Physicist Enrico Fermi produces the first nuclear chain reaction

Physicist Enrico Fermi produces the first nuclear chain reaction

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Enrico Fermi, the Italian-born Nobel Prize-winning physicist, directs and controls the first nuclear chain reaction in his laboratory beneath the bleachers of Stagg Field at the University of Chicago, ushering in the nuclear age. Upon successful completion of the experiment, a coded message was transmitted to President Roosevelt: “The Italian navigator has landed in the new world.”

Following on England’s Sir James Chadwick’s discovery of the neutron and the Curies’ production of artificial radioactivity, Fermi, a full-time professor of physics at the University of Florence, focused his work on producing radioactivity by manipulating the speed of neutrons derived from radioactive beryllium. Further similar experimentation with other elements, including uranium 92, produced new radioactive substances; Fermi’s colleagues believed he had created a new “transuranic” element with an atomic number of 93, the result of uranium 92 capturing a neuron while under bombardment, thus increasing its atomic weight. Fermi remained skeptical about his discovery, despite the enthusiasm of his fellow physicists. He became a believer in 1938, when he was awarded the Nobel Prize in physics for “his identification of new radioactive elements.” Although travel was restricted for men whose work was deemed vital to national security, Fermi was given permission to leave Italy and go to Sweden to receive his prize. He and his wife, Laura, who was Jewish, never returned; both feared and despised Mussolini’s fascist regime.

Fermi immigrated to New York City—Columbia University, specifically, where he recreated many of his experiments with Niels Bohr, the Danish-born physicist, who suggested the possibility of a nuclear chain reaction. Fermi and others saw the possible military applications of such an explosive power, and quickly composed a letter warning President Roosevelt of the perils of a German atomic bomb. The letter was signed and delivered to the president by Albert Einstein on October 11, 1939. The Manhattan Project, the American program to create its own atomic bomb, was the result.

It fell to Fermi to produce the first nuclear chain reaction, without which such a bomb was impossible. He created a jury-rigged laboratory with the necessary equipment, which he called an “atomic pile,” in a squash court in the basement of Stagg Field at the University of Chicago. With colleagues and other physicists looking on, Fermi produced the first self-sustaining nuclear chain reaction and the “new world” of nuclear power was born.

READ MORE: Atomic Bomb History

Physicist Enrico Fermi produces the first nuclear chain reaction - HISTORY

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=mc 2 ). 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.

10 Intriguing Facts About the World's First Nuclear Chain Reaction

Watch how the world's first controlled, self-sustaining nuclear chain reaction unfolded in this "brick" video by Argonne National Laboratory.

On December 2, 1942, the world’s first self-sustaining, controlled nuclear chain reaction took place paving the way for a variety of advancements in nuclear science.

The experiment took place at the University of Chicago’s football stadium under the direction of Enrico Fermi—a Nobel Prize-winning scientist.

Chicago Pile-1 was the world’s first nuclear reactor to go critical and fueled future research by the Energy Department’s national laboratories to help develop early naval and nuclear reactors.

Fifteen years to this historic day, America’s first full-scale atomic electric power plant went critical on December 2, 1957 as the nation began reaping the benefits of clean and reliable nuclear power.

Here are 10 intriguing facts you probably didn’t know about the world’s first controlled release of nuclear energy.

1. The experiment took place at 3:36 p.m. in a converted squash court at the University of Chicago’s abandoned Stagg Field in Chicago, Illinois.

2. Forty-nine scientists, led by Fermi, were present for the event. Leona Marshall was the lone female researcher.

3. The word “pile” was used in the first few years of the atomic age and gradually gave way to “reactor” to identify the key device that controls the nuclear fission reaction.

Drawing of CP-1, the world's first nuclear reactor.

4. The reactor was built with graphite blocks, some of which contained small disks of uranium.

5. Scientists monitored the reaction on instruments named after Winnie the Pooh characters—Piglet, Tigger and Pooh.

6. Scientist George Weil withdrew the cadmium-plated control rod unleashing the first controlled chain reaction.

7. The reactor had three sets of control rods. One was automatic and could be controlled from the balcony. Another was an emergency safety rod. The third rod (operated by Weil) actually held the reaction in check until it was withdrawn the proper distance.

8. The group celebrated with a bottle of Chianti that was poured into paper cups. Most of the participants signed the wine bottle’s label. This was the only written record of who had taken part in the experiment.

9. In the lead up to this experiment, a letter from Albert Einstein to President Franklin D. Roosevelt helped lead to the Manhattan Project—a government research project that produced the first atomic bombs. It was also the seed that grew into the modern U.S. Department of Energy national laboratory system.

10. The Energy Department’s Fermi National Accelerator Laboratory is named in honor of Enrico Fermi for his contributions to nuclear physics and scientific success at nearby University of Chicago.

Learn more about Argonne National Laboratory's legacy in nuclear science.

Physicist Enrico Fermi produces the first nuclear chain reaction - HISTORY

1853 - It has long been thought that the Earth is no more than a few tens of thousands of years old. Beginning in the 1820's, however, many geologists and biologists have come to believe that the Earth is much older than previously thought, perhaps in the hundreds of millions of years. (Darwin estimates the age of the Earth at 300 million years in the initial printing of Origin of Species .) These estimates are based on an increased awareness of how very slowly geological and biological processes such as erosion or evolution occur, and therefore how enormously old the Earth must be to accomodate them.

Prominent physicist William Thompson (also known as Lord Kelvin - degrees Kelvin are named after him) is firmly opposed to evolution. He begins to marshall theoretical evidence against Darwin. He performs classical thermodynamic calculations which prove that if the Earth were as old as Darwin and others claim, then it would have long since cooled to an inert rock and no geological activities such as volcanism or hot-water springs would be possible. Other physicists soon join the fray. Hermann Helmholtz, who only six years earlier had enunciated the principle of conversation of energy, calculates how much heat the Sun would radiate if its energy comes from slow contraction, thus converting gravitational potential energy into heat. He calculates an age of only 18 million years.

The enormous gap between geology and biology on the one hand, and theoretical physics on the other (as far as estimating the age of the Earth is concerned) will last for 50 years. In the face of the hard criticism from well-respected physicists, Darwin removes all mention of any specific age for the Earth in later printings of Origin of Species .

1896 - Henri Becquerel, a French physicist, reads of William Roentgen's experiments with X-rays, and learns that they can cause certain materials to fluoresce. ( Tech Note - The X-rays were only exciting spectral lines in the fluorescent materials, like the gas tubes I show in class except with X-rays instead of electricity.)

Becquerel wonders, do phosphorescent materials emit X-rays while they are glowing? ( Tech Note - They don't.) To test his idea, Becquerel obtains some materials which glow after being exposed to light, just like those magic decoder rings which they still put into cereal boxes. He conducts some experiments in which he first sets the materials out in the sun to start them glowing, then sets them on top of a photographic plate wrapped in black paper to see if they are emitting X-rays. Becquerel obtains some positive results, and some negative ones, which is confusing.

One day, when it's cloudy, he puts one of the minerals that has been giving him positive results into a drawer with an unexposed photographic plate - and then on a whim decides to develop it, expecting to see only a faint outline since the Sun was so dim that day. Instead, he accidentally discovers that the plate has become completely fogged even though the mineral had been barely exposed to light at all and wasn't glowing! The mineral happens to be potassium uranyl disulfate, and Becquerel eventually discovers that the uranium in this compound is the magic ingredient. All compounds with uranium in them will fog a photographic plate compounds without uranium will not. Becquerel therefore calls the new radiation "uranic rays".

Tech Note - The property which makes some compounds "glow in the dark" after being exposed to light has to do with their molecular structure, and has nothing at all to do with either X-rays or with radioactivity. In brief, some molecules exhibit a marked "time delay" between when they are excited by incoming light, and when they emit their molecular spectral lines. Instead of instantly releasing all their stored energy and going out after the power is removed, like a neon sign does, phosphorescent materials gently release their energy for some while after the stimulus has been removed. It was sheer accident that Becquerel was using a "glow in the dark" compound which happened to have uranium in it.

1897 - Ernest Rutherford, a physicist originally from New Zealand but working in Canada, investigates Becquerel's "uranic rays" and discovers that they are in fact a mixture of two components: a very heavy component which is easily absorbed by matter and has a positive charge and a much lighter, more penetrating component which is not so easily absorbed and has a negative charge. Rutherford calls these components and , after the first two letters of the Greek alphabet.

1898 - Pierre and Marie Curie, two French physicists who are studying Becquerel's "uranic rays", discover that thorium also gives off "uranic rays". They propose the new term "radioactivity" to describe elements which have the property of giving off rays. Working from samples of pitchblend, they isolate and discover two new elements which are much more intensely radioactive than uranium: the Curies name them polonium (after Marie's homeland of Poland) and radium (due to its highly radioactive power).

1899 - French chemist Andre Debierne, a close friend of the Curies, isolates yet another radioactive element from pitchblende. He calls it actinium, after the Greek word for ray.

Becquerel, who has continued to study "uranic rays", realizes that the b -particles of Rutherford are so much like electrons that they must be electrons, albeit electrons of very high energy.

The French physicist Paul Villard discovers that uranium is giving off yet a third component, one which is not affected by magnets and so is apparently uncharged. They are considerably more penetrating than either -particles or -particles, and Villard calls them (predictably) -rays, after the third letter of the Greek alphabet. Villard suspects that -rays are electromagnetic radiation of incredibly short wavelength, even shorter than X-rays. (He is right.)

Tech Note - We still use the terms "-particles", "-particles", and "-rays" to refer to the three forms of radiation, even though we know that - and -particles are really just helium nuclei (two protons and two neutrons) and electrons, respectively.

1901 - The Curies measure the energy being given off by radioactive elements, and discover that one gram of radium gives off the incredible amount of 140 calories per hour. As far as they can tell, this energy just magically goes on and on, undiminished, for month after month. The radium does not seem to be changing in any way. Where is all this energy coming from? Is conservation of energy being violated?

1903 - Ernest Rutherford is the first to realize that the long-standing dispute about the age of the Earth between biologists and geologists on the one hand, and physicists on the other, can be resolved if one assumes that the interior of the Earth contains slight traces of radioactive elements. The vast bulk of the Earth, and the poor thermal conductivity of the rocky materials that mostly constitute it, mean that even a small input of heat would be enough to keep it geologically active for far longer than the times calculated by William Thompson (who of course assumed that the Earth's interior was completely inert). Rutherford hypothesizes that the (apparently inexhaustable) energy produced by radioactive ores is in fact exactly that heat source, thus siding with the biologists and geologists concerning the age of the Earth.

Indeed, within only a few years, Rutherford and other physicists investigating radioactives ores come to the conclusion (based on the very long halflives of some of the isotopes they've found) that the age of the Earth may well be in the billions of years rather than the mere hundreds of millions. (They are right -- the currently accepted value for the age of the Earth is about 4.2 billion years.)

1906 - Rutherford discovers that -particles, when brought to a stop inside a container, become helium atoms. In other words, an -particle consists of two protons and two neutrons (which is the nucleus of a helium atom) moving at high speed. If and when the -particle is slowed down and captures a couple of electrons from somewhere, it becomes recognizable as ordinary helium.

The very high speed of the helium nuclei, and the high speed of the electrons ( rays) emitted by radioactive elements, and the high-energy electromagnetic radiation also emitted, and the heat measurements by the Curies, indicate that there is something going on in these elements which is very energetic indeed. But what? Rutherford does not realize that the answer has already been published by Einstein in 1905 (indirectly), in the form of E = mc 2 .

1909 - Eugene Marsden and Hans Geiger are two graduate students working with Ernest Rutherford in Manchester, England, where Rutherford has relocated. They perform a series of experiments in which -particles are shot into a gold foil. Contrary to expectations, most of the -particles go through the gold as if it wasn't there, but a few are deflected through large angles, and a very few even turn around and bounce straight back as though they have hit an impenetrable barrier. This leads Rutherford to propose the "solar system" model of the atom, in which the atom is essentially empty space but has a very small and incredibly dense nucleus. (See the Quantum Mechanics Timeline for more details.)

1913 - The British chemist Frederick Soddy and the American chemist Theodore Richards elucidate the concept of atomic weight. As people continued to study radioactivity, it had become increasingly clear that there were multiple varieties of elements. For example, there are both radioactive and nonradioactive versions of carbon. Soddy and Richards prove that the difference lies in the weight of the atomic nucleus - there can be different versions of the same element with different weights. The different versions are christened isotopes , from the Greek words meaning "same place".

Tech Note - The chemical properties of an element are determined solely by the number of protons in a nucleus, because it is the positively-charged protons which interact with the electron cloud around the nucleus, and it is the electron cloud which produces chemistry. Nuclei can also contain neutrons, which have about the same mass as protons but have no charge. Neutrons can thus affect the weight of a nucleus, and its radioactive properties, but have no effect on its chemical properties.

1915 - American chemist William Harkins notes that the mass of a helium atom is, in fact, not exactly four times that of a proton. It is slightly less. He states that the excess mass has been converted to energy via Einstein's E = mc 2 and that this is the source of nuclear energy.

1919 - Rutherford, still hard at work bombarding things with -particles (see 1897, 1906, 1909) succeeds in getting an -particle (i.e., a helium nucleus) to react with a nitrogen nucleus to produce a proton (i.e., a hydrogen nucleus) and an oxygen nucleus. Rutherford has brought about the first human-engineered nuclear reaction. Also, this makes him the first person in history to change one element into another.

1930 - British physicist Paul Dirac is trying to combine relativity and quantum mechanics. He succeeds, and the relativistic quantum equation is called the Dirac equation as a consequence. He notices that his equation predicts the existence of "negative" states for the electron and proton, and he thus predicts the existence of antimatter.

1931 - For over a decade, physicists have been wrestling with a very puzzling problem with -emission. The electrons emitted by -decay do not always have the same kinetic energy, unlike the particles emitted in -decay. Rather, the electrons come off with a bell-curve-type distribution of energies, which means that (1) energy is apparently not being conserved, and (2) the amount of missing energy varies in some probablistic way. It seems that some of the nuclear energy powering -decay is going someplace other than the emitted electron. But where? Elaborate attempts are made to detect heat or electromagnetic radiation coming from the samples -- but every effort fails. A few physicists begin to seriously wonder if perhaps -decay really does violate conservation of energy, and Niels Bohr goes so far as to work out a possible scenario for how the Sun's energy could be generated by massive energy non-conservation resulting from -decays.

German physicist Wolfgang Pauli and Italian physicist Enrico Fermi propose that b -decay is producing two particles which share the kinetic energy: an electron, and an unseen particle which Fermi christens as a neutrino , from the Italian for "little neutral one". The particle is assumed to be very light as well as neutral, allowing it to penetrate matter so easily that it is almost impossible to detect.

1932 - English physicist James Chadwick bombards beryllium with a -particles to knock out free neutrons, and thus becomes the first physicist to detect neutrons directly.

1932 - American physicist Carl Anderson is studying cosmic rays when he notices some tracks on his photographic plates that look exactly like electron tracks except that they are curving in the wrong direction. He realizes that that he has discovered a positively-charged electron, i.e., the antielectron predicted by Dirac. Anderson calls the new particle a positron .

