Work resumed at the conversion facility quickly after inspectors from the UN nuclear watchdog finished installing surveillance equipment there. Iran had suspended work at the plant and its other nuclear facilities in November to avoid UN sanctions and as a gesture in negotiations with the Europeans.
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"The uranium conversion facility restarted its work a few minutes ago," the official Islamic Republic News Agency reported.
The facility, 10 miles southeast of the historical city of Isfahan, converts raw uranium, known as yellowcake, into gas, the feedstock for enrichment.
In the next stage of the process -- which Iran has said it will not resume for the time being -- the gas is fed into centrifuges for enrichment. Uranium enriched to a low level is used to produce nuclear fuel and further enrichment makes it suitable for use in atomic bomb.
Iran has said its nuclear program is for peaceful purposes, but the United States accuses it of seeking to develop atomic weapons.