Pure oxide refractories are often monocrystalline and self-bonded. Which oxide brick is also known for use as a moderator material in certain nuclear reactor designs?

Introduction / Context:
Among pure oxide refractories, several have exceptional high-temperature properties and unique nuclear attributes. Beryllium oxide (beryllia), alumina (corundum), and thoria are advanced ceramics with high melting points and good thermal properties. In nuclear engineering, moderators slow fast neutrons to thermal energies without capturing too many neutrons.

Given Data / Assumptions:

• We compare oxide bricks by their nuclear relevance.
• “Moderator” implies strong scattering and very low absorption cross-section.
• Carborundum refers to silicon carbide, which is not an oxide and not a typical moderator.

Concept / Approach:
Beryllia (BeO) has very high thermal conductivity for a ceramic, high melting point, excellent thermal shock resistance, and—crucially—favourable neutron moderating properties with low neutron absorption. Graphite and heavy water are more famous moderators, but BeO has been used in specialized research and high-temperature reactor concepts. Corundum (Al2O3) and thoria (ThO2) are highly refractory but lack the optimal moderating characteristics that BeO offers.

Step-by-Step Solution:
Identify the oxide with moderator credentials: BeO (beryllia).Eliminate carborundum: it is SiC, not an oxide, and not a standard moderator.Corundum and thoria are refractories but not preferred as moderators due to less favourable neutron cross-sections.

Verification / Alternative check:
Nuclear materials references cite BeO as a potential moderator/reflector with low absorption and good thermal properties, though toxicity and cost limit widespread deployment.

Why Other Options Are Wrong:
Carborundum: not an oxide; primarily a structural/high-temperature semiconductor ceramic.Corundum/Thoria: refractory oxides but not used as mainstream moderators.

Common Pitfalls:
Assuming all high-melting oxides are suitable moderators; confusing reflectors with moderators.

Final Answer:
Beryllia