How can Iran build a nuclear bomb?

China has also gained experience with implosion bombs. In the 1960s, the People's Republic built two such bombs for its first nuclear weapons tests. The Chinese nuclear weapons program demonstrated that the non-nuclear components of such a bomb—that is, everything surrounding the bomb core made of highly enriched uranium—can be developed in parallel with the enrichment of the bomb core. Once sufficient weapons-grade uranium is available, the bomb can be assembled within three to five weeks.

An implosion bomb is detonated from the outside in. The goal is to compress the bomb core of highly enriched uranium and the neutron source contained within it as tightly as possible. This puts the uranium mass into a supercritical state, setting off an ultrafast chain of fission reactions.

The destructive power is maximized by maintaining these chain reactions long enough until the explosion. The tamper provides the delay. This prevents the bomb from exploding immediately after the first nuclear fission and thus "fizzles out." In a medium-sized bomb , 99.9 percent of the energy is released in the last 0.07 microseconds before the explosion.

The core of the bomb is uranium, which is responsible for uncontrolled chain reactions. However, such chain reactions are not possible with naturally occurring uranium. Natural uranium consists largely of the difficult-to-fissile uranium-238. Only fissile uranium-235 is suitable for chain reactions, but it only accounts for 0.7 percent of the weight of natural uranium. To obtain more fissile material, the uranium must be enriched.

Four to five tons of natural uranium are needed to produce the approximately 25 kilograms of highly enriched gaseous uranium needed for one bomb. The material must undergo several conversion steps to achieve this.

The enrichment of natural uranium into weapons-grade uranium in centrifuges is by far the most complex part of producing a nuclear bomb. The rest, namely the construction of the bomb itself, is challenging. But Iran may already have the necessary weapons-grade know-how. This knowledge can be acquired in parallel with the uranium enrichment. The implosion and neutron source can also be tested with natural uranium. The IAEA recently presented evidence that Iran conducted implosion tests in undeclared facilities as early as 2003.

Iran is also well advanced with enrichment. According to the IAEA, at the end of May the Islamic Republic already had a stockpile of over 400 kilograms of uranium enriched to 60 percent.

The highly enriched uranium obtained from the centrifuges is easy to transport. The uranium-containing gas can be filled into special gas cylinders. A few truckloads of gas cylinders would be enough to transport Iran's entire uranium stockpile. This makes the assumption that Iran had moved its uranium stockpiles to safety before the airstrikes seem plausible.

In the months before the war, Iran had increasingly fed 20 percent uranium into the centrifuges instead of natural uranium, thereby greatly accelerating the enrichment to 60 percent.

The initial enrichment effort, when using natural uranium as the starting material, is very high. However, the effort then levels off. Enriching from 60 to 90 percent is only a relatively small step.

The Institute for Science and International Security (ISIS), an American think tank, estimated in early June that Iran could produce enough weapons-grade uranium to build nine nuclear bombs at its enrichment facility in Fordow within three weeks. According to the ISIS analysis, Iran would have enough material for up to 22 bombs over five months.

But on June 22, the facilities in Fordow and Natanz were attacked with American bunker-busting bombs. Since then, it has been unclear how severely the centrifuges at both sites were damaged.

In 2015, Iran and the permanent members of the UN Security Council, plus Germany, signed a nuclear agreement. According to the US government, Tehran still lacked the necessary expertise to build a bomb. With the nuclear deal, Iran committed to refraining from acquiring nuclear weapons. In return, Washington eased its economic sanctions. Under President Trump, the US withdrew from the agreement in 2018 and reimposed sanctions against Iran. Since then, Tehran has accelerated the pace of its nuclear program.

While the Iranian leadership officially claims that nuclear weapons are un-Islamic and therefore taboo, the Tehran regime has simultaneously called for the destruction of Israel and repeatedly withheld information about its own nuclear program from the responsible UN agency, the IAEA. It is unclear whether Iran has actually committed to building nuclear bombs or whether it intends to use the advanced work toward that goal as leverage in international negotiations.

At the beginning of June, after the IAEA had published its latest report on the status of Iran's nuclear program, IAEA Director General Rafael Rossi, when asked how long it would take Iran to build a nuclear bomb, said something like: "More like months than years."

Following the recent Israeli and American attacks, such an estimate has become more difficult, especially because Iran suspended its cooperation with the IAEA on Wednesday. The US Department of Defense now estimates that the military strikes have set back Iran's nuclear program by up to two years. However, the Pentagon has not presented any evidence for this, relying instead on intelligence information. At the end of June, IAEA Director Rossi said that Iran could resume uranium enrichment within a few months.

It could be some time before clarity is achieved regarding the status of Iran's nuclear program. And ultimately, it will depend primarily on whether the rulers in Tehran muster the political will to build the bomb. Or hopefully not.