The move could mark a collision course with the United States. Iranian President Hassan Rouhani announced the move last week. President Trump told reporters on Sunday, “Iran better be careful.”

A week ago, Iran said it had breached a stockpile limit for low-enriched uranium that had been allowed under the 2015 nuclear deal. The International Atomic Energy Agency subsequently confirmed that Iran’s stockpile had breached the 300-kilogram limit.

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The Trump administration — which pulled out of the Iran deal in May 2018 — has suggested that the move showed Iran was never really serious about the 2015 agreement. “There is little doubt that even before the deal’s existence, Iran was violating its terms,” White House press secretary Stephanie Grisham said in a statement Monday.

Iranian officials have responded to the remarks incredulously. “Seriously?” Iranian Foreign Minister Mohammad Javad Zarif wrote the next day.

The emotional responses belie the fact that the uranium stockpile limit is a relatively minor detail of the nuclear agreement — some experts would even argue it is an insignificant one on a technical level. But its potential diplomatic impact should not be underestimated.

Why does uranium enrichment matter?

Uranium is a naturally occurring metal that can be used in both nuclear power plants and nuclear weapons. However, to make it useful for either, in most cases it must be enriched. This means that the percentage of uranium-235, a naturally occurring fissile uranium isotope, must be increased.

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In the uranium we find in the ground, the percentage of uranium-235 is less than 1 percent. However, it can be enriched by using centrifuge techniques to have a higher percentage of uranium-235.

Centrifuges can enrich uranium by rapidly spinning uranium hexafluoride gas. If uranium is enriched enough to be around 90 percent uranium-235, it can be used as fissile material in a nuclear weapon.

What does the Iran deal say about uranium enrichment?

The 2015 uranium deal, officially known as the Joint Comprehensive Plan of Action, placed limits not only on the amount of enriched uranium Iran could have, but also on how much the country could enrich its uranium.

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The text of the deal states specifically that for 15 years, Iran “will keep its uranium stockpile under 300 kg of up to 3.67 percent enriched uranium hexafluoride” and that any excess quantities can be sold off on the international market. Three hundred kilograms amounts to about 660 pounds.

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This limit is a far cry from the 22,046 pounds of higher-enriched uranium that Iran once possessed. But it also meant Iran would be allowed to have low-enriched uranium that could be used for commercial nuclear plants.

What has Iran done and what has it said it would do?

After warnings, Iran announced a week ago that it had exceeded the 300-kilogram limit of enriched uranium. This week, it announced that it has enriched uranium to 4.5 percent. Behrouz Kamalvandi, a spokesman for the Atomic Energy Organization of Iran, was quoted by state media as saying there were “no limitations or obstacles” to enriching at a higher percentage in the future.

Does this mean Iran could build a nuclear weapon?

Not right now. Even if the Iranians wanted to enrich uranium to the levels required to make a bomb, they would need a lot more uranium to start with.

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“It takes some 1,200 kilograms [2,645 pounds] of low-enriched uranium, if further enriched, to produce enough material for just one nuclear device,” Daryl G. Kimball, executive director of the Arms Control Association, told The Washington Post. “So there’s still a long way from having that quantity and even further away from having the bomb.”

It’s also notable that at the moment, Iran is enriching at only 4.5 percent. That’s a long way from the 90 percent needed for a bomb, and still far off the 20 percent enriched uranium that Iran says it needs to produce isotopes at a 52-year-old, U.S.-supplied research reactor in Tehran.

Is this a breach of the Iran deal?

Most analysts would say so. Yes, Iran cannot yet produce a weapon, but it could eventually. “This is a violation nonetheless, and it’s very worrisome,” Kimball said.

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But Zarif has argued against this, saying Iran’s partners in the nuclear deal were not living up to their side of the agreement. On Twitter, he pointed to a section of the accord that describes how Iran should respond when it feels like the other parties are not holding up their side.

Since the United States pulled out of the deal and reimposed sanctions, Iran has complained that it cannot reap the economic benefits promised under the deal because European companies are afraid of running afoul of the Trump administration.

Germany, Britain and France, three of the remaining parties to the deal, are working on a barter system that would allow European businesses to trade with Iran. However, the complex system has not been used, and it is unlikely to meet all Iranian demands.

What does this mean for the future of the deal?

European leaders have suggested that they view Iran’s moves as ultimately reversible and part of a negotiating tactic. On Tuesday, French President Emmanuel Macron released a statement that called on Iran to act “without delay” to put its enriched-uranium stockpiles back below the agreed-upon limit.

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The United States, meanwhile, has argued that Iran’s move shows it should not have been allowed to enrich uranium at all. “It was a mistake under the Iran nuclear deal to allow Iran to enrich uranium at any level,” Grisham said.

Groups such as the Foundation for Defense of Democracies, long-running critics of the Iran deal, have said Iran does not need to enrich uranium itself and should instead buy enriched uranium on the open market.

But some nonproliferation experts argue that the idea of zero enrichment would never work in practice. “Zero is a fantasy. It is a way of killing any possible deal by demanding the impossible,” Joe Cirincione, president of the Plough Shares Fund, tweeted on Monday.