Coolant selection: liquid metals such as molten sodium are preferred in which reactor type due to neutronic and thermal-hydraulic requirements?

Introduction / Context:
Reactor coolant choice must align with the desired neutron energy spectrum. Fast breeder reactors (FBRs) rely on a fast spectrum to convert fertile isotopes (e.g., U-238) into fissile material (e.g., Pu-239) while producing power. Any coolant or moderator that strongly slows neutrons is counterproductive in an FBR.

Given Data / Assumptions:

• Objective: maintain a fast neutron spectrum.
• Need high thermal conductivity and low neutron moderation.
• Coolant must operate at high temperature at near-atmospheric pressure.

Concept / Approach:
Liquid sodium has excellent thermal conductivity and heat capacity per unit volume, enabling compact cores with high power density. Crucially, sodium has a low moderating power and low neutron absorption (for Na-23), preserving the fast spectrum needed for breeding. Operating near atmospheric pressure reduces mechanical loading on the primary system compared with high-pressure water.

Step-by-Step Solution:
1) Identify spectrum need: fast → avoid strong moderators (e.g., light water).2) Select coolant with high heat-transfer capability and minimal moderation → liquid sodium fits.3) Conclude that FBRs prefer sodium or other liquid metals for neutronics and thermal reasons.

Verification / Alternative check:
Historical and modern fast reactors (e.g., EBR-II, BN series) have used sodium; some designs use lead or lead-bismuth for similar spectral reasons, trading chemistry and activation differences.

Why Other Options Are Wrong:
Homogeneous or graphite-moderated reactors are thermal designs; sodium would not add benefit.Enriched-uranium LWRs depend on strong moderation by water, not liquid metal.

Common Pitfalls:
Overlooking sodium–water reaction hazards in steam generators; confusing breeding (fuel cycle) with simple power production.

Final Answer:
Fast breeder reactor