How close is Iran to producing a nuclear bomb?

The central achievement of the Iran nuclear deal - keeping Tehran at arm's length from nuclear weapons - is eroding. The 2015 accord's many restrictions on Iran's atomic activities were built around one objective: to extend the 'breakout time' Tehran would need to produce enough fissile material for one atomic bomb - if it decided to do so - to at least a year from around 2-3 months.

Iran has contravened many of the deal's core restrictions but has said it will continue to cooperate with the International Atomic Energy Agency and its inspectors. The deal has imposed on Iran the most intrusive nuclear verification regime of any country, and it has not backed out of that yet.

In photo: Iranian President Hassan Rouhani speaking in the capital Tehran about the rising tensions between Iran and the United States. 

The deal limits Iran's stockpile of enriched uranium to 202.8 kg - less than half the amount it was producing per quarter before its accord with world powers, and a small fraction of the tonnes it possessed. This was the first of Iran's breaches last year, verified by the IAEA on July 1. The last quarterly IAEA report in November said the stockpile stood at 372.3 kg. It will have continued to increase since then.

In photo: Iran's Atomic Energy Organization shows the interior of the Fordo (Fordow) Uranium Conversion Facility in Qom, Iran.

The deal caps the fissile purity to which Iran can refine uranium to at 3.67 per cent, far below the 20 per cent it was achieving before the deal and the 90 per cent that is weapons-grade. Iran breached that cap on July 8. Since then, however, its enrichment level has remained steady at up to 4.5 per cent.

In photo: A view of the water nuclear reactor at Arak in Iran.

The deal only allows Iran to produce enriched uranium with about 5,000 first-generation IR-1 centrifuges at its Natanz plant. It can operate small numbers of more advanced - faster-producing, more durable and efficient - models there without accumulating enriched uranium. Iran had roughly 19,000 installed centrifuges before the deal.

The IAEA verified on Sept. 25 that Iran had begun enriching with advanced centrifuges, but in much smaller numbers than the IR-1s. 

Iran has brought online two 164-machine cascades of centrifuges that were dismantled under the deal and installed smaller clusters of other models. As those come online, its production of enriched uranium is likely to increase.

The Islamic Republic has yet to breach the cap on IR-1 centrifuges at Natanz.

In photo: Iran's Atomic Energy Organisation shows the nuclear water reactor of Arak.

The deal bans enrichment at Fordow, a site that Iran secretly built inside a mountain and was exposed by Western intelligence services in 2009. Centrifuges are allowed there for other purposes, like producing stable isotopes. Iran began enriching there on Nov. 9 but only with a small number of IR-1s.

In photo: Members of the media and officials tour the water nuclear reactor at Arak in Iran.

The breaches have eaten into the breakout time slightly, but estimates of the current breakout time vary. Many diplomats and nuclear experts also believe the starting point of one year is a conservative estimate.

In photo: Iran's Atomic Energy Organisation spokesman Behrouz Kamalvandi, government spokesman Ali Rabiei, and Deputy Foreign Minister Abbas Araghchi give a joint press conference at the presidential headquarters in Tehran

Even if Iran had accumulated sufficient fissile material, it would need to assemble a bomb, probably one small enough to be carried by its ballistic missiles. How long that would take exactly is unclear, but stockpiling enough fissile material is widely seen as the biggest hurdle in producing a weapon.

Both US intelligence agencies and the IAEA believe Iran once had a nuclear weapons programme that it halted. There is evidence suggesting Iran obtained a design for a nuclear weapon and carried out various types of work relevant to making one.

US intelligence experts, however, believe Iran has yet to demonstrate an intention to shatter the 2015 deal, three US government sources said, noting Tehran continues to grant the IAEA access to its declared nuclear facilities.

In photo: Iran's Supreme Leader Ayatollah Ali Khamenei delivering a sermon to the crowd during Friday prayers in the capital Tehran.