A Science Odyssey
People and Discoveries
photo of Enrico Fermi

Fermi creates controlled nuclear reaction

Photo by Bortzells Esselte, courtesy AIP Emilio Segre Visual Archives.

Enrico Fermi (1901-1954) left Italy in 1938 to receive the Nobel Prize for physics in Sweden. He never went back. He and his wife moved to the United States to escape Italy's increasing fascism and antisemitism.

Fermi, among others, realized that nuclear fission was accompanied by the release of colossal amounts of energy from the conversion of mass into energy (according to Einstein's mass-energy equation E=mc2). When scientists convinced President Roosevelt of this, Fermi was appointed to head a research team as part of a secret project to develop an atomic bomb. Fermi's task, however, was to create a controlled nuclear reaction; that is, to split the atom without creating a deadly explosion.

Theoretically, it was possible. During fission, a fast-moving neutron splits an atom's nucleus, which results in the release of energy and additional neutrons. These ejected neutrons can split other nuclei, which release other neutrons to split still other nuclei, and so on: a self-sustaining chain reaction. If this chain reaction went too fast, it became an atomic explosion, but under control it could produce a steady flow of energy. (If the chain reaction started with uranium, it also created a byproduct, plutonium, a better fuel for a nuclear weapon.)

At the University of Chicago, Fermi worked with a team to find a way to control the chain reaction. He did this by setting up the equipment -- atomic pile -- so that he could insert a neutron-absorbing material into the midst of the fission process to slow it down or stop it altogether. He found that rods made of cadmium would absorb neutrons. If the chain reaction speeded up, the cadmium rods could be inserted to slow it down and could be removed to accelerate it again.

By the end of 1942, the team was ready for its first test. The equipment was set up in a squash court at the University of Chicago. It was December 2. The moment was tense: if their theories and experiments until now proved wrong, they could blow up half of Chicago. A few of the rods were pulled out, and the reaction began. More rods came out. The reaction was self-sustaining. The team could increase or decrease the energy output by adjusting the rods. Fermi's idea had worked, and the first controlled, self-sustaining nuclear chain reaction -- the first controlled flow of energy from a source other than the Sun -- was achieved.

A coded message told the government of this success: "The Italian navigator has just landed in the new world."

Since then, Fermi's theory has been expanded and refined. Nuclear reactors have been built in many countries to supply energy for military uses such as nuclear submarines and civilian uses such as ordinary electricity. But incidents through the years showed the dangers of the process and of its waste products, and nuclear power lost much of its original popularity.

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