|
The First Atomic Pile: An Eyewitness Account Revealed by Some of the
Participants and Narratively Recorded On December 2, 1942, man first initiated a self-sustaining nuclear chain reaction, and controlled it. Beneath the West Stands of Stagg Field, Chicago, late in the afternoon of that day, a small group of scientists witnessed the advent of a new era in science. History was made in what had been a squash-rackets court. Precisely at 3:25 P.M., Chicago time, scientist George Weil withdrew the cadmium-plated control rod and by this action man unleashed and controlled the energy of the atom. As those who witnessed the experiment became aware of what had happened, smiles spread over their faces and a quiet ripple of applause could be heard. It was a tribute to Enrico Fermi, Nobel prizewinner, to whom, more than to any other person, the success of the experiment was due. Fermi, born in Rome, Italy, on September 29, 1901, had been working with uranium for many years. . . . Awarded the Nobel prize in 1938, he and his family went to Sweden to receive the prize. The Italian Fascist press severely criticized him for not wearing a Fascist uniform and failing to give the Fascist salute when he received the award. The Fermis never returned to Italy. From Sweden, having taken most of his personal possessions with him Fermi proceeded to London and thence to America where he remained {until his death in 1954}. The modern Italian explorer of the unknown was in Chicago that cold December day in 1942. 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 construction of this crude-appearing but complex pile (the name which has since been applied to all such devices) 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 were doing it, they'd be sure we are." In relation to the atomic bomb program, of which the Chicago pile experiment was a key part, the successful result, reported on December 2, formed on more piece for the jigsaw puzzle which was atomic energy. Confirmation of the chain reactor studies was an inspiration to the leaders of the bomb project, and reassuring at the same time because the Army's Manhattan Engineer District had move ahead on many fronts. Contract negotiations were under way to build production-scale nuclear chain reactors, land had been acquired at Oak Ridge, Tennessee, and millions of dollars had been obligated. Three years before the December 2 experiment it had been discovered that when an atom of uranium was bombarded by neutrons, and uranium atom sometimes was split, or fissioned. Later it had been found that when an atom of uranium fissioned, additional neutrons were emitted and became available for further reaction with other uranium atoms. These facts implied the possibility of a chain reaction, similar in certain respects to the reaction which is the source of the sun's energy. The facts further indicated that if a sufficient quantity of uranium could be brought together under the proper conditions, a self-sustaining chain reaction under given conditions is known as the critical mass, or more commonly, the "critical size" of the particular pile. . . . Further impetus to the work on a uranium reactor was given by the discovery of plutonium at the Radiation Laboratory, Berkeley, California, in March, 1940. This element, unknown in nature, was formed by uranium 238 capturing a neutron, and thence undergoing two successive changes in atomic structure with the emission of beta particles. Plutonium, it was believed, would undergo fission as did the rare isotope of uranium, U235. Meanwhile at Columbia Fermi and Walter Zinn and their associates were working to determine operationally possible designs of a uranium chain reactor. Among other things,they had to find a suitable moderating material slow down the neutrons traveling at relatively fast velocities. In July, 1941, experiments with uranium were started to obtain measurements of the reproduction factor (called "k"), which was the key to the problem of a chain reaction. If this factor could be made sufficiently greater than 1, a chain reaction could be made to take place in a mass of material of practical dimensions. If it were less than 1, no chain reaction could occur. Since impurities in the uranium and in the moderator would capture neutrons and make them unavailable for further reactions, and since neutrons would escape from the pile without encountering uranium 235 atoms, it was not known whether a value for "k" greater than unity could ever be obtained. Fortunate it was that the obtaining of a reproduction factor greater than 1 was a complex and difficult problem. If Hitler's scientists had discovered the secret of controlling the neutrons and had obtained a working value of "k," they would have been well on the way toward producing an atomic bomb for the Nazis. One of the first things that had to be determined was how best to place the uranium in the reactor. Fermi and Leo Szilard suggested placing the uranium in a matrix of the moderating material, thus forming a cubical lattice of uranium. This placement appeared to offer the best opportunity for a neutron to encounter a uranium atom. Of all the materials which possessed the proper moderating qualities, graphite was the only one which could be obtained in sufficient quantity of the desired degree of purity. The study of graphite-uranium lattice reactors was started at Columbia in July, 1941, but after reorganization of the uranium project in December, 1941, Arthur H. Compton was placed in charge of this phase of the work, under the Office of Scientific Research and Development, and it was decided that the chain reactor program should be concentrated at the University of Chicago. Consequently early in the 1942 the Columbia and Princeton groups were transferred to Chicago, where the Metallurgical Laboratory was established. . . . At Chicago, the work on sub-critical size piles was continued. By July, 1942, the measurements obtained from these experimental piles had gone far enough to permit a choice of design for a test pile of critical size. At that time, the dies for the