The rumor is that those pellets (20mm diameter, 20mm long) are the active elements of the fuel rods in many utility power generation reactors
Eh? Nonsense. First of all those (the pictures) aren't 20mm long, and they're not "pellets." They're "typical" globs of the shape created by surface tension when you let some liquid metal solidify on a flat surface (my guess would have been solder, too...)
Second, there are no reactor designs that use metallic (pure) plutonium, and few reactors that use plutonium as a fuel at all (Japan seems to have some reactors, including the troubled ones, based on MOX fuel rods - Mixed oxides of uranium and plutonium.) Most of the plutonium in the average reactor is a waste product produced when (non-fissile) U238 absorbs neutrons, and this requires a lot of additional processing to be concentrated enough to be useful as fuel.