because it becomes too viscous. BeF2 tends to form a glass unless enough Lewis-base fluorides are present to keep the mixture fluid. In FLiBe, BeF2 helps form the [BeF4]2− complex.

The chemistry of FLiBe is active at high temperatures and many reactions are reversible. If the salt is exposed to air, moisture can form oxides or hydroxides that promote corrosion. The salt’s redox potential governs which reactions occur, so materials in contact with FLiBe must be chosen to stay far from unfavorable redox conditions. Potentially corrosive fluorination reactions are a concern, so alloys like nickel, iron, chromium, molybdenum, tungsten, platinum, iridium, and carbon are considered. Hastelloy-N was a common corrosion-resistant material used in the MSRE.

Redox potential can be adjusted by applying a voltage with an inert electrode or by chemical methods such as sparging hydrogen and hydrogen fluoride into the salt, or by dipping a metal into it.

As a molten salt, FLiBe can serve as a high-temperature coolant without building up high vapor pressure, and its transparency allows easy visual inspection of contents and impurities. It does not react violently with air or water, unlike some metal coolants. It also has low hygroscopicity and low solubility in water.

FLiBe’s light elements (lithium, beryllium, fluorine) make it a good neutron moderator. To reduce neutron absorption by lithium, nearly pure Li-7 is used in reactor cooling, which lowers unwanted neutron capture and tritium production.

Beyond reactor use, FLiBe can dissolve fluoride salts of fissile and fertile materials (as in LFTR designs) and can serve as a coolant in molten-salt reactors. It has been proposed as a liquid blanket for tritium production and cooling in fusion concepts like MIT’s ARC reactor.