VIENNA (Reuters) -- Iran has effectively stopped expanding active uranium enrichment since September, diplomats said, while considering a big power offer to fuel a medical reactor if it turns over enriched material seen as an atomic bomb risk.
While Iran's stock of low-enriched uranium (LEU) has likely risen by 200-300 kilograms from 1,500 kilograms reported by UN monitors in August, the number of operating centrifuge machines at its Natanz enrichment plant has remained at about 4,600, they said.
Iran's potential enrichment capacity was much higher. It had installed at least 8,700 centrifuges in all by late September, diplomats said. A fresh figure was not yet available.
But it was unclear why almost half the centrifuges were not yet enriching, remaining idle or undergoing vacuum tests.
Diplomats and analysts said possible reasons ranged from technical glitches to politically motivated restraint, to avoid closing the door to diplomacy with world powers and provoking harsher international sanctions or even Israeli military action.
"The situation is now pretty much as it was in September," said a senior diplomat in Vienna, where the UN nuclear watchdog, the International Atomic Energy Agency, is based. Officials at Iran's IAEA mission were unavailable for comment.
Precise figures will come next week in a new IAEA report on its inspections and investigations in Iran, whose record of atomic secrecy has raised suspicions it is illicitly pursuing nuclear weaponry and drawn UN sanctions.
The IAEA's last report showed Iran was enriching uranium with about 300 fewer centrifuges than the almost 5,000 operating earlier in the year, the first such scaleback in three years. The report did not give possible reasons but diplomats said at the time Iran may have taken down centrifuges for maintenance.
Iran says it will refine uranium only to low levels needed for electricity, not to the high purity suitable for atom bombs.
The size of Iran's LEU reserve is of great interest to world powers since an IAEA-brokered draft deal calls on Iran to send some 75 percent of it abroad to be turned into fuel for a Tehran research reactor that makes isotopes for cancer treatment.
But diplomats say Iran has backpedalled from the basics of the deal. Iranian officials have said Tehran prefers to buy reactor fuel from foreign suppliers rather than part with its LEU, or at most swap small amounts of LEU for the reactor material on Iranian soil. They have called for more talks.
The United States and France, the other parties to the deal along with Russia, have vowed not to renegotiate the main conditions. They say Iran's proposals would leave intact enough LEU for conversion into nuclear explosive.
Iran has amassed enough LEU for one to two bombs, analysts say.
The IAEA is consulting on possible compromises to save the deal, including Iran placing its LEU under escrow in a friendly third country, like Turkey pending delivery of reactor fuel. Iran and Turkey discussed the idea in talks this week.
The impasse over fine print in the fuel deal has prevented follow-up talks on a broader solution to Iran's contested nuclear programme, with sanctions relief and trade benefits on offer to Tehran if it curbs enrichment as a whole.
IAEA To Report On Second Enrichment Site
The IAEA will also report next week on its initial visit to a second, hidden enrichment site that Iran revealed in September after, Western diplomats said, discovering that U.S., British and French spy services had detected it. IAEA Director-General Muhammad el-Baradei said in New York media interviews last week that his inspectors found "nothing to be worried about" in what he called a "hole in a mountain" without nuclear equipment or material.
Tehran has referred to the bunkered site near Qom as a fallback for its professed civilian enrichment program in case the much larger Natanz complex were bombed by a foe like Israel.
But Western diplomats and nuclear experts say the Qom site's planned capacity -- 3,000 centrifuges -- makes little sense as a standalone civilian enrichment centre since it would be too small to fuel a nuclear power station around the clock but ideal to yield fissile material for one or two bombs per year. Diplomats said it was too early for next week's report to draw conclusions as the IAEA would need time to compare plant documentation to be provided by Iran against their impressions of its layout and intelligence provided by Western powers.
"I'd be surprised if any [sinister] evidence was found there. Rather, the most important issue to be resolved is why this site exists at all, what is its chronology, is it plausible for [civilian] purposes?" said a senior Vienna diplomat.
Iran has said the plant under construction will not start operating before the end of next year.