Fast breeder reactor (FBR) essentials: which statement correctly reflects the coolant and moderation features of a typical FBR used for breeding fissile material?
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AUses graphite as a moderator to slow neutrons
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BUses light water as the primary coolant in the fast core
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CUses molten (liquid) sodium as the primary coolant and operates without a moderator
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DMust use only U-235 metal as fuel in the core
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EUses helium as a moderator and coolant simultaneously
Answer
Correct Answer: Uses molten (liquid) sodium as the primary coolant and operates without a moderator
Explanation
Introduction / Context:Fast breeder reactors are designed to maintain a fast-neutron spectrum and convert fertile isotopes (like U-238) into fissile ones (like Pu-239) while generating power. Their thermal–hydraulic and neutronic choices differ from thermal reactors and are optimized for breeding ratios and core physics.
Given Data / Assumptions:
- Fast spectrum is desired; moderators would thermalize neutrons and defeat this.
- Coolant selection must remove heat efficiently while preserving fast spectrum.
- Common FBR coolants: liquid sodium (sometimes sodium–potassium), occasionally lead or lead–bismuth in other fast designs.
Concept / Approach:Liquid sodium has excellent thermal conductivity, a high boiling point, and low neutron moderation/absorption, making it a classic FBR coolant. The absence of a moderator is intrinsic to preserving fast neutrons needed for breeding and for fission in plutonium-rich fuels. Fuel types vary (MOX, metallic fuels with Pu/U mixtures), so insisting on U-235-only fuel is incorrect for typical FBRs.
Step-by-Step Solution:Eliminate options involving moderators (graphite, helium as moderator) since FBRs are unmoderated.Eliminate light-water coolant: it moderates and is uncommon in fast-spectrum cores.Reject “U-235 only” fuel: FBRs commonly use Pu-bearing fuels (e.g., MOX).Select “molten sodium coolant and no moderator.”
Verification / Alternative check:Summaries of prototype and experimental FBRs (e.g., FBTR, Phénix, BN-series) consistently use sodium coolant and exclude moderators in the fast core.
Why Other Options Are Wrong:Graphite/light water/helium moderators: contradict “fast” spectrum.U-235-only fuel: not characteristic of breeder cores; Pu-rich fuels are standard.
Common Pitfalls:Assuming any reactor that breeds must be moderated. “Breeder” refers to fissile production, not to neutron energy regime; a fast spectrum is typical for high breeding gain.
Final Answer:Uses molten (liquid) sodium as the primary coolant and operates without a moderator