Fast breeder reactors (FBRs) in nuclear engineering: identify what they do NOT do from the list below.

Introduction / Context:
Fast breeder reactors are advanced nuclear systems designed to produce more fissile material than they consume. They operate with a fast neutron spectrum and a fertile blanket that breeds fissile isotopes. Knowing what FBRs do and do not do is a core concept in nuclear power technology.

Given Data / Assumptions:

• Standard liquid-metal-cooled fast breeder design (e.g., sodium-cooled).
• Fertile isotopes: U-238, Th-232. Fissile isotopes: U-235, Pu-239, U-233.
• Fast spectrum implies little or no moderator.

Concept / Approach:

FBRs deliberately avoid slowing down neutrons; fast neutrons increase breeding gain in blankets containing fertile material. Common fertile-to-fissile conversions include U-238 → Pu-239 and Th-232 → U-233. However, Th-232 itself is not fissile; it is fertile and must be converted to U-233 before it can sustain fission efficiently. FBRs commonly use liquid sodium because of its excellent heat transfer and low neutron moderation properties.

Step-by-Step Solution:

Verification / Alternative check:

Design literature distinguishes clearly between fissile driver fuel (e.g., Pu-239, U-235) and fertile blankets (e.g., U-238, Th-232) used for breeding.

Why Other Options Are Wrong:

use fast neutrons: Core principle of FBRs. convert fertile to fissile: The defining feature (breeding). use molten sodium: Common coolant choice in FBR practice. operate without a moderator: True for fast spectrum operation.

Common Pitfalls:

Confusing fertile with fissile; assuming thorium can directly serve as fissile material without prior conversion to U-233.

Final Answer:

use Th-232 as fissile fuel