An atomic tug of war

After the 12-day war between Israel and Iran in June 2025, which also saw American B2 bombers attacking Iran’s nuclear facilities and Iranian missiles targeting the American military base in Qatar, Israel declared a ‘historic victory’. U.S. President Donald Trump claimed that American strikes had “obliterated” the Iranian nuclear facilities. The Iranians kept tactical silence about the impact of the strikes on their nuclear plans, including the Fordow facility, which is built deep underground, beneath a mountain.

However, early assessments by the U.S. intelligence community, which were leaked to American media, claimed that Iran’s nuclear programme had not been destroyed by U.S. strikes, but set back by “a few months”. Even if the nuclear facilities were destroyed, there is no certainty that Iran’s stockpile of highly enriched uranium and all advanced centrifuges have been destroyed. There were reports, based on European intelligence assessments, that Iran had dispersed its enriched uranium well before the Israeli-American strikes. According to Rafael Mariano Grossi, head of the International Atomic Energy Agency (IAEA), the UN’s nuclear watchdog, Iran has the industrial and technological capacity to resume enriching uranium in a few months. This leaves the Iranian nuclear programme unresolved, at least from an Israeli point of view

After Mr. 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 2024, Iran came under increasing pressure — its so-called axis of resistance was humbled by Israel, it lost an ally in Syria when the regime of President Bashar al-Assad fell, and its economy was in serious trouble.

This article is from The Hindu e-book. Iran: Revolution in retreat

As the heat on Iran rose, Mr. Trump offered dialogue. “We can’t let Iran have a nuclear bomb,” he said in April. Faced with the threat of war in a moment of weakness, Iran agreed to engage the Americans diplomatically. However, Israel struck Iran on June 12, when Iran was technically in talks with the U.S. The attack has set back the diplomatic efforts. And Iran’s nuclear programme remains one of the most dangerous disputes in West Asia as tensions between Iran and Israel are heating up again.

Iran, a signatory of the Nuclear Non-Proliferation Treaty (NPT), ceased to cooperate with the IAEA after the 1979 revolution that brought down the Shah’s monarchy and led to the establishment of the Islamic Republic.

Ever since, the Republic, which turned away from the U.S., its former ally, and adopted a revolutionary foreign policy, 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. However, 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.

HIGH POTENTIAL: An Iranian security official, in protective clothing, walks inside the Isfahan nuclear facility. According to some estimates, Iran has enough 60% enriched uranium for five-to-eight nuclear warheads.
| Photo Credit:
AP

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.

Ramiffications of talks failure

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

Thus, Iran might have been in a position to develop a deployable warhead in a matter of months if it decided to do so. The U.S.-Israeli strikes clearly set back the programme. However, there is no evidence to suggest that the strikes had destroyed the nuclear programme.

As of now, there is no evidence to suggest Iran has decided to make a bomb. However, Iran has suspended cooperation with the IAEA post Israeli strikes. So the Iranian programme has effectively gone dark. There were also reports that Iran has started construction and repair works at Fordow. Besides, the European decision to trigger snapback sanctions on Iran — sanctions that were lifted as part of the 2025 deal — has further enhanced tensions between the Islamic Republic and the West. For Israel, the 12-day war did not destroy the Iranian regime. Nor did it tear out the Iranian nuclear programme — its declared goal. Beneath its rhetoric of victory, Israel, which is now asking the international community to stop Iran from getting a nuclear bomb, knows this all too well. It will only grow more paranoid, closely monitoring Iran’s every move, while Tehran replenishes its arsenal, readying itself to fight another day. The war is far from over.