With every passing week, there are suggestions that an agreement could be close on a new Iran nuclear deal.
But officials have spoken of obstacles remaining, with Iran saying on Monday that the UN's nuclear watchdog, the International Atomic Energy Agency, must drop a long-standing investigation into nuclear particles found at four sites it had not disclosed to inspectors.
There are concerns that time is running out for an agreement to be reached between Iran and global powers including the US, UK and EU.
Iran quickly increased its enrichment of uranium after the US withdrew from the 2015 nuclear deal while Donald Trump was president.
In February, IAEA chief Raphael Grossi said Iran’s enrichment of uranium to 60 per cent purity put it on the path to 90 per cent, which is enough to develop a nuclear bomb.
“A country enriching at 60 per cent is a very serious thing. Only countries making bombs are reaching this level,” Mr Grossi said.
“Sixty per cent is almost weapons grade, commercial enrichment is two, three per cent,” he told The Financial Times.
The National spoke to three experts about what comes next and how Iran could create a nuclear weapons programme if negotiations fail.
"The biggest risk of a no-deal is the dramatic acceleration of Iran’s nuclear programme, which could encourage other regional states to follow suit,” Sanam Vakil, deputy head of the Mena programme at Chatham House in London, told The National.
"Further regional instability and escalation of tensions between Israel and Iran are also to be expected."
Iran's mystery uranium
In November 2019, the IAEA said “man-made” particles of uranium were discovered at a site, later identified as a warehouse in Turquzabad, south of Tehran, that had not been declared by Iran as being involved in nuclear activity.
Iran said the particles had come from previously declared nuclear work elsewhere, which had been halted.
But the IAEA was not satisfied and has repeatedly asked Iran to explain what happened in writing.
Tehran said the inquiry, which has also looked at three other sites where uranium particles were detected, must be dropped before a deal can be reached.
Satellite images of Turquzabad from 2018 also showed that Iran may have sanitised the area by moving equipment in apparent anticipation of UN inspections in February 2019.
Experts say the presence of undeclared nuclear material suggests there was an attempt to hide work from inspectors. As a result, sceptics of a new deal have asked how Iran can be trusted, even with renewed UN inspections.
In early 2018, Mossad — or Iranians working on its behalf — raided a warehouse in Tehran that contained documents related to the country’s nuclear programme.
Within the archive, new information was found about previously hidden work at a known site called Parchin.
The IAEA requested access to Parchin previously, asking in 2012 for “clarification of unresolved issues in connection with Iran's nuclear programme, particularly those relating to possible military dimensions”.
Its question on “possible military dimensions” of Iran’s nuclear ambitions is at the heart of negotiations.
Under the 2015 nuclear deal, inspectors visited Parchin and found man-made nuclear particles and asked Iran for an explanation.
The nuclear archive revealed images of what had long been suspected at Parchin: specially constructed chambers for testing explosives designed to “implode” highly enriched uranium, creating a supercritical mass for a nuclear blast.
Nuclear scientist David Albright said the site may have been constructed with the help of Vyacheslav Danilenko, a former Soviet nuclear scientist who specialised in “explosive compression”, which is required to detonate nuclear warheads.
This follows a pattern of Iran obtaining knowledge to build nuclear devices from a global network of rogue scientists, including Pakistani physicist AQ Khan, who was accused of stealing information on how to make centrifuges to enrich uranium while working in the Netherlands in the 1980s.
Pakistan detonated a nuclear bomb in the 1990s. Khan is said to have passed the information on to Iran and North Korea.
How quickly could Iran develop a bomb?
The documents obtained by Mossad also showed research into a process known as warhead miniaturisation.
Experts are divided on whether this technical process presents a major hurdle for Iran.
After uranium is enriched to 90 per cent purity, the next step is to design a nuclear bomb small enough to fit on to the tip of a missile — a challenge that took US scientists about a decade to solve after atom bombs were dropped on Japan in 1945.
Those acts of mass destruction required bombs that weighed about 4,400 kilograms. They were so heavy they could barely fit on to the US’s largest bomber.
But by the 1950s, the US miniaturised the weapons to fit on to a missile, with the weapons weighing about 700kg.
Mr Albright said research carried out by Iran in the late 1990s and early 2000s lead to the development of a warhead that would require 25 kilograms of highly enriched uranium.
The weapon was not tested owing to a lack of enriched uranium at the time, so there are questions about whether it would work.
“Iran's design is for a miniaturised nuclear warhead to fit the Shabab 3 missile, a design worked out in the Amad Plan in the early 2000s,” he told The National.
The Amad Plan, revealed in detail in the captured documents from 2018, has been studied in detail by Mr Albright.
“However, Iran is not believed to have finished all the steps to create a reliable warhead for that missile, but it certainly can if it moves forward to make nuclear weapons,” he said.
Mr Albright said the plan envisioned Iran having the capability to produce five warheads, four deliverable by the Shahab 3 missile and one reserved for testing, a project led by Mohsen Fakrizadeh, a veteran of the Islamic Revolutionary Guard Corps.
Fakrizadeh was behind the effort to create a bomb and was assassinated in November 2021.
What if inspections resume?
In the event of a new deal, UN inspectors would return to Iran to verify that its nuclear programme could be broken down and limited to civilian research.
“If there is a deal — and at this point, despite the progress made in recent weeks, the prospects for finalising one remain very much — there would be a sequence by which Iran would roll back its nuclear programme in exchange for the easing of US sanctions," said Naysan Rafati, senior Iran analyst at the International Crisis Group.
Mr Albright said new inspections, as well as the focus of western intelligence agencies, would have a good chance of detecting whether Iran was telling the truth about its nuclear ambitions.
“Weaponisation programmes can be difficult to detect, but western intelligence prioritises Iran, so detection probability, while not 100 per cent, is pretty large. Iran would also need to make weapon-grade uranium. It could also use its 60 per cent stockpile directly,” he said.
“IAEA safeguards are expected to remain in place, allowing detection of diversion of enriched uranium, or breakout within a few weeks or a month,” he said, referring to Iran’s commitment to the Nuclear Non-proliferation Treaty.
“If Iran ends safeguards, equivalent to leaving the NPT, we all should probably conclude it is moving to build nuclear weapons and respond accordingly."