FACTBOX: What is uranium enrichment?

(Reuters) - Iran has started to install 6,000 advanced centrifuges at its uranium enrichment facility, President Mahmoud Ahmadinejad said on Tuesday, an expansion of nuclear work the West fears is aimed at building bombs.

Here are some key facts about enriching uranium:

* WHAT IT MEANS:

-- Enrichment is a process of increasing the proportion of fissile isotope found in uranium ore (represented by the symbol 'U') to make it usable as nuclear fuel or the compressed, explosive core of nuclear weapons.

* WHY URANIUM MUST BE ENRICHED:

-- Uranium is found naturally in a variety of forms but only a particular adapted form of the mineral can be used to create electricity or explosives.

-- This type, called U-235 to represent its mass, is present in only about 0.7 percent of mined ore while most of the rest is U-238, which has a slightly heavier mass.

-- To generate electricity, the concentration of U-235 must be increased to between 3 and 5 percent. It must be refined to levels over 80 percent to create the core of an atom bomb.

* TECHNOLOGIES: