In fast breeder reactors

Question

Examoogle · Accepted Answer

No moderator is used

Explanation: Fast breeder reactors (FBRs) operate using high-energy (fast) neutrons to sustain the chain reaction and breed fissile fuel from fertile material. Because fast neutrons are required to maintain this process, no moderator is used, as moderators would slow the neutrons down to thermal energies.

Working Principle: The chain reaction is maintained by neutrons with energy levels in the MeV range. Fast neutrons facilitate the capture of neutrons by fertile isotopes like $^{238}U$ to produce $^{239}Pu$, which is a fissile fuel. A liquid metal coolant, typically liquid sodium, is employed instead of a moderator because it has a very low neutron moderation cross-section and excellent heat transfer properties.

Key Formulas: $E = \frac{1}{2}mv^2$ — High kinetic energy of fast neutrons; $BR = \frac{	ext{Rate of production of fissile material}}{	ext{Rate of consumption of fissile material}}$ — Breeding Ratio definition

Key Points: FBRs use fast neutrons to maintain fission and convert fertile material into fissile fuel.; Liquid sodium is used as a coolant because it does not slow down neutrons effectively.; The breeding ratio is typically greater than 1, allowing the reactor to produce more fuel than it consumes.; Moderators like water or graphite would shift neutron energy to thermal ranges, preventing the breeding process.

Advantages: Extremely efficient utilization of Uranium resources; High operating temperatures leading to better thermodynamic efficiency

Disadvantages: High cost of liquid sodium coolant systems; Complexity in managing fast neutron radiation damage to materials

Applications: Sustainable power generation; Transmutation of long-lived nuclear waste

Additional Info: Standard coolants include liquid sodium or liquid lead-bismuth.. Option A (Heavy water) and Option C (Graphite) are moderators used in thermal reactors like CANDU or Magnox, not fast reactors.

Answer: B is correct — Fast breeder reactors require high-energy neutrons to sustain the breeding process, which is why no moderator is utilized.

Exam Tip: Remember: FBRs are 'fast' because they lack a moderator; Thermal reactors (like PWR/BWR) are 'slow' because they use water as both coolant and moderator.

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