In nuclear reactor engineering, what is the primary function of a moderator within the core? Choose the best description that reflects standard terminology (i.e., how the moderator affects neutron energies and fission probability), without focusing on other control or safety components.

Introduction / Context:
In a thermal nuclear reactor, the moderator is a core material that changes the energy spectrum of neutrons. Most fissions in fuels like uranium-235 happen much more readily with slow (thermal) neutrons than with fast ones. The moderator’s job is to reduce neutron kinetic energy efficiently while avoiding excessive neutron absorption, thereby sustaining a stable, controllable chain reaction.

Given Data / Assumptions:

• The reactor is a thermal system (e.g., PWR, BWR, PHWR, graphite-moderated types).
• Fission cross section for U-235 rises sharply as neutron energy drops from fast to thermal range.
• Common moderators: light water, heavy water, graphite; each has low absorption and good scattering characteristics.

Concept / Approach:
Elastic scattering with low-atomic-mass nuclei removes energy from neutrons in many small steps. An effective moderator has a small microscopic absorption cross section and a favorable scattering cross section so that neutron losses are minimized while the spectrum softens. The control of neutron population (and thus power) is primarily performed by control rods and soluble poisons; the moderator’s role is spectral, not regulatory.

Step-by-Step Solution:
Identify what sustains fission in thermal reactors: a supply of thermal neutrons.Relate moderation to neutron energy: repeated scattering slows neutrons.Separate functions: moderator sets spectrum; control rods set reactivity by absorption.Choose the statement that highlights slowing down neutrons with minimal capture.

Verification / Alternative check:
Standard reactor physics texts depict the “slowing-down region” of neutron life cycle, where moderators bring neutrons from MeV to eV energies before thermalization, maximizing thermal fission probability in U-235 or Pu-239 fuel regions.

Why Other Options Are Wrong:
Absorb neutrons: that would be a poison or control material, not a moderator.Control the chain reaction directly: that is the job of control rods/soluble boron.None of these / Increase energy: contradicts the definition of moderation.

Common Pitfalls:
Confusing moderation (energy reduction) with reactivity control (neutron removal). Also, assuming all moderators are equally effective; absorption penalties differ widely (e.g., heavy water versus light water).

Final Answer:
Slow down fast neutrons to thermal energies without capturing too many of them