It was the latest of numerous steps by Iran increasing weight on the U.S. President Joe Biden with the two sides in a standoff over who should move first to restore an agreement that was proposed to curb Iran's ability to develop a nuclear bomb, if it is so planned.
The agreement imposed boundaries on Iran's nuclear activities that it began breaching in 2019 in response to a U.S. withdrawal from the accord under Biden's predecessor Donald Trump, as well as the reimposition of U.S. sanctions upon the Islamic Republic that had been lifted under the deal.
The agreement simply lets Iran enrich with relatively antiquated first-generation IR-1 centrifuges at the underground Fuel Enrichment Plant (FEP) at Natanz, a commercial-scale enrichment facility. Last year Tehran started adding more advanced centrifuges there able to enrich much faster than the IR-1.
"On 31 March 2021, the Agency verified at FEP that: Iran had begun feeding natural UF6 into a fourth cascade of 174 IR-2m centrifuges," the International Atomic Energy Agency announced in the report dated Wednesday, referring to the underground Fuel Enrichment Plant and to uranium hexafluoride, the form in which uranium is supplied into centrifuges for enrichment.
Iran has notified the IAEA that it intends to use six cascades of IR-2m machines at the FEP to improve uranium up to 5% fissile purity. The report stated the remaining two cascades were fixed though not yet enriching. Installation of a planned second cascade of IR-4 machines had not yet started, it added.
"In summary, as of 31 March 2021, the Agency verified that Iran was using 5,060 IR-1 centrifuges installed in 30 cascades, 696 IR-2m centrifuges installed in four cascades and 174 IR-4 centrifuges installed in one cascade to enrich natural UF6 up to 5% U-235 at FEP," said the report, sent to IAEA member states.