Enrico Fermi Builds the First Nuclear Reactor

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Enrico Fermi Builds the First Nuclear Reactor

Overview

On December 2, 1942, a group led by Enrico Fermi (1901-1954) built and started up the world's first man-made nuclear reactor. This was a test to prove that a nuclear reactor could be built, and it paved the way initially for nuclear reactors that would produce plutonium for the atomic weapon to be dropped on Nagasaki, Japan, just three years later. In the following decades, more nuclear reactors would follow, producing plutonium and tritium for more advanced nuclear weapons as well as electrical energy for tens of millions of people, radioactive pharmaceuticals for medical diagnoses and treatment, and much more.

Background

The first known nuclear reactor actually goes much further back than 1942. In fact, nearly two billion years ago in what was to become the nation of Gabon in West Africa, a lucky set of circumstances led to the formation of a natural nuclear reactor in a bed of rich uranium ore. Far from the precisely engineered machines that are built today, the Gabon reactor (also called the Oklo reactor for the part of Gabon in which it lies) was a legitimate nuclear reactor that seems to have operated intermittently for several tens of millions of years.

Prior to the twentieth century atoms were thought to be indivisible. In fact, the word itself comes from the Greek a (meaning not) and tomos (meaning divisible). The latter word is also the root for the word "tome," as in microtome (a device to cut thin slices for microscopy). In investigations beginning with Henri Becquerel's (1852-1908) discovery of radioactive decay in 1896, many researchers eventually found that atoms were not indivisible, but consisted of smaller parts. James Chadwick's (1891-1974) discovery of the neutron in 1932 prepared the way for Fermi's 1934 proposal that neutrons could be used to split atoms. In 1938, the German physicists Otto Hahn (1879-1968) and Lise Meitner (1878-1968) became the first to knowingly cause uranium atoms to fission (Fermi had accomplished this a few years earlier, but thought he had instead created transuranic elements). In the process, scientists began to realize that an enormous amount of energy lay stored in the atomic nucleus. The secret to this was what may be the most famous equation in history: E = mc².

What researchers realized in the 1930s is that, when you split a uranium atom, the mass of the fission fragments is a little less than the mass of the original atom. The extra mass, when put into Albert Einstein's (1879-1955) equation, turned out to be exactly equal to the amount of energy released in a nuclear fission reaction. In fact, converting one atomic mass unit (about the mass of a neutron or proton) releases nearly 1000 MeV (or million electron volts), a tremendous amount of energy on the atomic scale. In effect, "burning" this very small amount of mass into energy gives us nuclear power.

In 1933, in the midst of these exciting discoveries, the Hungarian expatriate physicist Leo Szilard (1898-1964) realized in a flash of inspiration that it could be possible to create a self-sustaining nuclear chain reaction. As Szilard described it, he was in the process of starting to cross a street when he suddenly realized how a chain reaction would work. Realizing that a neutron is required to cause an atom to fission, or split apart, Szilard understood that any nuclear reaction would eventually die out because not all neutrons would cause fission. However, if an isotope could be found that released more than one neutron, it might be possible to have fissions go on indefinitely. This is precisely the principle behind a nuclear reactor—in a "critical" configuration, exactly the same number of neutrons are produced by fission as are lost, so the total number of neutrons in the nuclear reactor remains constant. Thus, all nuclear reactors are "critical" as a matter of course. A nuclear reactor that is not critical is simply not operating. Such a chain reaction was first shown experimentally in 1939 by French physicists Irène and Frédéric Joliot-Curie (1897-1956 and 1900-1958).

In 1941, Albert Einstein, at the behest of several concerned physicists, helped write and sign a letter to President Franklin Roosevelt, urging him to initiate a project to develop the atomic bomb before Nazi Germany could do so. At that time, the Nazi bomb was considered a very real possibility because of the large number of truly outstanding physicists and engineers in Germany at that time. This letter helped convince Roosevelt to begin the Manhattan Project, resulting in Fermi's reactor, several plutonium production reactors at Oak Ridge, Tennessee, and Hanford, Washington, and, of course, the first atomic weapons. Fermi's reactor, a pile of graphite blocks with precisely placed uranium spheres, was built in a squash court beneath the football stands at the University of Chicago. Assembled by hand, crude, dirty, with manual controls, it generated about enough energy to have lit a very small electric light bulb, if any way had existed to extract that energy. The reactor was more or less egg-shaped, 22 feet (6.7 m) high, 26 feet (7.9 m) across, and contained 6 tons of uranium oxide encased in 250 tons of graphite.

