Heavy water reactors use heavy water (D₂O) as a neutron moderator and coolant. Unlike regular water, heavy water contains a higher proportion of the isotope deuterium, which allows the reactor to efficiently slow down neutrons, making it possible to use natural uranium as fuel. This feature is beneficial for countries with limited access to enriched uranium. These reactors are known for their ability to sustain a nuclear chain reaction with less fuel enrichment. They are commonly used in various countries, including Canada, which has developed several heavy water reactors for both energy production and research purposes.