What is uranium enrichment?
An Iranian technician works at the Uranium Conversion Facility just outside the city of Isfahan. AP photoWorld powers are seeking concessions from Iran over its higher grade uranium enrichment at talks in Baghdad today.
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 is uranium enriched?
Uranium is found naturally in a variety of forms. By definition, the atoms of the element - chemical symbol U - have 92 protons but they can have varying numbers of neutrons, creating different isotopes with varying properties.
- U-235, a designation formed from the sum of 92 protons plus 143 neutrons, is the isotope prized for reactors and bombs - but it makes up only 0.7 percent of mined uranium, nearly all
of which is the heavier U-238, with three extra neutrons.
- 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.
- The two most popular production techniques require uranium, in a processed form known as "yellow cake", to be converted into a gas called uranium hexafluoride (UF-6) before enrichment.
- When gaseous uranium hexafluoride is pumped through a porous barrier, the lighter molecules formed with U-235 atoms traverse the pores at a quicker rate those bearing U-238. This is like smaller grains of sand passing through a sieve more easily than larger. The process has to be repeated about 1,400 times to get U-235 at a concentration of 3 percent of the UF-6.
- Used by Iran. Like the diffusion process, the centrifuge method exploits the slight difference in mass between U-235 and U-238. Uranium hexafluoride 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. By separating the outer and inner layers and repeating the process many times the concentration of U-235 in the mixture is increased.
Iran's enrichment progress
- Iran started enriching uranium to 20-percent purity of U-235 in 2010 after talks broke down on a nuclear fuel swap - under which other countries would have supplied the higher-grade fuel Iran wants for a medical research reactor. The technical leap required to enrich to 90 percent in order to build weapons is considerably reduced once engineers have been able to reach 20-percent purity, hence Iran's new expertise in 20-percent enrichment was particularly alarming to its adversaries.
- Iran said in June 2011 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. Iran's chief nuclear negotiator Saeed Jalili said last month that Tehran needed to enrich uranium to 20-percent purity for peaceful purposes.
- The previously secret Fordow site is underground, raising fears in Israel and its Western allies that Iran might soon be able to make bomb-grade uranium out of reach of air strikes.
- In February, Tehran proclaimed advances in nuclear know-how, including new centrifuges able to enrich uranium much faster. The advances involved a new line of uranium enrichment centrifuges and the loading of its first domestically produced batch of fuel into a research reactor that is expected to soon run out of imported stocks.
- At the same time Iran sent a letter to EU foreign policy chief Catherine Ashton expressing readiness to "hold new talks over its nuclear program in a constructive way."