The size of Iran's most contested uranium stockpile has declined significantly for the first time in four years following a landmark nuclear deal with world powers in November, the U.N. atomic agency reported on Thursday, Reuters reported.
As a result, Iran's holding of uranium gas enriched to a fissile purity of 20 percent - a relatively short technical step away from the level required for nuclear weapons - is now well below the amount needed for a bomb, if processed more.
The stockpile is closely watched: Israel, believed to be the Middle East's only nuclear-armed power, warned in 2012 that there would be a "red line" for possible military action if Iran amassed enough such refined uranium for a single bomb.
Iran agreed under a November 24 deal with six big powers to stop its 20 percent enrichment, which it began in 2010, and has since diluted some of the material to a lower concentration and converted some into less proliferation-prone oxide.
"That decrease has been quite important," a senior diplomat familiar with Iran's nuclear program said. "That progress has been quite substantial in terms of inventory."
Thursday's report by the International Atomic Energy Agency (IAEA) also showed, as reported by Reuters earlier this week, that Iran was meeting its commitments under last year's interim agreement with the six powers to curb its most sensitive nuclear work in exchange for some easing of sanctions.
"Things are progressing as planned," the diplomat said.
The IAEA report was issued to member states just hours after Iran and the six countries - the United States, France, Germany, Britain, China and Russia - wound up a first round of negotiations in Vienna aimed at a final settlement of the decade-old dispute over the nature of Tehran's nuclear activity. The next round was set for March 17.
U.N. nuclear inspectors are playing a critical role in monitoring that Iran is living up to its side of last year's six-month accord, designed to buy time for the negotiations on a comprehensive agreement over atomic activity Tehran says is entirely peaceful but the West fears may have military designs.
Iran's reserve of 20 percent uranium fell to 161 kg (354 pounds) in February from about 196 kg (431 pounds) in November, the IAEA said. About 250 kg (550 pounds) is needed for the core of one nuclear warhead, experts say.
However, the stock of low-enriched (LEU) - or 5 percent purity - uranium rose to 7,609 kg (16,740 pounds) from 7,154 kg (15,739 pounds) in November.
This arose apparently from a delay in building a plant to convert some of the material into an oxide form less suitable for processing into high-enriched bomb material.
Low-enriched - 5 percent - uranium can be used to fuel nuclear power plants, the stated purpose of Iran's nuclear program, but can also provide material for nuclear weapons if further refined to the 80-90 percent threshold.
Iran justified its 20 percent enrichment drive by saying it was meant to replenish the fuel supply of a Tehran medical research reactor, but Western officials were skeptical, saying Iran had refined far more than it required for such a purpose.
PARCHIN "BUILDING MATERIAL AND DEBRIS"
The interim agreement took effect on January 20 and the IAEA gave an update about its implementation on Thursday, together with a regular quarterly report on Iran's atomic activities.
Among other steps, Iran agreed under the deal to limit its LEU reserve and the new plant is meant to achieve that.
Diplomats and experts said this matter was of no immediate concern since Iran's commitment concerned the size of the stockpile towards the end of the deal, in late July, giving it time both to complete the facility and convert enough material.
While Iran a month ago stopped 20 percent enrichment, it is allowed under the November agreement to continue producing uranium refined to up to 5 percent.
Its LEU output even seems to have increased slightly, as the IAEA said Iran had started producing LEU with centrifuges - machines spinning at supersonic speed to increase the fissile concentration - previously used for 20 percent enrichment work.
Experts say Iran potentially has enough LEU for a few nuclear weapons if refined much further. Limiting its overall enrichment capacity will be one of the thorniest issues in the negotiations that began this week.
The IAEA also said Iran had told it about preliminary site selection for five new enrichment sites but that there would be no new such facilities during the half-year agreement with the powers.
If Iran were to implement these plans - first announced during the 2005-2013 presidency of nationalist hardliner Mahmoud Ahmadinejad - it would likely wreck any chance of a final treaty with the major powers. Ahmadinejad's moderate successor, Hassan Rouhani, has initiated a thaw with the West.
Iran had also informed the U.N. nuclear watchdog of a new planned research reactor.
The IAEA repeated its request to visit the Parchin military base, a site where it believes - based on Western intelligence information - that Iran may have conducted nuclear weapons-related explosives tests. Iran denies this.
The IAEA has indicated that it suspects Iran has tried to purge the site of any incriminating evidence and Thursday's report said it had observed via satellite imagery "possible building material and debris" there. It did not elaborate.