A Pressurised Heavy Water Reactor (PHWR) is a nuclear power reactor that forms the backbone of Stage I of India's three-stage nuclear power programme. The concept was planned by Homi J. Bhabha in the 1950s to secure long-term energy independence by utilizing India's vast thorium reserves. The PHWR was chosen because it could operate on natural uranium, which contains only about 0.7% fissile Uranium-235, thus avoiding the need for expensive uranium enrichment facilities. The first unit, Rajasthan 1 (RAPS-1), began commercial operation in 1973 as a joint Indo-Canadian venture based on the CANDU design. Following the 1974 Pokhran-I test, Canada terminated cooperation, leading India to indigenise and standardize the technology.
A PHWR works by using heavy water ($\text{D}_2\text{O}$) as both the neutron moderator and the primary coolant. The heavy water's superior neutron economy allows the chain reaction to be sustained with natural uranium. The reactor core uses a horizontal cylindrical tank called a calandria for the moderator, which is penetrated by pressure tubes carrying the fuel bundles and the hot, pressurized heavy-water coolant. This design enables a key feature: on-power refuelling, which allows fuel replacement without shutting down the reactor. The reactor generates electricity and produces Plutonium-239 as a by-product from the non-fissile Uranium-238.
The Plutonium-239 produced in the PHWRs is the essential fissile material for the Fast Breeder Reactors (FBRs) of Stage II, which will eventually produce Uranium-233 from thorium for the final Stage III. India has progressively developed indigenous designs, moving from the initial 220 MWe units to the standardized 540 MWe and the current fleet-mode construction of 700 MWe PHWRs (IPHWR-700). The 700 MWe design incorporates enhanced safety features like a dedicated Passive Decay Heat Removal System. Recently, the Sustainable Harnessing and Advancement of Nuclear Energy for Transforming India (SHANTI) Act, 2025, aims to allow private participation in the sector, with the indigenous 700 MWe PHWR technology being a focus for technology transfer to accelerate capacity expansion.