Fusion core

The fusion core was a section of Deep Space 9 and other space stations. The fusion core made up the bottom part of the lower core section of the station and housed six fusion reactors which provided power for the facility. In the consturction of Terok Nor the fusion core was built in the forth stage of the station's assembly, following the completion of the structure of the rest of the central core sections. In emergency situations the entire fusion core could be ejected from the station, as an overload in the core would obliterate anything within a hundred kilometers. 

History
Since Starfleet took over operation of Deep Space 9 they only maintained four of the six fusion reactors as the remaining two were rated borderline for safety reasons so were kept powered down. 

In April 2376 a trio of rogue Jem'Hadar fighters attacked DS9, they were destroyed before they could destroy the station themselves but four of the crew were able to beam aboard the station. The Jem'Hadar, led by Kitana'klan, remained shrouded and attempted to make their way to the stations fusion core to initiate an overload and destroy the station. Another Jem'Hadar, Taran'atar, also beamed to the station, but worked to stop the rogues, successfully killing three of them before they achieved their objective. 

Unfortunately he was not able to get to Kitana'klan before he was successful. By this time the station's personnel had also become aware of the situation and a team led by Kira Nerys and Elias Vaughn had also been tracking Kitana'klan. When they realised they were too late they, with Taran'atar, proceeded to the upper levels of the lower core where Kira initiated the lower core emergency separation. The fusion core was jettisoned, and 120km away from the station exploded, pushing the station out of it's position but not destroying it. The explosion could be seen from Bajor. 

Without it's fusion core the station was nearly powerless. Fortunately shortly after the incident an allied fleet arrived, in response to the initial Jem'Hadar attack. At the suggestion of Jean-Luc Picard each of the Federation vessels in the fleet donated an energy generator to the station. 

The generator network allowed the station to continue basic operations for a fortnight until a plan devised by Nog brought a new fusion core. A fleet of nine vessels were used to tow Empok Nor to the Bajor system, and shortly after the other station's fusion core was connected to DS9, restoring full power.