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% 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%; the Iranians say they have increased that to 3.5%. What worries the US 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% uranium 235. Even then, they would need 50kg 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." Satellite images suggest the enrichment plant at Natanz could house 50,000 centrifuges.
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."
To assess how close this takes the Iranians towards a nuclear reactor or potential bomb, he says we must know more about the centrifuges. The machines are unreliable, which makes their design and efficiency as important as their number.