Chat with us
General Studies 2026 GS Paper III 10 marks 150 words Compulsory Distinguish

GS Paper III — Q6

Distinguish between a Fast Breeder Reactor (FBR) and a thermal nuclear reactor. In the context of first indigenously developed…

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) 10 marks

हिंदी में प्रश्न पढ़ें

एक द्रुत प्रजनक रिएक्टर (एफ बी आर) और एक तापीय परमाणुक रिएक्टर के बीच भेद बताइए। कल्पक्कम में स्वदेशी रूप से विकसित प्रथम प्रारूपी (प्रोटोटाइप) एफ बी आर के संदर्भ में, 'क्रान्तिकता (क्रिटकलिटी)' पद की व्याख्या कीजिए। हमारे देश के स्वच्छ ऊर्जा भविष्य के लिये इसके निहितार्थ क्या हैं? (उत्तर 150 शब्दों में दीजिए)

Q6 of the 2026 UPSC Mains General Studies GS Paper III, as printed
The question as printed in the 2026 General Studies paper
Evaluate your answer to this question → See all 2026 General Studies questions

Model answer

Written by UPSC Answer Check against this question's marking rubric, to the 150-word length. UPSC does not publish answers for Mains — this is one way to score well, not an official key.

A Fast Breeder Reactor (FBR) uses fast, unmoderated neutrons and liquid sodium coolant to breed more fissile Plutonium-239 from fertile Uranium-238 than it consumes. In contrast, a thermal reactor (e.g., PHWR) uses a moderator like heavy water to slow neutrons, primarily consuming scarce Uranium-235 with negligible breeding.

Criticality denotes the operational state where a nuclear fission chain reaction becomes self-sustaining, meaning the rate of neutron production exactly balances neutron losses (k=1). At the 500 MWe Kalpakkam Prototype Fast Breeder Reactor (PFBR), achieving criticality signifies the successful indigenous validation of controlled, sustained fast-neutron fission.

Implications for Clean Energy: The PFBR is the linchpin of Stage-2 in Bhabha’s three-stage nuclear power programme, bridging Stage-1 (natural uranium PHWRs) to Stage-3 (Thorium-based reactors). By transmuting fertile thorium into fissile Uranium-233, it unlocks the vast thorium reserves in the monazite sands of the Kerala coast, providing an indigenous, carbon-free baseload energy source for India's long-term energy security.

What "Distinguish" is asking you to do

Name the property that separates the items and say which side holds it. Distinguish is marked exactly as differentiate is, with no difference in expectation, but its stems more often line up three terms rather than two — gender equality, gender equity and empowerment — and every pair in the set has to be separated.

Structure that answers it

The category they all sit in → the property dividing the first pair → the second pair → the third → why the boundary matters in practice

Where marks are lost

Separating the two obviously different items and leaving the middle term unplaced. A description of each side from which the line must be inferred is marked as description, not as a distinction.

All UPSC directive words, compared →

How this answer will be evaluated

Approach

Framework: GS3 Technology: why it matters, use, benefit, risk, policy. compare: paired headings or table > key differences > significance > conclusion Full marks: Clear distinction (fast vs thermal, breeding), precise definition of criticality, and strong link to fuel security.

Key points expected

  • FBR uses fast neutrons; thermal uses slow neutrons
  • FBR breeds fuel (Pu-239); thermal consumes fuel
  • Criticality defined as self-sustaining chain reaction
  • Implications: fuel security, reduced waste, clean energy

Evaluation rubric

Each sub-part is marked on its own, against the marks and word limit printed on the paper.

  1. The answer Distinguish FBR from thermal reactor, explain criticality at Kalpakkam, and assess clean energy implications. 10 marks · 150 words

    compare— paired headings or table → key differences → significance → conclusion

    Must cover

    • FBR uses fast neutrons; thermal uses slow neutrons
    • FBR breeds fuel (Pu-239); thermal consumes fuel
    • Criticality defined as self-sustaining chain reaction
    • Implications: fuel security, reduced waste, clean energy

    Loses marks

    • Confusing FBR with Pressurized Heavy Water Reactor (PHWR)
    • Defining criticality as 'critical failure' or accident
    • Ignoring the 'clean energy' aspect of implications

    Earns more

    • Mention of liquid sodium coolant in FBR
    • Reference to 500 MWe capacity of PFR
    • Mention of closed fuel cycle
    • Reference to indigenous development (BARC)

    Extra mark

    • Mention of specific date of criticality (Aug 2010)
    • Mention of specific fuel used (MOX fuel)

Practice this exact question

Write your answer and it is marked point by point against the model answer above — what you covered, what you missed, what you got wrong.

Evaluate my answer →

More from General Studies 2026 GS Paper III