A PXK reactor was a pergium-powered nuclear fission reactor that was commonly used by the Federation in the late 22nd and early 23rd centuries. During this time, PXK reactors were used to power many facilities, such as the Earth Outpost Stations along the Romulan Neutral Zone and mining colonies, such as Janus VI. However, by the 2240s, the PXK reactors had become antiquated and replaced by warp-powered reactors, yet many of the original reactors were still retained. (TOS episode: "The Devil in the Dark"; SCE eBook: Foundations, Book One)