A Pressurised Heavy Water Reactor (PHWR) is a type of nuclear power reactor that uses natural uranium as its fuel and heavy water (deuterium oxide, $\text{D}_2\text{O}$) as both its coolant and neutron moderator. The PHWR was a natural choice for the first stage of India's nuclear power program, which was envisioned by Dr. Homi J. Bhabha in the 1950s to secure long-term energy independence by utilizing India's vast thorium reserves. The PHWR solved the problem of India's limited domestic uranium reserves and lack of uranium enrichment facilities, as it can use unenriched natural uranium due to the excellent neutron-moderating properties of heavy water.
In a PHWR, the fission reaction begins when neutrons collide with the natural uranium fuel, causing the uranium atoms to split and release heat. The heavy water moderator slows down the neutrons, making them more effective at sustaining the chain reaction. The heavy water coolant, kept under pressure to prevent boiling, absorbs the heat and transfers it to a secondary system where ordinary water is heated to produce steam, which then drives a turbine to generate electricity. A key mechanism of the PHWR is its ability for on-power refuelling, meaning new fuel bundles can be loaded and used fuel unloaded without shutting down the reactor, which increases operational efficiency.
The PHWR is intrinsically connected to India's three-stage nuclear power programme, where it constitutes Stage I. The primary output of this stage is electricity and the byproduct Plutonium-239 ($\text{Pu}-239$), which is a fissile material that fuels the Stage II Fast Breeder Reactors (FBRs). The technology originated with the Canadian-designed CANDU (CANada Deuterium Uranium) reactor, and the first PHWR unit, Rajasthan 1, began commercial operation in 1973 with Canadian cooperation. Following the withdrawal of Canadian support in 1974, India launched extensive R&D to indigenise the technology, leading to the development of the indigenous IPHWR series.
Recently, the indigenous 220 MWe PHWR design is being modified and converted into Bharat Small Reactors (BSR), which are essentially the existing design with minimal improvements, to be transferred to the private sector for captive power generation. India has also successfully scaled up the design to the 700 MWe PHWR and has approved the construction of ten new 700 MWe PHWRs in a "fleet mode". The core concept of using natural uranium and heavy water remains the same, but safety features have been upgraded, such as the inclusion of passive heat removal systems in the 700 MWe PHWRs.