2026 · GS Paper 3 · Q6 · 10 marks
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)
In short
A fast breeder reactor uses unmoderated fast neutrons and sodium coolant to breed more fissile fuel than it burns, unlike a thermal reactor. Criticality at Kalpakkam's 500 MWe PFBR means a self-sustaining chain reaction, opening the route to thorium and India's 100 GW nuclear target for 2047.
Model answer
Ghissu Editorial · Verified 4 Oct 2026 · 207 words
The 500 MWe Prototype Fast Breeder Reactor at Kalpakkam, built by BHAVINI, opens the second stage of Homi Bhabha's three-stage nuclear programme; its core loading began in March 2024.
How a Breeder Differs and Goes Critical
FBR vs Thermal Reactor a: Fast Breeder Reactor b: Thermal Reactor → Fast neutrons :: Slowed neutrons → No moderator :: Heavy or light water → Liquid sodium coolant :: Water coolant → Plutonium-uranium oxide fuel :: Natural or enriched uranium → Breeds more fissile fuel :: Consumes fissile fuel
1. Meaning of criticality: A reactor is critical when each fission triggers, on average, exactly one more, so the chain reaction becomes self-sustaining at a steady power level. 2. Significance at Kalpakkam: First criticality proves that an Indian-designed, sodium-cooled fast core can be started and controlled without a moderator.
Fuel multiplied
By converting uranium-238 into plutonium, the FBR extracts far more energy from the same uranium while running on plutonium recovered from spent PHWR fuel.
Gateway to thorium
Later fast reactors with thorium blankets will breed uranium-233 for stage three, unlocking India's vast thorium reserves.
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Question from the official UPSC Civil Services (Main) Examination 2026 paper. Answer written by Ghissu's engine and checked against our editorial standards. Found an error? Tell us and we correct it within 48 hours.