The reactor controller is the main control block for the reactor. This block needs to be placed once somewhere on the walls. Interact with it to assemble or update the reactor.
The GUI shows the current status of the reactor, along with component temperatures and statistics. The GUI only shows the first layer of the interior.
Danger
Always monitor your reactor's heat levels. A sustained overheat will trigger a nuclear meltdown, leading to significant structural damage.
For detailed information on reactor mechanics, see the nuclear reactor guide.