It’s very, very difficult to make fuel for nuclear bombs because the elements are only found in gas. Gas like in vapor. How can the elements in a vapor be obtained? The gas in spun in a centrifuge and a tiny, tiny bit of the necessary material is obtained. That gas is put into more and more centrifuges. It’s called a cascade system. Finally a teeney, tiny bit of the material, say Uranium is produced from the gas.