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)

The question as printed in the 2026 General Studies paper

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Model answer

Drafted with AI 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. How we build rubrics and answers.

A Fast Breeder Reactor (FBR) uses fast, unmoderated neutrons, uranium-plutonium mixed oxide (MOX) fuel and liquid sodium coolant, and breeds more fissile Plutonium-239 from fertile Uranium-238 than it consumes. A thermal reactor (e.g., PHWR) slows neutrons with a moderator such as heavy water, mainly consumes scarce Uranium-235 and converts only part of the U-238 into plutonium (conversion ratio below 1).

Criticality is the state in which a fission chain reaction becomes self-sustaining: each fission generation produces exactly as many neutrons as are lost (k = 1). The 500 MWe Kalpakkam PFBR, designed by IGCAR and built by BHAVINI, attained first criticality on 6 April 2026, validating controlled fast-neutron fission indigenously.

Implications for Clean Energy: The PFBR launches Stage 2 of India's three-stage programme, multiplying plutonium for later thorium-blanketed breeders that convert thorium into fissile Uranium-233, unlocking India's large monazite thorium reserves. Its closed fuel cycle extracts far more energy per tonne of uranium and reduces long-lived waste, offering carbon-free baseload power for the 100 GW nuclear target (2047) and net zero by 2070.

How this answer will be evaluated

Framework: GS3 Technology: why it matters, use, benefit, risk, policy

What earns full marks: Clear distinction (fast vs thermal, breeding), precise definition of criticality, and strong link to fuel security.

Evaluation rubric

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

The answer Distinguish an FBR from a thermal reactor, explain 'criticality' with reference to the Kalpakkam PFBR, and state its implications for India's clean energy future. 10 marks · 150 words

compare— paired headings or point-wise contrast → key differences → significance → conclusion
Must cover
- FBR uses fast (unmoderated) neutrons; thermal reactor uses a moderator to slow neutrons
- FBR breeds more fissile Pu-239 than it consumes; thermal reactor is a net consumer of fissile material
- Criticality defined as a self-sustaining chain reaction (k = 1), linked to the PFBR milestone
- Implications: stage 2 of the three-stage programme, fuel security/thorium, clean baseload power
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 the PFBR
- Mention of closed fuel cycle and reduced long-lived waste
- Reference to indigenous development (designed by IGCAR, built by BHAVINI)
Extra mark
- Mention of the date of first criticality (April 2026)
- Mention of specific fuel used (MOX fuel)

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.

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