Coolants in nuclear reactors: Which molten metal is most commonly used for heat removal in fast reactors and experimental systems?

Introduction / Context:
Nuclear reactors require an efficient coolant to transport thermal energy from the core to heat-exchange equipment. Liquid metals are attractive because they have high thermal conductivity and operate at low pressure even at high temperatures. The question asks which molten metal is most commonly used, especially in fast breeder and research reactors.

Given Data / Assumptions:

• We are comparing practical reactor coolants among the listed metals.
• Focus is on historical and widespread use, not theoretical properties alone.
• Coolant must offer high heat capacity, good heat transfer, chemical manageability, and acceptable neutron economy.

Concept / Approach:
Liquid sodium has been the dominant choice for fast reactors (e.g., Phénix, BN series, EBR-II, PFBR) due to excellent heat transfer properties, low neutron moderation, low viscosity, and operation near atmospheric pressure. Alternatives like lead, lead–bismuth eutectic, and sodium–potassium alloy exist, but pure sodium is the most widely implemented molten metal coolant. The other options in the list are either impractical or obsolete as primary coolants.

Step-by-Step Solution:

Verification / Alternative check:
Many fast reactors worldwide (historical and current) have used sodium primary and secondary loops, validating its status as the common choice.

Why Other Options Are Wrong:

• Calcium: Highly reactive and not used as a mainstream reactor coolant.
• Mercury: Toxic, poor neutron characteristics, and thermophysical drawbacks.
• Zinc: Not adopted as a primary reactor coolant in practice.

Common Pitfalls:
Confusing sodium with lead–bismuth or NaK; while these are used in some designs, sodium has the broadest adoption history.

Final Answer:
Sodium