Seventy one years ago — on December 2, 1942, at 3:25 pm — Enrico Fermi and his team achieved the first controlled, man-made, self sustaining chain reaction in a simple reactor. In recognition of that historical event, several of my nuclear colleagues refer to December 2 as “Critmass” (short for critical mass).

The first nuclear reactor — CP-1 (Critical Pile number 1) — was actually a carefully constructed pile consisting of graphite bricks, uranium oxide pseudospheres, and uranium metal pseudospheres. The pile was built on a 30 by 60 foot squash court located under the stands at Stagg Field, the former home of the University of Chicago’s short-lived, but successful, football program.

The construction period was remarkably brief; the stacking process started on November 16, 1942, slightly more than two weeks before the criticality experiment.

In 2012, the Argonne National Laboratory produced a short history video that included interviews of Harold Agnew and Warren Nyers, two of the atomic pioneers who were part of the team that built the pile and produced the world-changing demonstration. ANL also has a Flickr page of CP-1 related images worth perusing.


At the time of the CP-1 experiment, excitement about the possibility of a self-sustaining chain reaction and the use of uranium 235 as a highly capable replacement for coal and oil had been building for about three years. Even though Ernest Rutherford had once dismissed expectations for the use of atomic energy as “moonshine”, talented physicists and chemists recognized that nuclear energy production was almost inevitable once they learned how neutrons could split uranium atoms and how that reaction itself produced additional neutrons. Atomic fission was first widely recognized in early 1939 when Lise Meitner and Otto Frisch properly interpreted the experimental results that Otto Hahn and Fritz Strassmann had reported.

That wide recognition was delayed by nearly five years from the first inkling of what happens when uranium is bombarded with neutrons. In September 1934, Ida (Tacke) Noddack, a chemist, had published a paper titled On Element 93 stating that Fermi’s claim of having produced new elements heavier than uranium during some experiments conducted in 1933 was not valid based on the test method that he reported. She recommended that future experiments test to see if what Fermi asserted were transuranics were, in fact, isotopes of known lighter elements like barium. Noddack suggested that Fermi’s results of at least five different decay half lives after bombarding uranium could be caused by uranium nuclei breaking in previously unknown ways.

When heavy nuclei are bombarded by neutrons, it is conceivable that the nucleus breaks up into several large fragments, which would of course be isotopes of known elements but would not be neighbors of the irradiated element.

Noddack’s interpretation was dismissed, leading to a series of confusing experimental results. That dismissal might have been because Ida Noddack was a chemist commenting on a physics experiment or perhaps because she was a little known woman in a field dominated by men.

Leo Szilard also anticipated the possibility of creating self-sustaining chain reactions in 1933. He filed a patent on the process, even though he was not sure which elements might split with a release of both energy and neutrons.

It is an unfortunate historical accident that Fermi’s successful initiation of an atomic fire occurred at a time when the world was being threatened by Hitler’s quest for total domination. Like Leo Szilard, many of the scientists who collaborated in a world wide burst of creativity to determine how to predictably release the vast amount of energy stored inside atomic nuclei were initially attracted by the idea of developing nuclear heat as an alternative to burning coal and oil. Though Szilard understood that rapid energy release might have military applications and he took steps to keep that knowledge out of Hitler’s hands, the patent itself shows that he anticipated power production because it describes a pile with a means of extracting heat to produce electrical power.

Since “there was a war on” and since several of the key experimenters — including Otto Hahn and Fritz Strassmann — with atomic energy were known to be actively working inside Germany, there was more immediate interest in secretly developing along the explosive path than in openly developing devices that could use controlled atomic chain reactions to produce reliable energy. Even before Fermi began building CP-1 in 1942, a veil of secrecy had been self-imposed by the mostly European scientists; that veil was made official US government policy as part of the Manhattan Project. Despite all of the prior interest in the possibility that chain reactions in uranium would be the atomic analog of fire, there were no contemporary public announcements of Fermi’s successful critical mass demonstration.

December 2 has been reinforced as an important date in nuclear energy by several additional developments. On December 2, 1957, just fifteen years after the CP-1 demonstration, the Shippingport nuclear power station achieved its initial criticality. On December 2, 1977, just twenty years after that power station first began operating, President Jimmy Carter issued an order to bring the final core of the Shippingport reactor to full power. That final core was known as the Light Water Breeder Reactor a demonstration project that eventually operated for five years and more than 27,000 effective full power hours. At the end of that period of operation, there was more fissile material in the core than there was when the core first started operating.

I now wish you a happy Critmass. I hope that this journey through history has been a pleasant addition to your day and to your optimism for the future of nuclear energy development.

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