Understanding the CANDU Reactor and its Components

The question asks about the specific substance that serves a dual purpose – acting as both the moderator and the coolant – in a Canada Deuterium Uranium (CANDU) nuclear reactor, which is known for using natural uranium as fuel.

Nuclear reactors like the CANDU type rely on the process of nuclear fission to generate heat, which is then converted into electricity. For fission to occur efficiently using natural uranium, the fast neutrons released during fission must be slowed down. This is the role of a moderator. A good moderator slows neutrons without absorbing too many of them.

Simultaneously, the fission process produces a large amount of heat. This heat must be removed from the reactor core and transferred to a power generation system (like a steam turbine). This is the function of the coolant. A good coolant has high thermal capacity and can be circulated safely through the reactor.

The Role of Heavy Water in CANDU Reactors

CANDU reactors are specifically designed to use natural uranium ($\text{U-238}$ with about $0.7\%$ fissile $\text{U-235}$) as fuel. Natural uranium does not contain a high enough concentration of $\text{U-235}$ to sustain a chain reaction with typical moderators like light water because light water absorbs too many neutrons. Therefore, a moderator that absorbs very few neutrons is required.

Heavy water ($\text{D}_2\text{O}$), where the hydrogen atoms are replaced by their heavier isotope deuterium, is an excellent moderator because deuterium absorbs far fewer neutrons than normal hydrogen. This low neutron absorption allows a chain reaction to be sustained even with natural uranium fuel.

In CANDU reactors, heavy water is also used as the primary coolant. It circulates through the fuel channels, removing the heat generated by fission. The heated heavy water then flows to heat exchangers where it transfers its heat to light water, producing steam to drive turbines.

Thus, heavy water effectively performs both crucial functions in the CANDU reactor: moderating neutrons and cooling the reactor core.

Examining Other Options

• Light water ($\text{H}_2\text{O}$): Light water is commonly used as both moderator and coolant in many reactors (like PWRs and BWRs). However, because normal hydrogen absorbs more neutrons than deuterium, reactors using light water as a moderator typically require enriched uranium fuel to sustain a chain reaction. CANDU uses natural uranium, so light water is not suitable as the moderator.
• Carbon dioxide ($\text{CO}_2$): Carbon dioxide can be used as a coolant in gas-cooled reactors, but it is not effective as a moderator.
• Sodium ($\text{Na}$): Sodium is used as a coolant, particularly in fast breeder reactors, because it has good heat transfer properties and does not moderate neutrons significantly (fast reactors need fast neutrons). It is not used as a moderator.

Based on the requirements of a natural uranium reactor and the properties of the substances, heavy water is uniquely suited to perform both the role of moderator and coolant in a CANDU reactor.

Conclusion

In a Canada Deuterium Uranium (CANDU) reactor, which is fuelled by natural uranium, the function of both the moderator and the coolant is performed by heavy water.

Revision Table: CANDU Reactor Components

Additional Information on Nuclear Reactor Types

Different types of nuclear reactors use different combinations of fuel, moderator, and coolant, which influences their design and operation:

• Pressurized Water Reactor (PWR): Uses enriched uranium fuel, light water as moderator and coolant.
• Boiling Water Reactor (BWR): Uses enriched uranium fuel, light water as moderator and coolant.
• CANDU Reactor: Uses natural uranium fuel, heavy water as moderator and coolant.
• Gas-Cooled Reactor (GCR): Can use natural or enriched uranium, graphite as moderator, and gas (like $\text{CO}_2$) as coolant.
• Fast Breeder Reactor (FBR): Typically uses mixed oxide fuel (plutonium and uranium), no moderator (uses fast neutrons), and liquid metal (like sodium) as coolant.

The choice of moderator and coolant is critical for reactor design, especially when using natural uranium fuel, as it directly affects neutron economy and the ability to sustain a chain reaction.