AGENCE FRANCE-PRESSE,
TEHRAN: Enrichment, the sensitive process the United Nations gave Iran until Friday to stop, takes low-grade uranium and refines it, turning it into a material that can power reactors — or a nuclear bomb.
The key difference is that reactor fuel only needs uranium that has been enriched to a low level, but 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 comprises the stable U-238 “heavyweight” isotope, and just 0.7 percent is the “lightweight” isotope, U-235.
It is the U-235 that interests scientists because it is fissile. Its nucleus can release energy by splitting into smaller fragments, which then smash into other atoms and so on.
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 gas centrifuge.
The UF6 is piped in a cylinder that is then spun at high speed. The rotation causes a centrifugal force that pushes the heavier U-238 isotopes towards the outside of the cylinder, while the lighter ones U-235 isotopes congregate at the centre.
The stream that is slightly enriched in U-235 is then drawn off and fed into the next enrichment stage.
When around five percent of the UF6 comprises U-235, the material is enriched enough to be turned into fuel for a civilian nuclear plant. The gas is allowed to cool and solidify before it is turned into fuel assemblies to be placed in reactors.
Iran says it has only enriched to 3.5 percent — the purity it requires for its planned nuclear plants — and on a limited scale.
To reach weapons-grade material, the enrichment level has to reach more than 90 percent and large quantities are also needed.
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 (33-55 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 from burning uranium.
Enrichment using the centrifugal method is half a century old. But it requires thousands of centrifuges, interconnected to form “cascades”, to concentrate the level of U-235 to military standards.
These 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 — that is widely viewed as a telltale of its ambitions 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 now seeking to use 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 International Atomic Energy Agency (IAEA) it planned to convert 37 tons of yellowcake into UF6 for a civilian enrichment programme. That, experts said, was enough to make one to several atomic bombs.
The country now says it has 110 tonnes of UF6.
Enrichment is only one of several other important 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. 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 power to provide power for its citizens when its fossil-fuel reserves run out, and to free up its reserves for export.
