CANDU reactor classification: which description correctly characterises a CANDU nuclear reactor?

Introduction / Context:
CANDU (CANada Deuterium Uranium) is a family of heavy-water-moderated, heavy-water-cooled power reactors. Their design allows the use of natural uranium fuel, providing flexibility in fuel supply and online refuelling capability.

Given Data / Assumptions:

• Design uses D2O as both moderator and coolant (in separate systems).
• Fuel enrichment: natural uranium (≈0.711% U-235).
• Pressure tubes and online refuelling are typical features.

Concept / Approach:
Heavy water’s low neutron absorption enables criticality with natural uranium, avoiding enrichment. The reactor employs pressure tubes carrying coolant and fuel channels, surrounded by a large moderator tank (calandria) filled with heavy water. This configuration is distinct from light-water and fast breeder designs.

Step-by-Step Solution:
Identify moderator/coolant: both are heavy water systems.Identify fuel: natural uranium.Match to the description that states all three traits.

Verification / Alternative check:
Technical summaries of CANDU highlight natural-uranium fuel and heavy-water moderation/cooling as core characteristics enabling online refuelling and good neutron economy.

Why Other Options Are Wrong:
Highly enriched uranium: not characteristic of CANDU baseline.Homogeneous: CANDU is a heterogeneous lattice of fuel channels and moderator.Fast breeder: CANDU is a thermal-spectrum reactor, not a fast breeder.

Common Pitfalls:
Assuming heavy water automatically implies fast spectrum; confusing “homogeneous” with “uses one fluid” rather than neutronic geometry.

Final Answer:
Natural uranium fuelled, heavy water cooled and moderated