CP-1 GOES CRITICAL
(Met Lab, December 2, 1942)
Events > The Plutonium Path to the Bomb, 1942-1944

• Production Reactor (Pile) Design, 1942
• DuPont and Hanford, 1942
• CP-1 Goes Critical, December 2, 1942
• Seaborg and Plutonium Chemistry, 1942-1944
• Final Reactor Design and X-10, 1942-1943
• Hanford Becomes Operational, 1943-1944

While arrangements were proceeding for the construction of full-size plutonium production reactors, critical questions remained about their basic design.  The Italian physicist Enrico Fermi hoped to answer some of these questions with CP-1, his experimental "Chicago Pile #1" at the University of Chicago.  On December 2, 1942, after a series of frustrating delays, CP-1 first achieved a self-sustaining fission chain reaction.  After the end of the war, Leslie Groves, commander of the Manhattan Project, described the first time CP-1 went critical as the single most important scientific event in the development of atomic power.  

An outsider viewing preparations for this historic moment would have been greeted by a strange sight.  In an abandoned squash court under the west grandstand of the University of Chicago's Stagg Field there lay a huge oblong pile of black bricks and wooden timbers, shrouded on all sides but one by gray balloon material.  (Security regulations had forbidden the engineers from explaining to Goodyear what the Army wanted with a giant square balloon.)  Workers machined bricks for the pile until their faces were so covered with graphite dust that they looked like coal miners.  They sang together to pass the time during their 12-hour shifts, and afterwards it took them half an hour to remove the graphite dust from their skin.  The dust also made the cement floor dangerously slippery.  

What an outsider would not have understood -- but what the men and women who would operate it certainly did -- was how dangerous this pile of wood and bricks really was.  The wooden timbers supported a lattice structure that contained over six tons of pure uranium metal, along with 34 more tons of uranium oxide. The almost 400 tons of black bricks in the assembly were graphite, placed there to serve as moderators; the bricks in two of every three layers had a nodule of uranium inside each of them.  The presence of so much "moderating" material might have sounded comforting to outsiders until they learned that the moderators were there to increase the amount of fission produced by the uranium.  The only things preventing a fission chain reaction from growing within the pile were a series of cadmium rods inserted into the pile's side to absorb the free neutrons emitted by the radioactive uranium.  Unlike most reactors that have been built since, this first one had no radiation shielding and no cooling system of any kind.  Fermi had convinced Arthur Compton that his calculations were reliable enough to rule out a runaway chain reaction or an explosion, but, as the official historians of the Atomic Energy Commission later noted, the "gamble" remained in conducting "a possibly catastrophic experiment in one of the most densely populated areas of the nation!"

Daily the pile grew, brick by brick.  Tests on the early afternoon of December 1st indicated that it was very close to being ready.  By that evening, the scientists present were convinced that if they withdrew the cadmium control rods the fission chain reaction in the pile would be self-sustaining.  Final preparations for the first test began.  The next morning most of the observers found themselves crowded together onto a balcony where squash spectators had once stood, ten feet above the floor on the north end of the room.  Fermi, Compton, Walter H. Zinn, and Herbert L. Anderson were grouped around an instrument console at one end of the balcony; from there they could operate one set of control rods.  The only person on the floor of the squash court was George Weil, the man who would physically withdraw the final control rod.  If the reaction threatened to grow out of control Weil could re-insert his control rod, and an automatic control rod would also insert itself if the reaction reached a certain pre-set level.  In case of emergency, such as Weil becoming incapacitated or failure of the automatic control rod, Norman Hilberry stood on the balcony with an improbable nuclear safety device: an axe.  In an emergency, he would cut a rope that ran up to the balcony, releasing another emergency control rod into the pile.  The last line of defense consisted of a "liquid-control squad" that stood on a platform, ready to flood the pile with a cadmium-salt solution.  Taken together, these safety precautions were a strange combination of the high-tech and the ad hoc.  

