Tuesday in Geneva, negotiators from six nations will sit down to talks with Iran over that country's nuclear program. At the heart of the negotiations are Iran's centrifuges: machines that can be used to enrich uranium for use in nuclear power plants, or for use in a bomb. This double role of centrifuges has negotiators in a bind.
"What the centrifuge basically is, is a long thin cylinder," says Jay Laughlin, head of operations at URENCO, a company with a huge uranium enrichment plant in New Mexico. A centrifuge spins at extremely high speeds, causing one kind of uranium, known as uranium-235, to concentrate at one end. This concentrated, or enriched, uranium is fed into another centrifuge, which concentrates it a little more, and so on.
The New Mexico plant looks like a giant Ikea box store, filled with centrifuges. "I can't tell you the exact number of centrifuges," Laughlin says. The precise number is classified. "I'll tell you it's in the thousands or, tens of thousands would be probably a closer estimate."
As the concentration of uranium-235 goes up, it starts to become useful. At around 5 percent, the uranium will give you a nice, steady nuclear burn perfect for a power plant. At 20 percent it can be used inside special reactors relied on for some types of research. At 90 percent uranium-235, the fuel will ignite in a flash it's a bomb.
The URENCO plant is perfectly legal. Regulators watch it carefully to make sure the fuel it creates never goes above that 5 percent mark.
But Iran is a different story. Iran started its centrifuge program in the 1980s using designs bought off the black market. For many years, the endeavor was completely secret.
"It looked like it was set up as a military program," says David Albright, a physicist and president of the Institute for Science and International Security, a think tank that tracks covert nuclear programs. Iran's program quietly designed and developed its centrifuges until 2002, when the international community discovered that Iran was building a big centrifuge plant.
But here's where the story takes a twist. Iran didn't try to cover up what it had done.
"Iran did something very clever," Albright says. "I mean it knew it was caught, and so it decided to get ahead of [the bad publicity] a little bit."
Iran is a member of the Nuclear Non-Proliferation Treaty, an international agreement that allows nations to develop nuclear technology as long as they don't build a bomb. So the nation's leaders did a big mea-culpa. They admitted they had developed their program in secret, but denied ever intending it as a weapons program.
"They've taken a hard line," Albright says. "They deny they ever thought about building nuclear weapons ... ever."
Inspectors from the International Atomic Energy Agency began visiting Iran's enrichment facilities regularly in the early 2000s. They counted centrifuges, took samples of uranium to make sure it wasn't being enriched beyond 5 percent, and tried their best to ensure that the program stayed peaceful.
But at the diplomatic level, things were still tense. Intelligence sources knew that parts of the program remained hidden. For example, a second enrichment plant was exposed in 2009.
Despite the tensions, the day-to-day inspections remained relatively low key. Olli Heinonen of Harvard University, who studied radiochemistry and nuclear materials analysis, worked for years as an inspector in Iran. He says that the people running the program day-to-day were friendly enough:
"They are just like any other person,' he says. "They are very often young, enthusiastic engineers. They just want to do a good job."
But their job, says David Albright, was to vastly expand Iran's enrichment capability without technically breaking the rules of the Nuclear Non-Proliferation Treaty. And they've done it well. "In the last 10 years, Iran has developed its centrifuge program greatly," he says.
Today Iran has tens of thousands of centrifuges. They've enriched lots of uranium to the legal limit of 5 percent. But here's the trick: If it decides to, Iran could, with very little effort, feed that uranium back through the centrifuges and enrich it to a concentration level of 90 percent.
In just four to six weeks, Albright says, Iran "could have enough weapon-grade uranium for a nuclear weapon." Others say it would take longer, but most agree that Iran is within months or a year of getting the material it needs for a bomb.
Of course, the engineers Heinonen met would deny that this was a goal of their program. "But I don't think the guy on the facility floor knows the true intention of the program," Heinonen says. "Maybe very few people in the country know."
This is the challenge facing negotiators: You can inspect facilities and watch centrifuges spin. But you can't know the minds of the people who run them.