A Pressurized Heavy Water Reactor (PHWR) is a type of nuclear reactor that uses heavy water, or deuterium oxide, as both a coolant and a neutron moderator. This design allows the reactor to use natural uranium as fuel, making it more efficient in utilizing available resources. The heavy water helps slow down neutrons, increasing the likelihood of fission reactions. In a PHWR, the reactor core is kept under high pressure to prevent the heavy water from boiling. This enables the reactor to operate at higher temperatures, improving thermal efficiency. PHWRs are commonly used in countries like Canada and India for electricity generation.