The CANDU reactor (Canada Deuterium Uranium) is a type of nuclear reactor that uses heavy water (deuterium oxide) as both a coolant and a neutron moderator. This design allows it to use natural uranium as fuel, making it more efficient and cost-effective compared to other reactor types that require enriched uranium. CANDU reactors are known for their safety features and flexibility, as they can be refueled while operating. They are widely used in countries like Canada and India, contributing significantly to their energy production and helping to reduce greenhouse gas emissions.