Part II: How did we enrich the uranium for Little Boy?
The hardest part about making the Hiroshima bomb was not designing it but producing enough highly enriched uranium for it.
How did we do that?
When our Manhattan Project got going in August 1942, the British knew more about how to enrich uranium than we did. At the end of 1940, one of the physicists in their MAUD atom bomb project, Franz Simon, had shown that U235 could be separated from U238 using a process called “gaseous diffusion.” James Chadwick, another British physicist, said that when he saw Simon’s report he realized that that “an atomic bomb was inevitable.” He began to have trouble sleeping.
But a gaseous diffusion operation would have to be huge, with big buildings containing ranks and ranks of special filters, It would require lots and lots of electricity. Uranium gas would be forced through the filters. The gas that came through a filter would have a tiny bit more U235 that the gas that went into it. Repeat. And repeat. Again. Again.
The British were being bombed by the Germans. An industrial project on the scale required to enrich uranium was not possible for them. So at the end of 1940, even before we Americans had joined the war against Japan and Germany, in a mission led by the Oxford chemist Henry Tizard, the British came over and told us what they knew about enriching uranium, among other important things, like the proximity fuse and a power source for radar. They asked for our help.
By August 1942, we had joined the war, and we established, in secret, the Manhattan Engineering District to build an atomic bomb. The first site to be established in what came to be referred to as the Manhattan Project was Site X, a big industrial plant in Oak Ridge, Tennessee, that would enrich uranium. It would be able to get power from the big new dams and electricity generators of the Tennessee Valley Authority. That would help.
Later that year the Scientific Division, Site Y, was established in the Jemez Mountains of New Mexico. At the secret Site Y designs for the bomb would be developed and tested.
The first method of enrichment tried at Site X was not gaseous diffusion but electromagnetic separation, a method that had been recommended by the American physicist at the University of California, Ernest Lawrence, who had invented the cyclotron. The cyclotron used big electromagnets to accelerate atomic and sub-atomic particles that had been given an electrical charge and make collide with other particles. At Site X, at a facility known as Y-12, charged particles of uranium would be created and accelerated in “racetracks” but not to cause collisions. Big machines called calutrons (as in “California”), with huge electromagnets in them, would cause the ever-so-slightly heavier U238 nuclei to take a slightly different path from the U235 particles. This would give us a way of enriching the uranium little by little with more and more U235. Construction of the first plant for electromagnetic separation at Site X began in February 1943.
Two months later, construction began at Site X on the massive plant that would use gaseous diffusion to enrich uranium. Scientists had been working for a while now to develop the special membranes that could be used in the cascades of filters.
Plants using the electromagnetic process went into operation at the end of 1943. It was soon apparent that this process wasn’t working fast enough. General Groves, the head of the Manhattan Project, decided to try a third method, thermal diffusion. The S-50 thermal diffusion plant began operation at Site X in September 1944. It didn’t work at all well. It managed to enrich by just a little bit, to less than 1% U235.
Finally, in February 1945, four months before the first bomb would be tested, the gaseous diffusion plant, K-25, was ready at Oak Ridge and operations commenced. The plant was a mile long, the world’s largest building at the time, larger even that the massive Pentagon the construction of which General Groves had just overseen.
Gaseous diffusion turned out to be more efficient than the other processes but it was still slow. J. Robert Oppenheimer, director of the Scientific Division of the Manhattan Project, decided we should use all three processes in tandem. Thermal diffusion would enrich the first little bit, gaseous diffusion would enrich to about 23%, and electromagnetic separation would bring the uranium up close to 90% U235, called “weapons grade.”
By the summer of 1945, Oppenheimer’s scheme allowed Site X to produce enough HEU for what was to be the Hiroshima bomb. You can see why the scientists decided not to waste any HEU in a test of the design they had developed for using it. They didn’t doubt it would work.
On July 16, the components of the Hiroshima bomb, Little Boy, were loaded onto the cruiser USS Indianapolis on the California coast and shipped out to the airbase we’d just built on Tinian Island in the South Pacific. From Tinian, our big new B-29 bombers could reach Japan. The B-29s were the only bombers we had at the time big enough to carry the atomic bombs. Little Boy was assembled on Tinian, loaded into a B-29 that had been named by the pilot Enola Gay after his mother, and on August 6, 1945, dropped on Hiroshima. The scientists and engineers at Site Y had been sure it would work. It did.
The hard part of making the Hiroshima bomb, the part the British wouldn’t have been able to do for themselves, the part the Germans and the Japanese didn’t really get started with during World War II, had been producing enough HEU for it.
After the war, gaseous diffusion became the preferred process for producing weapons-grade uranium. Today, the gaseous diffusion plants are all gone. In the 1950’s, a better method of enrichment was developed that used gas centrifuges. The Soviets and German scientists working for the Soviets were the first to develop it. The preferred technology today is the gas centrifuge.
Next: How is uranium enriched today?