FLiBe

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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]

Coolant[edit]

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.

Applications[edit]

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]

References[edit]