FLiBe

It served both purposes in the Molten-Salt Reactor Experiment (MSRE) at the Oak Ridge National Laboratory.

[2] Its appearance is white to transparent, with crystalline grains in a solid state, morphing into a completely clear liquid upon melting.

This made spectrophotometry a viable analysis tool, and it was employed extensively during the MSRE operations.

BeF2, which behaves as a glass, is only fluid in salt mixtures containing enough molar percent of Lewis base.

This moisture plays a negative role at high temperature by converting BeF2, and to a lesser extent LiF, into an oxide or hydroxide through the reactions and While BeF2 is a very stable chemical compound, the formation of oxides, hydroxides, and hydrogen fluoride reduce the stability and inertness of the salt.

Notably, its optical transparency allows easy visual inspection of anything immersed in the coolant as well as any impurities dissolved in it.

Unlike sodium or potassium metals, which can also be used as high-temperature coolants, it does not violently react with air or water.

[8] The low atomic weight of lithium, beryllium and to a lesser extent fluorine make FLiBe an effective neutron moderator.

Fluorine has a non-negligible cross section for (α,n) reactions, which needs to be taken into account when calculating neutronics.

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.