An actively cooled Reactor is a Reactor that uses an external coolant to dissipate its internal heat.
The coolant will be vaporized in the process, and the resulting vapor can be extracted from the Reactor to drive the rotors of one or more Turbines.
To build an actively cooled Reactor, you need to include at least two Fluid Ports in the multiblock: one configured to insert a coolant into the Reactor and one to extract the generated vapor from the Reactor.
The extracted vapor can then be piped into your Turbines to generate energy.