Pressurised Heavy Water Reactor: Natural Uranium, 700 MWe Fleet, Stage One
Pressurised heavy water reactor explained: natural uranium fuel, heavy water moderator, India's 20 PHWRs and the 700 MWe fleet behind stage one.
A pressurised heavy water reactor (PHWR) is a nuclear power reactor that runs on natural uranium and uses heavy water, water whose hydrogen atoms carry an extra neutron, both to slow neutrons down and to carry heat away. It is the workhorse of Indian nuclear power: 20 of the 24 reactors in commercial operation are PHWRs, making up 6,460 MW of the country’s 8,780 MW. The first, RAPS-1 at Rawatbhata in Rajasthan, started in 1972 on a Canadian design, and India now builds its own 700 MWe units in a fleet of ten.
Most readers arrive with two wrong ideas. The first is that heavy water is some exotic fuel. It isn’t the fuel at all; it’s the moderator and the coolant, and the fuel is ordinary uranium that has never been enriched. The second is that the PHWR is an old Canadian import India is moving past. The record says the opposite: India standardized the design, scaled it from 220 to 700 MWe, and the plutonium in PHWR spent fuel is what feeds the fast breeder reactor that opened the second stage of the nuclear program in April 2026.
What is a pressurised heavy water reactor?
A PHWR is a thermal reactor, which means it depends on slowed-down neutrons, and it belongs to the pressure-tube family. Its fuel sits in hundreds of tubes that run through a large tank of heavy water called the calandria. The table uses India’s 700 MWe design as the reference, with figures from NPCIL’s safety report and its list of plants.
| Fact | Detail |
|---|---|
| Full form | Pressurised Heavy Water Reactor (PHWR) |
| Fuel | Natural uranium, about 0.7% U-235, in bundles clad in Zircaloy-4 |
| Moderator and coolant | Heavy water (D2O), in two separate circuits |
| Core of a 700 MWe unit | 4,704 fuel bundles in 392 coolant channels, inside a calandria 7.8 m across |
| First PHWR in India | RAPS-1, Rawatbhata, Rajasthan, built with Canada on its CANDU design; started in 1972 |
| Unit sizes built in India | 100 and 200 MWe (RAPS-1 and 2), 220 MWe standard, 540 MWe (Tarapur 3 and 4), 700 MWe |
| Fleet in operation | 20 PHWRs, 6,460 MW of India’s 8,780 MW (NPCIL list) |
| Operator | Nuclear Power Corporation of India Limited (NPCIL), under the Department of Atomic Energy |
| Role in the three-stage program | Stage one: electricity from natural uranium, plutonium for stage two |
How does a PHWR work?
A PHWR splits uranium-235 atoms, slows the neutrons they release in heavy water so that they split more atoms, and carries the heat out through a closed heavy water loop that boils ordinary water into steam. The chain has five links, and each one is simple on its own:
- Fission. A U-235 nucleus captures a slow neutron and splits, releasing heat and two or three fast neutrons.
- Moderation. Fast neutrons are too quick for the next U-235 nucleus to catch. They bounce off deuterium nuclei in the moderator until they slow to thermal speed.
- Heat removal. Heavy water coolant flows through the pressure tubes around the fuel. In the 700 MWe design, it leaves the channel at 310°C under a nominal pressure of 100 kg/cm2.
- Steam. In the steam generators, the hot coolant boils ordinary water on the other side of the tube wall. The steam, at 44 kg/cm2 and 256°C, turns the turbine.
- Control. Absorber rods and the neutron balance are adjusted so that, on average, each fission causes exactly one more. That steady state is called criticality.
Why heavy water lets natural uranium work
Natural uranium is only about 0.7% U-235; the rest is U-238, which doesn’t split with slow neutrons. With so little fissile material, the reactor can’t afford to lose many neutrons. Ordinary hydrogen slows neutrons well but also swallows a share of them. Deuterium slows them almost as well and swallows far fewer. So a heavy water reactor can reach criticality on natural uranium, while a light water reactor has to enrich its fuel to a few percent U-235 to make up for the neutrons its water absorbs. The site’s note on uranium enrichment explains that step.
Think of the moderator as a crowd a runner has to push through to slow down. Ordinary water is a crowd that slows the runner but also tackles some of them to the ground. Heavy water slows them without the tackles. The picture holds for the neutron economy, but it breaks in one place: heavy water still captures a few neutrons, and some of that capture turns deuterium into radioactive tritium.
