Fast breeder reactors: which material is used as the principal fissile fuel in the core?

Introduction / Context:
Fast breeder reactors (FBRs) operate with a fast-neutron spectrum and are designed to produce more fissile material than they consume by converting fertile isotopes (typically U-238) into fissile isotopes (such as Pu-239). Understanding the choice of core fuel clarifies how breeding and high power density are achieved.

Given Data / Assumptions:

• Core operates without a moderator (or with minimal moderation) to maintain fast neutrons.
• Blanket contains fertile material (often U-238) for breeding.
• Coolant commonly a liquid metal (e.g., sodium) to preserve a fast spectrum.

Concept / Approach:
Plutonium-239 has favorable fission cross-sections in a fast spectrum and is typically the primary fissile in FBR cores, often as mixed-oxide fuel (MOX: PuO2-UO2). The surrounding blanket of U-238 captures fast neutrons to create additional plutonium, enabling a breeding ratio above one.

Step-by-Step Solution:
Identify which isotopes fission efficiently with fast neutrons: Pu-239 is well suited.Recognize common core fuel forms: MOX with significant plutonium content.Eliminate options not serving as primary fissile in FBRs (radium is non-fissile, neptunium is not standard core fuel, thorium is fertile not fissile).Select plutonium.

Verification / Alternative check:
Historical and contemporary FBR programs (e.g., BN-series, Phénix, Monju, PFBR) rely on plutonium-bearing fuels to achieve fast-spectrum performance and breeding.

Why Other Options Are Wrong:

• Uranium alone: Low-enriched uranium is optimized for thermal reactors, not fast cores without plutonium makeup.
• Radium: Not a reactor fuel.
• Neptunium: Can be transmuted but is not the standard FBR fissile base.
• Thorium: Fertile (breeds to U-233) but not fissile itself.

Common Pitfalls:
Confusing fertile with fissile materials; assuming fast reactors mirror thermal reactor fuel choices.

Final Answer:
Plutonium