If completed and implemented successfully, the accord would make any attempt by Iran to make a nuclear weapon -- which it denies wanting to do -- extremely difficult and easily detectable.
Despite Iran and six major powers having agreed on April 2 the main outlines of the accord in Lausanne, Switzerland, there are several potential problem areas to resolve in what will be a highly complex agreement:
- Sanctions -
The United States and European Union have committed to suspending economic sanctions they have imposed on Iran, although officials say that they will "snap back" into place if Iran violates the deal.
All past UN Security Council resolutions on the nuclear issue will be lifted and replaced by a new text endorsing the final deal and incorporating some UN sanctions such as those on conventional arms, missiles and asset freezes.
Iranian President Hassan Rouhani said a week after the Lausanne breakthrough that Iran would not sign a final agreement unless "all economic sanctions are totally lifted on the same day".
But Western officials say that they suspension will only happen once the UN atomic agency confirms Iran has taken the key nuclear-related steps under the deal. US Secretary of State John Kerry has said this would take six to 12 months.
- Numbers, timeframe -
According to a US fact sheet, Iran has committed to slashing the number of centrifuges enriching uranium -- which can render it suitable for power generation but also for a bomb -- to 5,060 from 19,000 at present, and for 10 years.
An additional 1,044 centrifuges will remain at the Fordo facility -- which is built into a mountain -- but for 15 years these will be used for purposes other than uranium enrichment, the US says.
The same document says that Iran will reduce its stockpile of low-enriched uranium -- enough for several bombs if further processed -- from 10,000 kilogrammes (22,000 pounds) to 300 kg, and not to expand it for 15 years.
Iran has however called the fact sheet a "mixture of facts and lies", and an April 2 joint statement by Iranian Foreign Minister Mohammad Javad Zarif and EU foreign policy chief Federica Mogherini was much vaguer.
They said only that "Iran's enrichment capacity, enrichment level and stockpile will be limited for specified durations". This may mean that the exact scope of the downsizing and the timeframe are yet to be nailed down.
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The statement did concur however that the Fordo site will no longer be used for enrichment and that a new reactor being built at Arak would be redesigned so that it does not produce weapons-grade plutonium.
- Inspections -
Iran and the six powers also need to work out the details of additional inspection work that the UN atomic watchdog, the International Atomic Energy Agency, would conduct under the final deal.
According to the US, this will include "new transparency and inspections mechanisms", greater access to uranium mines and monitoring a new procurement channel for Iran to acquire nuclear-related materials and technology.
Some of additional oversight measures would be covered by the additional protocol to Iran's existing inspections agreement with the IAEA, as well as the so-called modified code 3.1. The US says Iran will implement both of these.
More oversight is a particular thorny issue in view of allegations that the IAEA wants to investigate that before 2003, and possibly since, Iran conducted research into developing nuclear weapons -- claims that Iran denies.
Western officials stress that these claims of "possible military dimensions" need to be cleared up before sanctions can be lifted, but the IAEA's probe has been stalled since last August.
- Research and development -
A key area of concern is Iran's research into new types of nuclear equipment to replace the 1970s-vintage IR-1 centrifuge machines currently in use.
Critics, including US Republicans and Israel, fear that new machines could enable Iran to make material for a bomb much more quickly, particularly once restrictions on enrichment expire.
According to the US fact sheet, Iran will remove its 1,000 more advanced IR-2M centrifuges and place them under IAEA-monitored storage, while not using more advanced models to enrich uranium for at least 10 years.
Iran will however "engage in limited research and development with its advanced centrifuges, according to a scheduled and parameters which have been agreed to" by the six powers.
US State Department spokeswoman Marie Harf said on April 3 that the two sides "still have some R&D issues to work out, and those are among the most challenging, to be frank."