Nuclear Reactor Fundamentals – What Does a Moderator Do? In a thermal nuclear reactor, a 'moderator' primarily performs which role with respect to fast neutrons produced by fission?

Introduction / Context:
Safe and sustained fission in most power reactors requires an abundance of thermal (slow) neutrons. The component responsible for turning fast neutrons into thermal neutrons, without excessively removing them from the system, is the moderator. Recognizing this function is foundational for reactor physics and engineering design choices (e.g., water, heavy water, graphite).

Given Data / Assumptions:

• Thermal reactor using a moderator such as light water, heavy water, or graphite.
• Fast neutrons are born at a few MeV of energy from fission events.
• Goal is to increase probability of further fissions in fissile isotopes (e.g., U-235) which have higher cross-sections at thermal energies.

Concept / Approach:

A moderator reduces neutron energy primarily by elastic scattering, transferring kinetic energy from neutrons to moderator nuclei. Good moderators have low neutron absorption cross-sections and atomic masses conducive to effective energy loss per collision (e.g., hydrogen in H2O, deuterium in D2O, carbon in graphite). By slowing neutrons to thermal energies while avoiding significant capture, the moderator raises the likelihood that they cause further fissions, sustaining the chain reaction.

Step-by-Step Solution:

Verification / Alternative check:

Reactor design texts quantify 'moderating ratio' = slowing-down power / absorption cross-section; high values (e.g., D2O, graphite) indicate good moderators, confirming the role stated.

Why Other Options Are Wrong:

Absorbing neutrons suppresses reactivity; saying 'does not absorb and has no interaction' ignores scattering; acceleration of neutrons does not occur in moderators; conversion to protons via beta decay is not a moderation process.

Common Pitfalls:

Confusing moderators with control rods (which deliberately absorb neutrons) or reflectors (which return neutrons to the core).

Final Answer:

slows down (thermalizes) fast neutrons with minimal absorption