VIENNA - Iran recently understated by a third how much uranium it had enriched and U.N. nuclear inspectors are working with Tehran to ensure such a significant gap does not recur, diplomats said Friday.
The International Atomic Energy Agency believes the discrepancy was a technical mistake rather than subterfuge, but the matter is important given concerns, denied by Tehran, that it is secretly trying to develop nuclear weapons.
An IAEA report Thursday showed a significant increase in Iran's reported stockpile of low-enriched uranium (LEU) since November to 1,010 kg, which U.S. analysts said could be converted into enough high-enriched uranium for one bomb.
Even then, the technical steps needed to "weaponize" enrichment would probably take two to five years.
New figures in the report revealed that Iran had under-reported how much LEU it had amassed, raising new questions about the ability of the restricted inspectors' mission in Iran to keep track of Iranian nuclear advances.
The report said the 1,010 kg was based on an additional 171 kg from new production on top of 839 kg of previous output which inspectors had verified in November. But the last IAEA report at that time put the amount at 630 kg, based on Iran's estimate.
Diplomats familiar with the matter said the IAEA had concluded the discrepancy was due to faulty estimates that can arise from complexities in the phased enrichment process, not to any maneuver to divert LEU out of sight.
The report stressed that all nuclear material at Iran's underground Natanz enrichment plant, except for some waste and samples, was under regular IAEA containment and surveillance.
One inventory a year
But the diplomats said the verified LEU figure was based on an inventory check that inspectors perform only once a year.
In theory, this means there is a risk that any smuggling of enriched uranium out of Natanz for use at a secret site might not be noticed for some time.
U.N. inspectors are discussing with Iran how to improve its operating records to prevent any repeat of such large differences in accounting in future, the diplomats said.
"This doesn't mean nuclear safeguards aren't working. But the IAEA will have to do the inventories more often as the amounts increase," said David Albright, a senior non-proliferation analyst in Washington.
"All this reinforces the point that if Iran does divert material, it doesn't mean the IAEA knows right away. This could be delayed a number of weeks, particularly if Iran stalls on allowing them into the plant," he told Reuters.
"The (broader issue) is that Iran has crippled the IAEA's ability to detect undeclared activities."
Iran says it wants a nuclear fuel industry solely to meet growing electricity demand and has promised to preserve IAEA monitoring of its two declared uranium production centers.
But it has severely curbed IAEA movements since being hit with U.N. sanctions -- which it calls illegal -- for refusing to suspend enrichment and failing to open up to an IAEA inquiry into allegations of past atom bomb research.
It now bars IAEA access to plants developing new centrifuges and other enrichment machinery, and to a heavy-water reactor under construction. This means inspectors cannot verify that no parallel, military-oriented work is under way in Iran.