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Molten FLiBe flowing; this sample's green tint is from dissolved uranium tetrafluoride.

FLiBe is a mixture of lithium fluoride (LiF) and beryllium fluoride (BeF2). As a molten salt it is proposed as a nuclear reactor coolant, and two different mixtures were used in the Molten-Salt Reactor Experiment (MSRE).

The 2:1 mixture with proportions Li2BeF4 has a melting point of 459°C, a boiling point of 1430°C, and a density of 1.94 g/cm3. Its heat capacity is 4540 kJ/m3, 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).[1]

The eutectic mixture is slightly greater than 50% BeF2 and has a melting point of 360°C.[2]


As a molten salt it can serve as a coolant which can be used at high temperatures without reaching a high vapor pressure. Unlike sodium or potassium metals, which can also be used as high-temperature coolants, it does not violently react with air or water. FLiBe salt has low hygroscopy and solubility in water.[3]

Purified FLiBe. Originally ran in the secondary loop of the MSRE.

Nuclear properties[edit]

Ampoules of FLiBe with uranium-233 tetrafluoride: solidified chunks contrasted with the molten liquid.

The low atomic weight of lithium, beryllium and to a lesser extent fluorine make FLiBe an effective neutron moderator. As natural lithium contains ~7.5% lithium-6, which tends to absorb neutrons producing alpha particles and tritium, nearly pure lithium-7 is used to give the FLiBe a small cross section;[4] e.g. the MSRE secondary coolant was 99.993% lithium-7 FLiBe.[5]

Beryllium will occasionally disintegrate into two alpha particles and a neutron when hit by a fast neutron.


In the liquid fluoride thorium reactor (LFTR) it serves as solvent for the fissile and fertile material fluoride salts, as well as moderator and coolant.

Some other designs (sometimes called molten-salt cooled reactors) use it as coolant, but have conventional solid nuclear fuel instead of dissolving it in the molten salt.

See also[edit]