Radioactive Material can only be found after a nuke explodes. When stepped on, it gives you Radiation 3 (III). It is advised to stay away from these as they can kill you!(But not in peaceful)
As of now it is not crafted. The only way to make it is to explode a nuke. You CANNOT pick it up with a shovel,or pickaxe,even with the silk-touch enchantment. Surprisingly, the pickaxe can pick it up faster than a shovel.