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After separating the gaseous uranium-235 through many centrifuge steps, engineers use another chemical reaction to convert the gas uranium back into a solid metal, Klein said. That metal can then be shaped for use in either reactors or bombs.

Because each step only step only purifies the uranium gas mixture by a small amount, nations can only afford to run centrifuges that are engineered to the highest levels of efficiency, said Nietert. Otherwise, producing even a small amount of pure uranium-235 becomes prohibitively expensive.

And designing and making those centrifuge tubes requires a level of investment and technical know-how beyond the reach of many countries, said Nietert. The tubes require special types of steel or composites to withstand the extreme pressures of rotation, must be perfectly cylindrical to maximize efficiency, and are manufactured by specialized machines almost as difficult to build as the tubes themselves, said Nietert.

To put the endeavor of separating uranium into perspective, take the example of the building of the bomb that the United States dropped on Hiroshima. The United States needed 137 pounds (62 kg) of uranium-235 to make the bomb, according to “The Making of the Atomic Bomb” by Richard Rhodes (Simon & Schuster, 1995). Separating those 137 pounds from almost 4 tons of uranium ore took place in world’s largest building and used 10 percent of the entire country’s electricity, Nietert said. It took 20,000 people to build the structure, 12,000 people to operate the facility, and cost more than $500 million, in 1944, to equip it, according to “The Making of the Atomic Bomb.” That’s around $6 billion in 2009 dollars, according to the Bureau of Labor Statistics.