The pressurised heavy water reactor (PHWR) is a type of nuclear reactor that uses heavy water as its moderator and usually also as its coolant. The best-known design is the Canadian-developed CANDU (CANada Deuterium Uranium) reactor, which has been exported to several countries, including Argentina, South Korea and Romania. India has developed its own series of PHWRs based on CANDU technology.

CANDU reactors can use natural uranium as fuel. The reactor core consists of a large horizontal cylindrical vessel known as the calandria, typically about 7 metres long and 6 metres in diameter. Hundreds of horizontal pressure tubes pass through the calandria and form fuel channels containing short fuel bundles. The calandria is filled with heavy water, which acts as the moderator and is cooled by a separate circuit. Pressurised heavy-water coolant flows through the fuel channels and removes heat from the fuel. CANDU reactors can be refuelled while operating at full power.

Although the heavy-water coolant reaches temperatures of around 310 °C, the heavy-water moderator in a CANDU reactor is maintained at only about 70 °C.

Heavy water (D2O) is used as both the coolant and the moderator. It is an excellent moderator and absorbs far fewer neutrons than light water, providing the high neutron economy that enables CANDU reactors to use natural uranium fuel. The heavy-water coolant in the primary circuit is maintained at a pressure of about 10 MPa and leaves the fuel channels at a temperature of around 310 °C. The moderator is maintained at a much lower temperature by a separate cooling system. The hot primary coolant transfers its heat to light water in steam generators, where steam is produced to drive a turbine.