Purified solid FLiBe. (Source: Wikipedia.org)

Liquid salts, often fluoride, chloride, or nitrate, that can be used as a coolant in nuclear power plants, as a heat transfer fluid in thermal solar power plants, or as a medium in thermal storage. Molten salts are able to work at high temperatures at atmospheric pressure because they have a boiling point above 1,000 °C, are non-corrosive, and do not react with water or air, which makes their usage safe.

There are a number of nuclear reactors that use molten salts. Some (Advanced High-Temperature Reactor, AHTR) use them only for cooling, others (Molten Salt Reactor, MSR) dissolve fuel, uranium, or thorium in them. A typical salt used for nuclear reactors is a mixture of lithium fluoride (LiF) and beryllium fluoride (BeF₂) known as FLiBe.

In thermal storage, the most commonly used is a mixture of sodium nitrate and potassium nitrate. This mixture is also used as a heat transfer fluid in concentrating solar power plants.