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?

The distinction

A thermal reactor and a fast breeder reactor differ in the energy of the neutrons that sustain fission, and everything else follows from that.

• Neutron energy. A thermal reactor uses a moderator — light water, heavy water or graphite — to slow neutrons to thermal energies, where U-235 fission is most probable. A fast reactor has no moderator; it sustains the chain reaction with fast neutrons.
• Fuel. Thermal reactors burn natural or low-enriched uranium, using the 0.7 per cent U-235. A fast reactor uses a dense plutonium-uranium mixed oxide (MOX) core.
• Breeding. This is the point. A fast reactor is surrounded by a blanket of depleted uranium (U-238), which captures fast neutrons and transmutes into Pu-239. The reactor produces more fissile material than it consumes — a breeding ratio above one.
• Coolant. Water would moderate the neutrons, so fast reactors use liquid sodium, which has excellent heat transfer and does not slow neutrons — but reacts violently with water and air, which is the central engineering challenge.
• Power density is far higher, and the core is more compact.

What criticality means

Criticality is the state in which the fission chain reaction becomes self-sustaining: each fission produces on average exactly one further fission. It is expressed through the effective neutron multiplication factor k:

• k < 1, subcritical — the reaction dies out.
• k = 1, critical — steady, self-sustaining, controlled operation. This is the normal operating state of a reactor, not a hazard.
• k > 1, supercritical — power rises; used deliberately and in a controlled way to raise power level.

First criticality is therefore the milestone at which a newly built reactor first sustains its own chain reaction, and it marks the transition from construction to operation.

The PFBR at Kalpakkam

• The 500 MWe Prototype Fast Breeder Reactor, built by BHAVINI at the Kalpakkam Nuclear Complex, is India's first indigenously developed fast breeder.
• It is sodium-cooled and uses uranium-plutonium MOX fuel with a depleted uranium blanket.
• Final fuel loading began on 18 October 2025 after Atomic Energy Regulatory Board clearance.
• It attained first criticality on 6 April 2026, more than fifteen years behind the original schedule.
• Next comes phased power ascension, performance testing and safety validation before grid synchronisation and commercial generation.

Implications for a clean energy future

• It opens Stage II of the three-stage programme. Stage I pressurised heavy water reactors produce plutonium; Stage II fast breeders multiply it; Stage III uses U-233 bred from thorium. The three-stage blueprint has waited on this reactor for decades.
• Fuel multiplication. India's uranium reserves are modest and its thorium reserves are among the world's largest. Only a breeder economy converts that thorium into usable fuel, so this reactor is the bridge to energy independence rather than an end in itself.
• Waste profile. Fast reactors can burn long-lived minor actinides, reducing the radiotoxic lifetime of high-level waste.
• Baseload clean power that complements variable solar and wind, which matters for the net-zero-by-2070 pathway.

The honest caveats: sodium handling is unforgiving, the capital cost per MW is high, the doubling time for fissile inventory is long, and one prototype does not make a fleet. Detail on the fuel-loading stage is in our note on the PFBR core loading.

Conclusion

Criticality at Kalpakkam is not an electricity milestone but a fuel-cycle one. Its value lies in what it makes possible — a plutonium inventory large enough to eventually unlock India's thorium — and that payoff is measured in decades, not in megawatt-hours this year.