A power rod, pile rod, fuel rod, energy rod, pile core or core was the fuel source used power unit-type energy production facilities. The Constitution-class starship USS Enterprise used power rods as a fuel, as did the Robot civilization encountered by the Enterprise in 2266.
After a million years of operation the Robots' power unit had depleted its power rod supply and the Robots took remote control of the Enterprise hoping to cannibalise it for a new energy source. The Robots found themselves unable to access the Enterprise's power unit so using the threat of the life of his crew blackmailed Captain Kirk into retrieving power rod from it for them. Kirk however had other ideas and sabotaged the power rod, placing in it in a cobalt container, making it an explosive. Fifteen minutes later when the rod was placed in the Robots' power unit it exploded, destroying the unit and subsequently disabling the Robots. (TOS comic: "UK comic strips, first story arc")