India has a nuclear reactor that can make more fuel than it burns. Science explained

India's Prototype Fast Breeder Reactor at Kalpakkam has attained a historic milestone. The 500 MWe reactor, which produces more fuel than it consumes, puts India on course to harness its vast thorium reserves.

In Short

• India's Kalpakkam reactor goes critical, joins Russia in historic nuclear first.
• The PFBR breeds more fuel than it burns using plutonium.
• India holds 25 per cent of the world's known thorium.

Deep inside a facility on the Tamil Nadu coast, something remarkable happened this week. After decades of painstaking science, engineering setbacks, and quiet perseverance, a reactor came alive.

Not with a bang, but with the steady, self-sustaining hum of controlled nuclear fission.

The splitting of a heavy nucleus into two or smaller nuclei, releasing massive amounts of energy, is known as nuclear fission.

The Prototype Fast Breeder Reactor at Kalpakkam attained criticality on April 6, 2026, and India’s long nuclear dream moved one decisive step closer to reality.

Criticality is the precise state in a nuclear reactor where the chain reaction becomes self-sustaining.

This means each fission event produces exactly enough neutrons to trigger a subsequent one at a steady rate.

Prime Minister Narendra Modi announced the milestone, calling it a “defining step” in India’s nuclear journey and a decisive step towards harnessing the country’s vast thorium reserves.

But what exactly has India achieved, and why does it matter?

WHAT IS THE KALPAKKAM PFBR, AND WHY IS IT SPECIAL?

The PFBR has a capacity of 500 MWe, or megawatts electric, which is enough electricity to power roughly four to five lakh average Indian homes simultaneously.

Unlike conventional nuclear reactors, which use water as a coolant, or the fluid that carries away the intense heat generated inside the reactor, the PFBR circulates liquid sodium.

Sodium metal, kept molten at around 200 degrees Celsius, transfers heat far more efficiently than water and, critically, does not slow down the fast-moving neutrons that make this reactor special.

It runs on uranium-plutonium Mixed Oxide fuel, known as MOX. These are ceramic pellets made by blending uranium and plutonium oxides together.

The plutonium in these pellets comes from the spent, used-up fuel of India’s existing first-stage reactors.

This MOX fuel sits in the reactor core, the central chamber where nuclear splitting, or fission, takes place.

Surrounding that core is a blanket of uranium-238, the abundant but ordinarily non-reactive form of uranium that makes up 99 per cent of all natural uranium.

When the intense neutron bombardment from the core strikes this blanket, it converts the otherwise inert uranium-238 into fresh plutonium, which can be extracted and used as new fuel.

The reactor, in other words, makes more fuel than it burns. That is the defining trait of a breeder reactor, and no other type of power reactor in the world can do this at a commercial scale.

Once operational, India will become the second country, after Russia, to have a commercially operating fast breeder reactor.

That is an elite club with exactly one other member.

The PFBR was designed by the Indira Gandhi Centre for Atomic Research, or IGCAR, at Kalpakkam, and has been built completely indigenously with significant contributions from more than 200 Indian industries, including MSMEs.

WHAT DOES CRITICALITY MEAN?

Criticality is the operational heartbeat of a nuclear reactor.

It is the specific condition in which the chain reaction inside the reactor becomes perfectly self-sustaining: every fission event, every splitting of a fuel atom, releases neutrons, the subatomic particles that act as the bullets triggering further splits, and exactly enough of those neutrons survive to cause one more fission in turn.

Not growing out of control, not fading away, just steady.

Criticality in a nuclear reactor is when enough neutrons are produced by fission to replace those lost through leakage or absorption, ensuring the number of neutrons remains constant.

Attaining criticality is not the same as generating electricity; that comes later.

But it is the essential prerequisite, the moment the reactor proves it can sustain itself.

Once a sustained nuclear fission chain reaction is achieved, a series of low-power physics experiments will be conducted to further assess and understand reactor behaviour before the reactor is connected to the grid.

WHAT ARE INDIA'S THREE STAGES OF NUCLEAR POWER?

India's three-stage nuclear power programme was planned by physicist Homi Bhabha in the 1950s to secure the country's long-term energy independence, through the use of uranium and thorium reserves found in the monazite sands of coastal regions of South India.

Monazite sands are mineral-rich beach and inland sands concentrated along India's southern coastline, particularly in Kerala, Tamil Nadu, and Odisha, and they hold some of the world's largest thorium deposits.

The first stage involved pressurised heavy water reactors, which are large nuclear plants that use heavy water. This is a special form of water made with a heavier hydrogen isotope. It acts both as a coolant and as a moderator, the material that slows neutrons down to make fission easier.

These reactors run on natural uranium and produce electricity, but also generate plutonium as a by-product in their spent fuel.

That plutonium now becomes the starting fuel for the second stage, which is precisely what the PFBR represents. The third and final stage will use thorium, India's most abundant nuclear resource, as its primary fuel.

WHY DOES INDIA CARE SO MUCH ABOUT THORIUM?

India has only around one to two per cent of the global uranium reserves, but one of the largest shares of global thorium reserves at about 25 per cent of the world's known thorium reserves.

Uranium must largely be imported.

Thorium sits in India's own soil. According to the Indian nuclear establishment, the country could generate a staggering 500 GW, or gigawatts, of electricity for the next four centuries using only its economically extractable thorium reserves.

For context, India's entire current nuclear installed capacity is just 8.18 GW.

The catch is that thorium-232, the naturally occurring form found in Indian soil, is fertile but not fissile.

Fissile means a material whose atoms can be split directly to sustain a chain reaction.

Thorium cannot do this on its own. It first needs to be bombarded with neutrons inside a reactor, which converts it into uranium-233, a fissile fuel that can power third-stage reactors.

That conversion requires the kind of high-speed, fast neutron environment only a reactor like the PFBR can provide.

The conversion from thorium to uranium-233 is planned to be achieved in the second stage of the programme, which involves the commercial operation of fast breeder reactors.

WHAT HAPPENS NEXT?

Attaining criticality is the gateway, not the destination.

The reactor will now undergo a series of low-power experiments before being connected to the electrical grid.

India's nuclear energy mission aims to achieve 100 GW of electricity from nuclear power, with an additional 7.30 GW currently under construction or commissioning.

The Department of Atomic Energy has proposed the construction of additional fast breeder reactors at Kalpakkam after a year of successful PFBR operation, with further reactors planned beyond 2030.

This is not merely an energy story. It is the story of a 70-year vision, drawn up in the 1950s by one of India's greatest scientific minds, inching towards fruition, one neutron at a time.

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