Why Iran Needs So Many Centrifuges

Fifty-four thousand seems like a lot.

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Iranian scientists are trying to manufacture a more powerful centrifuge for enriching uranium, the country’s president announced on Thursday. Tehran has already set up 164 centrifuges and plans to install another 3,000 later this year as the government works toward its goal of 54,000. Why does Iran need so many centrifuges?

Because each one is so small. A centrifuge for enriching uranium consists of a thin vacuum tube with a rotor inside that spins around at tens of thousands of rpm. 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 standard nuclear weapon. *

Since each centrifuge is so small, this spin-and-repeat process can take a very, very long time. So far Iran has been relying on the P-1 design, which uses several aluminum rotor tubes connected by flexible joints. (The flexible joints allow you to increase the length of each device, which are still only about 2 meters long.) Now the Iranians are working on building an upgrade: The P-2 would be smaller than the P-1 (1 meter long), but its carbon fiber or maraging steel walls would enable it to spin much more quickly, which would in turn speed up the enrichment process.

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Explainer thanks David Albright of the Institute for Science and International Security.

Correction, April 19, 2006: This piece originally stated that the concentration of U-235 in uranium gas must be 80 to 90 percent to make a nuclear weapon. Although that’s the threshold for standard nuclear weapons, in theory you could make a weapon with concentrations over 20 percent. (Return to the corrected sentence.)