pressing of the uranium oxides were designed by Zinn and ordered made. It was a fateful step, since the entire construction of the pile depended upon the shape and size of uranium pieces. It was necessary to use uranium oxides because metallic uranium of the desired degree of purity did not exist. Although several manufacturers were attempting to produce the uranium metal, it was no until November that any appreciable amount was available. . . . Although the dies for the pressing of the uranium oxides were designed in July, additional measurements were necessary to obtain information about controlling the reaction, to revise estimates as to the final critical size of the pile, and to develop other data. Thirty experimental sub-critical piles were constructed before the final pile was completed. Meantime, in Washington, Vannevar Bush, Director of the Office of Scientific Research and Development, had recommended to President Roosevelt that a special Army Engineer organization be established to take full responsibility for the development of the atomic bomb, During the summer, the Manhattan Engineer District was created, and in September, 1942, Major General L.R. Groves assumed command. Construction of the main pile at Chicago started in November. The project gained momentum, with machining of the graphite blocks, pressing of the uranium oxide pellets, and the design of instruments. Fermi's two "construction" crews, one under Zinn and the other under Herbert L. Anderson, worked almost around the clock, V.C. Wilson headed the instrument work. Original estimates as to the critical size of the pile were pessimistic. As a further precaution, it was decided to enclose the pile in a balloon-cloth bag which could be evacuated to remove the neutron-capturing air. . . . The bag was hung with one side left open; in the center of the floor a circular layer of graphite bricks was placed. This and each succeeding layer of the pile was braced by a wooden frame. Alternate layers contained the uranium. By this layer-on-layer construction a roughly spherical pile of uranium and graphite was formed. Facilities for the machining of graphite bricks were installed in the West Stands. Week after week this shop turned out graphite bricks. This work was done under the direction of Zinn's group, by skilled mechanics led by millwright August Knuth. In October, Anderson and his associates joined Zinn's men. Describing this phase of the work, Albert Wattenberg, one of Zinn's group, said: "We found out how coal miners feel. After eight hours of machining graphite, we looked as if we were made up for a minstrel. One shower would only remove the dust in the pores of our skin would start oozing. Walking around the room where we cut the graphite was like walking on a dance floor. Graphite is a dry lubricant, you know, and the cement floor covered with graphite dust was slippery." Before the structure was half completed; measurements indicated that the critical size at which the pile would become self-sustaining was somewhat less than had been anticipated in the design. 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 they eagerly awaited moment drew nearer, the scientist gave greater and greater attention to details, the accuracy of measurements, and exactness of their construction work. . . . At Chicago during the early afternoon of December 1, tests indicated that critical size was rapidly being approached. At 4PM 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 would become self-sustaining. Both had agreed, however, that should measurements indicate the reaction would become self-sustaining 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. That night the word was passed to the men who had worked on the pile that the trial run was due the next morning. About 8:30 on the morning of Wednesday, December 2, the group began to assemble in the squash court. At the north end of the squash court was a balcony about ten feet above the floor of the court. Fermi, Zinn, Anderson, and Compton were grouped around instruments at the east end of the balcony. The remainder of the observers crowded the little balcony. R.G. Noble, one of the young scientists who worked on the pile, put it this way: "The control cabinet was surrounded by the "big wheels"l; the "little wheels' had to stand back." On the floor of the squash court, just beneath the balcony, stood George Weil, whose duty it was to handle the final control rod. In the pile were three sets of control rods. One set was automatic and could be controlled from the balcony. Another was an emergency safety rod. Attached to one end of this rod was a rope running through the pile and weighted heavily on the opposite end. The rod was withdrawn from the pile and tied by another rope to the balcony. Hilberry was ready to cut this rope with an ax should sometimes unexpected happen, or in case the automatic safety held the reaction in check until withdrawn the proper distance. Since this demonstration was new and different from anything ever done before, complete reliance was not placed on mechanically operated control rods. Therefore a "liquid control squad," composed of Harold Lichtenberger, W. Nyter, and A.C. Graces, stood on a platform above the pile. They were prepared to flood the pile with cadmium-salt solution in case of mechanical failure of the control rods. Each group rehearsed its part of the experiment. At 9:45 Fermi ordered the electrically operated control rods withdrawn. The man at the controls threw the switch to withdraw them. A small motor whined. All eyes watch the lights which indicated the rod's position. But quickly the balcony group turned to watch the counters, whose clicking stepped up after the rods were out. The indicators of these counters resembled the face of a clock, with "hands" to indicate neutron activity within the pile. Shortly after ten o'clock, Fermi ordered the emergency rod, called "Zip," pulled out and tied. "Zip Out," said Fermi. Zinn withdrew "Zip" by hand and tied it to the balcony rail. Weil stood ready by the "vernier" control rod which