A type of nuclear reactor that uses natural uranium as fuel and heavy water as moderator and coolant. Heavy water (D₂O, deuterium dioxide) is used for low neutron absorption, which allows the use of unenriched uranium. The cooling water is kept under pressure so that it does not boil and can reach higher temperatures, while the water used as moderator has its own circuit and is cooled to a low temperature of around 30 °C. The reactor vessel is a horizontal cylinder called a calandria in which the fuel bundles are placed horizontally. The fuel can be replaced during operation.

This type of reactor was developed in Canada and is also called CANDU (CANada Deuterium Uranium). In 2023, 47 pressurized heavy water reactors were in operation.