Harris Nuclear Power Plant

Coolants

The Coolant used to cool the reactor in most commercial reactors today is water. In this case, the water is dual purpose since it also serves a moderating function (i.e. it slows the neutrons down to thermal energy to increase the likelihood of fission). Desirable properties for a coolant include:

Low absorption cross section (so radiation levels produced during operation are lower)
Abundant and Inexpensive (since so much is needed)
Noncorrosive or low corrosivity (so the pipes and structures coming in contact with it stay intact)
High heat transfer coefficient so that heat can be picked up by the coolant and moves elsewhere
Low viscosity (so that not too much electrical power is needed to pump it)
Can be kept as a liquid to high temperatures, even if it is under pressure.

Coolants that have been used, in test or commercial applications, include:

Light (contains two atoms of hydrogen) or Heavy Water (contains one of two atoms of deuterium)
Liquid metal (e.g. sodium, potassium, or NaK [an alloy combination of sodium and potassium])
Liquid organics (e.g. ethanol, propane, pentane, benzene, heptane)
Air, helium, or carbon dioxide gases

Power plants that have used nonwater coolants have included:

EBR2 used metal coolants for over 20 years
The French Phenix and Superphenix used liquid metal coolants for a number of years
Public Service of Colorado's Fort Saint Vrain High Temperature Gas Cooled reactor used helium coolant.
The Magnox and Advanced Gas Reactors in the United Kingdom use carbon dioxide gas as a coolant.

Some excellent references on coolants:

Coolants for Nuclear Reactors, Rizwan Ahmed, UC Berkeley Nuclear Engineering Course paper NE 161, November 28, 1994
Nuclear Heat Transport, M.M. El-Wakil, American Nuclear Society, ISBN 0-89448-014-6
Nuclear Reactor Engineering, Samuel Glasstone and Alexander Sesonske, Van Nostrand Reinhold, ISBN 0-442-20057-9