Moderator identification: which of the following is not used as a neutron moderator in nuclear reactors?

Introduction / Context:
Moderators slow fast neutrons to thermal energies to increase fission probabilities in fuels like U-235. Suitable moderators have low neutron absorption and good slowing-down power (high scattering per absorption). Distinguishing moderators from coolants and absorbers is a foundational reactor engineering skill.

Given Data / Assumptions:

• Common moderators include light water, heavy water, graphite, and beryllium/beryllia.
• Some materials serve different roles (e.g., sodium is a coolant in fast reactors).
• Boron-bearing substances are strong absorbers and used for control/poisoning rather than moderation.

Concept / Approach:
A moderator should scatter efficiently while absorbing minimally. Light water, graphite, and beryllium satisfy this (with varying absorption). Molten sodium is chosen for liquid-metal cooling in fast reactors specifically to avoid moderation. Boron hydrides contain boron, a strong neutron absorber; while hydrogens can moderate, the boron presence makes such compounds unsuitable as practical moderators in power reactors.

Step-by-Step Solution:
List each material’s typical reactor role.Light water/graphite/beryllium: established moderators.Molten sodium: coolant in fast reactors; not a moderator.Boron hydride: absorber/poison characteristics dominate; not used as a moderator.Select the clearly correct “not a moderator”: molten sodium.

Verification / Alternative check:
Fast-spectrum sodium-cooled reactors avoid moderation to maintain fast neutrons; sodium’s scattering is weak and absorption is non-negligible compared to ideal moderators.

Why Other Options Are Wrong:

• Light water: Canonical moderator in PWR/BWR.
• Beryllium: Effective low-absorption scatterer used in some designs and reflectors.
• Boron hydride: Not a moderator in practice but an absorber; included here as a distractor highlighting absorber vs moderator distinction.
• Graphite: Classic moderator in many designs.

Common Pitfalls:
Confusing coolant role (sodium) with moderation; assuming all hydrogenous compounds are good moderators despite strong absorbers like boron present.

Final Answer:
Molten sodium