LONDON: Iran claims to have produced enriched uranium "to the 3.5 per cent level." That is pure enough to use as nuclear fuel, though nowhere near what would be needed to make a bomb. Experts say the bank of 164 centrifuges that the Iranians used is not enough to churn out significant amounts.

The centrifuges are needed because natural uranium is useless to feed nuclear reactors or to make bombs.

First the ore must be processed to extract the metal and 25,000 tonnes of ore yields 50 tonnes of metal. Less than 1 per cent of that is uranium 235, which can be forcibly split to release energy. The rest is uranium 238, its less volatile radioactive cousin.

To make reactor fuel and atomic bombs, the uranium 235 in the metal needs to be enriched. This is where the centrifuges come in. Taking advantage of the fact that uranium 235 is marginally lighter than uranium 238, the Iranians will have mixed the metal with fluorine, heated the mixture until it formed a gas (uranium hexafluoride) and spun it at high speed inside a thin metal cylinder.

Inside this centrifuge, the heavier uranium 238 molecules are flung towards the outer walls, which allows a stream of gas relatively rich in uranium 235 to be drawn off. By feeding this enriched stream into a second centrifuge, and then a third, and so on, the amount of uranium 235 in the original sample is increased.

At the start it is typically less than 1 per cent; the Iranians say they have increased that to 3.5 per cent.

What worries the United States is that, should the Iranians add more centrifuges, they may have the potential to enrich this fuel-grade uranium to weapons-grade uranium, which requires 80-90 per cent uranium 235. Even then, they would need 50 kilograms of this highly enriched uranium to achieve a viable atomic weapon.

Frank Barnaby, a nuclear physicist at the UK Atomic Weapons Establishment in the 1950s, said: "If they've enriched some uranium and measured the enrichment then that's quite a way down the line. But 164 centrifuges is negligible, you'd need thousands to get significant amounts of weapons grade uranium."

The domestic ore the Iranians are believed to be using would pose a problem, Dr Barnaby adds.

"The Iranian uranium is contaminated with molybdenum and other heavy metals and that would tend to gum up the centrifuges and limit the degree of enrichment to far below the weapons grade level."

Source: China Daily