Neutron moderation: which of the following is not a good moderating material for thermal reactors?

Introduction / Context:
Moderators slow down fast neutrons to thermal energies to sustain fission in thermal reactors. An effective moderator has high scattering effectiveness per absorption (low capture cross-section and suitable atomic mass) so that neutrons thermalize without being lost.

Given Data / Assumptions:

• Materials under consideration: concrete, boron, and 18/8 stainless steel.
• Comparison is with standard moderators such as light water, heavy water, graphite, and beryllium.

Concept / Approach:
Concrete is primarily a structural/shielding material; while it can slow neutrons, it also contains elements that absorb neutrons and is not an efficient, controllable moderator inside the core. Boron (notably B-10) has a very high neutron absorption cross-section and is deliberately used as a poison or control material, not as a moderator. Austenitic stainless steel (18/8) has relatively high absorption compared with classic moderators and is used for structural components, not for moderation.

Step-by-Step Solution:

Verification / Alternative check:
Standard reactor design texts list graphite, heavy water, and light water as preferred moderators; boron is explicitly used to absorb neutrons, not to moderate; steels are avoided for moderation roles.

Why Other Options Are Wrong:

• Graphite (option e) is an established high-quality moderator.

Common Pitfalls:
Assuming anything that slows neutrons suffices; ignoring absorption penalties that undermine neutron economy.

Final Answer:
All of the above