Uranium ore mined from the earth contains in excess of 99 percent of the more stable U-238 isotope, and just 0.7 percent of the U-235 isotope useful to nuclear engineers.

Uranium used in a nuclear reactor must contain three to five percent U-235, whereas the proportion must reach or exceed 90 percent for weapons-grade material.

There are two ways to enrich uranium, and for both the first step is to mill the ore into a concentrate called yellowcake, which is then converted into uranium hexafluoride gas (UF6).

During gas centrifuge enrichment -- the method used by Iran -- the UF6 is piped in a cylinder which is then spun at very high speed. The rotation pushes the heavier U-238 isotopes to the far end of the cylinder, while the lighter U-235 isotopes stay toward the center.

Some of the U-238 is extracted, and the process is repeated many times over until the sought-after level of enrichment is obtained.

Iran has said it has not enriched uranium beyond 4.8 percent.

This basic technique is half a century old, but it requires thousands of centrifuges connected in cascades to produce weapons-grade uranium, as well as highly sophisticated and precise equipment.

Tehran announced in February that it had successfully set up two cascades containing 164 centrifuges each at its nuclear facility in Natanz, and that two other cascades were in the final phase of installation.

Iran has remained vague as to exactly how many centrifuges are currently in operation, though it said earlier that it planned to have 3,000 by the end of last month.

A facility with 3,000 centrifuges could produce enough enriched uranium to produce one atomic bomb in about a year, according to experts.

Enrichment is only one of several hurdles to overcome before a country is considered nuclear-weapons capable.

Another is the electronic trigger, whose split-second timing is essential for unleashing the chain reaction necessary for a military device.

There is "weaponisation" -- putting the device into a missile or bomb that can be delivered to a target.

Little Boy, the Hiroshima bomb, used 64.1 kilos (141 pounds) of enriched uranium, although a device can also be built from between 15 and 25 kilos (33 and 55 pounds) of material, according to experts.

A bomb can also be made from as little as six kilos (13.2 pounds) of plutonium, a by-product of nuclear reactors.

The other method used for enrichment is called gas diffusion.

earlier related report
Iran defiant amid uncertainty on nuclear progress
Tehran (AFP) Apr 10 - Iran pledged on Tuesday to further expand its nuclear drive after announcing that its activities had entered an industrial phase, sparking new criticism from the West and renewed calls for negotiation. Shrugging off the latest warning from the United States, the head of Iran's atomic energy organisation reaffirmed Tehran's ultimate aim of installing 50,000 centrifuges to enrich uranium.

"The objective of the Islamic Republic of Iran is not just the installation of 3,000 centrifuges at the Natanz plant but we are doing everything to install 50,000 centrifuges," said Gholam Reza Aghazadeh, according to the state-run IRNA agency.

In a grand ceremony at the Natanz enrichment plant in central Iran on Monday, President Mahmoud Ahmadinejad announced that the country was now able to enrich uranium on an industrial scale.

Enrichment is the key sticking point in the standoff between Iran and the West as it can produce nuclear fuel but in highly extended form can also make the fissile core for an atomic bomb.

The United States, Israel and other countries suspect Iran is seeking nuclear weapons, but Tehran insists its nuclear drive is solely aimed at generating energy.

Ahmadinejad did not give figures on how far the nuclear programme has advanced, and diplomats in Tehran pointed to confusion over exactly how many uranium-enriching centrifuges it had installed.

"I did not understand what they were talking about," said one diplomat, who asked not to be named. "There seems a certain confusion about the technical aspects of the announcements.

"Ahmadinejad really meant that Iran had announced it has the capacity to pass into an industrial phase rather than it had actually got there."

Iran has so far only confirmed that it has two cascades of 164 centrifuges at Natanz. Monday's announcement was seen as signalling that it now has considerably more than this figure.

But the notion that Tehran has already reached its medium-term aim of installing 3,000 centrifuges "is simply not credible", said another Western diplomat.

Aghazadeh explained Tehran's reluctance to give any figures over the progress of its nuclear programme by saying it did not want to create "ambiguities".

"I did not want people to say that Iran has finished installing 3,000 centrifuges and everything has been completed now," he explained.

Russia, which has been building Iran's first nuclear power plant in the southern city of Bushehr, also warned against seeing any "breakthrough" in Ahmadinejad's announcement.

"We do not know of any recent technological breakthroughs in the Iranian nuclear programme that would change the character of the work in the field of enrichment," foreign ministry spokesman Mikhail Kamynin said in a statement.

In Berlin, the German presidency of the European Union said on Tuesday that Iran's claim to be producing enriched uranium on an industrial scale was a "direct contradiction" of two binding UN Security Council resolutions calling on Iran to cease all enrichment activities.

Iran's defiance of Western calls for it to suspend enrichment have already earned it sanctions under those resolutions.

"The presidency of the EU once again urges Iran to comply with the demands of the international community and to create the conditions for a return to the negotiating table and for a solution to the conflict surrounding the Iranian nuclear programme."

For his part, French Foreign Minister Philippe Douste-Blazy said in Paris that Iran's nuclear news was a "bad sign" and also called for a return to negotiations.

"I deplore the announcements made yesterday, which are a bad sign," he said in a statement. "I once again urge Iran to respect Security Council resolutions, which demand the suspension of all sensitive nuclear activities."

A team of inspectors from the IAEA arrived in Iran on Tuesday on a pre-planned week-long visit that will see them visit the Natanz nuclear plant, the Fars news agency reported.

Their confidential report is likely to contain the figures that were lacking from Ahmadinejad's announcement.

The United States lost little time in voicing its worries, with national security spokesman Gordon Johndroe saying on Monday: "We are very concerned about Iran's announcement that they entered an 'industrial stage' of nuclear fuel production."

But Iran was standing by its refusal to suspend enrichment.

"The suspension of enrichment is not acceptable either as a precondition to negotiations or as a result of such talks," Foreign Minister Manouchehr Mottaki told reporters.

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