FACTBOX - Iran's second nuclear enrichment plant

By Mark Heinrich

VIENNA (Reuters) - Iran has told the U.N. nuclear watchdog that it is building a second uranium enrichment plant, a disclosure Washington said was forced by Tehran's realisation that Western powers were about to blow the whistle on the site.

Here is a rundown of what is known about the nascent nuclear facility and what positions the key players have taken on it, based on U.S., British, French and U.N. diplomatic sources, Iranian officials and the International Atomic Energy Agency.

* Construction of the plant began in 2006, tucked away deep inside a mountain on a former missile base controlled by the elite Revolutionary Guards Corps. The site is around 160 km south of Tehran near the Iranian holy city of Qom, according to satellite pictures posted on the Internet.

* The site has space for about 3,000 centrifuges, the cylindrical machines that spin at supersonic speeds to enrich uranium, either to low concentrations suitable for power plant fuel or the high level that provides the core of an atom bomb -- depending on how the devices are calibrated.

That number of centrifuges is well below the tens of thousands needed to churn out industrial amounts of low-enriched uranium to keep a nuclear power plant running. But 3,000 could produce fissile material for one nuclear weapon a year if run nonstop at optimum capacity. This raises questions in Western intelligence minds whether the site was really for civilian purposes, as Iran has maintained since going public with it.

* The site is outfitted with security going beyond the norm of a civilian nuclear power plant, Western diplomats say.

* Asked why the facility was hidden underground, Iranian officials cite what they said was a constant risk of U.S. attack during the Bush administration and continued threats by Israel. Western analysts believe the Qom-area site could have served as a covert "ace in the hole" to preserve progress towards nuclear capability if its IAEA-monitored Natanz plant were bombed.

* Western powers suspect the Qom-area plant could have been configured to yield high-enriched uranium if it had remained secret and Iran chose to use it to make weapons. Analysts also wonder if a covert uranium conversion centre to supply the plant with UF6 gas for enrichment might also exist. Intelligence declassified last week made no reference to one.