Part III You Might Want to Know: How is uranium enriched today?
After World War II, the preferred process for enriching uranium to weapons-grade—the hard part of making a bomb—became gaseous diffusion.
In the 1950’s, the preferred method became the gas centrifuge. Gas centrifuges separate U235 from U238 by whirling a gas containing uranium inside a small column at very high speed—3600 times faster than the spin cycle on your washing machine, say. The heavier isotope, U238, collects on the inside walls of the column, the U235 at the center.
Even during the Manhattan Project, scientists had realized that centrifuges might be able to do this. They hadn’t tried to develop this method because they didn’t think they could get it working soon enough.
After World War II the standard design of the gas centrifuge was improved significantly by scientists in the Soviet Union. We learned about this improvement only in 1956 when a German-Austrian mechanical engineer named Gernot Zippe who had worked on the new design was released from Soviet captivity. He was surprised to learn that the West hadn’t yet figured out what his group had in the Soviet Union. Use of what came to be called the Zippe centrifuge soon became the preferred method of enriching uranium.
The good news about centrifuges is that they use much less energy than gaseous diffusion. The other good news, if you are trying to build a nuclear weapon in secret, is that ranks of centrifuges are harder to detect than is a big gaseous diffusion plant. That’s also bad news, of course, if you hope to prevent the spread of nuclear weapons.
China, Pakistan, India, North Korea all used centrifuges to enrich uranium for the bombs they surprised our CIA with. Iran used centrifuges and still does. But in the Joint Comprehensive Plan of Action treaty signed in 2015, Iran agreed not to enrich past 20%.
Quite a few countries enrich uranium now--Argentina, Brazil, China, France, Germany, India, Iran, Japan, the Netherlands, North Korea, Pakistan, Russia, the United Kingdom, and the United States. Some of these countries have enriched the uranium to weapons grade and built nuclear weapons. Countries that don’t see any reason to acquire bombs may still want to enrich uranium enough to use it in power reactors. Five percent enrichment is good enough for that.
Methods that use lasers to enrich uranium are being explored.
Today, producing highly enriched uranium is no longer the biggest obstacle to building an atomic bomb. Before World War II, no highly enriched uranium existed anywhere on earth. Today tons and tons of it are stored at Oak Ridge, Tennessee at a big secure—we hope--facility known as the National Security Complex. Some of the HEU there now was sent to us by Russia after the Cold War ended. They stopped doing that after a few years. They still have plenty.
How secure their HEU is, we don’t know. The other nuclear weapons states--Pakistan, India, North Korea, Britain, France, China, Israel--also have supplies of HEU. We don’t know how secure their supplies are either.
Today, if you decide you want to make a nuclear bomb that uses HEU as its fissile fuel, you can enrich the uranium yourself but you don’t have to, not if you can steal some or find someone who will sell it to you. Once you have the HEU, making a nuclear bomb as powerful as the Hiroshima bomb isn’t that hard.