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.
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.
- 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