In heavy-water reactor technology, heavy water (D2O) primarily serves which roles within the nuclear reactor system?

Introduction / Context:
Heavy water reactors, notably Pressurised Heavy Water Reactors (PHWRs), rely on deuterium oxide (D2O) to achieve efficient thermal-neutron spectra while allowing the use of natural or low-enriched uranium fuel. Understanding the dual role of D2O clarifies why these designs differ from light-water reactors.

Given Data / Assumptions:

• Heavy water has low neutron absorption and effective moderation properties.
• In PHWRs, D2O appears both in the moderator system and (often) in the primary coolant circuit.
• Question asks for the combined functional roles.

Concept / Approach:
Moderation slows fast fission neutrons to thermal energies to enhance fission probability in U-235. Deuterium has a small absorption cross-section, enabling high neutron economy. In PHWR layouts, D2O moderates in a separate calandria vessel and also circulates as coolant in pressure tubes, carrying heat from the fuel to the steam generators. It is not intended as a neutron “poison”; rather, its low absorption is a key advantage.

Step-by-Step Solution:
Identify moderation function: D2O slows neutrons with minimal losses.Identify cooling function: D2O removes fission heat in pressure tubes.Confirm dual role in canonical PHWR designs (e.g., CANDU lineage).Select “Both coolant and moderator.”

Verification / Alternative check:
Standard PHWR schematics show separate moderator calandria filled with D2O and a D2O primary heat transport system, confirming the dual functionality in many implementations.

Why Other Options Are Wrong:
Coolant only / Moderator only: incomplete—PHWRs commonly use D2O for both roles.Neutron absorber: contradicts low absorption property of deuterium.Gamma shield: shielding is mainly via dense materials and concrete, not D2O as a prime function.

Common Pitfalls:
Confusing PHWR (heavy water) with PWR/BWR (light water) where H2O acts as both coolant and moderator; in PHWRs, the moderator and coolant are both D2O but often in separated circuits.

Final Answer:
Both coolant and moderator