VIENNA - Iran has begun installing advanced centrifuges at its main uranium enrichment plant, a UN nuclear report said on Thursday, a defiant step likely to anger world powers ahead of a resumption of talks with Tehran next week.In a confidential report, the International Atomic Energy Agency (IAEA) said 180 so-called IR-2m centrifuges and empty centrifuge casings had been hooked up at the plant near the central town of Natanz.If operated successfully, such machines could enable Iran to significantly speed up its accumulation of material that the West fears could be used to devise a nuclear weapon. Iran says it is refining uranium only for peaceful energy purposes.