THE FIRST PILE by Corbin Allardice and Edward R
Total Page：16
File Type：pdf, Size：1020Kb
THE FIRST PILE by Corbin Allardice and Edward R. Trapnell (Originally written in 1946, this was the first compre hensive narrative account of the construction and operation of the world's first nuclear reactor. Though popular in approach and style, the story was written after extensive consultations with a number of the scientists directly associated with the great experi ment, much of it being constructed from their personal recollections of what happened on 2 December 1942. The account is reprinted here by permission of the United States Atomic Energy Commission.) On 2 December 1942, man first initiated a self- The modern Italian explorer of the unkown was sustaining nuclear chain reaction, and controlled it. in Chicago that cold December day in 1942. An out Beneath the West Stands of Stagg Field, Chicago, sider looking into the squash court where Fermi was late in the afternoon of that day, a small group of sci working would have been greeted by a strange sight. entists witnessed the advent of a new era in science. In the centre of the 30 by 60 foot room, shrouded on History was made in what had been a squash-rackets all but one side by a grey balloon cloth envelope, was court. a pile of black bricks and wooden timbers, square at the bottom and a flattened sphere on top. Up to half Precisely at 3. 25 p. m., Chicago time, scien of its height, its sides were straight. The top half tist George Weil withdrew the cadmium-plated control was domed, like a beehive. During the construction rod and by his action man unleashed and controlled the of this crude appearing but complex pile (the name energy of the atom. which has since been applied to all such devices) the As those who witnessed the experiment became standing joke among the scientists working on it was: aware of what had happened, smiles spread over their "If people could see what we're doing with a million- faces and a quiet ripple of applause could be heard. and-a-half of their dollars, they'd think we are crazy. It was a tribute to Enrico Fermi, Nobel Prize winner, If they knew why we are doing it, they'd be sure we to whom, more than to any other person, the success are. " of the experiment was due. In relation to the fabulous atomic bomb pro gramme, of which the Chicago Pile experiment was Fermi, born in Rome, Italy, on 29 September a key part, the successful result reported on 2 Dec 1901, had been working with uranium for many years. ember formed one more piece for the jigsaw puzzle In 1934 he bombarded uranium with neutrons and pro which was atomic energy. Confirmation of the chain duced what appeared to be element 93 (uranium is reactor studies was an inspiration to the leaders of element 92) and element 94. However, after closer the bomb project, and reassuring at the same time, examination it seemed as if nature had gone wild; sev because the Army's Manhattan Engineer District had eral other elements were present, but none could be moved ahead on many fronts. Contract negotiations fitted into the periodic table near uranium - where were under way to build production-scale nuclear Fermi knew they should have fitted if they had been chain reactors, land had been acquired at Oak Ridge, the transuranic elements 93 and 94. It was not until Tennessee, and millions of dollars had been obligated. five years later that anyone, Fermi included, realized he had actually caused fission of the uranium and that Three years before the 2 December experiment, these unexplained elements belonged back in the it had been discovered that when an atom of uranium middle part of the periodic table. was bombarded by neutrons, the uranium atom some times was split, or fissioned. Later, it had been Fermi was awarded the Nobel Prize in 1938 for found that when an atom of uranium fissioned, addi his work on transuranic elements. He and his family tional neutrons were emitted and became available for went to Sweden to receive the prize. The Italian further reaction with other uranium atoms. These Fascist press severely criticized him for not wearing facts implied the possibility of a chain reaction, sim a Fascist uniform and failing to give the Fascist salute ilar in certain respects to the reaction which is the when he received the award. The Fermis never re source of the sun's energy. The facts further indica turned to Italy. ted that if a sufficient quantity of uranium could be From Sweden, having taken most of his personal brought together under the proper conditions, a self- possessions with him, Fermi proceeded to London and sustaining chain reaction would result. This quantity thence to America where he has remained ever since. of uranium necessary for a chain reaction under given 41 conditions is known as the critical mass, or more