Factbox: What is uranium enrichment?

(Reuters) - A U.N. nuclear watchdog report said Iran has worked on developing a nuclear weapon design, and testing and other research relevant for nuclear arms, and some of the activities may still be going on.

Western powers want Iran to suspend uranium enrichment activity, which can produce fuel for nuclear power reactors or provide material for bombs if refined to a much higher degree.

Iran says its nuclear aims are peaceful.

Here are details about the process of uranium enrichment:

* WHY URANIUM MUST BE ENRICHED:

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

-- This type, called U-235 to represent its mass, makes up 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:

-- Used by Iran. 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 toward the cylinder's outer edge while U-235 collects around the center. Enriched U-235 is removed and put through the same process many times to raise its concentration.

* IRAN'S ENRICHMENT PROGRESS:

-- Despite a brief halt in late 2010, Iran's total output of low-enriched uranium (LEU) since early 2007 rose to reach 4.1 tons in May, up from 3.6 tons in February, suggesting steady work despite technical woes and possible cyber sabotage. LEU is uranium that is enriched to a concentration of U-235 lower than 20 percent.

-- Iran has also said it plans to install two new centrifuge cascades in a research and development facility at its main enrichment plant at Natanz, with more modern machines than the IR-1 model now in use.

-- Iran started enriching uranium to 20 percent purity in 2010 after talks broke down on a nuclear fuel swap -- under which other countries would have supplied the higher grade fuel Iran needs for a medical research reactor. The move took Iran's enrichment level closer to the 90 percent needed for making atomic bombs -- a worrying development for the West.

-- Iran said last June it would start transferring 20 percent enrichment work from Natanz to the Fordow site -- first revealed only in September 2009 -- and that it would increase the production capacity by three times.

Sources: Reuters/ Uranium Information Center www.uic.com/ Nuclear Policy Research Institute www.nuclearpolicy.org. (Reporting by David Cutler, London Editorial Reference Unit; Editing by Peter Graff)