Turbines have the ability to absorb the steam that is coming from water blocks that are heated by a fission reactor. It must be placed directly above the water blocks. As the turbine blades are turned by the steam, current is generated. A wire must be plugged in from the top of the turbine in order for it to produce electricity. Turbines may take a few seconds to turn as steam is produced, indicated by their sound and blade speed. Note that turbines generate a fairly high voltage of electricity so a thick wire is recommended.
Large Reactor Turbines
Putting 9 turbines in a 3x3 pattern flat on the ground and right clicking the turbine in the center with a wrench will turn the small reactor turbines into a large reactor turbine. The large turbine is more efficient and generates more power than the small turbine, so it is recommended to make one.
Fusion Reactor Tutorial