Deep inside the complex is the Clean Lab. It is a high-technology facility equipped to detect tiny levels of radiation. And it's where the IAEA inspectors bring their samples from Iran's nuclear sites for analysis.
Inside the Clean Lab, there is a hum from machinery surrounding a small, sealed-off chamber where visitors cannot go. Within that chamber, four people in spotless white suits are hovering around electronic consoles. Their shoes are left outside.
David Donohue is the head of the Clean Lab. He and his team are experts in the high-stakes game of determining the true nature of a country's nuclear program. The game pits radiation-detection equipment against official efforts to conceal activities that might lead to nuclear weapons.
Donohue says the Clean Lab has received some samples of uranium enriched to 50 percent from Iran.
"I think there were some particles seen with 50 percent, or between 30 and 50 percent enrichment, which is quite high," he says. "Of course, you need to have up to about 90-percent enrichment to make a nuclear weapon. But, even so, in a country which says they haven't enriched any uranium at all, to find these kinds of particles was a surprise and the [IAEA] is looking into the different explanations for that."
Iran denied it was trying to enrich uranium until an exiled opposition group exposed a secret underground pilot project near Natanz in 2002. Tehran later said the high levels of enrichment found at the site after it was opened to IAEA inspectors was caused by contaminated machinery imported from other countries.
Today, the nuclear crisis continues, as Tehran insists on its right under international treaties to pursue low-level uranium enrichment as part of a peaceful nuclear program. However, enrichment processes are hard to monitor, raising fears that they could be secretly applied to bomb making.
Iran's explanation of the origins of its highly enriched uranium has yet to be sufficiently proven to reassure the international community. So the IAEA has stepped up its inspection efforts to learn more about Iran's activities.
Donohue describes the sometimes cat-and-mouse nature of the inspection efforts. The IAEA experts like to target industrial and military sites equipped for the kind of high-precision machine work needed to produce nuclear components. But Tehran has at times barred inspection teams from sites for days. When the inspectors finally get access, they find only empty rooms -- and the battle of wits really begins.
"You go in with these swipe samples. You try to collect dust from places which haven't been cleaned lately, like the ventilation ducts or inaccessible places where they probably haven't cleaned very well -- or you sample the floor, or the walls, or something like that," Donohue says. "And you are just looking for these small traces of uranium [or] plutonium that would give you a clue what they did in there."
The task of detecting any radioactive material swept up in the samples falls to the nuclear engineers of the Clean Lab. Donohue says the laboratory has instruments capable of finding even the faintest traces.
"For this we need very expensive and very sensitive instruments, different kinds of mass spectrometers and an electron microscope, things like that, to target in on these few uranium particles that are there, to ignore all the millions of particles that are not of any interest," he says. "So, we have these instruments, they cost $1 million, some of them. And with this we are able to find the needle in the haystack, the five or 10 particles of uranium in a swipe [sample] that has a million particles of dirt."
If the Clean Lab team finds traces of radiation, it forwards the samples to other more specialized nuclear laboratories for further analysis. The other laboratories -- which can be in the United States, Russia, a European Union state, Japan, or Australia -- provide further information as to how and where the radioactive material was produced or used.
The IAEA hopes its inspections will ultimately provide the evidence for determining whether Iran does -- or does not -- have programs to develop nuclear weapons.
But so far, the agency says it is still far from being able to answer that question definitively.
IAEA Director-General Muhammad el-Baradei said this month that the inspectors are making "good progress" in understanding the nature of Iran's nuclear activities. He also called on Tehran to be "more transparent" with his agency if it wants to build confidence that Iran has only a peaceful nuclear program.