Iran has started enriching uranium at its underground Natanz plant with a second type of advanced centrifuge, the IR-4, the UN's nuclear watchdog said in a report reviewed by Reuters on Tuesday, in a further breach of Tehran's 2015 deal with major powers.
Iran accelerated breaches of the deal's restrictions on its nuclear activities in an apparent effort to put pressure on US President Joe Biden. The sides are locked in a stand-off over who should move first to save the deal.
Tehran's breaches began in 2019 in response to the US withdrawal from the deal and the reimposition of economic sanctions against Iran under Mr Biden's predecessor Donald Trump, who opposed the agreement and abandoned it.
Last year, Iran started moving three cascades, or clusters, of different advanced models of centrifuge from the surface plant at Natanz to its below-ground fuel enrichment plant.
Iran is enriching uranium underground using IR-2m centrifuges, although the deal only allows it to enrich in the plant using first-generation IR-1 machines.
"On March 15, 2021, the agency verified that Iran began feeding the cascade of 174IR-4 centrifuges already installed at [the Fuel Enrichment Plant] with natural UF6," the International Atomic Energy Agency said in the report to member states.
UF6 is uranium hexafluoride, the form in which uranium is fed into centrifuges for enrichment.
Iran has indicated that it now plans to install a second cascade of IR-4 centrifuges at the plant but installation of that cascade has yet to begin, the report said.
Iran has already increased the number of IR-2m machines, which are far more efficient than the IR-1 machines, installed at the underground plant.
"In summary, as of March 15, 2021, Iran was using 5,060IR-1 centrifuges installed in 30 cascades, 522IR-2m centrifuges installed in three cascades and 174IR-4 centrifuges installed in one cascade, to enrich natural UF6 up to 5 per cent U-235 at [the plant]," the report said.
Iran is enriching up to 20 per cent purity at its Fordow plant to the north-east of the city of Qom.