How close is Iran to building a nuclear weapon?

Heightened tensions between the United States and Iran, driven in part by the U.S. airstrike on Gen. Qassem Soleimani, brought new attention to Iran’s capacity for building a nuclear weapon.

After the Soleimani strike drew retaliatory attacks by Iran on U.S. military targets in Iraq, Trump warned Iran in a Jan. 8 address not to develop nuclear weapons.

"As long as I'm president of the United States, Iran will never be allowed to have a nuclear weapon," Trump said. He later reiterated, "Iran must abandon its nuclear ambitions and end its support for terrorism."

The question is how.

In May 2018, Trump withdrew the United States from an Obama-era, multi-country agreement that extended the time Iran would need to make a nuclear bomb, in exchange for an easing of sanctions on Iran. 

By agreeing to the deal, Iran committed to not pursuing nuclear weapons and faced practical obstacles if it broke that promise. 

Since the U.S. withdrawal, Iran has inched away from some of its commitments, including some announced after the strike on Soleimani. Meanwhile, there are no indications that Trump has made any progress on his pledge to renegotiate the agreement.

So how much has Iran done so far to break away from the nuclear agreement? And how far is Iran today from actually building a nuclear weapon?

Despite Iran’s straying from some aspects of the nuclear agreement, the deal is weakened but not entirely dead. Meanwhile, the "breakout" time — the amount of time Iran would need before it can produce a nuclear weapon — has shrunk, slowly but steadily.

Under the 2015 agreement, Iran pledged to forgo 97% of its stockpile of enriched uranium. It also said it would give up 14,000 of its 20,000 centrifuges — the machines used to enrich uranium — and agree to only enrich uranium to a level unsuitable for weapons for at least 10 years.

In addition, the deal also curbed Iranian production of plutonium, another element that can be used to make a bomb. The deal banned separating plutonium from spent fuel for 15 years and stipulated that Iran had to dismantle an existing facility. 

Meanwhile, the agreement significantly tightened international monitoring of Iran’s nuclear program.

If Iran had abided by these rules for 10 years, scientists say it would have taken the country at least 12 months to build a weapon once they left the agreement. That was designed to be enough "breakout" time for the U.S. and its allies to act before Iran could field a nuclear weapon.

For the first year after Trump pulled out of the nuclear deal, Iran continued to comply, said Matthew Bunn, a nuclear policy specialist at Harvard University’s Kennedy School of Government.

Then, he said, Iran began edging out of the deal, bit by bit.

Initially, this came through "small and easily reversible steps," such as going slightly above the cap of 300 kilograms of enriched uranium permitted, or going slightly above the 3.67% enrichment level permitted.

"Eventually they restarted development of advanced centrifuges, the most important actual step they have taken so far," Bunn said.

In response to the strike on Soleimani, Iran announced that it would no longer be bound by any of the agreement’s enrichment restraints. This is "serious, concerning, and a blow to the future" of the agreement, said Daryl G. Kimball, executive director of the Arms Control Association.

In a more conciliatory move, however, Iran also announced that it would not interfere with the international inspection regime.

This stance leaves the agreement "hanging by a thread," said Richard Nephew, a senior research scholar at Columbia University’s Center on Global Energy Policy. "If you are a very optimistic person, you can say that it is still a live agreement," he said. "I think that this is, though, a very optimistic reading."

Iran has also been silent about its intentions on other parts of the agreement. For instance, Iran has said nothing about its continuing adherence to restrictions on its Arak heavy water reactor and its commitment not to separate plutonium.

"While the Iranians are saying that they are effectively out of the deal, they are also explicitly saying they would come back to it if the United States did, and the Europeans are still trying to salvage as much of the deal as they can," Bunn said.

The "breakout" timeline has probably shrunk from its original 12 months, though it’s not clear by exactly how much.

Nephew suggested that the current breakout time is probably about 11 months, with the steps Iran has already taken dropping that on an ongoing basis.

"Iran probably will be within 2-3 months a nuclear weapon by the end of 2020, perhaps by the summer of 2021 if they slow down," Nephew said.

Bunn broadly agreed. "Iran could, if nobody interfered with them, produce enough material for a bomb in less than a year, and produce a bomb itself shortly thereafter," he said. "We’re talking somewhere in the three-to-nine month range, probably."

Still, he said that the caveat of "if nobody interfered with them" is a big one.

Once it became clear that an Iranian facility was starting to produce bomb material, Bunn said, "the question of bombing that facility would come up, and my guess is either the United States or Israel would do so. To avoid that, the Iranians, I would guess, are likely not to start producing weapon-grade material."

Iran, Kimball said, does not seem to be "dashing" toward a nuclear bomb. "Rather, it continues to try to pressure the United States and the Europeans to deliver on the sanctions relief Iran was promised when it entered into the 2015 agreement," Kimball said.