Pressurized heavy-water reactor (CANDU)

The term "CANDU" stands for Canadian Deuterium Uranium. A CANDU reactor  is like a PWR, but it does not use enriched uranium, and requires heavy water for coolant. Normal water has two hydrogen atoms and one oxygen atom. Heavy water is an isotope of hydrogen, called deuterium. Coolant is pressurized to stop it from flashing into steam to keep it liquid during operation. Powerful pumps circulate water through pipes, transferring heat that boils water in a separate, secondary loop. The resulting steam in the secondary loop drives the electricity-producing turbine generators