Iran’s Bushehr nuclear power plant has been in operation for almost ten years.Credit: Majid Asgaripour/AP/Shutterstock

Iran has accumulated 1,200 kilograms of enriched uranium — more than doubling the stockpile it had just three months ago, according to a statement from a senior official at the Atomic Energy Organization of Iran on 25 January.

That’s enough to build one atomic bomb, if the uranium is further refined to make it weapons-grade — a process that could take just two to three months , says David Albright, a nuclear-policy specialist at the Institute for Science and International Security in Washington DC. But building actual weapons would take much longer, he adds.

If confirmed, the rate of Iran's expanding uranium stockpile “shifts things dramatically”, Albright adds. But he and others caution that there is no evidence that Iran is rushing to build a bomb — at least not yet.

Tensions between Iran and the United States have escalated in the past month. On 3 January, a US drone strike killed Qasem Soleimani, the key architect of Iran’s regional military influence. In response, Iran shot missiles at US bases in Iraq.

The Joint Comprehensive Plan of Action (JCPOA), the 2015 deal between Iran and six global powers that limited its nuclear capabilities in exchange for the lifting of economic sanctions, is now in serious jeopardy. US President Donald Trump pulled out of the deal in May 2018, and Iran announced in May last year that it would resume uranium enrichment.

Nature talked to nuclear experts to find out how soon Iran can build a bomb, and whether this is likely to happen.

Has Iran tried to build nuclear weapons in the past?

Building nuclear weapons is expensive and requires technical expertise, such as enriching uranium. The fissionable isotope uranium-235, which makes up less than 1% of natural uranium, must be separated from uranium-238, which is by far the more common isotope.

Iran has a strong physical-sciences tradition, and has had an active nuclear programme for decades. The country has always maintained that this was purely for peaceful purposes, such as producing isotopes for medical use. But in the early 2000s, Iran appeared to have a crash programme to build at least five uranium fission bombs, according to US intelligence assessments and international observers.

Reports in the mid-2000s by the United Nations’ International Atomic Energy Agency (IAEA) suggested that Iran could be actively working to build a nuclear arsenal. That would be a violation of the 1968 Treaty on the Non-Proliferation of Nuclear Weapons (NPT), which Iran has signed. In 2003, bowing to international pressure, the country agreed to cut down its nuclear activities drastically — but not completely.

US president Dwight Eisenhower initials the first international agreement for the peaceful use of atomic energy, in 1955.Credit: Alamy

Did the 2015 deal reduce Iran’s nuclear capabilities?

As of 2015, the country had stockpiles of 11 tonnes of a uranium hexafluoride enriched to as much as 20% 235U. Weapons-grade uranium must be enriched to 90%. Uranium is commonly processed as uranium hexafluoride gas, which is separated by isotope in high-speed centrifuges, and Iran had more than 10,000 of these centrifuges. When the JCPOA was signed in July 2015, experts had estimated that the country was months — perhaps weeks — away from producing weapons-grade uranium.

But the JCPOA forced Iran to ship most of its stockpile abroad, and to mothball the majority of its centrifuges. The aim was partly to stretch the time Iran needed to stockpile enough fissile material for a bomb — known as ‘breakout time’ — to at least a year. The deal also subjected Iran to a stringent regime of IAEA inspections. In the following years, the agency periodically reported that Iran was fully complying with the deal.

The JCPOA was also a “big win” for global non-proliferation efforts, says Seyed Hossein Mousavian, who was the spokesperson for Iran’s nuclear negotiating team in 2003. “Over 200 nuclear scientists worked on the technical details for years,” says Mousavian, now a nuclear-policy specialist at Princeton University in New Jersey. As a consequence, he says, Iran’s inspection regime is more detailed than the one described in the NPT, which could make the 2015 deal a precedent and model for future disarmament accords.

What was the impact of the US withdrawal from the nuclear deal?

Despite its reported success at limiting Iran’s nuclear capabilities, some opponents of the deal complained that the JCPOA did not go far enough. In 2018, the United States unilaterally withdrew, imposing new, crushing economic sanctions on Iran. Mousavian says that Iranians felt cheated. The perception in the country is now that “you cannot negotiate with or trust the US” and the impact could be much wider. For example, it could make nuclear powers such as North Korea hesitant to come to the negotiating table, he adds.

Foreign minister Mohammad Javad Zarif has said that Iran is prepared to withdraw from the Treaty on the Non-Proliferation of Nuclear Weapons.Credit: NOEL CELIS/POOL/AFP/Getty

Does Iran now have enough enriched uranium to build nuclear bombs?

Last November, the IAEA found that Iran had accumulated around 550 kilograms of uranium hexafluoride that was “moderately enriched” to less than 4.5% 235U. It is unclear what material the Iranian official was referring to in his 25 January claim, but it is presumed to be 1,200 kilograms of moderately enriched uranium hexafluoride. If further enriched, this quantity could yield more than 30 kilograms of weapons-grade uranium, enough to build one fission bomb. The IAEA is expected to release its latest report on the Iranian nuclear programme, including its assessment of stockpiles, in early March at the latest.

But how quickly could Iran actually make a bomb once it has enough weapons-grade uranium?

Possessing fissile material is not enough: a country also has to master the design, and manufacture, of a bomb. In particular, uranium hexafluoride must be converted into uranium metal, which is not straightforward, says Richard Johnson, a proliferation specialist at the Nuclear Threat Initiative, a policy research centre in Washington DC.

A study prepared for the US Congress and updated last December suggests that by the time Iran partially froze its weapons development in 2003, it had not yet mastered all the skills necessary to build bombs — and that it probably did not make significant progress in later years.

According to Albright, some intelligence agencies estimate on the basis of this information that it could take the country about two years to make its first two bombs, if it wanted to do this.

If the nuclear deal is scrapped, will Iran be legally entitled to arm itself with nuclear bombs?

No. Because Iran has signed the NPT, it is committed to using nuclear technology exclusively for peaceful purposes. Members of the NPT must allow the IAEA to verify their compliance, or face consequences such as UN sanctions. However, Iran could withdraw from the NPT, as North Korea did in 2003, as it was becoming a nuclear power. Iranian foreign minister Mohammad Javad Zarif said on 20 January that the country is prepared to withdraw if its continued enrichment programme is reported to the UN Security Council.

So is Iran actively working towards a bomb?

“All the signs are that they are not,” says Zia Mian, a physicist and nuclear-policy expert at Princeton University. The country has complied with the rigorous IAEA inspection regime set out in the JCPOA, he says. This means that a nuclear-weapons programme is “either hidden so well that no one has been able to find it so far, or that there is no such crash programme”, he says. Albright agrees, saying that Iran could be stockpiling enriched uranium to increase its leverage in future negotiations. “You don’t see some of the indicators that would imply a well worked-out decision” to actually build bombs, he says.

“I would still maintain that Iran has not started a dash for the bomb,”adds Oliver Meier, an arms-control specialist at the Institute for Peace Research and Security Policy in Hamburg, Germany. “This would require a range of other activities, including on weaponization, that we have not seen.”

Iran’s officials continue to deny any intention to build nuclear bombs. But stepping away from its JCPOA obligations — and in particular, increasing stockpiles of enriched uranium — brings the country closer to having the capability to do so.

“I still think they are not racing for a bomb,” Johnson says, but he adds that the international community should remain vigilant. “The concern is there.”