India's three stage nuclear programme

The first principle

A reactor can only burn a fissile nucleus, one that splits when it absorbs a slow neutron, and nature gives us just one: uranium 235, which is 0.7 per cent of natural uranium. Thorium 232 and uranium 238 are fertile instead: they cannot sustain a chain reaction, but a neutron can turn them into fissile uranium 233 and plutonium 239. India has little uranium and one of the world's largest stocks of thorium, so Homi Bhabha, who founded the Department of Atomic Energy (DAE), laid out in the 1950s a chain of three kinds of reactor, each making the fuel for the next, which is the only route by which Indian thorium can become electricity. Fission itself is explained in the nuclear science basics note.

The core

Stage 1: pressurised heavy water reactors

• Natural uranium needs a very good moderator. Fission neutrons are fast, and they must be slowed to thermal speeds before uranium 235 captures them efficiently. Heavy water slows neutrons well and absorbs very few of them, so a reactor moderated by it can run on natural uranium, while ordinary water absorbs enough neutrons that light water reactors need uranium enriched to about 3 to 5 per cent.
• The pressurised heavy water reactor (PHWR) uses heavy water as both moderator and coolant, with fuel in horizontal pressure tubes that can be refuelled while the reactor runs. India began with a Canadian design at Rawatbhata and now builds its own 700 MWe design.
• The product that matters is plutonium. As the fuel burns, some uranium 238 captures neutrons and becomes plutonium 239, which stays in the spent fuel. Stage 1 therefore produces both electricity and the fissile material for Stage 2, provided the spent fuel is reprocessed to separate the plutonium.

Stage 2: fast breeder reactors

• A breeder makes more fuel than it burns. When a fast, unmoderated neutron splits a plutonium 239 nucleus, about three new neutrons are released on average. One is needed to continue the chain and some are lost by leakage or capture in other materials, and the rest are captured in a blanket of uranium 238 (making plutonium) or thorium 232 (making uranium 233) placed around the core. When more fissile atoms are made than consumed, the breeding ratio is above one.
• Fast reactors cannot use water. Water would slow the neutrons, so the coolant is liquid sodium, which carries heat well, stays liquid at atmospheric pressure over a wide range, and does not moderate. Its drawback is that it burns in air and reacts violently with water.
• The fuel is mixed oxide or carbide. The Fast Breeder Test Reactor at Kalpakkam, critical since 1985, proved a mixed plutonium uranium carbide fuel, and the 500 MWe Prototype Fast Breeder Reactor (PFBR) uses mixed uranium plutonium oxide.

Stage 3: thorium reactors

• Thorium becomes fuel in three steps. Thorium 232 absorbs a neutron and becomes thorium 233, which decays in about 22 minutes to protactinium 233, which decays with a half life of about 27 days to fissile uranium 233.
• Uranium 233 suits thermal reactors. It releases more neutrons per neutron absorbed in a thermal reactor than uranium 235 or plutonium 239 do, so a thorium and uranium 233 core can come close to sustaining its own fuel, which is the aim of Stage 3.
• The difficulty is radiation. Uranium 233 is always accompanied by a little uranium 232, whose decay chain emits hard gamma rays, so the fuel must be made remotely behind heavy shielding; the same property makes it unattractive for weapons.

Dimensions

• Energy security. India's known uranium can support only a modest fleet of heavy water reactors, while its thorium could support a far larger programme for centuries, but only after the breeders have built up enough fissile material.
• Time. Breeding is slow, since a fast reactor needs many years to make the extra plutonium for a new reactor, so the fast fleet must grow for decades before thorium carries a large share of power.
• Technological. India chose a closed fuel cycle, in which spent fuel is reprocessed and recycled rather than buried, so it had to master reprocessing, sodium technology and remote fuel fabrication on its own.
• Strategic. Reprocessing and plutonium also serve weapons, which is why the programme was isolated after 1974; non proliferation diplomacy, including the Nuclear Suppliers Group (NSG), is covered in GS2 International Relations.
• Economic. Fast reactors and thorium fuel cost more than proven light water reactors, so the programme buys fuel independence rather than the cheapest power today.
• Environmental. Recycling plutonium and uranium leaves less long lived waste per unit of electricity, though reprocessing creates high level liquid waste that must be vitrified.

