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.
This week Iran also said it has postponed indefinitely talks with Russia on a plan to enrich uranium on Russian territory, to allay fears that it would build an atomic weapon.
In a joint statement, Russia and France immediately called on Tehran to reconsider, and to halt enrichment work. The US has also criticized Iran for its recent enrichment activities.
"They're continuing to choose defiance and confrontation over cooperation and diplomacy," said White House press secretary Scott McClellan on Monday.
On Feb. 4, the International Atomic Energy Agency reported Iran to the UN Security Council, calling on it to stop all enrichment-related activities. Iran has retaliated by ending some of its cooperation with IAEA inspectors.
Iran has long maintained that its nuclear activities are solely for the purpose of nuclear power, and that enrichment for civilian purposes is not outlawed by the Non-Proliferation Treaty, of which Iran is a signatory.
"They say, 'we're within the treaty, we're within our rights,' " says Daryl Kimball, executive director of the Arms Control Association in Washington.
Enrichment of uranium is necessary because, as it comes out of the ground, uranium is just a lump of natural material.
But it is a natural material with potential. It's all a matter of isotopes, which are forms of the same element that have different numbers of neutrons in their nuclei.
Ninety-nine percent of uranium is the unexciting U-238 isotope. But the other 1 percent is the isotope U-235, which is slightly lighter, has three fewer neutrons, and can be split to release astounding amounts of energy.
The point of enrichment is to increase the percentage of U-235 - to the 5 percent level for fuel for nuclear power plants, and to the 90 percent level for nuclear weapons.
The first step is to take uranium ore and mill it. Then this crumbled material is heated to produce a gas, uranium hexafluoride.
In the centrifuge process - invented in the 1960s - this gas is then fed into a cylinder spinning at high speed. The heavier U-238 isotopes are spun to the outside, while the lighter U-235 stays closer to the vertical axis, where it can be drawn off.
This slightly enriched gas is then fed into another centrifuge, where it is enriched more. It is then drawn off, and fed into another, and so on down a line of thousands of spinning tubes.
Once it has reached the desired concentration, after much effort and use of electricity, the gas cools and solidifies, and the material is milled into a usable form.
Starting this process is much more difficult than simply continuing it. Thus production of a bomb core, which is 90 percent enriched, requires only about 25 percent more effort than producing 5 percent enriched fuel rods for civilian power reactors, estimates Mr. Albright of the Institute for Science and International Security.
It will take Iran some time to get its enrichment technology under control, says Albright, given that the experimental centrifuge cascade at Natanz has long lain idle. Moisture has penetrated the system, corroding pipes. Vacuum in the system must be secure for the cascade to work perfectly.
If Iran proceeds to a production cascade of thousands of centrifuges, it is now unlikely to build it at Natanz, says Albright. For security purposes, the Iranians would hide it in a new facility, perhaps among the many light industries clustered around Tehran.