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Iran's high-level enrichment of uranium is a key concern of world powers over a controversial nuclear programme which the Islamic republic insists is for peaceful purposes.
This process is expected to top the agenda at talks on Tuesday in Almaty between Iran's nuclear delegation and those of the P5+1 group of the United States, Russia, China, Britain, France and Germany.
Uranium enriched at high levels can be used in a nuclear weapon.
According to a February 21 report by the UN's nuclear watchdog, the International Atomic Energy Agency (IAEA), Tehran has declared enrichment to 3.5 percent and 20 percent purity at two facilities in central Iran:
- Natanz, near Isfahan:
Iran enriches uranium here to both 3.5 and 20 percent levels, using in total more than 12,000 IR-1 first-generation centrifuges, machines that spin uranium gas at supersonic speeds.
Monthly output of 3.5 percent-enriched uranium is some 230 kilos (500 pounds), using 8,892 of the centrifuges, and some 8.2 tonnes of purified uranium to this level have been produced since enrichment operations started in 2007.
Another 348 centrifuges at Natanz are used to further process some of that stock into 20 percent-enriched uranium.
Iran began in February to install more efficient IR-2m next-generation centrifuges at Natanz, a move which drew condemnation from world powers.
- Fordo, near Qom:
Iran has 696 IR-1 centrifuges at this fortified bunker facility, whose discovery in 2009 triggered alarm among world powers. Dug under a mountain to be protected from air strikes, the site is used to produce 20 percent-enriched uranium, with a potential capacity of up to 3,000 centrifuges.
Iran has produced 280 kilos of 20 percent-enriched uranium at Natanz and Fordo since February 2011, and currently makes another 15 kilos every month.
According to the IAEA's February report, Iran has a remaining stock of 5,974 kilos of 3.5 percent-enriched uranium and 167 kilos of 20 percent.
The rest of the 20 percent-enriched uranium has been converted into 50 kilos of fuel for a Tehran medical research reactor, rendering it unsuitable for further enrichment.