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Difficulty: Very hardDefense and Nuclear Technology

Consider the following statements regarding India's nuclear technology and three-stage nuclear power programme:

1. Pressurised Heavy Water Reactors (PHWRs), which form the backbone of Stage-1 of the programme, utilize natural uranium as fuel and heavy water (D2OD_2O) as both neutron moderator and coolant.
2. The Prototype Fast Breeder Reactor (PFBR) at Kalpakkam (Stage-2) operates using fast neutrons without a neutron moderator, employing liquid sodium as the heat transfer coolant.
3. In Stage-3 of the nuclear energy roadmap, Thorium-232 is designed to be used directly as a primary fissile fuel without requiring prior nuclear transmutation into Uranium-233.

Which of the statements given above is/are correct?

  1. 1 and 2 onlyAnswer
  2. B
    1 and 3 only
  3. C
    2 and 3 only
  4. D
    1, 2 and 3

Answer

1 and 2 only
The correct response is the option stating '1 and 2 only'. Stage-1 PHWRs use natural uranium fueled with heavy water (D2OD_2O) as both coolant and moderator. Stage-2 Fast Breeder Reactors operate without a moderator to maintain fast neutron flux for breeding Plutonium-239, using liquid sodium as the heat exchange medium. Statement 3 is false because Thorium-232 is fertile and cannot undergo direct nuclear fission without prior transmutation into Uranium-233.

Step-by-Step Solution

1
Evaluate Statement 1 regarding Stage-1 Pressurised Heavy Water Reactors (PHWRs).
Statement 1 is correct. PHWRs utilize unenriched natural uranium (≈0.7% U-235\approx 0.7\% \text{ U-235}) as fuel. Heavy water (D2OD_2O) serves as both moderator and coolant due to its extremely low neutron absorption cross-section.
Low neutron absorption by heavy water allows critical chain reactions to be sustained with natural uranium fuel.
2
Evaluate Statement 2 regarding Stage-2 Fast Breeder Reactor technology.
Statement 2 is correct. Fast Breeder Reactors like the PFBR at Kalpakkam do not use a moderator because high-energy (fast) neutrons are essential to convert fertile Uranium-238 into fissile Plutonium-239. Liquid sodium is used as a coolant because of its excellent thermal conductivity and low moderation properties.
Moderators slow down neutrons, which would prevent the fast-neutron breeding ratio from exceeding unity.
3
Evaluate Statement 3 regarding Stage-3 Thorium utilization.
Statement 3 is incorrect. Thorium-232 is a fertile isotope, not a fissile isotope. It cannot sustain a fission chain reaction on its own and must absorb a neutron to undergo nuclear transmutation (via Beta decay) into fissile Uranium-233 (232Th+n→233Th→β−233Pa→β−233U^{232}\text{Th} + n \rightarrow ^{233}\text{Th} \xrightarrow{\beta^-} ^{233}\text{Pa} \xrightarrow{\beta^-} ^{233}\text{U}).
Fertile materials cannot serve directly as primary fissile fuel without prior conversion.
4
Select the correct combination of true statements.
Statements 1 and 2 are true, making the combination '1 and 2 only' the correct choice.
Only statement 3 contains a conceptual error in nuclear physics.

Key Concept

India's Three-Stage Nuclear Power Programme (PHWR vs FBR vs Thorium Fuel Cycle)
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Topics

Defense and Nuclear Technology

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