In nuclear reactor engineering, which coolant is typically used in a fast breeder reactor (FBR) to remove heat while avoiding neutron moderation?

Introduction / Context:
Fast breeder reactors (FBRs) are designed to operate with a fast-neutron spectrum and to convert fertile isotopes (such as U-238) into fissile material (such as Pu-239). The choice of coolant is crucial, because many common coolants moderate (slow) neutrons, which would undermine fast-spectrum operation. This question reinforces why molten sodium is used widely in FBRs.

Given Data / Assumptions:

• Fast breeder reactor requires minimal neutron moderation.
• Coolant must provide excellent heat removal and good thermal-hydraulic performance.
• Chemical compatibility and corrosion behavior are manageable with proper materials and cover gas.

Concept / Approach:
Water (ordinary or heavy) is a strong neutron moderator, which is counterproductive in a fast reactor. Gas coolants like helium have very low moderation but comparatively low volumetric heat capacity, requiring high pressures and large flow rates. Liquid metals—especially molten sodium—offer high thermal conductivity and heat capacity with low neutron moderation, enabling compact cores and efficient heat transfer at near-atmospheric pressure.

Step-by-Step Solution:
Identify fast-spectrum requirement: minimize moderation.Evaluate coolants: H2O/D2O moderate strongly; helium has low moderation but low heat capacity.Liquid sodium: very high thermal conductivity and heat capacity, low neutron absorption and moderation.Conclusion: molten sodium is the standard FBR coolant.

Verification / Alternative check:
Historical and contemporary FBR programs (e.g., EBR-II, BN series, PFBR design choices) consistently use sodium for primary cooling due to its superior heat-transfer properties and suitable neutronic behavior in fast spectra.

Why Other Options Are Wrong:
Heavy water / ordinary water: strong moderators; unsuitable for fast-spectrum cores.Helium: acceptable spectrum-wise but limited by low density and lower heat capacity.Lead–bismuth: used in some concepts but less common than sodium in commercial FBRs.

Common Pitfalls:
Assuming high thermal efficiency demands water; spectrum requirements dominate.Ignoring sodium's chemical reactivity; while significant, it is managed by design and operating practices.

Final Answer:
Molten sodium