A diplomat said the Iranians have told the UN watchdog International Atomic Energy Agency (IAEA) here that "they intend to resume full conversion work on November 26."
That would be after a crucial November 24-25 meeting of the IAEA board of governors on Tehran’s nuclear program at which diplomats will consider whether to send Iran to the UN Security Council.
The announced work would be a second round of conversion. Iran has already processed 37 tonnes of ore.
Diplomats said the amount of uranium hexafluoride (UF6) gas the Iranians would have after processing 50 tonnes would be enough to make highly enriched uranium for up to 10 atomic bombs.
Iran had previously announced it would do more conversion but had not given a specific date, although it had been expected to begin the process earlier in November. The amount to be converted is also more than previously thought.
The diplomats also said reports that the UF6 is too contaminated to be put into the centrifuges that spin it into enriched uranium were wrong.
Iran is currently suspending enrichment work but "the current batch is good enough for a crash nuclear weapons program, if Iran doesn’t mind ruining a lot of centrifuges along the way," a Western diplomat said.
Iran says its nuclear program is a peaceful effort to generate electricity but the United States charges that Tehran is using this to hide secret work on developing atomic weapons.
Mark Fitzpatrick, a non-proliferation expert at the International Institute for Strategic Studies in London, told AFP that "my understanding is that the product (UF6) can be used to enrich uranium, although it will corrode centrifuge machines over time.
"But many of the centrifuge machines will crash upon start-up anyway, so contamination may be the least of Iran’s worries."
A non-Western diplomat said: "Fifty tonnes is considered to be a very large quantity and the feeding of such a quantity in the system clearly indicates that Iran believes in its capability to produce UF6 of good quality in the conversion process."
Iran is defying the international community by pushing ahead with uranium conversion.
Talks with the European Union aimed at securing guarantees that Iran is not secretly developing nuclear weapons collapsed in August when Tehran resumed conversion, nine months after suspending the work as a confidence-building measure.
On September 24, the IAEA passed a resolution calling on Iran to cease all nuclear fuel work, including conversion, and to cooperate fully in an IAEA investigation into its atomic program.
A diplomat close to the IAEA said the delay in Iran resuming conversion work "may also not be accidental. Something interesting may be going on."
The diplomat was referring to efforts to get the Iran-EU talks restarted, which include a Russian proposal to allow Iran to convert uranium but to do actual enrichment work in Russia.
The IAEA board meeting November 24 is to review progress since the September resolution.
Holding off on conversion, even while insisting on their right to do it, "is a chance for them (Iran) to do something positive without telling it through loudspeakers," a diplomat said.
But Fitzpatrick said: "I am not sure it means anything other than ensuring they get past yet another board meeting without being referred to the Security Council."
IAEA officials refused to comment.