Nuclear moderation — suitability of light elements Evaluate the statement: “Light-weight substances cannot be effective neutron moderators.” Is this statement valid for thermal reactors?

Introduction / Context:
Moderators in thermal reactors slow down fast neutrons to thermal energies where fission cross-sections of fuels like U-235 are high. The suitability of a material as a moderator is largely determined by its atomic mass and neutron-absorption characteristics.

Given Data / Assumptions:

• Thermal reactor context (e.g., PWR, BWR, CANDU, graphite-moderated).
• Goal: high scattering per collision, low absorption.

Concept / Approach:
Light nuclei (hydrogen in light water, deuterium in heavy water, carbon in graphite) are highly effective moderators. Their low mass enables large energy loss per elastic collision. Materials that are both light and have low neutron absorption make excellent moderators. Therefore, the statement that light substances cannot be effective moderators is false.

Step-by-Step Solution:
Identify candidates: H2O, D2O, graphite, Be.Note large fractional energy loss per collision for low-A nuclei.Confirm low absorption cross-section is desirable; D and C are particularly favorable.Conclude that light elements are among the best moderators.

Verification / Alternative check:
Most commercial thermal reactors use light water or heavy water as moderators, demonstrating practical effectiveness of light elements.

Why Other Options Are Wrong:
“True” contradicts theory and practice. Other choices limit the statement improperly.

Common Pitfalls:
Confusing moderator with reflector or coolant; some materials serve multiple roles but moderation quality hinges on scattering and absorption properties.

Final Answer:
False