Light water, composed of two hydrogen atoms and one oxygen atom (H₂O), is a common and versatile coolant utilized in nuclear reactors, including both pressurised and boiling water reactor designs.

Advantages:

• Light water serves as both an efficient coolant and moderator within the reactor.
• Widely available and cost-effective, light water proves to be an economical choice for reactor cooling systems.

Disadvantages:

• With a lower boiling point compared to some alternatives, light water imposes a restriction on the maximum operating temperature of the reactor.

Heavy water refers to deuterium oxide, D₂O, which is two deuterium atoms and one oxygen atom. Deuterium is double the mass of hydrogen, hence the name ‘Heavy Water.’  Heavy water has a higher density (higher than light water) and specific head capacity, thus making it efficient at absorbing and carrying heat in CANDU reactors.

Advantages:

• Heavy water has a higher density and specific head capacity than light water, thus the reactor can function at higher temperatures.
• Heavy water can be both a coolant and a moderator.

Disadvantages:

• Heavy water is expensive to produce, thus affecting the overall cost of operation.
• Deuterium is rare, increasing the cost and decreasing ease of use.

Helium (He) is a clear and odourless gas with the lowest boiling and freezing points of any other known substance. Helium has both high thermal conductivity and specific heat, along with its chemical inertness (un-reactiveness), Helium is an attractive coolant for high temperature gas cooled reactors.

Advantages:

• Due to its low boiling and freezing point, it remains a gas at all temperatures and pressures needed.
• Helium’s chemical inertness provides advantages in safety as it is highly resistant to corroding the reactor components over time.

Disadvantages:

• Helium has the weakest heat removal and retention properties.
• In the current market, Helium shortages can occur, resulting in it being harder to acquire and costing more to do so.

Carbon Dioxide, CO₂, is a colourless gas made of one carbon atom and two oxygen atoms. CO₂ is used as a coolant in advanced gas-cooled reactors. CO₂ has a high specific heat capacity and high thermal conductivity which makes it very efficient to transfer heat.

Advantages:

• CO₂ is less reactive than water when used in a cooling system, thus is inherently safer.
• CO₂ is more flexible surrounding operating pressures and temperatures, thus creating a more stable system.

Disadvantages:

• The use of CO₂ as a coolant results in a lower power density than water, thus requiring larger reactors and reduced efficiencies.
• CO₂ leaks are harder to detect without equipment as it is a clear gas, however infrared sensors are easy to use to detect these and provide rapid measurements.