That is because Western intelligence analysts and experts outside government have long suspected that if Iran wanted to produce fissile material for a nuclear weapon, it would have to do so at a hidden facility. Known Iranian nuclear sites are too closely watched to serve that purpose.
Two years ago, in its 2007 National Intelligence Estimate of Iran's nuclear intentions and capabilities, the US intelligence community concluded that "we assess with moderate confidence that Iran probably would use covert facilities – rather than its declared nuclear sites – for the production of highly enriched uranium for weapon."
Information made public so far makes it appear the new site has all the attributes this predicted covert facility would have.
1. It's hidden
First of all, it may be dug into the side of a mountain. The US government has not said exactly where the suspect plant is located, noting only that it is near the city of Qom. But outside experts have already produced satellite imagery (see a pdf of the pictures here) of a likely location that appears to be a tunnel dug into the side of a ridge on a military facility 30 miles north east of Qom.
2. It's small
A US official who briefed reporters last week said it is designed to hold about 3,000 uranium-enriching centrifuges. That is too few to produce low-enriched uranium for a nuclear power reactor.
"But if you want to use the facility in order to produce a small amount of weapons-grade uranium, enough for a bomb or two a year, it's the right size," said the US official.
3. The timing fits
In 2003, an Iranian dissident group revealed the existence of Iran's first clandestine enrichment site, at Natanz. Shortly thereafter, Tehran agreed to submit to tougher Internatonal Atomic Energy Agency scrutiny.
Then, in 2005, tunneling began at the secret site near Qom, imagery suggest. Caught once, Iran may have been trying its luck at a second attempt at concealment.
Iran's other nuclear facilities
The site at Qom is just one of Iran's network of nuclear facilities. Nonproliferation experts have concerns about each in varying degrees.
NATANZ was Iran's first centrifuge enrichment facility – far larger than the newly-discovered plant. Plans call for it to eventually hold 47,000 centrifuges, which Iran says will produce low-enriched uranium for civilian power reactors.
Iran began producing uranium at this facility in mid-April, 2007, according to a recent Congressional Research Service update on the status of the Iranian nuclear program.
As of May 31, 2009, Iran had produced an estimated 1,430 kilograms of low-enriched uranium at this site, according to CRS.
"This quantity of [low-enriched uranium], if further enriched, could theoretically produce enough [highly-enriched uranium] for a nuclear weapon," concludes CRS.
ARAK will be home to a heavy-water nuclear-energy reactor and a facility that produces the heavy water needed to moderate the nuclear chain reaction in such a reactor.
This plant is a proliferation concern because its spent fuel will contain plutonium that is better suited for use in nuclear weapons than the spent fuel from light-water reactors.
Iran says the Arak plant is intended to replace an outdated research reactor in Tehran that is fueled with low-enriched uranium. It will produce medical isotopes when it becomes operational, according to Iranian officials. According to CRS, the Arak reactor is likely to come on-line in 2013.
BUSHEHR is the site of a long-planned 1,000-megawatt light-water nuclear power plant. Begun in 1975 under the Shah, it was abandoned after the 1979 revolution. In 1995, Iran signed a contract with Russia to complete the facility, but the work has often been delayed.
US officials worry that plutonium could be separated from Bushehr's spent fuel, although, as previously noted, light-water reactors are generally considered to be more proliferation-resistant than heavy-water models.
Fuel loading at Bushehr is scheduled to take place this fall. Russia has argued that the project should not be a concern because it will be under close IAEA inspection.
Iran's missile tests: What do they tell the West?
How did Iran respond to the allegations that it was building a secret nuclear weapons facility. It tested a suite of ballistic missiles. Click here to read about what those tests mean for Iran and the West.
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