The U.S. and Iran have started talks about the latter’s controversial nuclear programme. After Donald Trump, in his first term, unilaterally withdrew the U.S. from the 2015 nuclear deal, officially known as the Joint Comprehensive Plan of Action (JCPOA), Iran had maintained that it would not hold direct talks with the U.S. There were multiple rounds of indirect talks in Vienna after Joe Biden became President in 2021, but those efforts were inconclusive. Iran, in this period, substantially accelerated its nuclear programme.

In recent months, Iran came under increasing pressure — its so-called axis of resistance has been humbled by Israel, it lost an ally in Syria, and its economy is in serious trouble. As the heat on Iran rose, Mr. Trump offered dialogue. “We can’t let Iran have a nuclear bomb,” he said last week in a joint press conference at the White House with the visiting Israeli Prime Minister, Benjamin Netanyahu. Faced with the threat of war in a moment of weakness, Iran has agreed to engage the Americans diplomatically.

Iran, a signatory of the Nuclear Non-Proliferation Treaty (NPT), ceased to cooperate with the International Atomic Energy Agency (IAEA) after the 1979 revolution. Ever since, the Islamic Republic faced allegations that it has been pursuing a clandestine nuclear programme. In 2022, the IAEA launched an investigation into Iran’s alleged nuclear activities. In November 2011, the agency reported that Iran appeared to have worked on designing an atom bomb. Iran has always maintained that its nuclear programme was for peaceful purposes. But its critics pointed to its stockpile of highly enriched uranium as evidence of the country’s clandestine designs.

Iran’s uranium enrichment story, however, is a long, complex one.

In natural settings, U-235, the uranium isotope that can sustain nuclear fission chain reactions, makes up around 0.7% of uranium. The rest is U-238. Before its use in nuclear settings, uranium is enriched to increase the concentration of U-235. Both low-enriched uranium (LEU) and high-assay low-enriched uranium (HALEU) are enriched up to 20% for use in different kinds of nuclear reactors. Highly enriched uranium (HEU) refers to enrichment beyond 20%. Weapons-grade uranium is typically 90% or more.

Centrifuges are the world’s enrichment technology of choice. These containers spin their contents at several thousand revolutions per minute. Because U-238 is slightly denser than U-235, the centrifugal force pushes it more towards the periphery. The feed is uranium hexafluoride (UF6) gas. Enrichment facilities have hundreds or thousands of centrifuges operating in cascades, with each cascade accepting as its feed the output of the previous cascade. At each step, more-enriched UF6 is passed to the next while the rest, called tails, is recycled or processed for long-term storage. Each centrifuge’s enrichment service is measured in separative work units (SWUs). Depending on the centrifuge design, producing 1 kg of weapons-grade uranium from natural uranium may need around 250 SWUs.

In 2006, Iran enriched uranium to about 3.5% using 164 IR-1 centrifuges, each of which delivers around 0.8 SWU/year. In 2010, the IAEA confirmed that Iran had enriched uranium to 19.75% using IR-1 centrifuges at the Natanz Fuel Enrichment Plant and in 2012 at the Fordow plant. By 2013, the country had a stockpile of about 7.6 tonnes of 3.5% LEU and 0.2 tonnes of 19.75% LEU gas.

Terms of the original deal

The 2015 Iran nuclear deal (JCPOA), between Tehran, the five permanent members of the UN Security Council, and the European Union, provided a short-lived solution to the nuclear crisis. The deal promised to remove international sanctions on Iran in return for the country removing most of its centrifuges, limiting enrichment to 3.67%, and capping its LEU stockpile at 300 kg, among other measures. Iran was fully compliant with the terms when Mr. Trump pulled the U.S. out of it in May 2018 and reimposed sanctions on Tehran. Iran has since accelerated its nuclear programme breaching the agreement, which saw the country enriching uranium to 60% at its plants.

This is crucial. If 126 SWUs are required to enrich uranium from 0.7% such that it yields 1 kg of 60% HEU plus 0.3% tails, only 2.2 SWUs are required to enrich 60% HEU to 1 kg of 90% weapons-grade level plus 20% tails (which is higher at higher enrichment). In other words, 60% HEU will have completed more than 90% of the work required to produce weapons-grade uranium. According to some estimates, Iran has around 70 kg of 60% HEU, sufficient for five to eight nuclear warheads.

While the number of SWUs decreases with more enrichment, the energy cost skyrockets. But Iran’s commitment suggests the centrifuges will not want for power.

Iran Watch has estimated that all centrifuges “presently installed in production mode” in Iran could produce 168-269 kg of 60% high-enriched uranium in “up to two weeks” (assuming 1% tails and 54% feed enrichment). The time to produce enough U-235 for one warhead may thus have dropped from around a year during the JCPOA to a few weeks today.

The IAEA suggests a “significant quantity” of 25 kg per warhead with a blast yield of 20 kilotonnes (to compare, Hiroshima was devastated by a yield of 13-16 kilotonnes). Newer designs could have the same yield with lighter cores. Iran may also assemble more weapons of lower yield.

Iran’s centrifuges also raise questions about how quickly it can assemble a bomb. Post-enrichment, engineers must convert the uranium in UF6 to metallic form and machine it into the bomb’s core. Second, they need to develop explosives, detonators, arming and firing systems, neutron initiators, explosive lenses, and launch and re-entry vehicles. And they need to conduct tests. The second set can be done in parallel with enrichment, however. According to data from the IAEA and the U.S. Office of the Director of National Intelligence, among others, Iran ran a programme in 1999-2003 during which it also focused on these activities.

Ramifications of talks failure

Harvard University Belfer Center for Science and International Affairs scholar Hui Zhang has written that if Iran’s steps towards its first nuclear weapon are like China’s in 1964, Iran will need “probably less than three weeks” between gaseous weapons-grade uranium and a bomb.

Thus, Iran may be able to develop a deployable warhead in a matter of months if it decides to do so.

As of now there is no evidence to suggest Iran has made that decision. But Iran’s growing stockpile of HEU and shrinking breakout time — the time taken to convert weapons-grade fuel into a bomb — have already set alarm bells ringing in Israel. Tel Aviv has made it clear that it will not hesitate to bomb Iran’s nuclear facilities. But for any such move, Israel needs America’s support. Mr. Trump has also threatened Iran with military action. But before that, he wants to try the path of diplomacy. If the talks collapse, there would be bombing, he threatened. “If it requires military, we’re going to have military. Israel will obviously be very much involved in that — it’ll be the leader of that.”