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 ? (Answer in 150 words)

Answer outline

Introduction

The Prototype Fast Breeder Reactor (500 MWe) at Kalpakkam, Tamil Nadu, designed by IGCAR and built and commissioned by BHAVINI, attained first criticality on 6 April 2026.

FBR versus thermal reactor

• Neutrons: thermal reactors use slow (thermal) neutrons, slowed by a moderator such as heavy or light water; FBRs use fast neutrons and have no moderator.
• Fuel: thermal reactors in India (PHWRs) use natural uranium; the PFBR uses uranium-plutonium mixed oxide (MOX) fuel, with plutonium recovered from reprocessed PHWR spent fuel.
• Breeding: an FBR core is surrounded by a uranium-238 blanket that fast neutrons convert into plutonium-239, so it produces more fissile material than it consumes; thermal reactors do not breed in this way.
• Coolant: thermal reactors are water cooled; the PFBR uses high-temperature liquid sodium.
• Fuel use: breeding and reprocessing let FBRs extract far more energy from the same uranium.

Criticality

• A reactor is critical when each fission, on average, causes exactly one more fission, so the chain reaction sustains itself at a steady level.
• First criticality is the first time this controlled chain reaction is started; for the PFBR it came on 6 April 2026 after the Atomic Energy Regulatory Board's safety review and clearance.
• Criticality is not the same as power generation: the reactor must still be raised to power before it supplies electricity.

Implications for clean energy

• Second stage of the three-stage programme: stage one PHWRs, stage two FBRs, stage three thorium reactors; the PFBR is designed to use thorium-232 in its blanket later to breed uranium-233 for stage three.
• Energy security from India's limited uranium and large thorium reserves; reprocessing and recycling spent fuel closes the second-stage fuel cycle.
• Low-carbon baseload power to support the Nuclear Energy Mission target of 100 GW by 2047.
• Challenges: sodium handling and safety, cost and delays, reprocessing capacity and public acceptance.

Conclusion

The PFBR's criticality moves the three-stage programme from plan to practice, though its value will depend on reliable operation and timely follow-on reactors.