Moderator identification: “heavy water” used to slow neutrons in certain reactors is best described as which substance?

Introduction / Context:
Moderators slow down fast neutrons to thermal energies where fission in certain fuels is more probable. Heavy water is a classic moderator used in specific reactor designs to achieve high neutron economy and enable the use of lower-enrichment or even natural uranium fuel.

Given Data / Assumptions:

• Term: “heavy water.”
• Application: neutron moderation in reactors.
• Chemical identity is sought.

Concept / Approach:
Heavy water is D2O (or primarily HDO in mixtures), where hydrogen is replaced by deuterium (D), an isotope of hydrogen with one proton and one neutron. Replacing protium with deuterium reduces neutron absorption, improving moderation efficiency compared to ordinary light water (H2O).

Step-by-Step Solution:
Identify heavy water: molecular water with deuterium in place of protium.Note performance: lower neutron absorption cross-section than light water.Therefore, define it as a compound of oxygen and deuterium.

Verification / Alternative check:
In practice, reactor-grade heavy water has high D2O concentration (> 99%) for optimum neutron economy.

Why Other Options Are Wrong:
“Highly purified water” describes demineralised H2O, not heavy water.“Water with heavy-metal salts” changes chemistry and does not define heavy water.“None of these” contradicts the correct definition.

Common Pitfalls:
Assuming “heavy” refers to impurities; confusing isotopic substitution with contamination.

Final Answer:
A compound of oxygen and deuterium