In reactor physics terminology, what does a moderator do to neutrons within the core?

Introduction / Context:
Moderators are central to thermal reactor design. Many fissile isotopes (such as U-235) have substantially higher fission cross sections for thermal (slow) neutrons than for fast neutrons. Therefore, thermal reactors employ a moderator to reduce neutron kinetic energy efficiently while minimizing neutron absorption.

Given Data / Assumptions:

• Typical thermal reactors (PWR, BWR, HWR) are considered.
• Moderator choices include light water, heavy water, and graphite.
• Goal is to enhance the probability of fission in the fuel with minimal losses.

Concept / Approach:
A moderator slows neutrons primarily through elastic scattering, transferring kinetic energy from neutrons to moderator nuclei. Effective moderators have low atomic mass (to maximize energy loss per collision) and low neutron-absorption cross sections (to preserve the neutron population). Heavy water and graphite excel in low absorption; light water moderates well but absorbs more than heavy water.

Step-by-Step Solution:
State function: reduce neutron energy from MeV to eV scale.Mechanism: multiple elastic collisions with moderator nuclei.Selection criteria: high scattering / low absorption, suitable thermal and mechanical properties.Outcome: higher thermal fission probability and sustained chain reaction.

Verification / Alternative check:
Cross-section data show the fission cross section of U-235 increases markedly at thermal energies; moderator presence shifts the neutron spectrum accordingly, improving reactivity in thermal designs.

Why Other Options Are Wrong:
Absorbs strongly: describes poisons or control materials, not moderators.Accelerates: not physically meaningful for neutrons in cores.Reflects: reflection is a separate function handled by reflectors; some moderators also reflect, but moderation specifically means slowing.Converts neutrons to protons: not a moderator function.

Common Pitfalls:
Confusing moderator with reflector or absorber.Assuming all moderators are equal; absorption penalties vary significantly (e.g., H2O vs D2O).

Final Answer:
Slows down the neutrons