Uranium 238, or u-238, is the most common isotope of uranium. If you shoot a neutron at it, it becomes u-239 and then something else called neptunium 239 and then plutonium 239, which is extremely fissile.
Slash: Man I'm bored. I wish I had some uranium 238 to blow something up.
Torch: Wtf? You wanna end up like that radioactive scout guy? Besides u-238 ain't the stuff used in atom bombs, it's u-235. You use u-238 to make plutonium.
Slash: Plutonium? Now you're talking! Get me some of that shit!