After rehearsals, Fermi at 9:54 a.m. ordered the electrically-operated control rods removed.  All eyes turned to the array of instruments indicating the pace of the fission reaction within the pile.  Shortly after 10:00, Fermi ordered the emergency control rod removed and tied to its rope.  At 10:37, Fermi ordered Weil to pull all but thirteen feet of the final rod out of the pile.  The pace of the audible clicking from the neutron counters (similar to Geiger counters) increased.  Over the next few hours, the pile inched its way toward criticality, Weil gradually removing more and more of the final rod while Fermi monitored his array of instruments.  William Overbeck continued to call out the neutron count over a speaker system while Leona Marshall, William Sturm, and Anderson (see the photograph above) recorded the readings from the instruments.  (Marshall, a physics graduate student, was the only woman present.)  At 11:25, Fermi ordered the automatic and emergency control rods reinserted for a final safety check.  Ten minutes later these were both removed in order for the experiment to resume.  The neutron counters immediately resumed their clicking, the pace growing and growing until a sudden "whrrrump!" filled the room.  The automatic control rod had slammed home into the pile, having been set too low during the safety check.  While everyone present took a few deep breaths, Fermi calmly called for lunch.  

By 2:00 p.m., everyone had resumed their places.  Fermi resumed the slow process of inching toward criticality, more and more of the control rod appearing as Weil slowly withdrew it from the pile.  Finally Fermi said to Compton "this is going to do it.  Now it will become self-sustaining."  Everyone waited as Fermi ran through some final calculations on his slide rule, turning it over occasionally to jot down some figures on its ivory back.  By this time the clickety-click of the neutron counter had become a steady hum, too fast for the ear to count.  At 3:25 p.m., Weil slid the rod back one more time.  As Fermi completed one final calculation his face broke into a broad smile and he announced "the reaction is self-sustaining."  There was a quiet ripple of applause in the room.  For the first time in history, humans had unleashed and controlled the power of the atom.  The reactor was generating about half a watt, barely enough to power a small light bulb.  

• Production Reactor (Pile) Design, 1942
• DuPont and Hanford, 1942
• CP-1 Goes Critical, December 2, 1942
• Seaborg and Plutonium Chemistry, 1942-1944
• Final Reactor Design and X-10, 1942-1943
• Hanford Becomes Operational, 1943-1944

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Sources and notes for this page.

The text for this entry is based upon, and portions were taken directly from, a press release, written by the Press Relations Section of the Manhattan Project, November 26, 1946 (to be released December 1, 1946) entitled "Background Material for Use in Connection with Observance of the Fourth Anniversary, December Second, of the Scientific Event of Outstanding Significance in the United States Program of Development of Atomic Energy"; this release is available on the University Publications of America microfilm collection President Harry S. Truman's Office Files, 1945-1953 (Frederick, MD: 1989), Part 3, reel #41/42; the press release itself is a government document.  See also John F. Hogerton, ed., "Chicago Pile No. 1 (CP-1)," The Atomic Energy Deskbook (New York: Reinhold Publishing Corporation, 1963, prepared under the auspices of the Division of Technical Information, U.S. Atomic Energy Commission), 97-98.  For "gamble" quote, see the History Office publication: Richard G. Hewlett and Oscar E. Anderson, Jr., The New World, 1939-1946: Volume I, A History of the United States Atomic Energy Commission (Washington: U.S. Atomic Energy Commission, 1972), 109. The terms "atomic pile" and "nuclear reactor" refer to the same thing.  The term "pile" was more common during early atomic research, and it was gradually replaced by "reactor" in the later years of the Manhattan Project and afterwards.  In this web site, the phrase "pile (reactor)" is used to refer to early, experimental piles, and "reactor (pile)" is used to refer to later production reactors, which had more elaborate controls and in general more closely resembled post-war reactors.  Much as the term "pile" gradually gave way to "reactor," "atomic" was gradually replaced by "nuclear."   The painting of CP-1 going critical and the drawing of the pile by itself are both courtesy the National Archives.  The photograph of the construction of CP-1 is courtesy Argonne National Laboratory (ANL); it is reprinted in Rachel Fermi and Esther Samra, Picturing the Bomb: Photographs from the Secret World of the Manhattan Project (New York: Harry N. Abrams, Inc., Publishers, 1995), 103.  Click here for more information on the photograph of "Met Lab" alumni.  The photograph of Enrico Fermi is courtesy the Department of Energy (via the National Archives).  The data printout is reproduced from Hewlett and Anderson, The New World, between pages 112 and 113.  The photograph of the Chianti is courtesy ANL.

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