There are three types of Coolant Cells, each used to craft the next.
When used in a Nuclear Reactor, the Coolant Cell serves simply as a large heat buffer. It does not dissipate heat on its own. It can be placed next to a Uranium Cell to collect heat or next to a Heat Exchanger to release heat.
A Coolant Cell cannot be placed into a Pressure Vessel Reactor until it is completely cool. A cool Coolant Cell can still be used in the Reactor Pressure Vessel as normal. If the Coolant Cell is removed from the Pressure Vessel Reactor, it must be cooled down before it can be used in a fluid reactor again. The Coolant Cell can be cooled by placing it into an EU Nuclear Reactor.
A fully heated Coolant Cell, like most other reactor components, will be destroyed.
Coolant Cells are also used to craft Overclocker Upgrades.