Iran enriches some uranium

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Iran announces that they've enriched some uranium:
If the Iranian declaration is correct, the enrichment and what appear to be rudimentary bomb-making documents that international inspectors have found in Iran suggest Iranians may now have most of the knowledge that Mr. Bush has sought to deny them.

At the least, they appear poised to be able eventually to expand enrichment on an industrial scale and, if they are determined to do so, enrich the uranium to levels necessary for an atomic weapon. But so far the quantities that the country has produced appear to be minuscule, and the enrichment level announced today 3.5 percent would work for producing power, not warheads.

International inspectors are stationed at Iran's main enrichment facility at Natanz, and presumably will be able to confirm or refute Iranian claims in coming days, assuming they have access to centrifuges.

Centrifuges are devices whose rotors spin very rapidly to enrich, or concentrate, a rare form of uranium known as uranium 235, which can then be used to fuel nuclear reactors or atom bombs. The 164 centrifuges Iran said it has strung together in a cascade are enough to test the technology, but with such a small number would take years to produce enough uranium for even one weapon.

Isotope separation is an incredibly expensive process because different isotopes are essentially chemically identical (though not 100%, see kinetic isotope effects). It's even worse in the case of uranium because the mass difference between U235 and U238 is only slightly over 1%. A lot of the uranium for the Manhattan project was separated by mass spectrometry.

For background here, natural uranium is .72% U235 and weapons-grade uranium is around 85% U235, though apparently you can get away with 20% (Wikipedia article here). According to Globalsecurity, you need about 1000-3500 centrifuges to produce enough material for a bomb, with more advanced designs needing less Uranium and consequently fewer centrifuges.