Noun, nuclear physics. A fuel rod is a tube packed with pellets of a fissionable material such as Uranium 235. Many rods are placed in a nuclear reactor where they undergo fission, releasing huge amounts of energy.
Noun, anatomical. A male penis. Placed in a vagina it is capable of releasing huge quantities of spermatozoa which look like little fish.
Dude 1: "Man last night was amazing!"
Dude 2: "How so?"
Dude 1: "My chick's reactor was hot so I stuck in my fuel rod, emptied it and she had a frickin' melt down like Three Mile Island!"