Here are details about the process of uranium enrichment as world powers began talks with Iran on Monday, hoping the meeting will lead to new negotiations over a nuclear programme the West believes is for making atomic bombs.
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 higher degree.
What is enrichment?
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 generate electricity or create explosives.
This type, called U-235 to represent its mass, is present in only about 0.7 per cent 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 per cent. It must be refined to levels over 80 per cent to create the core of an atom bomb.
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.
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.
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.
According to the International Atomic Energy Agency's last report in November, Iran temporarily halted low-level enrichment work at Natanz in mid-November, without giving a reason, but the number of centrifuge sets -- cascades -- in operation had still increased in the last few months.
According to the report Iran started producing small batches of 20 percent enriched uranium with 164 centrifuges at Natanz in February, fuelling Western fears that Iran aims to develop nuclear bombs.
In August, the IAEA said Iran had begun using a second cascade of centrifuge machines to make the work more efficient.