Pressure tubes and the calandria
The word “pressurised” applies only to the coolant. It flows at high pressure inside the tubes, so it stays mostly liquid at 310°C. The moderator sits in the calandria at low pressure and stays cool, between 53°C and 76°C in the 700 MWe design. That is the design choice that sets a PHWR apart from a pressurized water reactor, which holds its whole core in one thick steel pressure vessel. A PHWR spreads the pressure across hundreds of small tubes instead, and the calandria wall is just 32 mm of stainless steel.
On-power refuelling
Natural uranium burns out faster than enriched fuel, so the PHWR refuels a little at a time while it runs. Fuelling machines lock onto both ends of a channel, push fresh bundles in and take spent ones out, with the reactor at power. A light water reactor has to shut down to refuel. The trade-off is volume: a PHWR discharges more spent fuel for each unit of electricity than a reactor running on enriched fuel.
How India built its PHWR fleet
India imported the PHWR design once, at Rawatbhata, and has built every later unit itself, in four sizes. The history falls into four phases.
The Canadian start came first. RAPS-1 was built with Canada on the early CANDU (Canada Deuterium Uranium) design and started in 1972. After India’s nuclear test at Pokhran in May 1974, Canada suspended nuclear cooperation while it was still helping with the second unit. The Department of Atomic Energy finished RAPS-2 on its own, and it entered commercial operation on 1 April 1981. RAPS-1 has been shut since 2004 and is left out of the official capacity figure.
Next came the 220 MWe standard. MAPS-1 at Kalpakkam began commercial operation on 27 January 1984. Narora followed on 1 January 1991, and from Narora onwards the design was standardized with double containment and a suppression pool, a water pool that condenses steam in an accident. The same 220 MWe design went on to Kakrapar 1 and 2, Kaiga 1 to 4 and Rawatbhata 3 to 6.
The 540 MWe units at Tarapur were the first big step up. Unit 4 entered commercial operation on 12 September 2005, nearly a year before unit 3 on 18 August 2006. Tarapur’s older units 1 and 2 are boiling water reactors, not PHWRs.
The 700 MWe design is the current generation. Kakrapar 3 reached first criticality on 22 July 2020, the first Indian-designed 700 MWe PHWR to do so. It moved to commercial operation on 30 June 2023, and Kakrapar 4 followed on 31 March 2024. RAPP-7 at Rawatbhata reached criticality in September 2024, commercial operation on 15 April 2025 and full power on 10 February 2026.
| Generation | Units | Size (MWe) | First commercial operation |
|---|---|---|---|
| Canadian-assisted | RAPS-1 and 2 | 100 and 200 | RAPS-2: 1 April 1981 |
| Indigenous 220 | MAPS, NAPS, KAPS-1 and 2, Kaiga 1 to 4, RAPS-3 to 6 | 220 | MAPS-1: 27 January 1984 |
| Tarapur 540 | TAPS-3 and 4 | 540 | TAPS-4: 12 September 2005 |
| 700 MWe | KAPS-3 and 4, RAPS-7 | 700 | KAPS-3: 30 June 2023 |
The 700 MWe unit isn’t just a bigger 220. NPCIL’s safety report lists three changes worth knowing:
- Partial boiling: the coolant may carry up to 3% steam at the channel exit, which extracts more heat from the same channels.
- Passive decay heat removal: a system that keeps cooling the core through the steam generators without station power, designed to run 6 hours without make-up water.
- Steel-lined containment: the inner containment wall of pre-stressed concrete carries steel liners.
What fleet mode means for the 700 MWe PHWR
Fleet mode means building many reactors of one identical design as a single project, so that equipment is ordered in bulk, drawings are reused and crews move from one unit to the next. On 17 May 2017, the Government approved ten 700 MWe PHWRs as one project of 7,000 MW, and gave administrative approval and financial sanction in June 2017. A Rajya Sabha reply of July 2018 named the sites:
- Chutka 1 and 2, Madhya Pradesh;
- Kaiga 5 and 6, Karnataka;
- Mahi Banswara 1 to 4, Rajasthan;
- Gorakhpur (GHAVP) 3 and 4, Haryana.