Einstein who had found a haven at Princeton's Insti commonly, the "critical size" of the particular pile. tute for Advanced Studies. Bohr came to America, For three years the problem of a self-sustaining but the principal item he discussed with Einstein was chain reaction had been assiduously studied. Nearly the report of Meitner and Frisch. Bohr arrived at a year after Pearl Harbour, a pile of critical size was Princeton on 16 January 1939. He talked to Einstein finally constructed. It worked. A self-sustaining nu and J. A. Wheeler who had once been his student. clear chain reaction was a reality. From Princeton the news spread by word of mouth to neighbouring physicists, including Enrico Fermi at Years of scientific effort and study lay behind Columbia. Fermi and his associates immediately this demonstration of the first self-sustaining nuclear began work to find the heavy pulse of ionization which chain reaction. The story goes back at least to the could be expected from the fission and consequent fall of 1938 when two German scientists, Otto Hahn release of energy. and Fritz Strassmann, working at the Kaiser Wilhelm Institute in Berlin found barium in the residue mate rial from an experiment in which they had bombarded Before the experiments could be completed, uranium with neutrons from a radium-beryllium however, Fermi left Columbia to attend a conference source. This discovery caused tremendous excite on theoretical physics at George Washington Univer ment in the laboratory because of the difference in sity in Washington, D. C. Here Fermi and Bohr ex atomic mass between the barium and the uranium. changed information and discussed the problem of Previously, in residue material from similar experi fission, Fermi mentioned the possibility that neutrons ments, elements other than uranium had been found, might be emitted in the process. In this conversation, but they differed from the uranium by only one or two their ideas of the possibility of a chain reaction began units of mass. The barium differed by approximately to crystallize. 98 units of mass. The question was, where did this element come from? It appeared that the uranium Before the meeting was over, experimental atom when bombarded by a neutron had split into two confirmation of Meitner and Frisch's deduction was different elements each of approximately half the obtained from four laboratories in the United States mass of the uranium. (Carnegie Institution of Washington, Columbia, Johns Hopkins, and the University of California). Later it Before publishing their work in the German sci was learned that similar confirmatory experiments entific journal Die Naturwissenschaften, Hahn and had been made by Frisch and Meitner on 15 January. Strassmann communicated with Lise Meitner who, Frederic Joliot-Curie in France, too, confirmed the having fled the Nazi-controlled Reich, was working results and published them in the 30 January issue of with Niels Bohr in Copenhagen, Denmark. the French scientific journal, Comptes rendus. Meitner was very much interested in this phe On 27 February 1939, the Canadian born Walter nomenon and immediately attempted to analyse math H. Zinn and Leo Szilard, a Hungarian, both working ematically the results of the experiment. She at Columbia University, began their experiments to reasoned that the barium and the other residual ele find the number of neutrons emitted by the fissioning ments were the result of a fission, or breaking, of uranium. At the same time, Fermi, and his associ the uranium atom. But when she added the atomic ates, Herbert L. Anderson and H. B. Hanstein, masses of the residual elements, she found this total commenced their investigation of the same problem. was less than the atomic mass of uranium. The results of these experiments were published side- by-side in the April edition of the Physical Review and There was but one explanation: the uranium showed that a chain reaction might be possible since fissioned or split, forming two elements each of ap the uranium emitted additional neutrons when it proximately half of its original mass, but not exactly fissioned. half. Some of the mass of the uranium had disap peared. Meitner and her nephew O.R. Frisch These measurements of neutron emission by suggested that the mass which disappeared was con Fermi, Zinn, Szilard, Anderson, and Hanstein were verted into energy. According to the theory advanced highly significant steps toward a chain reaction. in 1905 by Albert Einstein in which the relationship „ Further impetus to the work on a uranium re of mass to energy was stated by the equation E = mc actor was given by the discovery of plutonium at the (energy is equal to mass times the square of the speed Radiation Laboratory, Berkeley, California, in March of light), this energy release would be of the order of 1940.