Thermal (slow-neutron) reactor versus fast breeder reactor: which statement correctly contrasts their neutron energy usage and typical characteristics?

Introduction / Context:
Nuclear reactors are often categorised by the neutron spectrum in which most fissions occur. Thermal reactors rely on slow (thermalised) neutrons, typically using moderators such as light water, heavy water, or graphite. Fast breeder reactors operate with fast neutrons and are optimised to convert fertile isotopes (e.g., U-238) into fissile ones (e.g., Pu-239).

Given Data / Assumptions:

• Thermal reactors: moderated spectrum; fuel commonly low-enriched uranium.
• Fast breeders: minimal or no moderation; liquid-metal coolants such as sodium are common.
• Comparisons are qualitative and typical rather than absolute for every design.

Concept / Approach:
Slow neutrons have higher fission cross-sections for isotopes like U-235, enabling stable chain reactions with moderation. Fast systems exploit higher-energy neutrons to achieve breeding ratios greater than one, often resulting in higher power density cores but stricter material and heat removal constraints.

Step-by-Step Solution:
Identify the defining neutron spectrum for thermal reactors: slow/thermalised.Recognise fast breeders use unmoderated or weakly moderated fast neutrons.Select the statement aligning with these fundamentals.

Verification / Alternative check:
Design documents and introductory reactor physics texts consistently define thermal reactors by their moderated neutron spectrum; this is independent of specific fuel cycles or coolants used.

Why Other Options Are Wrong:

• Uses faster neutrons: Opposite of thermal definition.
• Higher power density for thermal vs fast breeder: Generally fast cores achieve higher power density.
• Requires less fuel at same power: Breeders are designed for fuel efficiency and conversion.
• Converts more fertile material: Breeders outperform thermal systems in conversion ratio.

Common Pitfalls:
Assuming coolant choice alone determines neutron spectrum; it is moderation and core design that dominate spectrum characteristics.

Final Answer:
A thermal reactor uses slower neutrons for fission