22 August 2006

Uranium enrichment: how to make an atomic bomb

Cosmos Online
SYDNEY, 22 August 2006: Uranium 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 make an atomic bomb.
Uranium enrichment: how to make an atomic bomb

A bank of uranium gas centrifuges. Credit: Uranium Information Centre

SYDNEY, 22 August 2006: Uranium 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 make an atomic bomb.

While nuclear power stations can be fuelled with relatively low-grade uranium fuel, an atom bomb requires a much more highly enriched version.

When uranium ore is dug out of the ground, it is almost entirely composed of the relatively long-lived U-238 isotope. Only a very small fraction is made up of the unstable U-235, which is the isotope that can undergo spontaneous fission.

This is the process where the atomic nucleus splits, releasing enough energy in the process to coerce its neighbours into similarly splitting, resulting in a runaway chain reaction. It’s this chain reaction – controlled within the bounds of a nuclear reactor – that generates power. The same reaction, when left unhindered, produces the catastrophic effects of a nuclear explosion.

The goal, therefore, is to beef up the percentage of U-235 in uranium ore so that there is enough of it to induce and maintain a chain reaction.

The first step is to mill the ore into a concentrate called yellowcake, which is typically composed of 70 to 90 per cent triuranium octaoxide (U3O8). This is then converted into uranium hexafluoride gas (UF6) ahead of enrichment.

One of the popular methods of uranium enrichment is by gas centrifuge, which is the technique being pursued by Iran.

The uranium hexafluoride gas 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 per cent, the uranium is enriched enough to be used as fuel for civil nuclear reactors.

Iran says it has not enriched uranium beyond 4.8 per cent 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 per cent and be produced in large quantities.

Little Boy, the bomb dropped on Hiroshima, used 64.1 kgs of enriched uranium. Atomic bombs can also be built using less uranium, down to around 15 to 25 kgs of material, according to experts.

A bomb can also be made from a much smaller amount – as little as six kgs – of the more potent plutonium, which is produced as a by-product of nuclear reactors.

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 and are not useful for other industrial or scientific purposes. 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 U.N. nuclear watchdog it planned to convert 37 tonnes of yellowcake into UF6 for a civil enrichment program. That, experts said, was enough to make one or more atomic bombs.

The country now says it has 110 tonnes of uranium hexafluoride used in enrichment.

Iran said Monday it was also planning to start up a plant in the city of Arak to produce heavy water for use in a different sort of nuclear reactor.

Heavy water is just like regular water (H20) except it combines oxygen with deuterium instead of normal hydrogen. Unlike hydrogen, which normally consists of a single proton and one electron, deuterium contains a proton and a neutron plus one electron.

Heavy water can be used as a nuclear moderator, controlling the rate of fission in the reactor. This type of reactor can be used to produce plutonium, which presents a proliferation risk.

The UN nuclear watchdog is concerned about the risk of diversion of nuclear materials as the Iranian research reactor could produce 8 to 10 kilograms of plutonium a year, enough to make at least two nuclear bombs.

Although plutonium production and uranium enrichment present a serious proliferation issue, they are 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.

with Agençe Presse-France

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