GENEVA (Reuters) - An interim deal to restrain Iran's nuclear program aims to make it harder for the Islamic state to build any bomb but may still leave it, at least for now, with enough material for several nuclear warheads if refined to a high degree.
In a sign of how far Iran's nuclear activity has advanced in a few years, the deal under discussion in Geneva this week appears unlikely to achieve a central goal of an abortive one in 2009: reduce its stockpile of low-enriched uranium (LEU) to below that needed for one nuclear bomb, if processed more.
While details of the text being negotiated in Geneva by senior officials from Iran and six world powers have remained secret, it seems to focus mostly on halting Iran's higher-grade enrichment and neutralizing that material.
That is because enrichment to a fissile concentration of 20 percent - compared to the 3.5 percent usually required for nuclear power plants - represents most of the work needed to reach weapons-grade uranium of 90 percent.
The aim of the Geneva talks, a senior U.S. official said, is to make sure that Iran's program "does not advance, and in some cases is even rolled back", to give time for negotiations on a final settlement of the decade-old nuclear dispute.
Under this "first step" arrangement, diplomats say, Iran would have to stop 20 percent enrichment, convert its existing stocks of around 200 kg (440 pounds) of such uranium gas to an oxide form or "downblend" it to reduce the enrichment level.
Tehran could also be asked to produce less 3.5 percent enriched uranium - which it says it needs for a planned network of nuclear power plants - by operating fewer centrifuges used to refine the material by spinning at supersonic speed.
But diplomats have made little specific mention of Iran's growing LEU stocks, which have increased four-fold since 2009 to an amount Western experts believe would be enough for four bombs or more if refined to weapons-grade.
Still, the proposed agreement could double Iran's breakout time, defined as how long it would take it to produce fissile material for one weapon, said Shashank Joshi, an Iran expert at the Royal United Services Institute (RUSI) in London.
Cliff Kupchan, Middle East analyst at risk consultancy Eurasia Group, offered a similar assessment and said he believed it would be "very good deal" for the United States.
"Any breakout would be much more detectable," he said.
They did not give any specific estimates for how long Iran - which says its nuclear program is for peaceful purposes only - would need to assemble a bomb, before or after a deal.
U.S. President Barack Obama has said that any agreement in Geneva would buy additional months in terms of Iran's capacity to break out of the nuclear Non-Proliferation Treaty regime and "weaponise" enrichment, if it chose to do so.
But Olli Heinonen, a former chief U.N. nuclear inspector, questioned the suggestion that the timeline could be pushed back months.
"TIME ON THE CLOCK"
The U.S. Institute for Science and International Security (ISIS) last month warned of steadily shortening breakout times if Iran's nuclear program advanced further.
It said Iran could make one so-called Significant Quantity for a bomb - 25 kg of highly-enriched uranium (HEU) - in as little as one to 1.6 months if it used its 20 percent stock.
More would be required if Iran only had its lower-refined material, but it could still do it in 1.9-2.2 months, ISIS said, and Tehran could produce enough weapons-grade uranium for four bombs with its existing amount of LEU.
The estimates excluded the effort needed to produce a usable nuclear weapon itself - that is, fitting HEU into a missile cone with the means to launch it toward a target.
"This extra time could be substantial, particularly if Iran wanted to build a reliable warhead for a ballistic missile,"the ISIS report said.
"However, these preparations would most likely be conducted at secret sites and would be difficult to detect."
Iran's LEU stockpile stood at 7,154 kg in early November, according to the latest report on its nuclear program by the International Atomic Energy Agency, an increase of about 380 kg in the last three months even though the Islamic Republic stopped expanding its enrichment capacity during the period.
In late 2009, when the IAEA brokered the fuel swap accord with Iran, the amount was about 1,800 kg. The accord would have obliged Tehran to ship out 1,200 kg and get 20 percent reactor fuel in return from abroad, leaving it short of the more than 1,000 kg required for a nuclear weapon.
The fuel swap plan was envisioned by the West as a way to reduce mistrust and help pave the way for broader negotiations on Tehran's nuclear program, like this week's talks.
But that confidence-building step collapsed after Iran backed away from its terms. Shortly thereafter, in early 2010, it started enriching uranium to 20 percent, the part of its nuclear program that the powers now want it to stop.
Perhaps with such past developments in mind, the U.S. official said any agreement this week should "give us some time on the clock" for talks on a more far-reaching settlement to ensure that Iran's nuclear program is peaceful.
Nuclear expert Ali Vaez of the International Crisis Group think-tank said he believed a deal in Geneva would address all aspects of Iran's nuclear program in order "to freeze it in its tracks" and therefore it was likely to address the LEU issue as well.
Given that the agreement would lengthen Iran's breakout time, the absence of any significant reduction in the size of the stockpile "shouldn't pose a problem", Vaez said.
(Editing by Mark Heinrich)