The Coolant Cell is an item created by putting a Water Cell in an Extractor. It is most often used in Nuclear Reactors to keep them from exploding, but also extends to IC2 Upgrades and Hydration Cells as well.
Nuclear Reactor Usage
This cell can cool the reactor, absorbing up to 10.000 heat units from uranium cells, while cooling itself down by 1 unit of heat/tick. When the "charge" on a Coolant Cell is low, leave it in a Nuclear Reactor that is generating no heat (at all - no Uranium cells) and it will regain the "charge" over a period of time. It will not recharge whilst in a chest. Stored heat can be drawn out by an Integrated Heat Disperser to prevent an overburdened cooling cell from melting.
Coolant Cells do not stack