Reactor types and moderators: heavy water (D2O) is used as the moderator in which reactor type?

Introduction / Context:
Moderators slow down neutrons to thermal energies to enhance fission probabilities in certain fuels. Heavy water (D2O) is an exceptionally effective moderator because of its very low neutron absorption while maintaining good moderating power.

Given Data / Assumptions:

• We are matching reactor designs to their characteristic moderators.
• PWRs and BWRs employ ordinary light water as coolant and moderator.
• CANDU (Canada Deuterium Uranium) designs use heavy water as moderator—and often as coolant.

Concept / Approach:
Heavy water’s low absorption enables operation on natural uranium fuel. CANDU reactors exploit this to avoid enrichment while achieving adequate reactivity with pressure tubes and separate moderator calandria. Other listed designs rely on different moderator and coolant strategies.

Step-by-Step Solution:
Identify which standard design explicitly specifies D2O moderation.Recognise CANDU as the canonical heavy-water system.Select CANDU reactor.

Verification / Alternative check:
Public design descriptions consistently list D2O moderation for CANDU; PWR/BWR employ H2O; fast reactors avoid moderation and use liquid metals like sodium for cooling; graphite-moderated types use graphite, not D2O.

Why Other Options Are Wrong:

• PWR/BWR: Light water moderators.
• Molten-sodium fast reactor: Minimal moderation; sodium is coolant.
• Graphite-moderated CO2-cooled: Graphite is the moderator.

Common Pitfalls:
Assuming all water-cooled reactors use the same moderator; overlooking the critical difference between light and heavy water in neutron absorption.

Final Answer:
CANDU reactor