This handout image supplied by the IIPA (Iran International Photo Agency) shows a view of the reactor building at the Russian-built Bushehr nuclear power plant as the first fuel is loaded, on August 21, 2010 in Bushehr, southern Iran. Getty Images

Iran has now exceeded its internationally-agreed stockpile limit of low-enriched uranium, Foreign Minister Mohammad Javad Zarif and the International Atomic Energy Agency confirmed Monday, breaching a key tenet of the 2015 nuclear deal that the President Donald Trump administration abandoned last year. The move comes amid rapidly escalating tensions between Washington and Tehran and against the backdrop of an Iranian economy buckling under the weight of U.S. sanctions, which had previously been lifted under the Obama-era deal in exchange for limits on the Islamic Republic's nuclear program.

Zarif said Iran has surpassed 300kg (661 pounds) of uranium hexafluoride (UF6), the equivalent of 202.8kg of low-enriched uranium, Iran's limit under the nuclear deal. Uranium enriched to the low level of 3.67% fissile material, allowed under the deal, is the initial step in a complex process that could, over time, enable Iran to accumulate enough highly-enriched uranium to build a nuclear warhead, according to experts. Foreign Ministry spokesman Abbas Mousavi, however, said that Iran's breaches of the deal were "reversible". But what does exceeding a certain amount of low-enriched uranium actually mean? How much closer does this bring Iran to nuclear bomb-making capability?

A bomb is still 'a long way off'

Nuclear experts interviewed by CNBC say this is far less threatening than it sounds. "Even once Iran crosses the 300kg threshold, they are still a long way off from having a stockpile sufficient to produce a bomb," Anne Harrington, professor of international relations and a specialist in nuclear nonproliferation at Cardiff University in Wales, told CNBC in an email. That's because low-enriched uranium is only 3.67% U-235 ⁠— the uranium isotope needed to create a nuclear weapon ⁠— and is impractical for use in a weapon, Harrington explained. "At 3.67% they would need to stockpile approximately three times the current limit to have enough material for one bomb, and that material would need to be further enriched."

You still need to fit it into a device, you need precision engineering to design the core, you also need something to deliver [a bomb] with. So they are not a nuclear armed country, even if they enrich further. Jan Ruzicka Lecturer in Security Studies, Aberystwyth University, Wales

The minimum threshold for a crude nuclear weapon is 400kg of uranium enriched to 20% U-235, but weapons grade uranium is 90% U-235, the professor elaborated. "So while increasing their stockpile of low-enriched uranium puts Iran closer to producing weapons grade material, there is still a lot of work to do to get there," she told CNBC. Rather than indicating an intent to weaponize their program, Harrington said, "what the Iranian leadership appears to be doing is signaling to the Trump administration and its domestic constituencies that it remains committed to its right to enrich."

A significant move ⁠— but how dangerous is it?

Jan Ruzicka, a lecturer in security studies at Aberystwyth University in Wales, says he "could clearly see this escalating further into something nasty and dangerous." "But does this get Iran closer to the bomb?" he asked. "Technically speaking, yes in some ways, but it's still a ways to go from where they are ⁠— even if they overstep this threshold."

"It's significant, yes, but dangerous I'd say potentially not immediately with a view to the nuclear bomb." Ruzicka and Harrington see Iran roughly a year out from having enough material for an atomic weapon, but they say any timeline is dependent on many complicated factors. Still, significant engineering expertise and additional time is required to make an actually deliverable bomb. "You still need to fit (the highly-enriched uranium) into a device, you need precision engineering to design the core so you can actually control and spark the reaction, you also need something to deliver it with," Ruzicka explained. "So they are not a nuclear armed country, even if they enrich further." Aniseh Tabrizi, an Iran expert and research fellow at London's Royal United Services Institute, agrees. "All these measures are pretty reversible, and the increase of the stockpiles in itself is not a matter of concern, even if it is of course a violation of the JCPOA (the Iran deal's formal acronym) and entails some sort of response," Tabrizi said.

"The Iranians are trying to gain some sort of leverage on the nuclear front," she added, "showing that they're not going to sit and wait for pressure to increase without doing anything ⁠— they're actually responding and therefore increasing the cost of the decision that the U.S. has made."

How did we get here?

Trump reimposed heavy economic penalties, most significantly on Iran's massive oil sector, last year in response to what his administration calls "malign activity" in the Middle East including support for terrorist proxies like Hezbollah and ballistic missile testing. In May, his administration ended the granting of waivers to the remaining importers of Iranian oil with the aim of slashing the country's crude exports to zero. Iran's economy is in severe recession and its inflation is expected to exceed 40% this year. While the other signatories to the nuclear deal ⁠— France, Germany and the U.K. as well as Russia and China ⁠— have tried to keep the agreement alive, Tehran says that unless they can shield it from the punishing effect of the American sanctions and revive trade, particularly in oil, Iran will continue to renege on its JCPOA obligations. Zarif said Monday that Iran would reverse exceeding its stockpile limits "once European signatories to the nuclear deal meet their obligations." But engaging in that trade would mean falling foul of harsh U.S. secondary sanctions.

The Europeans have endeavored to come up with a mechanism to trade with Iran that avoids using dollars. But the success of this mechanism, called INSTEX, is dubious and so far, it's not enough for the Iranians. EU spokesperson Maja Kocijancic expressed "concern" at Iran's decision. "Noting that Iran has remained compliant for 14 months after the U.S. withdrawal from the JCPOA, as verified by the IAEA, we urge Iran to reverse this step and to refrain from further measures that undermine the nuclear deal," Kocijancic said Monday.

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