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:

-- The two most popular production techniques require uranium ore, known as "yellow cake", to be converted into a gas called uranium hexafluoride (UF-6) before enrichment.

DIFFUSION METHOD: When gaseous uranium is pumped through a porous barrier, the lighter U-235 atoms traverse the pores at a quicker rate than U-238. This is like smaller grains of sand passing through a sieve quicker than the bigger ones. The process has to be repeated about 1,400 times to get U-235 at a concentration of 3 percent of the UF-6.

CENTRIFUGE METHOD: Like the diffusion process, the centrifuge method exploits the slight difference in mass between U-235 and U-238. Uranium gas is fed into a cylindrical centrifuge. It spins at supersonic speeds, causing the heavier U-238 to move towards the cylinder's outer edge while U-235 collects around the centre. Enriched U-235 is removed and put through the same process many times to raise its concentration.

-- Around 1,500 centrifuges running non-stop for months would be needed to make the 20 kg (45 pounds) of highly-enriched uranium needed for one crude warhead.

* CONCERNS ABOUT IRAN:

-- Iran kept its centrifuge enrichment program hidden from the watchdog International Atomic Energy Agency for 18 years until 2003, and laser enrichment research secret for 12 years.

-- Analysts say they believe Iran aims to gradually replace its start-up "P-1" centrifuge with a new generation it has adapted from a "P-2" design, obtained via black markets from the West and able to enrich uranium two-three times faster than its older counterpart.

Sources: Reuters/ Uranium Information Centre www.uic.com/ Nuclear Policy Research Institute www.nuclearpolicy.org.