Pressurised Water Reactor (PWR) Technology vs India's PHWR

A nuclear power plant works like a coal power plant with a different heat source. Instead of burning coal, it splits uranium atoms to make heat. This heat turns water into steam, and the steam spins a turbine to make electricity. The two most important reactor designs for UPSC are the Pressurised Water Reactor (PWR), the most common type in the world, and the Pressurised Heavy Water Reactor (PHWR), the backbone of India's nuclear power.

How does any nuclear reactor make heat?

Uranium-235 atoms split when hit by a slow neutron (a tiny particle found in the centre of atoms). This splitting is called fission. Each fission releases a lot of heat and two or three new neutrons. These neutrons split more atoms. This is a chain reaction. A reactor keeps this chain reaction steady and controlled, like keeping a gas stove on a steady flame.

The key parts of every reactor

• Fuel: uranium, usually as ceramic pellets packed in metal tubes (fuel rods).
• Moderator: a material that slows down fast neutrons so they can split U-235 easily. Water, heavy water or graphite is used.
• Coolant: a fluid that carries heat away from the core to make steam.
• Control rods: rods of neutron-absorbing material (such as boron or cadmium) that are pushed in to slow or stop the reaction.
• Containment: a thick concrete and steel building that keeps radiation inside if something goes wrong.

How does a PWR work?

A PWR uses ordinary water (called "light water") as both moderator and coolant. The water in the reactor is kept at very high pressure, about 15-16 megapascals (roughly 150 times normal air pressure). This stops it from boiling even at around 300°C, just as a pressure cooker lets water get hotter without boiling away. This hot water flows through a steam generator.

There, it heats water in a separate loop, which turns to steam and drives the turbine. So there are two loops: a primary loop (radioactive) and a secondary loop (not radioactive).

Why does a PWR need enriched uranium?

Ordinary water absorbs some neutrons. So a PWR needs fuel with more U-235 than natural uranium has. Natural uranium has only about 0.7% U-235. PWR fuel is enriched to about 3-5% U-235. Countries without enrichment plants must buy this fuel from suppliers, which is why fuel supply deals matter.

How does a PHWR work?

A PHWR uses heavy water as moderator and coolant. Heavy water (D2O) is water in which the hydrogen atoms are the heavier isotope, deuterium. Heavy water absorbs very few neutrons. So a PHWR can run on natural uranium, with no enrichment needed. The fuel sits in hundreds of horizontal pressure tubes instead of one large pressure vessel. A PHWR can also be refuelled while it is running (on-power refuelling), so it does not need to shut down for refuelling.

PWR vs PHWR: side by side

• Moderator and coolant: PWR uses light (ordinary) water; PHWR uses heavy water.
• Fuel: PWR uses enriched uranium (about 3-5% U-235); PHWR uses natural uranium (about 0.7% U-235).
• Core: PWR has one large steel pressure vessel; PHWR has many horizontal pressure tubes.
• Refuelling: PWR must shut down, roughly every 12-18 months; PHWR refuels while running.
• Examples: PWR includes the Russian VVER, the French EPR and the US AP1000; PHWR includes the Canadian CANDU and India's 220 MWe and 700 MWe designs.

Where did India's PHWR path come from?

India's nuclear programme was planned by Homi J. Bhabha as a three-stage programme. India has little uranium but huge thorium reserves. Stage 1 uses natural uranium in PHWRs. Stage 2 uses plutonium from Stage 1 in fast breeder reactors. Stage 3 will use thorium. India's first PHWR, at Rawatbhata in Rajasthan (RAPS-1), was built with Canadian help and started in 1973.

After Canada stopped cooperation following India's 1974 test, India made PHWR technology fully its own. India's first indigenous 700 MWe PHWR, at Kakrapar in Gujarat, was connected to the grid in January 2021. In 2017, the government approved 10 more 700 MWe PHWRs to be built in "fleet mode". The 500 MWe Prototype Fast Breeder Reactor at Kalpakkam reached first criticality on 6 April 2026, taking India into Stage 2.

India's PWRs and other types

India's only large operating PWRs are at Kudankulam in Tamil Nadu. They are Russian VVER-1000 reactors, with more units under construction. France's EPR design (a PWR) is planned for Jaitapur in Maharashtra, and the US AP1000 (a PWR) for Kovvada in Andhra Pradesh. India's first nuclear plant, Tarapur (1969), uses two Boiling Water Reactors (BWRs), a third type in which water is allowed to boil inside the reactor.

India's position

As of 2026, India has a little over 8 GW of nuclear capacity (published figures range from about 8,180 MW to 8,780 MW), giving roughly 3% of its electricity. The government has set a goal of 100 GW of nuclear capacity by 2047. Both PHWRs (for self-reliance) and imported PWRs (for large capacity) are part of this plan.

Commonly confused concepts

• Heavy water vs light water: heavy water has deuterium instead of ordinary hydrogen. It is not radioactive; it is simply heavier and rarer.
• PWR vs BWR: in a PWR, water does not boil in the core because of high pressure. In a BWR, water boils directly in the core and the steam goes straight to the turbine.
• Fission vs fusion: fission splits heavy atoms like uranium (used in today's power plants). Fusion joins light atoms like hydrogen (how the Sun works; still experimental on Earth, for example ITER).
• Criticality vs grid connection: criticality means the chain reaction has become self-sustaining. Grid connection comes later, when electricity is first sent to the power grid.

Issues, criticism and the way forward

• PWRs: dependence on foreign enriched fuel and foreign vendors, high cost and the need for heavy pressure-vessel manufacturing.
• PHWRs: need large amounts of heavy water and produce more used fuel per unit of electricity.
• Common issues: high upfront cost, long construction times, public fears after Chernobyl (1986) and Fukushima (2011), land acquisition and spent fuel storage.
• Way forward: Generation III+ designs with passive safety, small modular reactors, and steady fleet-mode building.

Concepts to Know

• Isotope: atoms of the same element with different numbers of neutrons, for example U-235 and U-238.
• Generation III+ reactor: a modern reactor design with improved safety, such as passive cooling and a "core catcher" that traps molten fuel in a severe accident.
• Passive safety: safety systems that cool the reactor using gravity or natural circulation, with no electric power or operator action needed.
• Fleet mode: building many reactors of the same design together, to save time and money.
• Spent fuel: fuel removed from a reactor after use; it is highly radioactive and must be cooled and stored safely.