Tech Note - Electrons and positrons are exactly alike, except that they have opposite charges and opposite quantum numbers. That, and one other little thing. If an electron and a positron touch, they instantly annihilate each other in a flash of g -rays. In other words, they are both converted into pure energy. This is why positrons don't last very long after they're created.

Star Trek Note - All particles have antiparticles, so there are also negatively charged antiprotons and so forth. Federation starships are supposedly powered by matter-antimatter reactions, which is probably why they are always blowing up so spectacularly. If Jordi lets his antimatter spill out of its magnetic container, he's in big trouble.

1934 - Frederic Joliot and his wife Irene Curie, daughter of Marie Curie, bombard aluminum with a -particles to produce phosphorus-30, the first artificially radioactive element.

1935 - Japanese physicist Hideki Yukawa proposes that the neutrons and protons in atomic nuclei are held together by an intensely powerful force which he calls the strong force. Working with the Dirac theory, he realizes that the fundamental forces must be carried by quanta, i.e., they cannot exist as classical "lines" of force. The only way for such quanta to exist and still be compatible with classical physics is if they "steal" their energy by popping in and out of existence so fast that conservation of energy is not violated because it is masked by the Heisenberg Uncertainty Principle. (In other words, the Uncertainty Principle applies even to empty space -- how do you know it's truly "empty", when the Principle won't let you measure its energy exactly?) Yukawa predicts that the strong force is "carried" by what he calls an "exchange particle". From the known sizes of atoms, and by assuming that the exchange particle usually moves near the speed of light, he calculates that it should have a mass about 200 times that of the electron.

1938 - It is now widely recognized that the calculation made by Hermann Helmholtz over 60 years ago, deriving an age of about 18 million years for the Sun, is far off the mark for exactly the same reason that Thompson's calculation for the age of the Earth was so far off: both the Earth and the Sun have nuclear energy sources. But the question remains: how does nuclear energy power the Sun? Its enormous energy yield is far too great to be created by traces of radioactive elements, as on Earth.

German-American physicist Hans Bethe calculates in detail how nuclear fusion, rather than nuclear fission, can power the Sun. He deduces a three-step sequence which we now call the proton-proton chain:

  1. Two protons collide so violently that a nuclear transformation takes place. One of the protons is converted into a neutron, and fuses with the other proton to form a deuteron, i.e., "heavy" hydrogen, 2 H. To conserve charge and lepton number, an antielectron and a neutrino are emitted. The neutrino escapes from the Sun, but the antielectron immediately annhilates with an electron, releasing energy.
  2. The deuteron collides with a high-energy proton and the two fuse to form 3 He. The mass of 3 He is slightly less than that of 2 H and a proton separately, and the excess mass is converted to high-energy gamma rays.
  3. Two energetic 3 He atoms collide and in the resulting nano-nuclear fireball, an a -particle ( 4 He atom) and two protons emerge. The mass difference before and after the collision is considerable: it releases approximately double the energy of the first two steps combined. The energy manifests itself primarily in the kinetic energy of the after-products, i.e., as heat.
  4. The net effect of the chain is that four hydrogen atoms have been converted into one helium atom, and 0.7% of the original mass of the hydrogen has been converted to energy. This corresponds to 175 million kilowatt-hours of energy from each kilogram of hydrogen.

1938 - Austrian physicists Otto Hahn and Lise Meitner bombard uranium with neutrons and discover nuclear fission. In short, uranium is a very large atom with over 230 protons and neutrons, so whacking it with a neutron "bullet" can cause it to split in two. Meitner, who is Jewish, flees to Sweden when Germany invades Austria and prepares a paper with the help of her nephew, physicist Otto Frisch. Frisch tells Bohr (see 1913) about the paper, who in turn spreads the word in the U.S. during a conference held in January, 1939.

1939 - Hungarian physicist Leo Szilard, having fled Nazi-occupied Europe for the U.S., learns of nuclear fission and realizes that it could be utilized to produce a chain reaction. He immediately begins a campaign to convince American scientists that they should voluntarily keep their nuclear research secret, so that the Nazis cannot learn from it. He is largely successful.

1940 - American physicists Edwin McMillan and Philip Abelson bombard uranium with neutrons to produce plutonium. Uranium is element number 92, and plutonium is element number 93, so McMillan and Abelson are the first physicists to produce a new element. In his efforts to isolate the plutonium, Abelson begins to develop methods to separate rare radioactive isotopes from their more common brethren. He has taken the first step towards producing enriched uranium.

1941 - Acting partly in response to a letter signed by Albert Einstein and other prominent physicists, warning of the danger should Nazi Germany discover nuclear fission, President Franklin D. Roosevelt signs a secret order which starts the Manhattan Project.

1942 - Enrico Fermi (see 1931), who by now has fled Fascist Italy for the U.S., is made the principal scientist responsible for producing a chain reaction for the Manhattan Project. Working in a secret laboratory located beneath the stands of the University of Chicago's football stadium, Fermi and his team construct the world's first nuclear pile (so called because it is literally a huge pile of carefully-arranged uranium, graphite, and cadmium blocks). At 3:45 pm on December 2, it is allowed to go critical for just a few seconds, proving that practical utilization of nuclear energy is possible. As a safety measure, three young physicists are standing on scaffolding above the pile with buckets of water containing dissolved cadmium salts -- they are told that they should pour their water into the pile if the reactor begins to have a run-away reaction. (In fairness, I must note that the pile also had a more conventional automatic shut-off device. But given that nobody had ever cranked up a reactor before, the team thought it best to play it safe.)

