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What does 20% enrichment mean?

Just how important is that 20% nuclear enrichment figure? For starters, 20% constitutes highly-enriched uranium and was the level at which Iran was enriching uranium between 2010 and 2013, before the JCPOA was agreed. Its activities led Iran to be hit with the toughest sanctions ever coordinated by the U.S. and EU together.

Under the nuclear deal, by contrast, Iran was only allowed to enrich to 3.67% U-235. U-235 is the isotope of uranium that can sustain a fission chain reaction.

“Iran appears to be trying to maximize its leverage with the Biden administration in the hope that the U.S. will agree to re-enter, rather than attempt to renegotiate, the JCPOA,” Anne Harrington, professor of international relations and a specialist in nuclear non-proliferation at Cardiff University in Wales, told CNBC.

Behnam Ben Taleblu, a senior fellow at the Foundation for Defense of Democracies, shared Harrington’s prediction. “By upping the nuclear ante, Tehran is hoping to create a crisis, a crisis which it hopes the U.S. will defuse with premature sanctions relief.”

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, nuclear experts told CNBC. Still, they explained, reaching 3 to 4% enrichment equates to roughly two-thirds of the work done toward that 90% figure, as any increases beyond that amount disproportionately speed up breakout time.

Biden has expressed his aim to return to some form of the 2015 deal;

some of his top foreign policy picks were the original negotiators and architects of the deal. But Tehran is not likely to make it an easy return, demanding compensation for the economic damage it suffered under the last few years of sanctions and other U.S. concessions.

“This is a double edged sword, both for the Iranians and the Biden administration,” said Sanam Vakil, deputy head of Chatham House’s MENA Program. The Iranians are wary of returning to the table too quickly, while for Biden, “the optics are quite difficult to justify giving in to the Islamic Republic and giving in to pressure tactics, particularly in light of the past criticism of the deal.”

By ramping up the leverage with the nuclear moves and the South Korean tanker seizure, “Iran wants to remind the international community, and particularly the Biden administration, that Iran doesn’t only have the diplomatic path,” Vakil said.

“There is this impulse inside the country to use pressure to extract concessions.”