Impact

The most immediate and obvious impacts of Fermi's nuclear reactor was that it allowed the Manhattan Project to go forward and build a working atomic bomb. It made possible the plutonium production reactors and verified the physics calculations that bomb designers were using. Had the pile not gone critical at the predicted configuration, some minor recalculations would have been necessary. Had it not gone critical at all, it is likely that no nuclear weapon would have been built. Although this is potentially the most significant impact of all, it is discussed in greater detail in another essay and will not be discussed further here.

The next most apparent impact of this first artificial nuclear reactor is, of course, the construction of large civilian and military nuclear reactors, used for a variety of purposes. Military nuclear reactors are primarily used for propelling submarines and large surface ships, although several early attempts were made to develop small, portable nuclear reactors that could be used by Army units to produce power or desalinate seawater. There has also been a limited use of nuclear reactors in spacecraft. This should be clearly differentiated from the use of radioactive materials, including plutonium, in radioisotopic thermal generators (RTGs), such as those used in the Galileo and Cassini spacecraft. RTGs make use of heat generated via radioactive decay to produce electricity, but no nuclear chain reactions ever occur.

The civilian use of nuclear power is more varied, encompassing nuclear power reactors, research reactors, and isotope production reactors. Nuclear power reactors, of course, generate electrical energy for use by communities and industry alike. While the use of nuclear power for electrical energy generation is hotly debated in most nations with such reactors, several nations rely quite heavily on them for energy. These nations include Japan, France, the United States, and several republics that were formerly part of the Soviet Union. China began a massive program of nuclear reactor construction in the late 1990s in an effort to bring electrical power to most of its population, as have some other countries.

Nuclear reactors are also used extensively to manufacture radioactive isotopes for use in research and medical treatment. A very large fraction of genetic sequencing studies depend on radioactive labels attached to DNA in the lab, for example, and without radioactive materials, many of which are manufactured in nuclear reactors, much research in the medical, biological, and biotech arenas would stop. Nuclear medicine and radiation therapy also depend heavily on isotopes produced in nuclear reactors to help diagnose and treat a variety of illnesses, including many forms of cancer.

Finally, nuclear reactors are used for a variety of other research projects. For example, by bombarding a geologic specimen with neutrons in the core of a small research reactor, one can determine its chemical composition using what is called neutron activation analysis. This makes use of the fact that, by absorbing a neutron, a stable atom can be transformed into one that is radioactive. Identifying the new radioactive isotopes provides enough information to determine which elements gave rise to them. In other types of geologic work, nuclear reactors can be used to help determine the age of rocks. These dating methods, called Argon-Argon dating and fission track dating, use very different properties of atoms subjected to a neutron flux, and are both powerful and accurate ways to gain valuable geologic information.

Finally, the impact of nuclear reactors on the environmental movement has been profound. Or, perhaps a better way to put it is that the nuclear power industry has generated a long-lasting and often acrimonious debate about its potential environmental impacts, a debate that was fueled by the accidents at Three Mile Island and Chernobyl.

In a sense, the impact of these accidents is out of proportion compared to the risk actually posed by the nuclear power industry. At Three Mile Island, although the nuclear reactor was destroyed, it released only a relatively small amount of radioactivity to the environment and, in fact, nobody offsite received any more radiation than they would have from a typical series of x rays. Chernobyl, while a serious accident (although NOT a nuclear explosion, which is physically impossible in a civilian nuclear reactor), happened in a reactor plant design that is not allowed to be built anywhere except in Russia and other former Soviet Union states, making a recurrence unlikely. What contributed to the severity of the Chernobyl accident was the absence of an outer "containment" structure to keep radioactive materials from entering the environment. Such structures are required on all nuclear reactors in other nations. Therefore, holding Chernobyl up as an example of what could occur in a Canadian or French nuclear power plant is not a valid comparison. This is not to minimize these accidents, just to point out that the debate over the environmental effects of nuclear power may be largely built on misunderstanding or misinterpretation of the facts that are available.

P. ANDREW KARAM