1945 - On July 16, just before dawn, the world's first atomic bomb is detonated at a test site in the desert 60 miles northwest of Alamogordo, New Mexico. Fermi makes an instant estimate of its power by tossing some paper bits into the air at the time of ignition, then observing how far the bits are blown by the blast. (Fermi was about 10 miles from ground zero.) This event follows three years of frenzied labor at secret facilities located in Hanford, Washington Oak Ridge, Tennessee and Los Alamos, New Mexico.

Barely a month later, atomic bombs nearly obliterate Hiroshima and Nagasaki, killing over 100,000 people. The Empire of Japan surrenders shortly afterwards. (The photo is of Nagasaki, Japan, on August 9, 1945.)

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In 1942, the abandoned squash court located underneath the disused football stadium at the University of Chicago was little more than an eye sore. But where students saw remnants of squash games past, physicist Enrico Fermi saw an ideal place for an experiment, the results of which would change the trajectory of World War II and usher in a new, fraught geopolitical era.

The reinforced brick room was perfectly sized to hold a neatly stacked pile of 40,000 graphite bricks, some containing uranium, others drilled with holes designed to fit 14 -foot long cadmium-coated tubes.

A worker stands next to graphite blocks that formed the backbone of Chicago Pile-1, a primitive nuclear reactor.

On December 2, Fermi and nearly 50 fellow scientists piled into the bleachers. Geiger counters in hand, they watched the readings skyrocket as the neutron-absorbing tubes were removed one by one. Without the cadmium buffers, neutrons from splitting uranium atoms were unrestrained, free to crash into other uranium atoms, releasing even more neutrons that caused even more collisions.

When the last tube was removed at 3:25 pm, the pile was sustaining a steady stream of atomic energy. This was no longer a squash court. This was home to the world’s first manmade nuclear reactor and the provenance of the Atomic Age.

Today marks the 75 th anniversary of the Chicago Pile-1 chain reaction, a scientific breakthrough that made nuclear power and weaponry possible. It also opened up entire new avenues of research in medicine, engineering, and aeronautics. Though that initial reaction only generated about half a watt of power, the event marked a turning point. Later developments would give humankind access to unprecedented levels of power while forcing us confront whether and how it should be used.

“They had basically created an entirely new energy source,” says Rachel Bronson, president and CEO of the Bulletin of the Atomic Scientists. “They had created fire in some ways.”

In the process, the minds behind the Chicago Pile-1 broke cultural and political barriers, she adds. Fermi was an Italian immigrant, and Hungarian refugees played crucial roles in the project, including Leo Szilard , who came up with the idea of a nuclear chain reaction, and Eugene Wigner , who would later share a Nobel Prize for his contributions to atomic research.

“So many of the big issues that we’re grappling with—how to manage nuclear power, what kind of funding should go into research and development, what should our immigration policy be, this was all swirling around the Manhattan Project in 1942,” Bronson says.

While those questions loomed in the background of the Chicago Pile experiments, Fermi’s team stayed focused on two immediate goals—one, figure out how to control nuclear energy before Germany, and two, prevent the reaction from spiraling out of control. Given that the safety controls were primitive by today’s standards and mostly relied on a few cadmium tubes to prevent a nuclear explosion, the risk was very real.

“We could have very easily lost Chicago,” says Peter Kuznick, director of the Nuclear Studies Institute at American University in Washington, D.C.

Chicago Pile-1 was build beneath the stands of Stagg Field at the University of Chicago, located in the heart of the city.

Fermi’s team was well aware of the destructive potential of their research. Even while constructing the Chicago Pile, Szilard believed that the experiments “would go down as a black day in the history of mankind .” Their experiments also helped usher in an era in which scientists were more outspoken about how their work was used. Following World War II and into the Cold War, physicists routinely argued for the restriction or elimination of nuclear arms. Such activism around nuclear issues is another legacy of Fermi’s chain reaction, Kuznick says.

Fermi’s team probably never envisioned that their radioactive pile of graphite bricks would lead to cancer-spotting imaging technologies or devices that can help find hidden tombs in ancient Egyptian pyramids. But as they sat in those University of Chicago bleachers, listening to the ever-increasing clicks of their Geiger counters, they knew that something big was happening, says Alex Wellerstein, assistant professor of science and technology studies at the Stevens Institute of Technology.

“They definitely thought they were on the cusp of a new world with their experiment,” he says. “They knew it was just the beginning.”

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Harnessing fission

As part of the Manhattan Project effort to build an atomic bomb during World War II, Szilard worked together with physicist Enrico Fermi and other colleagues at the University of Chicago to create the world’s first experimental nuclear reactor.

For a sustained, controlled chain reaction, each fission must induce just one additional fission. Any more, and there’d be an explosion. Any fewer and the reaction would peter out.

