The main circulating pump was a retardation mechanism used in PXK pergium reactors which prevented the reactor from going supercritical.
In 2267, the silicon-based lifeform known as a Horta stole the circulating pump from the reactor used to power life support on the Janus VI pergium mining colony, forcing USS Enterprise Chief Engineer Montgomery Scott to construct a makeshift replacement pump. Unfortunately, Scott noted, the replacement pump would not last long, and it was critical that Captain Kirk find the pump that the Horta had stolen. Eventually, Spock was successful in making contact with the Horta via mind meld, and the pump was returned. (TOS: "The Devil in the Dark")