In nuclear reactor physics, what key property should an effective moderator (used in a thermal reactor) possess regarding neutron absorption?

Introduction:
Moderators in thermal nuclear reactors slow down fast neutrons to thermal energies, greatly increasing the probability of fission in isotopes like U-235. Understanding the required properties of a moderator is essential for safe, efficient reactor design and operation.

Given Data / Assumptions:

• We are referring to thermal reactors, not fast reactors.
• Moderator function is to reduce neutron energy primarily via elastic scattering.
• Question focuses on absorption characteristics, not phase or mechanical form.

Concept / Approach:
An ideal moderator has a high scattering cross section and a very low absorption cross section. Low absorption prevents loss of neutrons that are needed to sustain the chain reaction, while scattering on light nuclei (e.g., hydrogen in water, carbon in graphite) efficiently reduces neutron energy.

Step-by-Step Solution:
Identify moderator role: slow fast neutrons to thermal energies.Desirable property: low neutron absorption (i.e., a poor absorber) to minimize neutron losses.Select the option that states “poor absorber of neutrons.”

Verification / Alternative check:
Common moderators (light water, heavy water, graphite) are chosen precisely because they scatter well and absorb little compared to strong absorbers like cadmium or boron compounds.

Why Other Options Are Wrong:

• Good absorber: This would poison the reactor, reducing reactivity.
• “Must be a solid” or “must be a liquid”: Phase is not the defining criterion; both graphite (solid) and water (liquid) are used successfully.
• High atomic number: Heavy nuclei are less efficient at moderating; light nuclei work best.

Common Pitfalls:
Confusing moderators with control materials (e.g., cadmium, boron) whose purpose is to absorb neutrons and control reactivity.

Final Answer:
Poor absorber of neutrons