VIENNA: VIENNA: Iran has started enriching uranium with one of three cascades, or clusters, of advanced IR-6 centrifuges recently installed at its underground enrichment plant at Natanz, a report by the UN atomic watchdog to member states seen by Reuters said on Monday.Iran is using the cascade of up to 174 machines to enrich uranium to up to 5% purity, the confidential report said. Of the other two IR-6 cascades at the underground plant, one was undergoing passivation, a process that precedes enrichment, and the other had not yet been fed with nuclear material, it added.