Nobel Prize winner Enrico Fermi led the project (Argonne National Laboratory, CC BY-NC-SA)

In earlier studies, Fermi had found that uranium nuclei would absorb neutrons more easily if the neutrons were moving relatively slowly. But neutrons emitted from the fission of uranium are fast. So for the Chicago experiment, the physicists used graphite to slow down the emitted neutrons, via multiple scattering processes. The idea was to increase the neutrons’ chances of being absorbed by another uranium nucleus.

To make sure they could safely control the chain reaction, the team rigged together what they called “control rods.” These were simply sheets of the element cadmium, an excellent neutron absorber. The physicists interspersed control rods through the uranium-graphite pile. At every step of the process Fermi calculated the expected neutron emission, and slowly removed a control rod to confirm his expectations. As a safety mechanism, the cadmium control rods could quickly be inserted if something started going wrong, to shut down the chain reaction.

Chicago Pile 1, erected in 1942 in the stands of an athletic field at the University of Chicago. (Argonne National Laboratory, CC BY-NC-SA)

They called this㺔x6x25-foot setup Chicago Pile Number One, or CP-1 for short – and it was here they obtained world’s the first controlled nuclear chain reaction on December 2, 1942. A single random neutron was enough to start the chain reaction process once the physicists assembled CP-1. The first neutron would induce fission on a uranium nucleus, emitting a set of new neutrons. These secondary neutrons hit carbon nuclei in the graphite and slowed down. Then they’d run into other uranium nuclei and induce a second round of fission reactions, emit even more neutrons, and on and on. The cadmium control rods made sure the process wouldn’t continue indefinitely, because Fermi and his team could choose exactly how and where to insert them to control the chain reaction.

A nuclear chain reaction. Green arrows show the split of a uranium nucleus in two fission fragments, emitting new neutrons. Some of these neutrons can induce new fission reactions (black arrows). Some of the neutrons may be lost in other processes (blue arrows). Red arrows show the delayed neutrons that come later from the radioactive fission fragments and that can induce new fission reactions. (MikeRun modified by Erin O’Donnell, MSU, CC BY-SA)

Controlling the chain reaction was extremely important: If the balance between produced and absorbed neutrons was not exactly right, then the chain reactions either would not proceed at all, or in the other much more dangerous extreme, the chain reactions would multiply rapidly with the release of enormous amounts of energy.

Sometimes, a few seconds after the fission occurs in a nuclear chain reaction, additional neutrons are released. Fission fragments are typically radioactive, and can emit different types of radiation, among them neutrons. Right away, Enrico Fermi, Leo Szilard, Eugene Wigner and others recognized the importance of these so-called “delayed neutrons” in controlling the chain reaction.

If they weren’t taken into account, these additional neutrons would induce more fission reactions than anticipated. As a result, the nuclear chain reaction in their Chicago experiment could have spiraled out of control, with potentially devastating results. More importantly, however, this time delay between the fission and the release of more neutrons allows some time for human beings to react and make adjustments, controlling the power of the chain reaction so it doesn’t proceed too fast.

Nuclear power plants operate in 30 countries today. (AP Photo/John Bazemore)

The events of December 2, 1942 marked a huge milestone. Figuring out how to create and control the nuclear chain reaction was the foundation for the 448 nuclear reactors producing energy worldwide today. At present, 30 countries include nuclear reactors in their power portfolio. Within these countries, nuclear energy contributes on average 24 percent of their total electrical power, ranging as high as㻈 percent in France.

CP-1’s success was also essential for the continuation of the Manhattan Project and the creation of the two atomic bombs used during World War II.

Remembering the Chicago Pile, the World’s First Nuclear Reactor

December 2, 1942, was the coldest day in Chicago in almost fifty years. That frigid afternoon, a crew of men and women—many of them hailing from countries an ocean away, where the Second World War raged—gathered under the viewing stands of the University of Chicago’s Stagg Field to light a secret fire. They were members of the Metallurgical Laboratory, an organization that had existed only since that January, and were attending to their creation, a dusty collection of graphite, uranium, and scientific equipment that they called the Pile. Today, we know it as something different: the world’s first nuclear reactor.

The Chicago Pile deserved its low-tech name. It was a stack of forty thousand graphite blocks, held together in a wooden frame, twenty-five feet wide and twenty feet tall. Inside about half of the blocks were holes containing small amounts of uranium oxide inside a few others were nuggets of refined uranium metal, the production of which was still a novel process. The Pile had few safety features. The scientists’ only protection against radiation came from a set of cadmium control rods, designed to be inserted and removed by hand, along with untested theories and calculations. As one governmental report later put it, “there were no guidelines to follow and no previous knowledge to incorporate.” Neither university nor city officials were told that an experiment that even its creators judged as risky was taking place in the heart of the second-largest city in the United States.

The experiment itself was something of an anticlimax. The Pile was started up, brought to criticality (the point at which a nuclear reaction becomes self-sustaining), then shut down half an hour later, before its growing heat and radioactivity became too dangerous. The Metallurgical Laboratory experimented with it for a few months before disassembling and reconstituting it—now with radioactive shielding—at a site somewhat more removed from the city, where it became known as Chicago Pile-2. Ultimately, the reactor ran for over a decade before it was finally dismantled and buried in the woods.

