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The multiple targets of Israel’s sweeping Friday morning attack on Iran included air fields, missile bases and members of Iran’s senior military leadership.

But behind it all, the Israeli government said, lies a singular objective: impeding Iran’s ability to build a nuclear arsenal.

That has been a long-running goal, notwithstanding uncertainty about whether Iran intends to take the final step toward making bombs or instead use its capacity to do so as a bargaining chip.

What is less disputed is that Iran does have what it takes to get to that point, and relatively soon.

“It’s a credible risk from a technical point of view,” said Jeremy Whitlock, an independent consultant and former section head with the International Atomic Energy Agency, the United Nations nuclear watchdog organization based in Vienna, Austria.

Iran officially suspended its nuclear weapons program in 2003 and has said its current program is aimed at developing civilian nuclear power. But the way it has conducted that program – particularly after 2018, when U.S. President Donald Trump, during his first term, withdrew from an agreement designed to put safeguards around Iran’s nuclear activities – strongly suggests it has a different aim in mind.

On Thursday, the IAEA declared Iran in breach of its nuclear non-proliferation obligations. A key issue is Iran’s apparent stockpiling of the material needed to enable weapons production with relative ease.

“The biggest impediment to getting a weapon is to have access to the nuclear material itself,” Dr. Whitlock said, “because that’s the hardest thing to get your hands on.”

In Iran’s case, the material of choice is enriched uranium. (Other countries, including North Korea, have also made nuclear weapons that use plutonium.)

Enriched uranium does not occur naturally but has to be processed.

First, natural uranium is mined and then milled to separate it from other minerals.

At that point, it consists almost entirely of the isotope U-238, which is only modestly radioactive and unable to initiate the kind of runaway chain reaction that is needed for a nuclear weapon.

However, about 0.7 per cent of natural uranium comes in the form of the isotope U-235. As physicists discovered in the 1930s, U-235 does have the necessary properties to make a devastating bomb, if enough of it can be quickly brought together in one place.

It is this challenge that turns the exercise of building a nuclear weapon from a scientific puzzle into an industrial-scale enterprise with a footprint big enough to be targeted by opposing countries.

The key tool for gathering U-235 is a centrifuge – a rapidly spinning cylindrical container – which takes in uranium in a gaseous form. As the gas is whipped around, the slightly less massive U-235 isotopes drift toward the centre where they can be siphoned off.

The process is repeated many times, using a long chain of centrifuges called a “cascade” which serve to make the uranium that is increasingly more enriched with U-235. To step up production, many cascades can run in parallel.

Most commercial power-generating nuclear reactors (Canada’s are the exception) require uranium with a U-235 content of 3 to 5 per cent. When this ratio is achieved, the uranium is typically converted from a gas into solid uranium dioxide that can be used for fuel pellets.

That’s not good enough for a weapon, however.

“If you’re making a bomb, you would not want uranium dioxide, you’d probably want uranium metal,” said Markus Piro, an associate professor at McMaster University in Hamilton, Ont. and an expert in nuclear fuel engineering.

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For a weapon, uranium metal also needs to be much more highly enriched – over 90 per cent is the often-quoted benchmark – though weapons can be made with lower concentrations of U-235. (The uranium used in the first atomic bomb dropped in Hiroshima, Japan, was about 80 per cent enriched on average.) In general, higher enrichment also means less mass is needed to set off the explosion, so the device becomes easier to launch on a missile.

The key to weapons grade enrichment is many more centrifuges, which is why one objective of the cancelled Iran nuclear agreement was to put a cap on how many the country could use in its enrichment facilities.

“That would be a physical limitation to the maximum enrichment that they’d be able to achieve,” Dr. Piro said.

Iran is known to have two sites where uranium is enriched. One of those, in the city of Natanz, is reported to have been damaged by Israeli strikes. The other, called Fordow, is located deep under a mountain and much harder to hit.

Iran has reported no increase in radiation following the attacks. If an enrichment facility is hit and uranium is released, the contamination risk would likely be much lower and less severe than what might occur from damage to a working nuclear reactor, where there are more dangerous and radioactive fission byproducts.

Last month, the IAEA estimated that Iran had just over 400 kilograms of 60 per cent highly enriched uranium. One challenge for those monitoring the situation is that enrichment gets easier as the concentration of U-235 increases. If the estimate is correct, it may only be a relatively small step for Iran to then proceed to make 90 per cent highly enriched uranium in enough quantity to make about 10 bombs.

Israel has tried in the past to slow Iran’s progress with uranium enrichment. This is thought to include the development of Stuxnet, a computer malware uncovered in 2010 whose purpose was apparently to damage Iranian centrifuges by causing them to spin too rapidly.

Edwin Lyman, a physicist and director of nuclear power safety with the Union of Concerned Scientists in Washington D.C., said that while Stuxnet caused a media sensation, it may have only delayed Iran’s enrichment program by a year or two at best.

And after the current attacks, Iran will have more incentive to show that it can continue in spite of Israel’s efforts.

“This situation could and should have been resolved through diplomatic means,” Dr. Lyman said. “And there’s still the opportunity to do so.”