The logic is the one a factory uses. The first unit of a design pays for the learning, and every repeat should come cheaper and faster. It works only if the repeats actually arrive on schedule, and here the record is mixed. A Rajya Sabha reply of March 2026 blamed recent delays and cost overruns on land acquisition, rehabilitation, clearances, contractors’ cash-flow problems, the Covid-19 pandemic and design changes made after the Fukushima accident. The same reply put the pipeline at 18 reactors of 13,600 MW, to be completed progressively by 2031-32.
Where the PHWR sits in the three-stage program
The PHWR is stage one of the three-stage program the Department of Atomic Energy follows, the plan associated with Homi Bhabha for a country with limited uranium and large thorium reserves. PHWRs turn natural uranium into electricity. Their spent fuel is then reprocessed for plutonium, which fuels fast breeder reactors in stage two. Stage three uses uranium-233 bred from thorium. The site’s note on the three-stage nuclear programme covers the full chain.
The link is now live. The 500 MWe Prototype Fast Breeder Reactor at Kalpakkam, designed by IGCAR and built by BHAVINI, reached first criticality on 6 April 2026, as covered in the site’s report on the Kalpakkam criticality. A Rajya Sabha reply of 23 July 2026 said the first stage “has been successfully achieved” and that India entered the second stage with that criticality. The same reply noted that thoria pellets have been used in the initial cores of operating PHWRs, a small early bridge to stage three.
PHWR, light water reactor and fast breeder compared
The three reactor types a reader meets in Indian nuclear news differ in fuel, moderator and coolant. The comparison below uses the Russian-designed VVERs at Kudankulam as the light water example.
| Feature | PHWR | Light water reactor (VVER, Kudankulam) | Fast breeder (PFBR, Kalpakkam) |
|---|---|---|---|
| Fuel | Natural uranium | Enriched uranium | Plutonium-uranium mixed oxide |
| Moderator | Heavy water | Ordinary water | None; uses fast neutrons |
| Coolant | Heavy water | Ordinary water | Liquid sodium |
| Pressure boundary | Hundreds of pressure tubes | One large pressure vessel | Low-pressure sodium pool |
| Refuelling | While running | During shutdown | During shutdown |
| Stage of the program | Stage one | Outside the three stages (imported) | Stage two |
Why the PHWR matters, and where it falls short
The PHWR matters because it let India make nuclear power without depending on anyone for enriched fuel, and because stage two needs its plutonium. The strengths hold up on the record:
- No enrichment step: natural uranium goes straight into fuel bundles.
- A domestic supply chain: the 700 MWe design was drawn up, built and commissioned by Indian agencies and industry.
- High availability: on-power refuelling removes the refuelling outage.
- A path to stage two: PHWR spent fuel is the plutonium source for fast breeders.
- A base for small reactors: the 220 MWe design is the starting point for Bharat Small Reactors.
The criticisms are just as real. Heavy water is costly to produce, and losses have to be tightly managed because irradiated heavy water carries tritium. The larger volume of spent fuel means more to store and reprocess. Building times have run long, as the delays listed in Parliament show. And natural uranium still has to come from somewhere; the site’s note on uranium mining in India sets out how thin domestic supply is.
Neither side wins outright. Calling the PHWR outdated ignores that it is the only reactor India designs, builds and fuels end to end. Calling it the whole answer ignores its build times and its fuel limits. It is wise to see it as the reliable first stage it was meant to be, with fast breeders and small reactors built on top of it.
Pressurised heavy water reactors today
India’s PHWR program stood at a turning point in 2026. The latest figures come from Parliament replies and department releases:
- Capacity: a Rajya Sabha reply of 12 March 2026 put installed nuclear capacity at 8,780 MW from 24 plants, excluding RAPS-1. Twenty of those plants are PHWRs.
- Pipeline: the same reply listed KAPP-5 and 6, RAPP-9 and 10 and NAPP-3 and 4, all 2 x 700 MW, as planned by 2035 on the road map to 100 GW by 2047.
- RAPP-8: light water commissioning of the eighth Rawatbhata unit was in progress as of February 2026.
- Nuclear Energy Mission: the Union Budget 2025-26 set aside Rs 20,000 crore for small modular reactors. A Lok Sabha reply of 11 March 2026 said the lead units of the 220 MWe BSMR-200 and the SMR-55 will come up at Tarapur.
