Dawn of the Atomic Age: The First Nuclear Reactor

In December 1942, the Atomic Age began, in secret, in a racquetball court under the bleachers of an un-used football stadium in the middle of Chicago. It was here, as part of the Manhattan Project to build an atomic bomb, that a team of physicists put together the first nuclear reactor and produced the first self-sustained fission chain reaction.

A lab notebook from the CP-1 reactor, on display at the Chicago History Museum

In October 1939, President Franklin Roosevelt received a letter from Albert Einstein, who was then probably the most famous scientist in the world. Einstein described the experiments in nuclear fission that had been recently reported from Germany in which uranium atoms were split by neutrons, releasing large amounts of energy. This could, he noted, lead to the development of a uranium bomb of enormous power. Einstein urged FDR to begin an American effort to develop an “atomic bomb” before the Germans did. The result was the Manhattan Project.

As the work began, Manhattan Project physicists discovered a new element, plutonium, that was even better bomb material than uranium. Producing plutonium in usable amounts, however, would require a “nuclear reactor”–a sustained chain reaction that would constantly bombard natural uranium with neutrons to convert it into plutonium. Chain reactions occur when a uranium atom is fissioned by a neutron and during the process ejects more neutrons which go on to produce other fissions. If this reaction happened over an extremely short period of time, the result was a nuclear explosion. But if the rate of the reaction could be slowed and controlled, the result was a steady release of neutrons and energy–a  nuclear reactor. These neutrons could then be used to turn uranium into plutonium. The project was assigned to Italian physicist Enrico Fermi in Chicago.

In 1934, Fermi was conducting experiments in Rome by bombarding uranium (the heaviest element found in nature) with neutrons of different speeds. In one of his test runs, he discovered that the uranium, element number 92, had absorbed one of the neutrons and become transformed into an unknown element, number 93, later named neptunium. He was awarded the 1938 Nobel Prize in Physics for his discovery of “trans-uranic elements”, and when the Mussolini regime granted him permission to travel to Sweden to receive the Prize, Fermi took the opportunity to defect to London and go with his Jewish wife to the United States.

In 1940, Fermi and his team began a series of experiments at the University of Chicago to determine how to build a nuclear reactor. The chain reaction would not start until there was a sufficient amount of uranium present, a point known as “criticality”. Fermi determined that neutrons were most efficient at converting uranium to plutonium if they had already been slowed down, and that collisions with carbon atoms worked well at reducing the neutrons to the proper speed. He also found that the elements cadmium and indium would absorb neutrons, and that he could use this to control the rate of fission reactions. So he obtained a supply of graphite bricks to serve as a “moderator”, and long rods made with cadmium/indium as “control rods”.

The basic idea of the nuclear reactor has not changed much since Fermi’s original concept. The nuclear fuel consists of uranium metal or uranium oxide, in an amount large enough to reach criticality and produce a chain reaction. This nuclear fuel core is surrounded by a “moderator” which slows down the neutrons to make the fission process more efficient. Fermi’s reactor had such a low power output that it did not use any cooling system; modern high-powered  reactors usually use water to serve as both a moderator and a coolant. The rate of the fission reaction is governed by cadmium/indium control rods, which are withdrawn from the core to increase the power and inserted to weaken or shut it down. At the time of Fermi’s experiments, the dangers of nuclear radiation were not well-understood and there was no radiation shielding. Modern nuclear reactors are surrounded by heavy containment vessels.

By 1941, Fermi was ready to begin construction of a working reactor. Although some of his colleagues were alarmed at the potential results of an accidental runaway reaction, Fermi was so confident in his calculations that he boldly planned to build his reactor right there at the University of Chicago, in the middle of one of the largest and most densely-inhabited cities in the world. In the frantic race to build The Bomb before the Nazis did, such risks were considered to be acceptable. The reactor (or “atomic pile” as it was then called) was given the designation CP-1 (for “Chicago Pile Number One”).   Since no one had built a nuclear reactor before, nobody knew which geometry was best. The original plans called for a giant sphere, but later calculations showed that this was larger than would actually be needed. So the design was changed to a cube of 24 feet per side. The reactor was, literally, a pile. It was made by alternating solid layers of graphite bricks over layers of uranium metal and/or uranium oxide pellets placed between graphite bricks. The long control rods were inserted through the lattice from the back. As construction went on, more calculations determined that the full cube wasn’t needed either, so the top half of the pile was sloped inwards to form a dome, giving the finished reactor a shape like a beehive. The completed construction contained 40,000 graphite bricks and 5 tons of uranium pellets, arranged in 57 layers.

On December 2, 1942, Fermi’s team of 49 people were ready to turn the reactor on. In a series of steps, the control rods were retracted, measurements taken, and the equipment examined. Finally, at 3:25 in the afternoon,  the reaction went critical, the fission reactions became self-sustaining, and the first nuclear reactor began producing energy (although, at an output of just a few dozen watts, it was very weak).  For the next 28 minutes, Fermi and his team confirmed that they could vary and maintain the rate of reaction by adjusting the control rods, then they shut down the reactor.

To celebrate their success, the entire team sipped Chianti from paper cups, then each signed their name to the empty bottle. A coded message was sent to the Manhattan Project commanders in Washington DC: “The Italian navigator has landed in the New World.” Within the next year, giant new reactors were being constructed at Hanford WA for the sole purpose of producing plutonium for atomic bombs.

Today, the spot where Fermi’s CP-1 reactor was built is part of the University of Chicago’s library. It has been designated a National Historical Landmark and is listed on the National Register of Historic Places. The lab notebook used during the December 2 test is now on display in the Chicago History Museum. The CP-1 itself was dismantled in 1943, moved to the site of the current Argonne National Laboratory, and rebuilt in expanded form as CP-2. It was run for a time as an experimental reactor, then was decommissioned and buried in place.


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