What we know about Iran’s nuclear programme

For decades, Iran’s nuclear ambitions have been at the centre of tensions with the US and its allies, raising concerns that Tehran could eventually build atomic weapons. Israel has long considered a nuclear-armed Iran as an existential threat.

A 2015 landmark international accord placed limits on Iran’s nuclear activities in exchange for sanctions relief. US President Donald Trump withdrew from that deal in his first term.

During Mr Trump’s second term, efforts to negotiate a new agreement collapsed when the US and Israel bombed Iranian nuclear facilities in June 2025. The US and Iran had renewed talks in 2026 before the US and Israel launched airstrikes on targets across Iran on Feb 28.

Why is there concern about Iran’s nuclear programme?

Mr Trump said that the US began “major combat operations” to eliminate imminent threats from the Islamic Republic’s regime, including to “ensure that Iran does not obtain a nuclear weapon”.

The strikes were launched a day after inspectors from the International Atomic Energy Agency – the United Nations nuclear watchdog – reported that Iran was conducting regular and unexplained activity at the uranium-enrichment sites that were bombed in June.

Iran has always maintained that its nuclear program is peaceful and for energy purposes. President Masoud Pezeshkian told the UN General Assembly in September that “Iran has never sought, and will never seek, to build a nuclear bomb”.

However, just a day before Israel began its strikes in June, the IAEA censured Iran, saying it had breached its responsibilities to cooperate with inspectors and that the agency was unable to determine whether the country’s nuclear program is “exclusively peaceful”.

The IAEA accounts for gram-level changes in uranium inventories worldwide to ensure the material is not being diverted for weapons. Since the June 2025 attacks, Iran has blocked the IAEA from verifying the size and location of its stockpile of near-bomb-grade uranium, prompting the return of sweeping UN sanctions. 

What was the last confirmed status of Iran’s uranium stockpile?

The last time IAEA inspectors were able to access Iran’s uranium reserves, they determined it had amassed 441kg of uranium enriched to 60 per cent – a more than 50 per cent increase from February 2025. If processed further, that is enough material to power around a dozen nuclear bombs.

While the current status of Iran’s uranium stockpile is unclear, it retains the technical know-how for enrichment, which could allow the country to rebuild its nuclear programme with relative ease.

What is highly enriched uranium?

Natural uranium consists mainly of two isotopes: U-238 and U-235. The latter is key to the fission reaction necessary for both nuclear power and weapons, but it occurs in low concentrations in raw uranium ore.

The material therefore needs to be enriched to increase the concentration of U-235, which is done using thousands of centrifuges spinning at supersonic speeds to separate the isotopes.

A threshold of 3.7 per cent is required to fuel most nuclear-power plants. Anything above 20 per cent is defined as “highly enriched uranium” because at that stage, it requires special handling and getting to weapons-grade level is a relatively quick process.

The typical concentration of uranium for nuclear weapons is 90 per cent. Uranium enriched to 60 per cent can still be used in a crude bomb, albeit one with less power and reliability.

Iran has previously said it’s prepared to limit its enrichment of uranium to levels required for non-military purposes, but that it won’t stop entirely.

Is having weapons-grade uranium enough for a nuclear bomb?

An upgrade to 90 per cent-enriched uranium is not technically challenging; just a couple hundred centrifuges could achieve this in a matter of weeks or months.

But for the next step in the process, casting the uranium into a metal that can be used in a bomb, Iran would need to replace the capacity destroyed at its Isfahan facility in the June 2025 strikes. Iranian media reported that the site was also targeted in the February attack.

In addition to the fissile material, Iran would require a bomb mechanism and the means of delivering it. It is likely that Iran already has the technical know-how to produce a simple gun-assembly implosion device, such as the one the US dropped over Hiroshima, Japan, in 1945.

To strike a remote target, Iran would need a warhead that is small enough to ride atop one of its ballistic missiles and could survive re-entry into the Earth’s atmosphere. It has not conducted tests that would suggest it knows how to make a nuclear warhead.

Iran carried out studies on how to assemble such a device until 2003 but according to US intelligence reports, it probably hasn’t resumed that work. Estimates for how long Iran might need to complete the necessary activity range from four months to two years. Its most powerful ballistic missile has an estimated range of as much as 5,000km.

What do we know about Iran’s remaining capabilities to enrich uranium?

Whether enrichment can still be carried out at Iran’s two known facilities, Fordow and Natanz, after they were attacked last year is an unanswered question.

Satellite imagery confirmed major damage at the surface level from the US bunker-busting bombs dropped in June. However, it was unclear whether the deeply buried operations were impacted and it remains to be seen how they’ve fared after the latest strikes.

The main enrichment site in Natanz, located in the center of the country, included structures that were more than 40m below the surface, protected by a steel and concrete shell that researchers estimated to be eight metres thick. Fordow was even more heavily fortified, built into the side of a mountain and believed to be buried about 60 to 90m underground.

While Mr Trump said that Iran’s nuclear programme was destroyed by the US strikes in 2025, there was consensus among experts that the country retained key capabilities. Preliminary analysis by the Pentagon estimated Iran’s programme was set back by one to two years. A firm picture is unlikely to emerge until IAEA inspectors can physically verify the damage to the sites.

It is possible that Iran could further refine its remaining highly enriched uranium at a facility that is unknown to the outside world. In mid-June, the Atomic Energy Organization of Iran said a third enrichment plant had been built at an unspecified, secure location.

There is precedent for Iran operating secret nuclear facilities. Both Natanz and Fordow were built clandestinely and IAEA inspectors were only allowed in after the sites were nearly completed. BLOOMBERG