Iranian President Mahmoud Ahmadinejad, center, visits the Natanz Uranium Enrichment Facility. (AP)/US News And World ReportThe Science Of Uranium Enrichment -- The Guardian
Gas centrifuges are used to enrich uranium to make either fuel for nuclear reactors or fissionable material for nuclear weapons. To make enough fuel for a commercial reactor takes tens of thousands of centrifuges, which must be connected together and timed to run simultaneously.
Raw uranium ore as dug from the ground contains less than 1% Uranium-235, the type of uranium atom that can easily be split to produce energy. The rest is a heavier form, Uranium-238. Enrichment plants produce either low enriched uranium for nuclear power plants, which contains around 5% Uranium-235, or high enriched uranium for nuclear weapons, which contains 85%-90% Uranium-235.
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Update #1: Is Iran trying to build a nuclear bomb? -- Short Sharp Science
Update #2: Iran’s Secret Nuke Sites: Will the World Finally Get Serious? -- The Danger
My Comment: I have a bachelor's degree in Physiology/Physics, with a focus on nuclear physics. For the past week I was preparing a post that explains the physics and rational for why Iran's nuclear facilities are actually facilities whose sole function is to produce weapons grade uranium.
Ian Sample at the Guardian has beaten me to the punch, with a simple, short, and concise explanation on the science of uranium enrichment. Read it all.