PXK reactor
From Memory Alpha, the free Star Trek reference.
The PXK reactor was a pergium-powered nuclear reactor used during the 2210s. By the 2240s, though, it was basically out of common use.
Despite this, the subterranean Janus VI colony continued to use it to power its life support systems. Though the reactor was outdated and antiquated, it never gave any trouble until stardate 3196 in 2267, when the mother Horta stole, the one absolutely vital piece of machinery in the reactor, the reactor's main circulating pump, in an effort to force the colonists off the planet. Without the pump, the reactor would go supercritical in ten hours and irradiate half of Janus VI.
USS Enterprise chief engineer Montgomery Scott was able to cobble together a temporary replacement that lasted long enough for Captain James T. Kirk to make contact with the Horta and recover the missing part. (TOS: "The Devil in the Dark")