The Pile was not an abstract scientific achievement. It was part of a much larger plan, conceived under the auspices of the Manhattan Project, to build a fleet of industrial-sized nuclear reactors—not for the generation of electrical power (that would come much later) but to produce plutonium, a fuel for nuclear weapons. Virtually overnight, the University of Chicago had become a major wartime contractor. (One of its many government contracts, by itself, doubled the school’s budget.) Data from the Pile would inform the design of later reactors, including the one that furnished the plutonium for history’s first nuclear-weapons test, known as Trinity, and the atomic bomb dropped on Nagasaki.

Wartime secrecy and suspicion suffused every aspect of the Metallurgical Laboratory’s work. The U.S. military had deemed some of its staff, including Arthur Compton, its Nobel Prize-winning director, security risks. Other members of the project, including the gadfly physicist Leo Szilard and even the eminent Enrico Fermi, were considered “enemy aliens,” because the countries from which they had fled were under Fascist rule. Vannevar Bush, the scientist-administrator who coördinated much of the early work on the Manhattan Project, appealed to the military to let these concerns slide. Rather than letting nuclear experts roam free, wouldn’t it be better, he suggested, “to take in and put under thorough control practically every physicist in the country having background knowledge of the subject”?

Eventually, the government addressed its security concerns by opening a new facility in a more isolated location, where the truly sensitive work could be done. This became the Los Alamos laboratory, in New Mexico. Though many of the Chicago team’s most trusted scientists made the journey to Los Alamos, others stayed—or were kept—behind. They did not, however, remain idle. Having completed the majority of their jobs in the early part of the Manhattan Project, and unburdened by the challenges of actually building the bomb, they had time to reflect on the social and political problems posed by the new technology. A report on this topic, chaired by James Franck, a Nobel Prize-winning physicist from Germany who had worked on chemical weapons in the previous war, concluded somewhat heretically that the first atomic weapons should not be dropped on cities without warning. The Franck Report elicited some discussion at higher levels of the Manhattan Project, but no plans were changed on account of it. Eventually, after the war, it was released to the public, with some alterations made by the military. One line that was scratched out of every copy of the report, but is just visible in originals by holding it up to the light at the right angle, argued that, should the United States be the first country to use nuclear weapons in war, it “might cause other nations to regard us as a nascent Germany.”

Not all of the Chicago scientists’ thoughts were so dark. Members of the Metallurgical Laboratory also wrote reports about the peaceful benefits of the atom, imagining a new field of science and technology, which they dubbed “nucleonics,” ushering in medical breakthroughs and new energy supplies in the wake of the Second World War. They recommended the creation of a national-laboratory system, to insure that organizations such as the Metallurgical Laboratory could exist in peacetime, and lobbied vigorously for what they considered wise policy on atomic weapons. The Bulletin of the Atomic Scientists of Chicago and the Federation of Atomic Scientists (later the Federation of American Scientists) both emerged out of this political awakening, and a movement for social responsibility on behalf of scientists was born. The Pile team turned out to be better at building reactors that changing public policy, but its legacy of activism and public engagement reverberates in today’s discourse about climate change.

After the war had ended and the world had come to appreciate the power that had been unleashed, the University of Chicago installed a bronze plaque commemorating the Pile. It read, “On December 2, 1942, man achieved here the first self-sustaining chain reaction and thereby initiated the controlled release of nuclear energy.” In a rejected suggestion, the university press director proposed that a phrase be added to the end: “for better or worse.”

The Plan B decision to build at UChicago

The University wasn’t the original site for the historic experiment though. In early 1942, Compton identified a promising plot of land while on a horseback ride in a forest preserve about 25 miles southwest of Chicago. But by late October, workers constructing the buildings in the so-called Argonne Forest went on strike, and it soon became clear that the site wouldn’t be ready until year’s end.

Fermi suggested to Compton that he could demonstrate the controlled chain reaction safely on campus—under Stagg Field, the long-abandoned, crumbling home of the former Big Ten football powerhouse. And if something were to go wrong, “I will walk away—leisurely,” Fermi once wrote. As a safeguard, a series of control rods would be installed to prevent a runaway reaction.

“According to Fermi’s calculations, which I carefully checked…it should take some minutes for the reaction to double its power," Compton wrote in his memoir. “If this proved correct, there would be ample time for adjustments, and the reaction would be under full control.”

Compton at the outset had predicted a nuclear chain reaction would be achieved by Jan. 1, 1943. With time of the essence, Compton told Fermi to proceed without informing UChicago President Robert Maynard Hutchins. Compton felt Hutchins, a trained jurist and former Law School dean, “was in no position to make an independent judgment of the hazards involved.”

“As a responsible officer of the University, according to every rule of organizational protocol, I should have taken the matter to my superior. But that would have been unfair,” wrote Compton. “Based on considerations of the University’s welfare, the only answer he could have given would have been—no. And this answer would have been wrong. So I assumed the responsibility myself.”

Path to criticality

The self-assured Fermi gave Compton little cause for concern. In September, Fermi began a series of multi-hour weekly lectures at Eckhart Hall on the UChicago campus, where he described the measurements that would determine when the pile would go critical.

