Fusion reactor

A fusion reactor was a type of power-generation devise using nuclear fusion of hydrogen isotopes into helium. Fusion reactors were commonly employed to generate power in the starships and facilities of many races.

Fusion reactors were used to power the impulse engines on Federation starships. In a each impulse engine had three impulse fusion reactors to provide the power. 

Fusion reactors were also used to provide power for large facilities. The Delphic Expanse spheres were powered by seven twelve kilometer long fusion reactors. Cardassian space stations also used fusion reactors. The fusion core of a Nor-class facility held six fusion reactors, which were more than enough to power the entire station. 

Since Starfleet took over operation of Deep Space 9 they only maintained four of the six fusion reactors as the other two were rated borderline for safety reasons. 

In April of 2376 the Jem'Hadar Kitana'klan attempted to destroy DS9 by initiating an overload in the station's fusion reactors. He was successful in starting the overload, but fortunately station personnel were able to jettison the fusion core before it exploded, saving the station.