VIENNA - Tehran's nuclear programme will shrink significantly under a framework deal to make any Iranian moves towards building an atom bomb virtually impossible for years - but the devil is in the detail.
Iran has agreed with six world powers to curb its nuclear activity in three main areas: the size and grade of its uranium stockpile, the number of centrifuges that enrich uranium, and the maximum fissile purity of the product of these machines. "The approach outlined will effectively prevent Iran from building a nuclear bomb for an extended period of time," said Robert Einhorn, senior fellow at the US-based Brookings institution.
Still, some details have yet to be determined and the pact will take effect only if a final deal is agreed by June 30, a big "if" which can still scupper an agreement.
Here is an overview of the known details based on a US fact sheet and their potential pitfalls: CENTRIFUGES At marathon talks this week in Lausanne, Iran agreed it will operate only around 5,000 centrifuges out of 6,100 installed machines, which is less than half of its current capacity.
According to the latest report by the International Atomic Energy Agency (IAEA) on Feb. 19, Iran had around 19,500 installed centrifuges of which it was operating around 10,200.
While the number of centrifuges has been in the spotlight, enriching uranium is also a question of what kind of centrifuges are used to spin at supersonic speed to purify uranium.
Iran will not be able to enrich uranium with centrifuges that are much more efficient than the so-called IR-1 model. However, it will still be allowed to use the more modern models for research and development (R&D) purposes.
Iran is supposed to submit a detailed long-term R&D plan to the IAEA, but it is not clear how and when this will happen.
STOCKPILES AND ENRICHMENT
Since a 2013 accord with the six powers, Iran has stopped enriching uranium above 5 per cent purity - no material of a higher grade than this is normally used in nuclear power plants. It has "downblended" or further processed its 20-per cent enriched uranium stockpile.
Western countries see the step from 20-per cent to 90-per cent purity - the level needed for a bomb - as a relatively small one.
Iran's stockpile of low-enriched uranium is to be cut to 300 kg of 3.67-per cent purification from what the United States said was now 10,000 kg. According to the IAEA, Iran had around 8,000 kg in February.
One technical detail that might be a sticky point in any final deal is the so-called tails - the byproduct of enriched uranium extracted from a centrifuge.
Setting the concentration of tails affects the separative power of a centrifuge. "Tails concentration needs to be determined," said one expert and former IAEA employee.
Under the deal, Iran's "breakout" time - the time it would need to acquire enough fissile material for one weapon - would be extended to at least one year, for a duration of at least 10 years. The US fact sheet said it is now thought to be two to three months.
Experts say a bomb needs at least 25 kg of 90-per cent enriched uranium, or 250 kg of 20-per cent enriched uranium.