The fusion reaction subsystem was one of the engineering systems aboard Federation starships. This subsystem was responsible for generating the impulse power of these ships. The fusion reactors were an integral component of this subsystem. On Galaxy-class starships, the subsystem which generated the main impulse power was located on decks 23 to 25. The location of this subsystem was labeled on the schematic of the secondary hull of the USS Yamato in 2365. (TNG: "Contagion" , okudagram)