CP-1 at 80: The events of December 2, 1942

December 1, 2022, 12:01PMNuclear News

On the eve of the 80th anniversary of the first controlled nuclear chain reaction, Nuclear Newswire is back with the second of three prepared #ThrowbackThursday posts of CP-1 coverage from past issues of Nuclear News.

On November 17, we surveyed the events of 1942 leading up to the construction of Chicago Pile-1, an assemblage of graphite bricks and uranium “pseudospheres” used to achieve and control a self-sustaining fission reaction on December 2, 1942, inside a squash court at the University of Chicago’s Stagg Field.

Today we’ll pick up where we left off, as construction of CP-1 began on November 16, 1942.

A photo of CP-1 scientists taken on December 2, 1946, on the steps of Eckhart Hall at the University of Chicago, to commemorate the fourth anniversary of the achievement (published in the November 2002 issue of Nuclear News). Back row (left to right): Norman Hilberry, Samuel Allison, Thomas Brill, Robert Nobles, Warren Nyer, and Marvin Wilkening. Middle row: Harold Agnew, William Sturm, Harold Lichtenberger, Leona Woods Marshall, and Leo Szilard. Front row: Enrico Fermi, Walter Zinn, Albert Wattenberg, and Herbert Anderson. (Photo: ANL)

“They’d think we are crazy”: “The First Pile,” published in the November 2002 issue of NN (p. 34) was an edited and adapted version of an essay by the same title written in the fall of 1946 by Corbin Allardice and Edward R. Trapnell, two public information officers for the Atomic Energy Commission who conducted postwar interviews with more than a dozen scientists involved in the project to produce the first narrative account of the experiment. They describe the scene:

An outsider looking into the squash court where Fermi was working would have been greeted by a strange sight. In the center of the 30-by-60-foot room, shrouded on all but one side by a gray balloon cloth envelope, was a pile of black bricks and wooden timbers, square at the bottom and a flattened sphere on top. Up to half of its height, its sides were straight. The top half was domed, like a beehive. During the construction of this crude appearing but complex pile (the name that was applied to all such devices for the first few years of the atomic age, but which gradually gave way to “reactor”) the standing joke among the scientists working on it was: “If people could see what we're doing with a million-and-a-half of their dollars, they'd think we are crazy. If they knew why we are doing it, they'd know we are.”

Just over two weeks of construction: The pile was constructed around the clock, with a team divided into two 12-hour shifts: a day shift under Walter Zinn and a night shift under Herb Anderson. Allardice and Trapnell described it thus:

Day after day the pile grew toward its final shape. And as the size of the pile increased, so did the nervous tension of the men working on it. Logically and scientifically they knew this pile would become self-sustaining. It had to. All the measurements indicated that it would. But still the demonstration had to be made. As the eagerly awaited moment drew nearer, the scientists gave greater and greater attention to details, the accuracy of measurements, and exactness of their construction work. Guiding the entire pile construction and design was the nimble-brained [Enrico] Fermi, whose associates described him as “completely self-confident but wholly without conceit.”

So exact were Fermi's calculations, based on the measurements taken from the partially finished pile, that days before its completion and demonstration on December 2nd, he was able to predict almost to the exact brick the point at which the reactor would become self-sustaining.

But with all their care and confidence, few in the group knew the extent of the heavy bets being placed on their success. In Washington, the Manhattan District had proceeded with negotiations with E. I. duPont de Nemours and Company to design, build, and operate a plant based on the principles of the then unproved Chicago pile. The $350,000,000 Hanford Engineer Works at Pasco, Wash., was to be the result.

At Chicago during the early afternoon of December 1st, tests indicated that critical size was rapidly being approached. At 4:00 p.m., Zinn’s group was relieved by the men working under Anderson. Shortly afterwards, the last layer of graphite and uranium bricks was placed on the pile. Zinn, who remained, and Anderson made several measurements of the activity within the pile.

They were certain that when the control rods were withdrawn, the pile would become self-sustaining. Both had agreed, however, that should measurements indicate the reaction would become self-sustaining when the rods were withdrawn, they would not start the pile operating until Fermi and the rest of the group could be present. Consequently, the control rods were locked and further work was postponed until the following day.

Neutron intensity during startup of the first self-sustaining chain reaction, on December 2, 1942, as recorded by a galvanometer (published in the November 2002 issue of Nuclear News). (Source: ANL)

December 2: For a description of the momentous day, we revisit “CP-1: The Story of the First Nuclear Reactor,” published in the November 1992 issue of NN (p. 67) to mark the 50th anniversary:

The team began to gather at around 8:30 on the morning of Wednesday, December 2. On the balcony at the end of the squash court, Fermi, Zinn, Anderson, and [Arthur H.] Compton grouped around the instrumentation. More people crowded onto the balcony. . . .

Three types of control rods had been devised. One rod was electrically operated. The second was the emergency rod, called the “Zip” rod. A third rod, manually operated, was marked in feet and inches to indicate how far it was extending into (or out of) the pile. . . .

At about 9:45, Fermi ordered the electrically operated rod removed. Everyone switched attention to the instrumentation—neutron counters and a nearby recorder to trace neutron activity within the pile. Shortly after 10:00, the Zip rod was removed. Then [George L.] Weil began pulling out the manual rod. He pulled the rod out in six-inch or one-foot intervals, while Fermi calculated the neutron activity on his six-inch pocket slide-rule. They worked slowly and deliberately. But at 11:35, just as the counters began to work faster and faster, the automatic rod slammed home. The safety point at which it would automatically shut down had been set too low.

It seemed time for a lunch break.

The team reassembled at the site at 2:00 p.m. The automatic rod was reset, and Weil stood ready at the manual rod. Once again, he withdrew the rod foot by foot, then in smaller increments. Just before 3:30, as Weil drew the rod out one more foot, Fermi announced, “This is going to do it. Now it will become self-sustaining.”

Those on the balcony strained to see the recorder. Fermi worked with his slide rule for a time, then finally closed it. “The reaction is self-sustaining,” he stated. “The curve is exponential.”

The world's first man-made nuclear chain reaction operated for 28 minutes. The upward movement of the pen of the recorder left a straight line—there was no indication that it would level off. Estimates would later indicate that the pile reached a power level of half a watt. But power wasn't important; what was important was that the chain reaction had been demonstrated.

More to come: Look for part 3 next week, when Throwback Thursday will return to discuss the legacy of CP-1. As always, ANS members can explore the Nuclear News archives for free.

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