In practice

Where each stage stands

• Stage 1 is mature. India has 24 operating reactors with 8,780 MW of capacity (Nuclear Power Corporation of India Limited, NPCIL, September 2026), most of them heavy water reactors. The indigenous 700 MWe PHWR is now built in series: Kakrapar 3 and 4 and Rawatbhata 7 are in commercial operation, further units at Rawatbhata are being commissioned, and ten more were sanctioned in fleet mode in 2017, so that one standard design is built repeatedly.
• Stage 2 has begun. The PFBR at Kalpakkam, designed by the Indira Gandhi Centre for Atomic Research (IGCAR) and built by Bharatiya Nabhikiya Vidyut Nigam Limited (BHAVINI), began core loading in March 2024 and reached first criticality on 6 April 2026 after clearance from the Atomic Energy Regulatory Board (AERB). It still has to raise power and reach commercial operation, so its 500 MWe is not yet in installed capacity.
• Stage 3 is at the design stage. The Bhabha Atomic Research Centre (BARC) has designed a 300 MWe Advanced Heavy Water Reactor (AHWR), moderated by heavy water and cooled by boiling light water, to run on thorium with uranium 233 or plutonium. The only operating reactor in the world fuelled by uranium 233 is KAMINI (Kalpakkam Mini reactor), a small research reactor at Kalpakkam.

Imported fuel and the thorium base

• The 2008 opening. The NSG waiver of September 2008 and India's safeguards agreement with the International Atomic Energy Agency (IAEA), in force since May 2009, let India import uranium for reactors placed under safeguards, easing a fuel shortage that had kept Stage 1 reactors below capacity.
• Thorium resources. India's coastal and inland placer sands hold about 13.07 million tonnes of monazite in situ (Lok Sabha answer, August 2023), mostly in the beach sands of Kerala, Tamil Nadu, Odisha and Andhra Pradesh.

Issues and way forward

• Slow breeders. The PFBR took about two decades from construction to criticality, and the planned follow on fast reactors depend on its performance; steady operation of the PFBR and a firm sanction for the next units are the test of Stage 2.
• Limited reprocessing capacity. Every stage depends on separating plutonium and uranium 233 from spent fuel, so reprocessing and fuel fabrication plants must expand in step with the reactor fleet.
• Thorium waits on plutonium. Until enough fissile material has been bred, thorium can only be used in small amounts, so thorium fuels that use other fissile drivers in existing reactors are also being studied.
• Construction delays. Heavy water reactors have often taken far longer than planned, and fleet mode with standard designs and bulk orders is the remedy.
• A larger, mixed fleet. The national target of 100 GW by 2047, the opening to private operators and the small reactor programme are in the nuclear power note; they add uranium based reactors alongside the three stage chain rather than replacing it.

Common confusions

• Fissile and fertile. A fissile nucleus (uranium 235, plutonium 239, uranium 233) splits with a slow neutron and can sustain a chain reaction, while a fertile nucleus (uranium 238, thorium 232) cannot, but becomes fissile after absorbing a neutron.
• Moderator and coolant. A moderator slows neutrons so they cause fission, while a coolant carries heat to the turbines; heavy water does both in a PHWR, while a fast reactor has a coolant but deliberately no moderator.
• Breeder and fast reactor. A fast reactor uses unmoderated neutrons, while a breeder is any reactor that makes more fissile fuel than it burns; India's fast reactors are designed to breed, but a fast reactor can also be run as a burner.
• Criticality and commercial operation. First criticality is the moment a chain reaction first sustains itself at very low power, while commercial operation comes later, after power is raised step by step and the plant is tested and connected to the grid.

For the exam

• Keyword bank: fissile and fertile nuclei, three stage programme, pressurised heavy water reactor, natural uranium, plutonium 239, fast breeder reactor, breeding ratio, liquid sodium coolant, blanket, uranium 233, Advanced Heavy Water Reactor, closed fuel cycle, reprocessing, monazite, energy security.
• Previous year questions: Practise the previous year questions on this topic →
• Use in an answer:
  • Introduction with data: India has 24 reactors with 8,780 MW of nuclear capacity, and the Prototype Fast Breeder Reactor at Kalpakkam reached first criticality on 6 April 2026, opening the second stage of the programme Bhabha planned in the 1950s.
  • Introduction with a source: India's placer sands hold about 13 million tonnes of monazite (Lok Sabha answer, 2023), and the three stage programme exists because this thorium can be used only after it is turned into uranium 233 inside a reactor.
  • Conclusion: The three stage plan trades time for fuel independence, and the steady operation of the fast breeder is now what decides whether India's thorium becomes electricity in this century.
  • Conclusion: Imported uranium and new reactor types can add capacity quickly, but only the closed fuel cycle turns India's own thorium into a lasting energy resource.