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).
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.
 Nuclear properties
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; e.g. the MSRE secondary coolant was 99.993% lithium-7 FLiBe.
Beryllium will occasionally disintegrate into two alpha particles and a neutron when hit by a fast neutron.
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
- http://www.ornl.gov/~webworks/cppr/y2001/pres/122842.pdf CORE PHYSICS CHARACTERISTICS AND ISSUES FOR THE ADVANCED HIGH-TEMPERATURE REACTOR (AHTR), Ingersoll, Parma, Forsberg, and Renier, ORNL and Sandia National Laboratory
- http://www.fusion.ucla.edu/apex/meeting4/5sze0798.pdf FLIBE: WHAT DO WE KNOW?, Sze and Wang, 1998, Argonne National Laboratory
- Engineering Database of Liquid Salt Thermophysical and Thermochemical Properties
- The Pea and the Beach-Ball
- In Czech: ORNL part of nuclear R&D pact