Its fuel is dissolved in fluoride coolant salt typically consisting of some of lithium, beryllium, sodium or potassium fluorides. Composition is chosen according to desired properties like viscosity or melting point (usually the mixture has lower melting point than a single salt).

    This mixture has not only low reactivity with air and water but it’s also very stable under intense irradiation moreover it allows high operating temperatures (up to 1000°C) at very low pressure (near atmospheric). Boiling point always reaches more than 1200°C.