Fast breeder reactor basics: which statement correctly characterises a fast breeder reactor (FBR)?

Introduction / Context:
Fast breeder reactors (FBRs) are designed to work with high-energy (fast) neutrons and breed more fissile material than they consume by converting fertile isotopes in a surrounding blanket. Their physics and design choices differ substantially from thermal reactors that employ moderators.

Given Data / Assumptions:

• “Fast” implies no moderator slowing down neutrons.
• Coolants are typically liquid metals (e.g., sodium) for excellent heat transfer at low pressure.
• Fuel is often mixed-oxide or metallic/plutonium-bearing, not natural uranium alone.

Concept / Approach:
A moderator would defeat the purpose of a fast spectrum by thermalising neutrons. Therefore, an FBR operates without a moderator and relies on design choices (core geometry, coolant, materials) that preserve neutron energies. Claiming natural uranium fuel with heavy water moderation describes a PHWR, not an FBR. Water-cooled graphite-moderated configurations describe thermal reactors (e.g., certain historical designs), not fast breeders.

Step-by-Step Solution:
Identify the essential feature: absence of a moderator in fast reactors.Eliminate options that include moderators or thermal-reactor pairings.Select the statement affirming no moderator is used.

Verification / Alternative check:
Standard reactor classifications list FBRs as unmoderated, often sodium-cooled, with U-238 blankets to breed Pu-239, confirming the chosen statement.

Why Other Options Are Wrong:
Natural uranium with heavy water: PHWR, not an FBR.Both/Neither: do not match defining characteristics.Water + graphite: thermal reactor pairing, not fast.

Common Pitfalls:
Equating “breeder” with any reactor that produces fissile material; spectrum matters. Fast breeders specifically avoid moderators to exploit fast-neutron fission and breeding.

Final Answer:
Operates without a moderator (fast spectrum)