- Bharat Small Reactors: these are to be based on the proven 220 MWe PHWR, for use as captive plants near industry. They are distinct from the BSMR-200, a new small modular design.
- The law: the SHANTI Act, 2025 received Presidential assent on 20 December 2025 and was notified the next day. It replaces the Atomic Energy Act, 1962 and the Civil Liability for Nuclear Damage Act, 2010 and opens the sector to private participation under licence. Its rules had not been notified as of February 2026.
The site’s notes on nuclear energy in India, small modular reactors and the SHANTI Act take each of these further, and the Department of Atomic Energy note explains who runs what. NPCIL’s list of plants is the place to check commercial operation dates.
How to study pressurised heavy water reactors for exams
The PHWR sits in GS Paper III under science and technology and under energy infrastructure, and in Prelims under science and technology. Questions rarely name the PHWR on its own; they test it through the three-stage program and the fast breeder. Mains 2017 GS Paper III asked candidates to “Give an account of the growth and development of nuclear science and technology in India. What is the advantage of fast breeder reactor programme in India?” A good answer to that question starts with the PHWR. On the Prelims side, 192 of the 1,403 questions in the site’s Prelims question bank are tagged Science and Technology.
Revise these until they’re automatic:
- Fuel: natural uranium, about 0.7% U-235, with no enrichment.
- Heavy water: D2O is the moderator and the coolant, never the fuel.
- RAPS-1: Rawatbhata, Canadian CANDU design, started 1972, shut since 2004.
- 540 MWe: Tarapur 3 and 4, with unit 4 in commercial operation first, in 2005.
- 700 MWe: Kakrapar 3 critical on 22 July 2020; Kakrapar 3 and 4 and RAPP-7 in commercial operation.
- Fleet mode: ten 700 MWe units approved on 17 May 2017, at Chutka, Kaiga, Mahi Banswara and Gorakhpur.
- Capacity: 8,780 MW from 24 plants as of March 2026; target 100 GW by 2047.
Five confusions cost marks:
- PHWR or PWR. Kudankulam’s VVERs are pressurized light water reactors on enriched fuel. The PHWR runs on natural uranium with heavy water.
- Tarapur’s two reactor types. Units 1 and 2 are boiling water reactors from 1969; units 3 and 4 are 540 MWe PHWRs.
- Criticality or commercial operation. Kakrapar 3 was critical in 2020 but commercial only in 2023. Read which milestone a question asks for.
- BSR or BSMR. Bharat Small Reactors adapt the 220 MWe PHWR; the BSMR-200 is a separately designed small modular reactor.
- RAPS-1 in the count. The official 8,780 MW leaves RAPS-1 out.
The PHWR is the hinge of India’s nuclear story, not a side topic. An aspirant who can explain why heavy water lets natural uranium work will find the fast breeder, the thorium stage and the small-reactor push falling into place as consequences. Learn the mechanism once, attach the dates to it, and the rest of the nuclear syllabus stops being a list.
Frequently Asked Questions
What is a pressurised heavy water reactor in simple words?
A pressurised heavy water reactor is a nuclear reactor that runs on natural, unenriched uranium and uses heavy water to slow neutrons and carry heat. The fuel sits in pressure tubes that pass through a tank of heavy water called the calandria. India uses it as the first stage of its three-stage nuclear program.
Why does a PHWR use heavy water?
Heavy water slows neutrons without absorbing many of them, because deuterium captures far fewer neutrons than ordinary hydrogen. That saving is what lets a reactor reach criticality on natural uranium, which is only about 0.7% U-235. A light water reactor loses more neutrons to its water, so it needs enriched fuel.
How many PHWRs does India have?
India has 20 PHWRs in commercial operation among its 24 reactors, according to NPCIL’s list of plants. Together they account for 6,460 MW of the 8,780 MW installed capacity reported to the Rajya Sabha in March 2026. The figure leaves out RAPS-1, which has been shut since 2004.
What is a 700 MWe PHWR?
The 700 MWe PHWR is India’s largest indigenous reactor design, with 392 coolant channels holding 4,704 fuel bundles. It allows limited boiling at the channel outlet and adds a passive decay heat removal system and steel-lined containment. Kakrapar 3 was the first to reach criticality, on 22 July 2020.
What are fleet mode reactors?
