Coolants and moderators: heavy water (D2O) is preferred over ordinary water in some reactors primarily because it acts as an efficient moderator while also serving as a coolant. Which statement best reflects this advantage?

Introduction / Context:
Reactor designers choose coolants and moderators to optimize neutron economy and heat removal. Heavy water (D2O) is prized in certain thermal reactors, such as pressurized heavy water reactors (PHWRs), because it is a superb moderator that slows neutrons with minimal parasitic capture while also functioning as a coolant to carry heat away from the core.

Given Data / Assumptions:

• D2O has a much lower thermal-neutron absorption cross-section than H2O.
• It remains a liquid under pressure at typical reactor operating temperatures.
• We compare moderation effectiveness and practical coolant behavior.

Concept / Approach:
An ideal moderator slows neutrons efficiently via scattering while absorbing very few, thereby enabling a self-sustaining chain reaction even with fuels of low fissile enrichment (including natural uranium). Heavy water meets this goal better than light water. As a coolant, D2O exhibits broadly similar thermophysical properties to H2O, making it usable in conventional heat-removal circuits when appropriately pressurized.

Step-by-Step Solution:

Verification / Alternative check:
PHWR designs demonstrate high conversion ratios and the ability to use natural uranium due to D2O moderation. Operating conditions still require pressurization to raise boiling temperature, similar to light-water designs.

Why Other Options Are Wrong:

• (b) Incorrect: D2O does not eliminate the need for pressurization; its normal boiling point is only slightly above H2O.
• (c) Incorrect: D2O undergoes radiolysis; it is not immune to radiation damage (though rates differ from H2O).
• (d) Incorrect because (b) and (c) are not fully true as stated.
• (e) Incorrect: D2O’s key benefit is moderation, not merely corrosion effects.

Common Pitfalls:
Assuming heavy water allows high-temperature operation without pressure; ignoring radiolysis management common to water coolants.

Final Answer:
Acts both as an efficient coolant as well as a moderator.