CANDU stands for CANada Deuterium Uranium. The CANDU reactor is a pressurized-heavy water,(PHWR) natural uranium (i.e. unenriched) power reactor; hence, it needs a more efficient moderator than most other power reactor designs - in this case heavy water (D2O, deuterium oxide). This means that they can be operated without expensive fuel enrichment facilities. Most less-developed countries find this attractive because they cannot afford the enrichment facilities, and cannot be assured of access to enriched uranium.

See Light-Water Reactor, Pressurized-Water Reactor, Reactor.