A type of pressurized heavy water-cooled reactor developed in Canada. Heavy water (D₂O; water with a deuterium isotope instead of hydrogen) is used as a coolant and as a moderator because it absorbs almost no neutrons. This allows the use of natural uranium as a fuel without the need for costly enrichment. The CANDU reactor vessel is a horizontally placed cylinder called a calandria. Horizontal pressure tubes with short fuel bundles pass through it. The moderator is cooled by a separate circuit to 30 °C because its moderating properties worsen with rising temperatures. The construction of calandria allows CANDU to be refueled under power.