Q+A: What threat does Iran's nuclear work pose?

(Reuters) - Turkey said on Wednesday that Iran had promised to stop refining uranium to 20 percent purity if world powers agree to a proposed nuclear fuel swap. Iran escalated its enrichment activity in February, compounding suspicions that it wants to stockpile potential material for nuclear weapons.

The following looks at the possible dangers behind the work:

WHAT ARE IRAN'S MOTIVES?

Iran said it started enriching to higher levels after growing frustrated with the lack of movement on a U.N.-brokered plan for big powers to provide it with fuel rods made from low-enriched uranium for a medical reactor.

The powers accused the Islamic Republic of reneging on the October 2009 agreement to ship out what then would have been around two-thirds -- 1.2 tons -- of its known low-enriched uranium (LEU) stockpile to be turned into reactor fuel. This would have prevented Iran retaining enough of the material to fuel a nuclear weapon, if refined to about 90 percent purity.

In May, Iran tried to revive the plan with the diplomatic help of Turkey and Brazil. But major powers said the agreed swap had lost value by then because Iran's LEU reserve had grown and removing 1,200 kg now would leave Iran enough potential material for a nuclear weapon. They have also voiced alarm about the higher enrichment.

WHAT IS IRAN'S NUCLEAR PLAN?

Iran's LEU production continues around the clock, albeit at a pace well below full capacity to minimize the breakdowns that have afflicted its 1970s-vintage centrifuges.

The International Atomic Energy Agency (IAEA) said in a May 31 report Iran had stockpiled 2.4 tons of LEU, enriched to the 3.5 percent level typically needed to run civilian power plants.

Iran has been refining some of this stock up to 20 percent, the concentration needed by the Tehran medical reactor, producing up to 3-5 kg per month. It has tweaked 164 of its estimated 4,000 operating centrifuge machines to do this and installed a second cascade to improve the efficiency of the work. The additional machines could also allow it to expand capacity or raise the enrichment level further.

The IAEA said Iran had produced around 5.7 kg of 20 percent-enriched uranium by early April and would have another batch ready soon.

The research reactor, which makes isotopes to treat more than 850,000 Iranian cancer patients, is expected later this year to use up the last of the 115 kg of fuel that Iran imported from Argentina in 1993.

IS THERE A PROLIFERATION RISK?

Yes, depending on Iran's ultimate intentions. Iran now has enough LEU for 1-2 bombs if highly enriched. No one knows yet how much of this it plans to enrich further. But once at the 20 percent mark, Iran could advance to the 90 percent weapons-grade level in mere months since low-level enrichment is the most time-consuming and technically difficult stage of the process.

WHY IS THE WEST SUSPICIOUS?

With Iran's history of nuclear secrecy and restricting U.N. inspections, just enriching to 3.5 percent has stirred mistrust abroad since Iran will not have any operating nuclear power plants using LEU for many years, despite plans on the table.

Iran also lacks the technical ability to convert 20-percent pure uranium into the fuel rods needed for the Tehran reactor, and might need years to develop it. This means Iran would not be able to preserve an uninterrupted fuel supply to the reactor.

Only France, one party to the U.N. draft deal, and Argentina are known to possess the immediate technology. So analysts ask why Iran would enrich uranium well above its stated needs, except to lay the groundwork for producing bomb-grade uranium.

Another cause for skepticism, analysts say, is that Iran's targeted monthly output of 20 percent uranium is at least double the medical reactor's projected requirements.

On the other hand, given that Iran's centrifuges have often malfunctioned before, Iran may not be able to produce a notable amount of higher-grade material in the coming year.

SO IS IRAN CLOSING IN ON A NUCLEAR WEAPON?

Not yet. Significant technical and practical hurdles remain. Iran has pledged anew to keep all enrichment work including the 20 percent project under International Atomic Energy Agency monitoring. So any illicit attempt to escalate to 90 percent enrichment would not escape notice, unless done at a secret site, for which Western intelligence services have their eyes peeled. Last year they unearthed a second embryonic Iranian enrichment site at Qom, not yet operational, that had been hidden from the IAEA since 2006.

Once Iran has produced highly-enriched uranium (HEU), it would have to be transformed from gaseous into metal form, fitted with reflectors and a trigger system, then miniaturized to squeeze into the nose cone of a Shahab-3 ballistic missile.

All of these steps could take two or more years for Iran to master, analysts estimate. Then-U.S. intelligence chief Dennis Blair assessed in February that Iran will not be technically able to "weaponize" nuclear material before 2013.

Another factor slowing Iran's advances may be U.N. sanctions banning trade that would benefit its nuclear program. It has been forced to rely increasingly on indigenously manufactured components likely to be of inferior quality.

However, Blair reported: "Iran's technical advancement strengthens our ... assessment that it has the scientific, technical and industrial capacity to eventually produce nuclear weapons, making the central issue its political will to do so."

WHAT ABOUT THE PLAN FOR 10 MORE ENRICHMENT PLANTS?

Iran says it will build 10 more enrichment sites over the next year, thumbing its nose anew at U.N. Security Council resolutions demanding a nuclear suspension to foster talks.

But, given that Iran needed a good decade to hone limited enrichment capacity at one site, Natanz, diplomats and analysts believe there is no way Tehran can get 10 more up and running in the foreseeable future.

(For take-a-look, click on [nLDE5BD2ES])

(Compiled by Mark Heinrich and Sylvia Westall)