Fleet mode reactors are ten identical 700 MWe PHWRs approved as a single 7,000 MW project on 17 May 2017. Building one design repeatedly lets NPCIL order equipment in bulk and reuse designs and crews. The approved sites are Chutka, Kaiga, Mahi Banswara and Gorakhpur in Haryana.
How is a PHWR different from a fast breeder reactor?
A PHWR uses slow neutrons, natural uranium fuel and heavy water, while a fast breeder uses fast neutrons with no moderator, plutonium-based mixed oxide fuel and liquid sodium coolant. The two are linked, because plutonium recovered from PHWR spent fuel feeds the breeder. India’s Prototype Fast Breeder Reactor reached first criticality on 6 April 2026.
Is Kudankulam a PHWR?
No. Kudankulam’s operating units are Russian-designed VVERs, a type of pressurized light water reactor that runs on enriched uranium with ordinary water as moderator and coolant. Every other operating Indian power reactor except Tarapur 1 and 2, which are boiling water reactors, is a PHWR.
What are Bharat Small Reactors?
Bharat Small Reactors are small reactors to be based on India’s proven 220 MWe PHWR design, meant for captive power near industry. They are separate from the BSMR-200, a new 220 MWe small modular reactor that BARC and NPCIL are designing. Both form part of the push under the Nuclear Energy Mission announced in the Union Budget 2025-26.
Practice Questions
Prelims
1. Consider the following statements about pressurised heavy water reactors: 1. They use enriched uranium as fuel. 2. Heavy water serves as both moderator and coolant. 3. They can be refuelled while operating. Which of the statements given above is/are correct?
- (a) 1 and 2 only
- (b) 2 and 3 only
- (c) 1 and 3 only
- (d) 1, 2 and 3
Answer: (b) PHWRs run on natural uranium, which heavy water’s low neutron absorption makes possible.
2. Consider the following statements: 1. India’s first 700 MWe PHWR to reach criticality was at Kakrapar in Gujarat. 2. The units at Kudankulam are pressurised heavy water reactors. Which of the statements given above is/are correct?
- (a) 1 only
- (b) 2 only
- (c) Both 1 and 2
- (d) Neither 1 nor 2
Answer: (a) Kakrapar 3 reached criticality on 22 July 2020; Kudankulam runs Russian-designed VVER light water reactors.
3. Which of the following sites was NOT among those named for the ten 700 MWe PHWRs approved in fleet mode in 2017?
- (a) Chutka
- (b) Mahi Banswara
- (c) Kudankulam
- (d) Kaiga
Answer: (c) The fleet sites are Chutka, Kaiga, Mahi Banswara and Gorakhpur in Haryana.
4. In India’s three-stage nuclear power program, pressurised heavy water reactors mainly serve to:
- (a) breed uranium-233 from thorium
- (b) produce electricity from natural uranium and plutonium for fast breeder reactors
- (c) enrich uranium for light water reactors
- (d) burn thorium directly as the main fuel
Answer: (b) Stage one PHWRs yield plutonium in spent fuel, which fuels stage two fast breeders.
5. In a pressurised heavy water reactor, the high pressure is maintained mainly in:
- (a) the calandria that holds the moderator
- (b) the pressure tubes that carry the coolant past the fuel
- (c) a single large steel reactor vessel
- (d) the containment building
Answer: (b) The moderator in the calandria stays at low pressure; only the coolant in the tubes is at high pressure.
Mains
- Distinguish between a Fast Breeder Reactor (FBR) and a thermal nuclear reactor. In the context of first indigenously developed prototype FBR at Kalpakkam, explain the term “criticality”. What are its implications for clean energy future of our country? (10 marks, 150 words) Previous year: Mains 2026, GS Paper III.
- Explain how the use of heavy water allows a pressurised heavy water reactor to run on natural uranium. What are the advantages and drawbacks of this design for India? (10 marks, 150 words)
- Trace the evolution of India’s PHWR program from RAPS-1 to the 700 MWe design. What does it reveal about India’s indigenous nuclear capability? (15 marks, 250 words)
- What is meant by building nuclear reactors in fleet mode? Assess the progress of the ten 700 MWe PHWRs approved in 2017 and the reasons for delays. (15 marks, 250 words)
- With India targeting 100 GW of nuclear capacity by 2047, examine the role that PHWRs and Bharat Small Reactors can play alongside the SHANTI Act, 2025. (10 marks, 150 words)