I'm presuming you mean the Nuclear warhead item? If so once you've produced one via 10 plutonium or 30 enriched uranium. You'll also need a rocket (the one used as ammo for the rocket launcher weapon) the two items, rocket and warhead can be combined by hand or in a assembling machine to make a nuclear rocket fired from the rocket launcher. This causes MASSIVE damage and has a large damage radius so be prepared to run once you fire it.
