In 1942, a "secret city" sprang up in a Tennessee valley, as Allied scientists raced to beat the Germans at developing enriched uranium to fuel the first atomic bomb.

A Desperate Race for Atomic Weapons

By the early 1900s, physicists had discovered a strong nuclear force that holds the nucleus of an atom together. This force is far stronger than gravity or electricity. Scientists began to realize that if its power could be harnessed, the "strong force" would provide an unlimited supply of energy--and could be used to create the most powerful weapons imaginable.

As Hitler persecuted European Jewish scientists in the 1930s, many of them, including Albert Einstein, immigrated to the United States and England. In 1939, these scientists learned that two of their colleagues back in Germany had succeeded in creating the first nuclear fission reaction.

The reaction used uranium, a radioactive element that is found underground in mineral ores. The Allies feared what could happen if Hitler had access to atomic power, especially since he had gained control of the Belgian Congo, where uranium is mined.

In 1942, partly through Einstein's influence, President Roosevelt authorized the top-secret Manhattan Project, headed by Gen. Leslie R. Groves and physicist J. Robert Oppenheimer. The Project's goal was to develop an atomic bomb before the Germans could do so.

Three Secret Cities

The U.S. Government bought thousands of acres of land and built three different secret laboratory sites: Oak Ridge, Tennessee, for making uranium fuel; Hanford, Washington, where plutonium fuel would be made; and Los Alamos, New Mexico, where both kinds of fuel would be turned into atomic bombs.

All three sites were located far from major cities, making it easier to keep them a secret, while at the same time protecting the public from catastrophic accidents. One advantage of the Oak Ridge site was the nearby Tennessee Valley Authority's hydroelectric dam system--because lots of power was needed for processing uranium into fuel.

A Nuclear Chain Reaction

Most uranium mined out of the ground has an atomic mass of 238. That means the total number of protons and neutrons in the nucleus of one uranium atom adds up to 238. But a tiny fraction of natural uranium has an atomic mass of only 235--it has the same number of protons, but three fewer neutrons.

This U-235 is unstable. When a uranium-235 atom gets hit with a fast-moving particle, it can fission, or split apart, spitting out more particles that can cause more fission reactions. If there's enough U-235 around, a chain reaction occurs, eventually causing a huge explosion.

The trick to making the chain reaction work is building a higher concentration of U-235 than in the natural uranium mix. To make this happen, scientists have to "enrich" the amount of U-235, using a separating process.

Three Enrichment Methods

During the last years of the war, three different methods for enriching the uranium were tested at different Oak Ridge plants:

• gaseous diffusion
• liquid thermal diffusion
• electromagnetic separation

In the gaseous diffusion plant, code-named K-25, uranium gas was pumped through miles of membranes full of microscopic holes that separated the faster, lighter U-235 atoms from the heavier and slower U-238 atoms. Another process, called liquid thermal diffusion, was developed at plant S-50.

The third enrichment method, tested at the Y-12 plant, used electromagnetic fields to deflect the heavier atoms from the lighter ones. This method concentrated the enriched fuel that had already gone through the two diffusion processes, making it even more pure. The result was uranium with about 90 percent U-235.

At a fourth Oak Ridge plant, called X-10, scientists built a huge reactor to create plutonium, a man-made element that could also be used for bombs. After experiments with the X-10 reactor, scientists actually manufactured the plutonium at the Hanford, Washington site.

Great Loss, Great Potential

The first plutonium bomb was tested at the Trinity Test Site, near Alamagordo, NM, in July, 1945. The test succeeded. The scientists didn't bother to test a uranium bomb--they knew it would work.

Enriched uranium fuel made at Oak Ridge went into the "Little Boy" bomb that was dropped on Hiroshima, Japan, on August 6, 1945. A plutonium bomb, called "Fat Man," was dropped three days later on the city of Nagasaki. As a result, Japan surrendered and WWII ended.

Even before seeing the total destruction caused by the two bombs, many of the scientists who worked to make them began to regret pursuing atomic weapons. They worried that once a single atomic bomb was dropped, a nuclear arms race would begin. They were right: At least eight countries, including North Korea, now have tested atomic weapons and others are developing them.

Nuclear Power In Hindsight and in the Future

After the war, it turned out that the Germans were much farther from making atomic weapons than the Allied scientists had thought. But, if the Allies had not created them, would some other country have done so later? The matter is still under debate.

Meanwhile, many of those who had worked for the Manhattan Project used their new understanding of atomic energy for non-defense technologies. Nuclear power plants and radiation treatments for cancer are two examples. Research on these and other areas, such as biofuels and nanotechnology, are just some of the projects in the works at what is now called the Oak Ridge National Laboratory.

Learn more about the history of the first atomic bomb at The American Museum of Science and Energy in Oak Ridge, Tennessee.

See information about taking a guided tour of the Oak Ridge site here.