Moderator versus reflector materials in reactors: Is the statement “materials that make good neutron moderators do not make good neutron reflectors” correct?

Introduction / Context:
In reactor physics, moderators slow down (thermalise) fast neutrons, while reflectors return leaking neutrons back into the core. Many practical materials can serve as both to varying degrees, and understanding this overlap is important in core design and shielding decisions.

Given Data / Assumptions:

• Common moderators: light water, heavy water, graphite, beryllium.
• Common reflectors: graphite, beryllium, water, steel (less effective as moderator).
• Assessment is qualitative regarding scattering and absorption traits.

Concept / Approach:

Good moderators have high scattering cross-section and low absorption cross-section to efficiently slow neutrons. Good reflectors also rely on high scattering with low absorption to bounce neutrons back. Therefore, some excellent moderators (graphite, beryllium) are also excellent reflectors; water moderates well and reflects reasonably, albeit with higher absorption than heavy water or beryllium.

Step-by-Step Solution:

Verification / Alternative check:

Many reactor designs use graphite or beryllium as both moderator and reflector regions. Heavy water reactors also use D2O as moderator and reflector, confirming the overlap in function.

Why Other Options Are Wrong:

“Correct” contradicts empirical design. Conditional statements about fast reactors or heavy water are unnecessary; the general claim is false. Saying moderators are always reflectors is too absolute; some moderators reflect poorly depending on geometry and absorption.

Common Pitfalls:

Assuming mutually exclusive roles; forgetting that reflection is largely multiple scattering while moderation is energy loss per collision—both benefit from similar cross-section characteristics.

Final Answer:

Incorrect