Fast breeder reactor coolants: why is molten sodium commonly selected as the primary coolant in fast breeder reactors?

Introduction / Context:
Fast breeder reactors operate with a fast neutron spectrum, requiring coolants that do not moderate neutrons while efficiently transporting heat from the core to the heat exchangers. Liquid metals, particularly molten sodium, satisfy these requirements and have been used extensively in experimental and prototype breeder systems.

Given Data / Assumptions:

• Sodium has high thermal conductivity and a high boiling point at near-atmospheric pressure.
• Fast reactors avoid moderators to maintain a fast spectrum for breeding (e.g., from U-238 to Pu-239).
• Coolant selection should minimize neutron absorption and moderation while maximizing heat removal.

Concept / Approach:
Molten sodium efficiently removes heat because of its excellent thermal properties and allows operation at high temperatures without high system pressure. Its low neutron moderation maintains fast-spectrum conditions, which is critical for breeding performance. Additionally, sodium's low vapor pressure contributes to plant safety margins relative to pressure excursions.

Step-by-Step Solution:

Verification / Alternative check:
Operational experience from sodium-cooled fast reactors demonstrates effective heat transport, with design attention to chemical reactivity of sodium with water/air and the need for intermediate loops to isolate sodium from steam generators.

Why Other Options Are Wrong:

• (a) Incorrect: sodium is a poor moderator, which is actually desirable for fast reactors.
• (b) Incorrect: breeding occurs in fertile material (e.g., U-238), not in the coolant.
• (d) Incorrect: coolant does not directly increase reaction rate; it affects temperature and feedbacks but not by design to raise fission rate.
• (e) Incorrect and irrelevant to reactor physics.

Common Pitfalls:
Confusing moderation needs of thermal reactors with fast reactors; ignoring sodium's chemical reactivity and the design measures taken to mitigate it.

Final Answer:
Because it enables rapid heat removal due to high thermal conductivity and high boiling point.