A reactor core is the device that powers a Romulan planetary base and Cardassian space stations.

According to false information feeded to the defector Admiral Alidar Jarok, on stardate 43462.5, it would take only 48 more hours till the reactor core on a base on Nelvana III would be online, completing the construction of the base. (TNG: "The Defector")

In 2369, Commander Benjamin Sisko noted that if the hull breaches continue it could happen that one affect the reactor core and they could loose the half station. (DS9: "Q-Less")

After been taken over by the Saltah'na energy spheres in 2369, the Klingon first officer Hon'Tihl placed a thalmerite device under a crossbeam of the reactor core of his Vor'cha-class attack cruiser, the IKS Toh'Kaht, to destroy the ship. (DS9: "Dramatis Personae")