A Heavy Water Reactor (HWR) is a type of nuclear reactor that uses heavy water (D₂O) as both a coolant and a neutron moderator. Heavy water contains a higher proportion of the isotope deuterium, which allows the reactor to operate with natural uranium fuel, making it more efficient in some cases compared to other reactor types. HWRs are known for their ability to produce plutonium, which can be used in nuclear weapons. They are commonly used in countries like Canada and India, where they play a significant role in the development of nuclear energy and technology.