Iranian President Mahmoud Ahmadinejad (center) visits a uranium enrichment facility in Natanz, Iran, in 2008. Enriching uranium requires many fast-spinning centrifuges, arranged in what's called a cascade.
Iran's government on Thursday made clear it has no interest in direct talks until the U.S. eases sanctions that have been squeezing Iran's economy. But the Obama administration isn't budging and says the ball is in the Iranians' court.
The suspicion that Iran wants to make a nuclear weapon is the rationale for the sanctions as well as for veiled threats of U.S. or Israeli military action if those sanctions fail.
Iran's perceived nuclear aspirations are also the subject of a global effort that keeps popping up in the news: the game of "nuclear keep-away" to keep Iran from buying or manufacturing centrifuges, the machines that make uranium suitable for a bomb.
"We call it the long pole in the tent," physicist David Albright, a former United Nations weapons inspector, tells All Things Considered host Robert Siegel. "Getting the wherewithal to make the weapon-grade uranium or the separated plutonium is harder than learning how and assembling everything you need to know to make the nuclear weapon itself."
Separate Your Isotopes
Most uranium is useless for nuclear fuel or weapons. Less than 1 percent of it is the light, radioactive isotope uranium-235 that's used for power plants and bombs.
But if you gasify uranium with fluorine and put it in a rapidly spinning centrifuge, you can separate the nuclear wheat from the chaff. As the centrifuge spins, the heavier, nonradioactive U-238 moves to the centrifuge wall, while the U-235 moves toward the center, explains Houston Wood, a University of Virginia engineering professor who worked on gas centrifuges at the U.S. Department of Energy.
Ultimately, two streams are removed: one of enriched U-235, the second depleted in U-235. And when you take the material that has more 235 and conduct the process over and over again, you eventually get uranium that is 90 percent U-235: weapons-grade uranium.
Build Lots Of Centrifuges — Fast Ones
Getting that weapons-grade product takes thousands of centrifuges, spinning as quickly as possible and arranged in what's called a cascade.
How fast they can go depends on what they're made of. An aluminum centrifuge can only spin at about 350 meters per second before it bursts, explains Wood. Another material, maraging steel, can spin at 450 meters per second.
Even more appealing is carbon fiber, used to make products like golf club shafts and bicycle frames. Centrifuges made of carbon fiber can reach speeds of 1,000 meters per second.
That's where the game of nuclear "keep away" comes in. Much of the global effort is aimed at blocking Iran from getting the highly specialized materials needed to make those fast-spinning centrifuges.
Gather The Right Materials
"Iran successfully bought 70 tons of maraging steel, enough in theory for over 30,000 centrifuges, from Britain in the 1990s," says Albright. "The Brits caught them eventually and kept them from buying more."
The Department of Justice says the Iranians are still shopping for maraging steel. One of the department's hottest cases involves Iranian citizen Parviz Khaki, who the U.S. says tried to buy 20 tons of the material, allegedly with the help of a Chinese citizen.
While the steel has many "innocent" uses, says DOJ prosecutor Steven Pelak, the attempted transaction just didn't add up. In particular, the Chinese company trying to buy the material was a toy company — and "toy companies don't need 20 tons of maraging steel," Pelak says.
Khaki and the Chinese citizen were indicted in July and are now in custody in the Philippines, fighting extradition to the U.S.
Carbon fiber centrifuges spin even faster than those of maraging steel, but making the fiber is a complex process, says Zsolt Rumy, who runs a St. Louis company that makes carbon fiber. It also involves navigating a specialized industry where people know people.
In December, the Justice Department arrested an Iranian-American named Reza Hamid Hashemi in New York. The U.S. says Hashemi had been shopping for a type of carbon fiber used mostly by aerospace companies but also for centrifuges.
The Global Game Of 'Keep-Away'
U.S. sanctions against Iran have made the global game of nuclear keep-away easier. The measures are so broad, even shopping for a carbon fiber winding machine to make golf club shafts could run afoul of them.
But how successful can the U.S. really be at keeping these specialized products out of hands it deems dangerous?
In the case of Iran, Albright says the odds are 50-50. "What you see is Iran's procurements are often detected and stopped. ... In the end, though, what you're trying to do is buy time. You're trying to make it harder for them to succeed," he says.
"You want to disrupt or create bottlenecks in their ability to develop centrifuges and deploy them. You want to limit the number they can build," he says. But ultimately, he says, "you can delay their program, but you can't stop it."