The fission of a single ²³⁵U nucleus releases about 200 MeV (3.2 × 10⁻¹¹ J) of energy. However, in a reactor with a thermal power of 3,000 MW, approximately 10²⁰ nuclei undergo fission every second. The resulting heat must be continuously removed from the reactor core to prevent the fuel from overheating. The heat is then used to produce steam or otherwise transferred to a power conversion system to generate electricity. The most common reactor coolant is water, but other coolants include carbon dioxide, helium, liquid sodium, lead and molten salts.

Schematic diagram of a horizontal steam generator used in a VVER nuclear power plant.

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Schematic diagram of a horizontal steam generator

Helium is a chemically inert gas used as a coolant in high-temperature gas-cooled reactors. Commercial helium is obtained mainly from natural gas deposits containing elevated concentrations of this noble gas.

Water

In light water reactors, water serves as both the moderator and the coolant. In pressurised water reactors (PWRs), the primary coolant is maintained at a pressure of about 15—16 MPa, preventing it from boiling at temperatures of around 300 °C. In boiling water reactors (BWRs), the coolant operates at a lower pressure of about 7 MPa and boils directly in the reactor core to produce steam.

Gases

Helium is used as a coolant in high-temperature gas-cooled reactors. It is chemically inert, remains gaseous at reactor operating temperatures and can operate at very high temperatures. In some advanced reactor concepts, hot helium may be used in a direct Brayton cycle to drive a gas turbine. Carbon dioxide is used as the coolant in British Magnox and advanced gas-cooled reactors (AGRs).

In a large PWR, around 20 m³ of water may pass through the reactor core every second, typically heating up by several tens of degrees Celsius.

Liquid Metals

Lead or lead-bismuth eutectic can be used as a coolant in fast reactors. These liquid metals have little moderating effect on neutrons and, unlike sodium, do not react vigorously with water or burn in air.

Fast reactors require coolants with little moderating effect in order to maintain a fast neutron spectrum. Liquid sodium is the most widely used liquid-metal coolant in fast reactors. It has excellent heat-transfer properties and a high boiling point, allowing the primary system to operate at relatively low pressure. Lead and lead-bismuth eutectic are alternative liquid-metal coolants. Unlike sodium, they do not react vigorously with water or burn in air, but they present other challenges, including corrosion of structural materials.

Molten salts

Molten salts can also be used as reactor coolants. Depending on the reactor design, the fuel may be dissolved directly in the molten salt or the salt may serve only as a coolant. Depending on the reactor design, heat from the primary coolant is transferred through one or more intermediate circuits to a steam or gas power conversion system.

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The temperature at the centre of a fuel pellet in a light water reactor can exceed 1,000 °C during normal operation, while the surrounding coolant remains at about 300 °C.

Zirconium alloys commonly used for fuel cladding have melting temperatures of around 1,850 °C.