Neutron moderation in reactors: Slowing down fast neutrons is desirable because slow (thermal) neutrons are generally more effective at inducing fission in common reactor fuels.

Introduction / Context:
Most commercial reactors (e.g., light-water reactors) are thermal reactors that employ moderators to slow neutrons. Understanding why thermal neutrons are preferred illuminates core design, material selection, and safety.

Given Data / Assumptions:

• Fuel contains fissile isotopes such as U-235 or Pu-239.
• Moderator materials (water, heavy water, graphite) are present in thermal reactors.
• Goal: maximize probability that a neutron causes fission rather than escaping or being captured non-productively.

Concept / Approach:
Fission cross-sections of many fissile nuclides are much higher for thermal (slow) neutrons than for fast neutrons. Moderation increases the likelihood of fission per neutron, enabling a sustained chain reaction with lower enrichment and more compact cores.

Step-by-Step Solution:

Verification / Alternative check:
Cross-section data tables show orders-of-magnitude increase in fission probability for U-235 at thermal energies versus MeV energies, affirming the desirability of moderation in thermal reactors.

Why Other Options Are Wrong:
“Disagree” ignores the fundamental cross-section behavior that underpins thermal reactor operation.

Common Pitfalls:
Assuming all reactors are thermal. Fast reactors avoid moderation intentionally, but the statement specifically addresses why slowing down is desirable in typical thermal designs.

Final Answer:
Agree