Uranium enrichment is enormously complicated. It is the most difficult technological challenge in the nuclear fuel cycle. This is due to the fact that the two uranium isotopes have the exact same physical properties, and therefore, one cannot use any chemical procedures to separate them. The only difference between the two isotopes is their weight, where U235 is about one percent lighter than U238, because it has three fewer neutrons in its nucleus.
The most common method to enrich uranium is called Cascade Gaseous Centrifuge. Here's how it works: The uranium oxide-the yellow cake-is combined with fluoride and turned into a gas. This gas-uranium hexafluoride (UF6), commonly referred to as "hex"-is fed into a series of vacuum tubes, each about 6 feet long and 8 inches in diameter. At the center of each tube there's a rotor. When the rotor is spun rapidly-at twice the speed of sound-the heavier gas molecules with U238 congregate towards the cylinder's outer edge. There's a corresponding migration of U235 gas molecules near the center. This enriched gas is siphoned off from the center and moved forward to the next centrifuge while the depleted gas is sucked out from the edge of the tube and sent back to the previous one. The process of moving the material from one centrifuge to the next and on to another is known as "cascade." At each stage the hex is slightly enriched over the previous one. It takes about twenty stages to achieve 4% percent enrichment, which is suitable for nuclear power plants.
What about weapon's grade? The exact same process, if repeated over and over again, will produce 90% or more enriched uranium which is ready for weaponizing.
Iran has constructed an elaborate and sophisticated underground uranium enrichment plant at Natanz about four hours drive south of Tehran. The Iranians have kept a tight lid over this facility until recently. Some intelligence agencies believe that the Iranian's centrifuge technology is borrowed or stolen from that of the Russians. Others believe they are based on Pakistani designs and procured from A.Q. Khan.