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The Chicago Pile

January 24, 2014

Scientific discoveries have often derived from piles. As a personal example, my office desk has always been piled high with journal articles important enough to copy and save, but not important enough at the time to actually read. Then, I spill my coffee, frantically remove some piles, and accidentally uncover a paper that leads to a new series of experiments.

Paper pile on office desk

Piled higher and deeper.

The usual warning is that desktop computers need proper cooling, so their ventilation holes should never be blocked.

Today, data centers run their servers hot, since it's cheaper to replace them than pay the energy cost for cooling.

(Photo of a corner of the author's desk.)

Possibly the first observation of a scientific phenomenon in a pile was when early man noticed that piles of decaying vegetation generated heat. Compost piles will regularly reach internal temperatures of 150°F (65°C). This is because of the action of thermophilic bacteria, some of which can survive at 250°F (120°C).

Then there's the story of the discovery of stainless steel in a pile of rejected alloy specimens. The story is possibly apocryphal, but such stories are usually much more interesting than the real history. As the story goes, English scientist, Harry Brearley, was doing routine testing of different alloy compositions in order to find one with a greater erosion resistance. After a while, he noticed that his pile of rejected specimens had all rusted, except for one. This chromium-containing specimen marked the invention of stainless steel.

Another famous pile is Alessandro Volta's voltaic pile, which was the first electric battery. Volta's pile was an interleaved assemblage of copper and zinc discs with felt pads soaked in salt water inserted between alternate disc pairs. The salt water electrolyte created electrochemical cells between each set of copper and zinc discs, and these produced a voltage which was multiplied according to the number of disc pairs on the pile.

I would claim that the most famous pile is the Chicago Pile, sometimes called the Chicago Pile-1 (CP-1) to distinguish it from its successors. This was the world's first nuclear reactor, although you wouldn't know it by looking at it. It was a pile of graphite blocks containing uranium fuel, the graphite acting as a neutron "moderator."[1-2]

CP-1 was one of the world's first "Big Science" experiments, and it was part of the Manhattan Project. Its principal investigators were physicists Enrico Fermi; and, Leó Szilárd, who had conceived the idea of a nuclear chain reaction. The reactor achieved a sustained nuclear reaction at about 3:25 PM on December 2, 1942.[1]

Artist's recreation of the Chicago pile's first sustained reaction on December 2, 1942.

An artist's recreation of the Chicago pile's first sustained reaction on December 2, 1942, at 3:22 PM. The world's first nuclear reactor was housed in the Racquets Court under West Stands of Stagg Field, the University of Chicago. No photographers were present, possibly for security reasons. This recreation was done fifteen years after the fact. (National Archives and Records Administration image, via Wikimedia Commons.)

This milestone was communicated to James Conant, chairman of the National Defense Research Committee, by Arthur Compton, who was head of the University of Chicago Metallurgical Laboratory in which Fermi and Szilárd's experiment took place. Compton and Conant used the following impromptu code,
Compton: The Italian navigator has landed in the New World.
Conant: How were the natives?
Compton: Very friendly.[3,5]
The Italian navigator was Fermi, but first mate Szilárd was not mentioned. I guess there was nothing in the code book for a portly Hungarian.

The pile, which produced only enough power to light a light bulb, was 19 feet high, 24 feet wide, and 24 feet long, containing 385.5 tons of graphite, along with 46.5 tons of uranium metal and oxides.[1] Preparatory to starting the pile reaction, there were about thirty subcritical experiments, and all this attention to detail was vindicated when a neutron-absorbing control rod made of indium, cadmium, and silver was withdrawn from the reactor core, allowing the controlled chain reaction.[2]

Aside from the control rod, there was an additional safety mechanism of buckets filled with a cadmium salt solution which could be poured onto the reactor to stop the reaction. Fermi let the reaction proceed for eighteen minutes, and then the control rod was re-inserted.[2] The Chicago Pile was a prototype for reactors to be used in the production of plutonium for bombs.

The University of Chicago CP-1 reactor team

The University of Chicago CP-1 reactor team. Enrico Fermi is front row left, while Leó Szilárd is rightmost in the center row.

The sole woman in the photograph, Leona Woods, was also a physicist.

(Via Wikimedia Commons.)

One interesting thing about the photograph of the CP-1 team, as shown above, is the presence of a woman, Leona Woods (August, 1919 - November, 1986, also known by her married names, Leona Woods Marshall and Leona Woods Marshall Libby). Even today, women are a rarity in the STEM fields (science, technology, engineering, and mathematics). As Barbie so famously stated, "Math class is tough!". I wrote about this in two previous articles, Sexism in Science, September 28, 2012, and Math is Tough, but Boys are Tougher, August 6, 2012.

Leona Woods received a B.S. degree in chemistry from the University of Chicago in 1938, at the young age of 19. She had decided to continue her education in physics, so she became a graduate student of Robert Mulliken. A paper on her thesis work, "On the Silicon Oxide Bands", was published in the Physical Review.[6] As she was finishing her thesis, one of Fermi's colleagues discovered her facility with vacuum equipment, so she was recruited into the CP-1 project to develop boron trifluoride neutron detectors.

Woods was married in 1943, and she became pregnant. She told Fermi, but they decided it would be best for her to hide her condition, lest she be barred from working in a radiation laboratory. After giving birth to a son, she returned to work a few days later. After her war work, she led a peripatetic life, working for the Institute for Advanced Study, Brookhaven National Laboratory, New York University, the University of Colorado Boulder, the RAND Corporation, and UCLA. Woods published more than 200 scientific papers, proving that the "weaker sex" is surely a misnomer.

Figure caption

Leona Woods Marshall in 1946 at the University of Chicago.

This photograph is from the Leona Woods Marshall Libby biography in the book, Uranium People, pp. 182-183.

(Argonne National Laboratory photograph, via Wikimedia Commons.)


  1. History of Argonne Reactor Operations, US Department of Energy (PDF File).
  2. Daniel Terdiman, "How the nuclear age burst onto the world from a squash court," CNET, July 3, 2013.
  3. Argonne's Nuclear Science and Technology Legacy - The Italian Navigator Lands, Argonne National Laboratory Web Site.
  4. Chicago Pile 1 (CP-1) 70th Anniversary, December 2, 1942 - December 2, 2012, Argonne National Laboratory Web Site.
  5. A similar impromptu code was used to announce the successful test of centimetric radar at the MIT Radiation Laboratory. Radar signals had been reflected from the spire of the Christian Science temple across the Charles River, and the coded message was, "We have seen Mary Baker Eddy with one eye," the one eye being the radar dish antenna. See Philip J. Hilts, "Last Rites for a 'Plywood Palace' That Was a Rock of Science," The New York Times, March 31, 1998, and my previous article about Lee Davenport.
  6. L. H. Woods, "On the Silicon Oxide Bands," Physical Review, vol. 63, nos. 11-12 (June, 1943), pp. 426-430.

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