The reactor room was the location within main engineering on Constitution-class refit starships where the ship's dilithium crystals were contained, and also where the crystals connect to the matter-antimatter reaction that created the conditions necessary for warp drive.
The room's purpose was to house the dilithium crystals and a point for engineers to control the power distribution network related to the ship's warp drive.
23rd century design Edit
On Constitution-class starships, the room was a partitioned section within main engineering located near the vertical warp core. The room was isolated in case of radiation leaks with transparent glass. A circular doorway served as the room's entrance, which was controlled by a console directly opposite the entrance way. Inside the room itself there were several computer consoles and modules that served to control the dilithium power levels and indeed the matter-antimatter reaction. Within the room in the center a cylinder housing contained the dilithium crystals and a circular tube served as access to the ship's warp core.
After several attacks by Khan Noonien Singh, who had commandeered the USS Reliant, main engineering aboard the USS Enterprise was severely damaged. The dilithium control room was leaking antimatter radiation through the crystal housing and the crystal housing itself was damaged. In order to avert a serious problem, the ship's chief engineer entered the chamber and deactivated the warp drive before succumbing to radiation poisoning and requiring medical treatment. Since Khan initiated the Genesis Device, the Enterprise was unable to escape without warp drive. In a heroic sacrifice, Captain Spock made his way to engineering and entered the chamber. He then released the build-up of antimatter radiation into the room. This was done in order to repair the damaged warp drive by releasing the radiation itself and then re-routing power back to the crystal chamber to bring the warp drive back online.
Although Spock succumbed to the lethal levels of radiation and died within the room, his body was removed from the chamber. (Star Trek II: The Wrath of Khan)