Spinning Wheel, Got to Go 'Round

There are 6 currently unclassified methods for enriching Uranium, each more technically challenging than the previous, with *tiny* increases in production.

Slate has a really good article on the basic physics of what's going on in Iran around their enrichment program.

When uranium gas enters the tube and whips around at high speeds, the useful U-235 isotopes get separated out from their heavier cousins, the U-238 isotopes. The speed of the rotor determines how effectively a centrifuge can enrich uranium, and a rotor's maximum speed depends in part on the dimensions of the centrifuge. Since it's hard to maintain a balanced spin in a long tube, engineers have to keep the devices to less than a few meters in length. That means they can enrich only a few grams of material at a time.

Even those few grams need lots of trips through the centrifuge to reach the weapons-grade level. (For more on how enrichment actually works, click here.) Enrichment plants get the job done by connecting hundreds of small centrifuges together, in clusters called "cascades." The uranium gas enters one centrifuge after another, with the enriched gas that comes out of the first getting siphoned off and directed into the next. By routing the enriched product back through the cluster of centrifuges over and over again, you can keep increasing its U-235 content. Given enough time—and enough spins—you'll reach the 3 to 5 percent concentration you need to run a light-water reactor, or the 80 to 90 percent you need to make a nuclear weapon.