Plutonium Ingots are used to make the most powerful power sources. They are made from Uranium Ingots in the Ore Furnace.
Note: In Version 1.0.5 there is no recipe for making Uranium Ingot which makes this resource impossible to make for the time being. There is a temporary fix for this that can be found on the Bugs Glitches and Exploits page.
3 Uranium Ingot