The CANDU (Canada Deuterium Uranium) reactor is a Canadian-invented heavy water reactor that uses natural uranium fuel, pressurized heavy water as a coolant and unpressurized heavy water as a moderator. It contains horizontal pressure tubes that circulate the fuel and coolant, transferring heat to steam generators before entering turbines to generate electricity. Unlike other reactors, CANDU uses unenriched uranium and heavy water, allows refueling during operation, and controls reactions using absorber rods inserted into the core.