This level name is unconfirmed. If you know of the correct name, please discuss it on the talk page before renaming it.
The Reactor Core (marked as "RC" in SGA on the map) is level number 5 in Keen 2. It is one of only two levels (besides the Tantalus Control Centers) necessary to complete the game (the other being the Bottom Galley). Passing it gives access to the entire front half of the Vorticon Mothership.
This is the first level in Keen 2 to feature a bridge, which, in this case, goes over a deadly pit of glowing things (plasma), presumably the reactor itself. It can be activated by moving a switch with the same key used to activate the pogo stick. The bridge switches look exactly like the Tantalus Ray activation switches.
The Reactor Core is a fairly easy level to beat, with only two Vorticon Grunts and the plasma pit being the only dangers. It is very small, and Keen only has to go down and then up again, avoiding some annoying Scrubs.
This is also one of the few levels (the other being Weaponry A) to be partially in perpetual darkness (different from the darkness obtained by turning off the lights in some levels). This may be due to the purpose of this level, which appears to be the place where the ship's fuel is converted into usable energy, for the ship's electrical systems and thrusters. The reactor may need to be not so illuminated in order to function properly.