A liquid or gaseous substance designed for heat dissipation from a reactor or from a device that needs to be cooled. In the production of energy in a thermal power plant, the heat generated must be dissipated. It does not matter whether its source is fission, fusion, or biomass combustion. For this purpose, a liquid or gaseous coolant is usually used, which circulates in a closed circuit around the heat source (e.g., a reactor) and removes the heat generated. The heat is then either used directly or converted into electricity, for example in a steam turbine or thermoelectric generator. The most common coolant is water, but helium, CO2, molten sodium, lead, or fluoride salts can also be used.
In addition to transferring heat in power generation, coolant is used to keep a variety of instruments or components at the desired temperature. Many detectors operate properly only within a certain temperature range. In thermonuclear fusion research, superconducting coils are necessary, which retain their properties only at temperatures below 4 kelvin. If very low temperatures are needed, liquid nitrogen or helium are used as coolants.