was marked to show the number feet and inches which remained within the pile. At 10:37 Fermi, without taking his eyes off the instruments, said quietly: "Pull it to 13 feet, George." The counters clicked faster. The graph pen moved up. All the instruments were studied, and computations were made. "This is not it," said Fermi. "The trace will go to this point and level off." He indicated a spot on the graph. In a few minutes the pen came to the indicated point and did not go above that point. Seven minutes later Fermi ordered the rod out another foot. Again the counters stepped up their clicking, the graph pen edged upwards. But the clicking was irregular. Soon it leveled off, as did the thin line of the pen. The pile was not self-sustaining -- yet. At 11 o'clock, the rod came out another six inches; the result was the same: an increase in rate, followed by the leveling off. Fifteen minutes later, the rod was farther withdrawn and at 11:25 was moved again. Each time the counters speeded up, the pen climbed a few points. Fermi predicted correctly every movement of the indicators. He knew the time was near. He wanted to check everything again. The automatic control rod was reinserted without waiting for its automatic feature to operate. The graph line took a drop, the counters slowed abruptly. At 11:35, the automatic safety rod was withdrawn and set. The control rod was adjusted and "Zip" was withdrawn. Up went the counters, clicking, clicking, faster and faster. It was the clickety-click of a fast train over the rails. The graph pen started to climb. Tensely, the little group watched and waited, entranced by the climbing needle. Whrrrump! As if by a thunderclap, the spell was broken. Every man froze -- then breathed a sigh of relief when he realized the automatic rod had slammed home. The safety point at which the rod operated automatically had been set too low. "I'm hungry," said Fermi. "Let's go to lunch." Perhaps, like a great coach, Fermi knew when his men needed a "break." It was strange "between halves" respite. They got no pep talk. They talked about everything else but the "game." The redoubtable Fermi, who never says much, had even less to say. But he appeared supremely confident. His "team" was back on the squash court at 2:00PM. Twenty minutes later, the automatic rod was reset and Weil stood ready at the control rod. "All right, George," called Fermi, and Weil moved the rod to a predetermined point. The spectators resumed their watching and waiting, watching the counters spin, watching the graph, waiting for the settling down, and computing the rate of rise of reaction from the indicators. At 2:50 the control rod came out another foot. The counters nearly jammed, the pen headed off the graph paper. But this was not it. Counting ratios and the graphs scale had to be changed. "Move it six inches," said Fermi at 3:20. Again the change -- but again the leveling off. Five minutes later, Fermi called: "Pull it out another foot." Weil withdrew the rod. "This is going to do it, "Fermi said to Compton, standing at his side. "Now it will become self-sustaining. The trace will climb and continue to climb. It will not level off." Fermi computed the rate of rise of the neutron counts over a minute period. He silently, grim-faced, ran through some calculations on his slide rule. In about a minute he again computed the rate of rise. If the rate was constant and remained so, he would know the reaction was self-sustaining. His fingers operated the slide rule with lightning speed. Characteristically, he turned the rule over and jotted down some figures on its ivory back. Three minutes later he again computed the rate of rise in neutron count. The group on the balcony had by now crowded in to get an eye on the instruments, those behind craning their necks to be sure they would know the very instant history was made. In the background could be heard William Overbeck calling out the neutron count over an annunciator system. Leona Marshall (the only girl present), Anderson, and William Sturm were recording the readings from the instruments. By this time the click of the counters was too fast for the human ear. The clickety-click was now a steady brrrrr. Fermi, unmoved, unruffled, continued his computations. "I couldn't see the instruments," said Weil. "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." Fermi closed his slide rule. "The reaction is self-sustaining," he announced quietly, happily. "The curve is exponential." The group tensely watched for twenty-eight minutes while the world's first nuclear chain reactor operated. The upward movement of the pen was leaving a straight line. There was no change to indicate a leveling off. This was it. "O.K., "Zip' in," called Fermi to Zinn, who controlled that rod. The time was 3:53PM. Abruptly, the counters slowed down, the pen slid down across the paper. It was all over. Man had initiated a self-sustaining nuclear reaction -- and then stopped it. He had released the energy of the atom's nucleus and controlled that energy. Right after Fermi ordered the reaction stopped, the Hungarian born theoretical physicist Eugene Wigner presented him with a bottle of Chianti wine. All through the experiment Wigner had kept this wine hidden behind his back. Fermi uncorked the wine bottle and sent out for paper cups so all could drink. He poured a little wine in all the cups, and silently, solemnly, without toasts, the scientists raised the cups to their lips -- the Canadian Zinn, Compton, Anderson, Hilberry, and a score of others. They drank to success -- and to hope they were the first to succeed. A small crew was left to straighten up, lock controls, and check all apparatus. As the group filed from the West Stands, one of the guards asked Zinn: "What's going on, Doctor, something happen in there?" The guard did not hear the message which Arthur Compton was giving James B. Conant at Harvard, by long distance telephone. Their code was not prearranged. "The Italian navigator has landed in the New World," said Compton. "How were the natives?" asked Conant. "Very friendly." |