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:

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

Power plants that have used nonwater coolants have included:

Some excellent references on coolants:

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