AGENCE FRANCE-PRESSE,
Tehran: Enrichment, the sensitive process that Iran vowed on Monday was “no longer possible” to stop, takes low-grade uranium and refines it into a material that can power reactors — or an atomic bomb.
The key difference is that reactor fuel needs uranium that has been enriched only to a low level, whereas an atom bomb requires a much more highly enriched version.
When uranium ore is dug out of the ground, more than 99 percent of it is made up of the more stable U-238 isotope, and just 0.7 percent of it consists of the U-235 isotope that is useful to nuclear engineers.
The goal, therefore, is to beef up the percentage of U-235 so that there is enough of it to induce a chain reaction.
The first step is to mill the ore into a concentrate called yellowcake. This is converted into uranium hexafluoride gas (UF6) ahead of enrichment.
One of the two methods of enrichment is that chosen by Iran, which is by gas centrifuge.
The UF6 is piped in a cylinder which is then spun at high speed. The rotation causes a centrifugal force that leaves the heavier U-238 isotopes at the outside of the cylinder, while the lighter U-235 isotopes are left at the centre.
The process is repeated many times over through a cascade of centrifuges to create uranium of the desired level of enrichment.
When the U-235 level reaches around five percent, the uranium is enriched enough to be used as fuel for civil nuclear reactors.
Iran says it has not enriched uranium beyond 4.8 percent and only on a limited scale.
To be used as the fissile core of a nuclear weapon, the uranium has to be enriched to more than 90 percent and be produced in large quantities.
Little Boy, the Hiroshima bomb, used 64.1 kilos (141 pounds) of enriched uranium, although a device can also be built from between 15 and 25 kilos (33 and 55 pounds) of material, according to experts.
A bomb can also be made from as little as six kilos (13.2 pounds) of plutonium, a by-product of nuclear reactors.
Iran said Monday it was planning to start up a plant in the city of Arak to produce heavy water for use in a different sort of nuclear reactor.
The UN nuclear watchdog is concerned about the risk of diversion of nuclear materials as the reaserch reactor could produce 8-10 kilograms (about 20 pounds) of plutonium a year, enough to make at least two nuclear bombs.
Enrichment using the centrifuge method is half a century old. But it requires thousands of centrifuges connected in cascades to produce weapons-grade uranium.
The machines and their components are highly specialised.
When a country starts to buy large numbers of them on the black market — as Iran was reported to have done several years ago — it raises suspicions that it is trying to develop a nuclear weapon.
Iran has installed 164 centrifuges at a pilot plant in Natanz, and a senior official has said Tehran wants to install 3,000 centrifuges within the next year.
Iran is also trying to develop advanced P2 centrifuges — devices that are capable of making weapons-grade uranium more efficiently than the P1 technology currently in use.
In 2004, Iran told the UN nuclear watchdog it planned to convert 37 tonnes of yellowcake into UF6 for a civil enrichment programme. That, experts said, was enough to make one or more atomic bombs.
The country now says it has 110 tonnes of UF6.
Enrichment is only one of several hurdles to overcome before a country is considered nuclear-weapons capable.
One is the electronic trigger, whose split-second timing is essential for unleashing the chain reaction necessary for a military device.
Another is weaponisation — putting the device into a missile or bomb that can be delivered to a target.
Iran is a major exporter of oil and has vast reserves of natural gas. It contends it needs nuclear energy to provide power for its citizens when its fossil fuel reserves run out, and to free up its reserves for export.
