Once it masters the difficult technique, a nuclear bomb is not far away.
To begin enriching raw uranium into fissile material, as Iran now may have done, is to take a fateful step down the path of nuclear capability.
That's because it is perhaps the most difficult aspect of developing a nuclear power - or weapons - program. Centrifuge enrichment is a sort of technological ballet, requiring thousands of thin tubes to spin at outrageous speeds, each feeding a thin stream of uranium gas along to a neighboring tube, until the gas reaches the end of the cascade line.
And once a nation has mastered the art of enriching uranium for a power plant, it does not take much more effort to increase the concentration of fissile elements to the level required for bombs. Thus, Iran may already have collected almost everything it requires, if it wants to become a member of the nuclear weapons club.
"I don't see that they need anything more except knowledge and experience, and they'll get that by running the [enrichment] cascade," says David Albright, president of the Institute for Science and International Security in Washington.
On Tuesday, Javad Vaeidi, a senior Iranian nuclear negotiator, said that Iran has resumed small-scale uranium enrichment, defying preventive efforts by the United States and much of the rest of the world. Mr. Vaedi, deputy Secretary of the Supreme National Council, told reporters in Tehran that work has resumed at Iran's main enrichment plant at Natanz. The work is preliminary, and the amount of enriched uranium it might produce would be small, as Iran's existing enrichment cascade is only an experimental one.
To produce usable amounts of enrichment material, Iran must still construct a larger facility, said Vaedi.
"We need time to have 60,000 centrifuges," he said.