The 1942 Experiment That Changed Our World
Beneath the abandoned west stands of the University of Chicago's Stagg Field, on a cold December day in 1942, a small group of scientists huddled on a squash court balcony, about to alter the course of human history.
Their leader, the brilliant Italian physicist Enrico Fermi, was about to conduct an experiment that would alter the course of human history. There were no flashing lights or giant machines; their device, a crude stack of graphite and uranium, was ominously codenamed Chicago Pile-1 (CP-1).
At 3:25 p.m., as a final control rod was withdrawn, the instruments clicked and their needles climbed—the world's first controlled, self-sustaining nuclear chain reaction had been achieved1 4 . This moment marked the dawn of the Atomic Age, a profound leap in human capability that would usher in both terrifying weapons and the promise of limitless energy.
Italian physicist who led the Chicago Pile-1 experiment and later won the Nobel Prize in Physics.
The story of CP-1 begins with the discovery of nuclear fission in 1938. Scientists found that when a neutron struck the nucleus of a heavy atom like uranium, the nucleus could split into two lighter elements. This process, called fission, released a staggering amount of energy and, crucially, two or three additional neutrons.
These newly released neutrons held the key. If they could be made to split other uranium nuclei, which would then release more neutrons to split even more nuclei, a self-sustaining chain reaction would occur. Controlling this cascade was the grand challenge. A controlled chain reaction could produce steady heat; an uncontrolled one, an immense explosion1 .
The experiment was conducted under the umbrella of the Manhattan Project, the Allied effort to build an atomic bomb during World War II. The fear that Nazi Germany was pursuing the same goal lent an intense urgency to the work1 .
Fermi's experiment was a masterpiece of practical engineering. With the simple materials at hand, his team demonstrated one of the most profound physical phenomena of the 20th century.
The term "nuclear reactor" didn't yet exist; Fermi's team used the more descriptive word "pile." The construction of CP-1 was a meticulous, hands-on process2 4 .
The experiment took place in a converted squash court underneath the university's football stadium. The high ceiling of the court was necessary to accommodate the wooden frame of the pile1 4 .
The pile was constructed from 40,000 graphite bricks, some of which held over 19,000 pieces of uranium metal and uranium oxide fuel1 . Graphite's role was to act as a "moderator," slowing down the neutrons released by fission, making them more likely to be captured by other uranium nuclei and continue the chain reaction2 .
The crew, led by Fermi's principal superintendent Walter Zinn, built the pile layer by layer2 . They stacked two layers of solid graphite bricks, followed by a layer of bricks containing the uranium fuel lumps. The pile grew into a large, slightly flattened spherical shape, nearly 25 feet wide1 2 .
Fermi incorporated a simple but effective safety system. He directed Zinn to machine channels into some graphite bricks to hold long, wooden strips wrapped with cadmium foil. Cadmium is a potent neutron absorber. When inserted, these control rods would halt the chain reaction by "soaking up" the vital neutrons. The pile had several sets of these rods: one automatic, one emergency safety rod, and one manual rod that was carefully withdrawn to start the reaction2 4 .
25 feet wide
Nearly spherical shape
400 tons
Graphite and uranium
On December 2, 1942, the team gathered for the final test. Scientist George Weil stood ready to withdraw the final manual control rod, inch by inch, while others monitored the neutron levels on instruments whimsically named after Winnie the Pooh characters4 .
As Weil slowly pulled the rod, the clicking of the neutron counters quickened into a steady roar. The radiation level was climbing exponentially. At 3:25 p.m., Fermi directed Weil to insert the rod back into the pile. The clicking immediately subsided. The experiment was over. It had been a complete success2 4 .
| Date of Experiment | December 2, 19421 |
|---|---|
| Location | Squash court under Stagg Field, University of Chicago1 |
| Lead Scientist | Enrico Fermi1 |
| Key Construction Lead | Walter Zinn2 |
| Purpose | Achieve the first man-made, self-sustaining nuclear chain reaction1 |
| Energy Produced | Approximately 0.5 watts (initial test)1 |
For 28 minutes, the pile had sustained a chain reaction, producing about half a watt of energy. While this was too weak to power even a single light bulb, its scientific importance was immeasurable1 2 . It proved that nuclear energy could be controlled, paving the way for both nuclear power and further weapons research.
The CP-1 experiment was remarkable for its mechanical simplicity. It was built from readily available materials, each serving a specific and vital function.
Served as a moderator, slowing down high-speed neutrons released by fission so they could be efficiently captured by other uranium nuclei to sustain the chain reaction2 .
Nuclear fuel. The uranium atoms underwent nuclear fission (splitting), releasing the energy and neutrons that powered the chain reaction1 .
Structural support. Held the massive, 400-ton weight of the graphite and uranium pile in a stable, layered structure1 .
The immediate legacy of CP-1 was its crucial role in the Manhattan Project, providing the scientific certainty needed to build the reactors that would produce plutonium for atomic bombs1 . However, its long-term impact has been far more complex and wide-ranging.
The principles demonstrated by CP-1 are the same ones that power nuclear energy plants, which provide a significant source of carbon-free electricity globally4 .
The field of nuclear medicine also owes its birth to this experiment. The research efforts led to the use of radioactive isotopes for diagnosing and treating diseases, most notably in the fight against cancer1 .
The experiment directly contributed to the development of atomic bombs, with devastating consequences for Hiroshima and Nagasaki, and initiating the nuclear arms race.
Nuclear technology introduced challenges of radioactive waste disposal and the risk of accidents, creating long-term environmental and safety concerns.
The scientists involved, deeply aware of the destructive power they had unleashed, founded the Bulletin of the Atomic Scientists and its iconic Doomsday Clock to warn humanity of the existential dangers of nuclear weapons and other global threats1 .
The first controlled chain reaction was more than a scientific experiment; it was a point of no return for humanity. In that humble squash court, we unlocked a fundamental force of nature, harnessing the power that fuels the stars themselves. The legacy of CP-1 is a dual one, a testament to our brightest ingenuity and a warning of our most terrifying capabilities. It gave us the potential for clean energy and advanced medicine, but also the shadow of nuclear annihilation. As we continue to navigate the challenges and opportunities of the Atomic Age, the story of Chicago Pile-1 remains a powerful reminder of the immense responsibility that comes with profound knowledge.