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 ?
Approach · directive: “distinguish / explain / what”
What it asks · Contrast fast breeder and thermal reactors, explain criticality with reference to the Kalpakkam PFBR, and assess what the milestone means for India's clean-energy future.
The question has 3 parts — answer each
- Distinguish between a Fast Breeder Reactor and a thermal nuclear reactor
- Explain 'criticality' in the context of the Kalpakkam PFBR
- What are the implications of the milestone for India's clean-energy future
Open with · The 500 MWe PFBR at Kalpakkam attained first criticality on 6 April 2026, taking India into the second stage of Homi Bhabha's three-stage nuclear programme.
Cover
- Neutrons: thermal reactors slow neutrons with a moderator (light or heavy water); FBRs use unmoderated fast neutrons.
- Fuel: thermal reactors burn natural or enriched uranium; the PFBR uses uranium–plutonium MOX fuel with a uranium-238 blanket, breeding more plutonium-239 than it consumes.
- Coolant: PHWRs use heavy water; FBRs use liquid sodium, allowing a compact, high-power-density core.
- Criticality: fissions become self-sustaining — each fission on average triggers one more; first criticality marks the start of a controlled chain reaction.
- Implications: far better use of scarce uranium, closed fuel cycle with less long-lived waste, and a bridge to thorium (Th-232 to U-233) for stage three.
- Clean-energy future: low-carbon baseload power for net zero by 2070 and the 100 GW nuclear target for 2047; indigenous technology capability.
- Challenges: sodium handling and safety, long delays and costs, reprocessing capacity, public acceptance.
Close with · The PFBR proves an indigenous closed-fuel-cycle capability; timely scale-up of fast reactors will decide whether thorium can power India's clean future.
Add value (verified)
- DAE confirmed that the PFBR reached first criticality — the start of a controlled fission chain reaction — on 6 April 2026. Department of Atomic Energy — PFBR attains first criticality (7 April 2026) ↗“the 500 MWe Prototype Fast Breeder Reactor (PFBR) has successfully attained first criticality (start of controlled fission chain reaction) on 6th April 2026”
- DAE explains that fast neutrons convert uranium-238 in the blanket into plutonium-239, so the reactor produces more fuel than it consumes. Department of Atomic Energy — PFBR attains first criticality (7 April 2026) ↗“Fast neutrons convert fertile Uranium-238 into fissile Plutonium-239, enabling the reactor to produce more fuel than it consumes.”
Question: UPSC's CS (Main) 2026, GS Paper III — paper ↗. Approach: Minimalist IAS, checked 30 Sept 2026 (how we verify) — UPSC publishes no model answers. ·
Model answer · 223 words (UPSC limit 150) · Minimalist IAS
The 500 MWe Prototype Fast Breeder Reactor at Kalpakkam attained first criticality on 6 April 2026, opening stage two of Homi Bhabha's three-stage nuclear programme.
FBR versus thermal reactor
- Neutrons: a thermal reactor slows neutrons with a moderator (light or heavy water); an FBR has no moderator and fissions with fast neutrons.
- Fuel: thermal reactors burn natural or enriched uranium; the PFBR uses uranium-plutonium MOX fuel with a uranium-238 blanket and breeds more plutonium-239 than it consumes.
- Coolant: PHWRs use heavy water; FBRs use liquid sodium, allowing a compact, high-power-density core.
Criticality
- A reactor is critical when each fission, on average, triggers exactly one more, so the chain reaction sustains itself. First criticality at Kalpakkam marks the start of a controlled fission chain reaction, the base for all later power operation.
Implications for a clean-energy future
- Fuel security: far better use of scarce uranium, and a closed fuel cycle with less long-lived waste.
- Thorium bridge: fast reactors can breed thorium-232 into uranium-233, opening stage three and India's large thorium reserves.
- Climate goals: low-carbon baseload power towards net zero by 2070 and the 100 GW nuclear target for 2047, on indigenous technology.
- Caveats: sodium safety, delays and cost, reprocessing capacity and public acceptance.
The PFBR proves an indigenous closed-fuel-cycle capability; scaling fast reactors on time will decide whether thorium can power India's clean future.
Written by Minimalist IAS from facts checked at source (how we verify) — a little fuller than exam length, so every part of the question is covered; in the hall, keep the structure and trim the detail. UPSC publishes no model answers: compare your structure and coverage with this, then write your own.