At a remote site 200 miles south of Tehran, Iranian scientists are learning more about the basic means to build a nuclear weapon every day.
The facility – named Natanz, after the nearest town – is where Iran has begun the process of producing fissile material. Thousands of thin, vertical tubes spin at outrageous speeds, atom by atom enriching raw uranium gas into more useful material.
Iranian officials say Natanz will make low-enriched uranium to use in civil power plants. And the just-released assessment by US intelligence agencies concludes that Iran has indeed put its covert weapons program on hold.
But developing the technology to enrich uranium is perhaps the most difficult step in a nuclear weapons – or civilian power – program. According to administration officials and outside experts, it is possible that Tehran has simply decided it does not need to proceed with actual bomb work, at least for now.
"Iranian leaders appear to have recognized that by staying within the rules they can acquire capabilities sufficient to impress their own people and intimidate their neighbors, without inviting tough international sanctions or military attack," concludes George Perkovich, director of the nonproliferation program at the Carnegie Endowment for International Peace, in an assessment of the US National Intelligence Estimate's (NIE) revelations.
As of now the US intelligence has high confidence that Iran has not produced enough highly enriched fissile material for a nuclear weapon. The earliest it would be able to do so is probably within the 2010 to 2015 time frame, according to the new NIE.
And if Iran does decide to develop nuclear weapons, scientists would most likely use centrifuge technology, which they are currently working on at Natanz.
"Iranian entities are continuing to develop a range of technical capabilities that could be applied to producing nuclear weapons," says the NIE.
Iran has long claimed that its enrichment program is intended for civilian purposes. Iranian officials say they only want to learn how to produce fissionable fuel for power plants, as they are allowed to do under terms of the Nuclear Non-Proliferation Treaty.
But in the past, Iranian officials have engaged in what UN weapons inspectors consider to be suspicious behavior in regard to their enrichment effort. For instance, Tehran has built and secretly operated centrifuges, the spinning tubes which are the heart of the enrichment technology Iran has chosen.
For this and other reasons the International Atomic Energy Agency in 2003 declared Iran in violation of its UN nuclear safeguards agreement.
The Bush administration has vowed to continue to press for further UN sanctions designed to pressure Iran into abandoning enrichment altogether.
"Iran's uranium and plutonium programs are still a concern for US security and are still operating in violation of binding UN Security Council resolutions," write Jon Wolfsthal and Jon Alterman, senior fellows at the Center for Strategic and International Studies in an analysis published in December.
As the US intelligence assessment makes clear, Iran has made progress in its enrichment efforts, but it must still surmount daunting technical problems before it can operate Natanz's machinery on full throttle.
The gaseous diffusion-enrichment process Iran uses appears to be modeled after one perfected by the European enrichment consortium Urenco. It uses centrifuge tubes which spin at 1,500 revolutions per second or higher.
At that speed the uranium hexafluoride gas within the tube begins to separate into isotopes of different atomic weight. The lighter and more fissionable U-235 remains near the center, while the heavier U-238 – which makes up 99 percent of the natural feedstock – is thrown to the outside.
By collecting the gas in the center, then pumping it into another tube, and another, and so on, it is gradually enriched to a usable level. Low-enriched uranium, suitable for power plants, is about 20 percent U-235. The high-enriched uranium needed for bombs has a U-235 concentration of 80 percent or higher.
Today Iran has about 3,000 centrifuges installed in buildings at the Natanz complex. That's up from only 300 a year ago.
But running centrifuge cascades is a difficult sort of high technology ballet. The tiniest wobble can destroy a tube spinning at such high speeds. Data on Iran's usage of hexafluoride gas shows its enrichment plant is running at a fraction of capacity.
As of mid-August 2007, Iran had produced only about 70 kilograms of low-enriched uranium, according to David Albright, a physicist and president of the Institute for Science and International Security. At full speed, 3,000 centrifuges should be able to produce 90 kilograms of LEU in a month.
Once Iran accumulates 700 to 800 kilograms of low-enriched material, it would have a breakout capability, concludes Albright in an assessment in the current issue of the journal Arms Control Today. It could take that material and put it back into the centrifuges and keep upping its concentration, producing enough HEU for a bomb within a few months.
Yet given the tenaciousness with which Iran has proceeded with enrichment, insisting on its abandonment might not work, according Matthew Bunn, a senior research associate at Harvard University's Managing the Atom Project.
"It is time to begin thinking about the least bad non-zero [centrifuge] options might look like," concluded Bunn in a presentation made to government nuclear scientists in November. "They may offer the lowest risks to US security of the many bad options now available."