The Egg-Boiling Experiment


On Wednesday, December 2, 1942, Chicago’s weather turned frigid — ten degrees Fahrenheit and a howling wind. Underneath the unheated west stands of the University of Chicago’s football field a group of men, oblivious to the cold, worked diligently around a scaffolding that surrounded an incredible pile of bricks. The shape of the brick pile’s 57 layers resembled a doorknob: “A flattened rotational ellipsoid 25 feet wide at the equator and 20 feet high from pole to pole.” The first layer of bricks contained “dead” graphite: lacking uranium. The next two layers contained bricks drilled and filled with uranium. One layer dead and two alive alternated to the top of the pile. Ten slots remained open for the “control rods.”

Up on the balcony overlooking the doubles squash court where the brick pile stood was a short man with black hair that had receded evenly to the top of his head. He looked like a high school math teacher, but he was a world renown physicist. Winner of the 1938 Nobel Prize for physics, the 41-year-old Italian Enrico Fermi had created this incredible experiment in brick together with Leo Szilard a Hungarian physicist. Szilard was a leading figure in launching the Manhattan Project, America’s quest for the atomic bomb. The brick pile was a key to the bomb — an attempt to create a nuclear reactor.

The Fermi-Szilard brick pile known as “the egg-boiling experiment” was based on a theory that a nuclear chain reaction could be created and controlled. While Szilard felt unsure about the experiment’s success, he doubted some colleagues who feared the chain reaction might result in an explosion. Yet, the night before the experiment Szilard crossed the campus to Culver Hall to seek out his friend physiological psychologist Henrich Kluver. Szilard had eaten dinner, but he asked Kluver to join him for a second dinner, “just in case.” Kluver asked “Just in case of what?”

Over dinner, Szilard told Kluver about the next day’s experiment. Szilard believed the experiment might fail, but if it worked “too well” an explosion might result. Kluver asked Szilard if he doubted his colleagues conclusions. “Not at all,” Szilard answered. “But even the greatest theoretical physicists cannot be absolutely certain. So I felt that a second dinner was in order.”

On the morning of December 2, inquisitive Manhattan Project members lined the squash court’s balcony. Down on the floor, the scientists put on their graphite-stained lab coats and took their positions. Walter Zinn stood by his invention, the ZIP rod. This device would stop the chain reaction if the neutron intensity became to great. George Weil took up position at the last of the control rods. All eyes looked to Fermi. The Italian physicist ordered all the rods removed except the last one. Fermi told Weil to pull the last thirteen-foot rod out by increments. The neutron counters broke the silent air with loud and rapidly escalating clicks — the neutrons were bombarding the final rod. Fermi checked the counters and his slide rule; he looked up and said, “I’m hungry. Let’s go to lunch.” All the rods went back into the pile and everyone went to lunch.

When they returned at 2:20 P.M., the scientists resumed their positions; Szilard stood next to Fermi. At Fermi’s order Weil again pulled out all the rods but one. The last one was pulled out to the previous mark of seven feet. Thirty minutes crawled by as Fermi checked his calculations. Satisfied with the numbers, he said to Weil, “This time take the control rod out twelve inches. This is going to do it. Now it will become self-sustaining.”

Fermi’s assistant Herbert Anderson remembered: “At first you could hear the sound of the neutron counter, clickety-clack, clickety-clack. Then the clicks came more and more rapidly, and after a while they began to merge into a roar; the counter couldn’t follow anymore…. Suddenly Fermi raised his hand. ‘The pile has gone critical,’ he announced…. Then everyone began to wonder why he didn’t shut the pile off. But Fermi was completely calm. He waited another minute, then another, and then when it seemed that the anxiety was too much to bear, he ordered ‘ZIP in’.”

No one applauded, although Fermi grinned. One of the physicists pulled out a bottle of Bertolli Chianti that he had held in a paper bag behind his back. Fermi pulled the cork and everyone drank silently from paper cups. They all stared at the ugly brick pile that had just become the world’s first nuclear reactor.

After everyone departed, Fermi and Szilard found themselves standing alone. “I shook hands with Fermi,” Szilard recalled, “and I said, I thought this day would go down as a black day in the history of mankind.”