What It Was Like to Witness the World’s First Self-Sustained Nuclear Chain Reaction

Scientists achieved the first self-sustained nuclear chain reaction 75 years ago on Dec. 2, 1942, on an underground squash court beneath the University of Chicago’s Stagg Field. The brains behind the discovery was Enrico Fermi, a leader of the Manhattan Project who had won the 1938 Nobel Prize in Physics for work that led to the discovery of nuclear fission. Below is a narrative of that fateful day, from David N. Schwartz’s new biography of Fermi, The Last Man Who Knew Everything:

On the morning of Wednesday, December 2, 1942, Chicago was in the grip of a cold snap. The previous day the high was thirty-two degrees Fahrenheit, but when Fermi awoke the next morning the temperature had dropped to zero. Leona Libby accompanied him to the pile, where they took some measurements of reactivity to compare with the measurements taken the night before. Anderson, who had been up late, arrived next, and the three made the short walk to Libby’s apartment, where she cooked pancakes. Then they returned to the squash court to begin the day’s historic work.

The process began about midmorning. The crowd overlooking from the balcony grew as the morning progressed and eventually included Zinn, Anderson, Szilard, Wigner, and several dozen other physicists who played a role in the pile’s construction. At 9:45 a.m., Fermi instructed three of the safety rods to be withdrawn. Immediately, the counters started clicking in response to neutron production, and Fermi watched as the production leveled off. Shortly after 10:00 a.m., having satisfied himself that his predictions to this point were correct, Fermi called out “Zip!” Zinn, who was responsible for the zip rod, now withdrew it completely and set it above the pile, hanging on its rope. Again the clicking of the counters began to race. Again the clicking leveled off.

Fermi instructed George Weil, who was manning the last control rod in the pile, to pull it “to thirteen feet,” halfway out of the pile. The rod had been marked carefully to allow its operator to know exactly how much of it remained inside the reactor. The counters rose dramatically in activity. Fermi not only knew that the pile was subcritical but also was able to point to the spot on the graph where the pen would begin to level off. Level off it did. After a few minutes of calculation, Fermi instructed Weil to withdraw the rod another foot. The counters picked up, but then leveled off again. Fermi fiddled with his slide rule, doing some quick calculations, and according to Wattenberg, “seemed pleased” that the neutron production was developing in the way Fermi predicted it would. Weil and Fermi repeated this process, six inches at a time. “Every time the intensity leveled off, it was at the values [Fermi] had anticipated for that position of the control rod,” Wattenberg later recalled. At 11:25 a.m., the intensity of the neutron production increased to the point at which an adjustment of the instrumentation scale was required, an adjustment Fermi oversaw with Wilson. As a test, Fermi asked for the safety rods to be reinserted in the pile, and the intensity dropped dramatically. He then asked Zinn to remove all the safety rods, and the reactor started up again, the counters ticking wildly for a moment before rather suddenly, at 11:35 a.m., a loud crash startled those watching. The instrumentation had recorded a level of intensity that tripped the mechanism holding a safety rod in place; the rod had come crashing down into the pile, bringing the reactivity to a complete halt. It was, however, a level of intensity that was still below criticality.

When Fermi understood the cause of the crash, he smiled with relief and announced to the group, “I’m hungry, it’s time for lunch.” All the control rods were reinserted in the pile, locked in, and the group braved the freezing cold to walk to the main university dining room at Hutchinson Commons. In the splendor of a glorious Gothic replica of the Great Hall at Oxford’s Christ Church College, they had a quiet lunch and spoke of anything except what they had just witnessed.

They returned to the squash court at about two o’clock, and Fermi asked the team to return the safety rods up to their positions prior to lunch. Over the next hour or so, Weil withdrew the control rod gradually, according to Fermi’s instructions. Each time, the instruments would chatter away and then level off. At about 3:25 p.m. Fermi ordered a full foot of additional withdrawal. As Weil followed Fermi’s instructions, Fermi turned to Compton. “This is going to do it,” he assured Compton, who joined the group after lunch, with a wide-eyed Craw- ford Greenewalt in tow. For Greenewalt, this was a moment he would remember for the rest of his life. “Now it will become self-sustaining,” Fermi explained. “The trace will climb and continue to climb,” he said, referring to the line being drawn across the graph drum attached to the counters. “It will not level off.”

He was right. The counters picked up speed and this time did not level off. The clicking became a high-pitched whine. The line traced on the graph paper moved ever upward. Fermi took some measurements, fiddled with his slide rule again.

“I couldn’t see the instruments,” Weil later said. “I had to watch Fermi every second, waiting for orders. His face was motionless. His eyes darted from one dial to another. His expression was so calm it was hard. But suddenly, his whole face broke into a broad smile.”

“The reaction is self-sustaining,” Fermi announced. “The curve is exponential.” Still he did not order the reactor shut down. Not yet. He continued to study the graph and the instruments, monitoring the exponential production of neutrons. He gave no indication of next steps.

Richard Watts, a member of Wilson’s instrumentation team, memorialized the moment in the log book: “We’re cookin’!”

The several dozen witnesses grew increasingly tense, but Fermi was calm. The team atop the reactor, led by Sam Allison, was alert to any sign of danger, ready at a moment’s notice to flood the pile with the cadmium solution. At one point Leona Libby approached Fermi and whispered, “When do we become scared?” Fermi didn’t answer. His attention was entirely on the instruments.

Twenty-eight minutes into criticality, he decided he had witnessed enough. “Zip!” he called out to Zinn, and Zinn dutifully released the safety rods into the pile. At 3:53 p.m., the world’s first controlled fission chain reaction came to a complete halt.

The room was quiet. Fermi was elated, but said little. He was silent even as Leona Libby accompanied him home at the end of the day. Wigner had brought a bottle of chianti for the occasion. Those in attendance shared the wine out of paper cups, Fermi first. Later most of those present signed the straw cover surrounding the bottle. Perhaps the most famous chianti in history, it now resides in the archives at Argonne Lab. No toasts were made, no dramatic speeches offered. History had been made, but the future looked grim. Everyone understood that this was a major step toward the development of a fission weapon. Szilard recounts that he shook Fermi’s hand in congratulations but warned that this would go down as a black day in human history.

Excerpted from The Last Man Who Knew Everything: The Life and Times of Enrico Fermi, Father of the Nuclear Age by David N. Schwartz. Copyright © 2017 by David N. Schwartz. Available from Basic Books, an imprint of Perseus Books, a division of PBG Publishing, LLC, a subsidiary of Hachette Book Group, Inc.