Fast breeder reactor (FBR) essentials: which statement correctly reflects the coolant and moderation features of a typical FBR used for breeding fissile material?

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