When he started building chain-reacting piles at Columbia University after his arrival in January 1939, Fermi would don a lab coat and worked alongside football players enlisted to move the 50- to 100-pound bricks of graphite. &ldquoWith Fermi, it was the work that made the physics worthwhile. He wanted to wrestle with nature himself, with his own hands,&rdquo wrote physicist Herbert Anderson, who ran the night shift in Chicago&rsquos pile program, in a 1974 Bulletin essay. &ldquoHe liked to have someone to work with. He liked the companionship the work went faster that way.&rdquo

This artist's conception shows the UChicago students, scientists and day laborers working on Chicago Pile-1. (Photo courtesy of National Archives and Records Administration)

There were no blueprints for the Chicago pile. Instead, machinists and scientists reported on the daily progress of construction to Fermi. Two crews formed: One pressed uranium oxide power into 22,000 spheres the size of baseballs. The other used a wood planer to mill about 400 tons of graphite into rectangles, which were then drilled to create holes to hold the uranium.

&ldquoWe found out how coal miners feel,&rdquo wrote Wattenberg in the Bulletin. &ldquoOne shower would remove only the surface graphite dust. About a half-hour after the first shower, the dust in the pores of your skin would start oozing.&rdquo

By late fall, dozens of smaller test piles had provided proof-of-concept for Fermi&rsquos larger experiment. But CP-1, 20 times larger than its predecessors, would require even larger amounts of uranium and graphite in purer forms.

On Nov. 16, two 12-hour shifts began to construct the pile&mdashwork that would continue non-stop over the next 15 days. By the evening of Dec. 1, they had constructed the reactor, which resembled a 57-layer graphite cake, wrapped in wood and studded with hundreds of uranium raisins toward the center that would serve as the nuclear fuel for the reaction.

By the morning of Dec. 2, Chicago Pile-1 was ready.

Photograph taken in November 1942 during construction of the first nuclear reactor. Chicago Pile-1 consisted of 57 layers cost an estimated $2.7 million and contained 380 tons of graphite, 40 tons of uranium oxide and six tons of uranium metal. (Photo courtesy of Argonne National Laboratory)

Enrico Fermi

Under the west stand of the University of Chicago’s squash courts in Stagg Field, sits a plaque. It reads: “On December 2, 1942, man achieved here the first self-sustaining chain reaction and thereby initiated the controlled release of nuclear energy.” How did the squash courts at the University of Chicago became the site of the first self-sustaining nuclear chain reaction? The story begins in Italy in 1915.

In Rome that year a 14 year old boy, grieving the death of his older brother, sought distraction in books. Roaming the Campo de Fiori he happened upon two antique volumes of elementary physics. Our world was never to be the same. The boy was Enrico Fermi, and he would become the man who in 1942 performed the first self-sustaining nuclear chain reaction at the University of Chicago’s squash courts.

Fermi’s interest in physics was intense. At 19, he entered the University of Pisa, where, by some accounts, he shortly began instructing his teachers. At the tender age of 25, he became a professor of theoretical physics at the University of Rome. In 1934, Fermi almost discovered nuclear fission—the process that was used in the first atomic bomb—while conducting experiments in the radioactive transformations that resulted when various elements were repeatedly bombarded with neutrons. However, Fermi missed this opportunity because the sheet of foil he used to cover his uranium sample, which would have created fission, was too thick. It blocked the fission fragments from being recorded and went unnoticed. Though Fermi failed to discover fission, he did discover that passing neutrons through a light-element “moderator,” such as paraffin, slowed them down and in turn, increased their effectiveness. This discovery was instrumental in generating the heat needed by a nuclear reactor to generate electricity. In 1938 Fermi was awarded the Nobel Prize for his work.

Fermi traveled from Italy to Sweden to obtain his Nobel medal and never returned home. Italy’s fascist and anti-Semitic climate increasingly disturbed him. Like many European scientists of the period he left Europe and settled in the United States, taking employment at the University of Chicago. Others at the university were working on the atomic bomb. Fermi’s task was to find a way to control the chain reaction that resulted from fission. His answer was to create a nuclear reactor, which Fermi, whose English was still poor, called simply a “pile,” so that, theoretically, he could insert a neutron-absorbing material into the midst of the fission process to control its speed.

In December 1942 Fermi and his team were prepared to test their reactor. Due to space considerations, the “pile” was set up in the university’s squash court. The test did not occur without some concern. Up to that very moment Fermi’s notions about controlling fission were based entirely on theory, not practice. If he was wrong, Chicago could be blown away. The test began. At first, just a couple of rods were removed. Gradually, Fermi pulled more. Finally, it was apparent—Fermi and his team had created a self-sustaining nuclear reaction—the first controlled flow of energy from a source other than the sun. A coded message told the government of this success: “The Italian navigator has just landed in the new world.”

Final Years and Death

Fermi continued his work at the Institute for Nuclear Studies at the University of Chicago, where he turned his attention to high-energy physics and led investigations into the origin of cosmic rays and theories on the fantastic energies present in cosmic ray particles.

By 1954, Fermi was diagnosed with incurable stomach cancer, and spent the remaining months of his life in Chicago, undergoing various medical procedures. He died in his sleep on November 28, 1954, at his home in Chicago, Illinois.

Watch the video: Enrico Fermi e lenergia nucleare. Biografia e scoperte


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