The technology, known as a mixer-settler, is used to separate the uranium from the mined ore to produce concentrated uranium oxide - also known as yellowcake - which can then be converted and enriched in nuclear fuel work.

  

The United States accuses Iran of trying to master the civil nuclear fuel cycle as a cover for a military programme to obtain atomic weapons, butTehran denies this.

 

Iran announced in 2003 its intention to start extraction of uranium from a mine at Saghand, in the province of Yazd, which has an estimated 1.5 million tonnes of uranium ore.

 

Stumbling block

 

Once refined, the yellowcake is usually sent to a facility for conversion into uranium hexafluoride gas and subsequently for enrichment before making it to a nuclear reactor.

 

In a highly enriched form, uranium can be used in the core of an atomic bomb.

 

In August, Iran ended its freeze on the conversion of yellowcake to uranium hexafluoride gas (UF6) at a facility near the central city of Isfahan. So far, it has agreed to maintain a freeze of enrichment at a plant in Natanz.

  

Iran's enrichment ambitions have proved a serious stumbling block in talks with the European Union on its nuclear programme.

 

Russia has proposed that Iran enrich uranium on its soil, but an Iranian official said on Monday that the plan was flawed.

Ali Larijani, the secretary of Iran's security council, said on national television: "It is an idea, not a structured proposal, we don't see it as mature and it has serious problems."