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From the 1946 book, Dawn Over Zero.
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THE end of 1942 was marked by four outstanding developments in the Atomic Bomb Project. One of these, of course, was the spectacular production of the first self-sustaining chain reaction, which led directly to the second—the authorization to proceed with the construction of the plutonium plants at Hanford.
At the same time it was decided to experiment with several different methods and thus lessen the possibility of ending up in a blind alley. Accordingly, orders were issued to proceed with the construction in Tennessee of the ectromagnetic and gaseous diffusion plants for concentrating U.235.
The electromagnetic method is based on the principle that electrically charged atoms, named ions, describe a curved path as they move through a magnetic field.
Atoms of different mass and the same electric charge, when moving with the same speed through the magnetic field, follow different circles, and the path of the heavier atoms, because of their greater momentum, has a longer radius than the path of the lighter atoms.
The atoms are most separated after traversing half of their respective circles, at which point they can be collected in specially designed containers.
The gaseous diffusion method is based on the principle that at any given temperature the molecules of a lighter gas move faster than the molecules of a heavier gas.
The apparatus developed for the electromagnetic separation of U.235 is known as the calutron, a name representing a contraction of California University’s cyclotron. It is a gigantic offspring of the prewar mass spectrometer, the laboratory apparatus used mainly for determining the relative abundance of the isotopes of the elements, as well as for detecting the existence of isotopes.
But while basically the calutron is designed on the principle of the mass spectrometer, the two are about as far apart as a modern transport airliner is from the ‘Wright brothers’ first airplane.
The calutron consists of four principal parts:
• a source for the production and acceleration of uranium ions (that is, uranium atoms stripped of an electron and thus carrying a positive charge of electricity);
• a large magnet to make the ions follow a curved path of different radii;
• collectors in which the separated ions are deposited;
• a tube, chamber, or tank, pumped down to low pressure, in which the ions travel their different semicircular paths from the source to the collector. The tank is placed between the pole faces of the magnet.
Solid or liquid compounds containing the atoms to be separated must first be vaporized.
The ions are produced in the source by an electric arc running through the vapor. They are then accelerated by a high-voltage system and made to travel at constant speed along curved paths in the magnetic field.
Upon arrival at the collectors the ions are neutralized; that is, they give up their electric charge, and solid material is deposited.
A high vacuum must be maintained in the tank in which the ions travel, to reduce the number of gas molecules present, as these would collide with the ions and deflect them from their path, resulting in contamination and the collection of less U.235.
Credit for this remarkable transmutation of a laboratory instrument into a giant industrial plant, which in the course of three years increased the output of U.235 billions of times, is mainly due to Professor Lawrence.
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|Y-12 electromagnetic separation plant at Oak Ridge Tennessee.|
The electromagnetic method had been ruled out on the basis of three principal limitations that were believed to be inseparable from the process itself and therefore impossible to overcome. These limitations, it was universally believed, would make it impossible to obtain U.235 in appreciable quantities. Attention was therefore focused on other methods.
Dr. Lawrence had at his disposal the largest magnets in the world, incorporated in his giant cyclotrons, and he is the sort of man to whom the word “impossible” constitutes a challenge. So in November 1941, without any financial assistance from any Government agency, he proceeded to rip his original cyclotron apart and put its 85-ton magnet to use in a giant mass spectrometer, alongside of which all existing apparatus of its kind were mere toys.
Within three months Dr. Lawrence had produced an amount of U.235 thousands of times greater than had ever been concentrated before, at a ten times faster rate. This quantity was sufficient to be useful in determining the properties of the material and to demonstrate that the electromagnetic method of separation held possibilities of ultimate success.
During the course of this preliminary work the Research Corporation made a grant of $5,000 to the University of California Radiation Laboratory, directed by Dr. Lawrence. In December 1941 the National Defense Research Committee, headed by President Conant, offered a Government contract to underwrite this phase of the research, and the grant from Research Corporation was returned.
After the preparation of the first sample, experiments were pushed day and night to increase the output of the equipment. By March 1942, alterations had raised the production rate for short periods by a factor of 500.
By May 26, 1942, the great 184-inch magnet, largest in the world, was turned on for the first time on the concentration of U.235. Its completion as the world’s largest cyclotron had been indefinitely postponed some months previously in favor of its conversion into a giant mass spectrometer, the greatest by far ever built. The Rockefeller Foundation made a grant of $60,000 for the conversion, as a contribution to the Radiation Laboratory’s war research.
This giant showed, by midsummer of 1942, that the electromagnetic method was practical, and that a large enough electromagnetic plant could have a critical bearing on the war and inestimable implications for the future.
By the fall of 1942, plans for a small pilot plant to be built at Berkeley, California, were approved. It soon became evident, however, that time would not permit this conventional intermediary development between laboratory and production plant.
Plans for the pilot plant were therefore abandoned and all efforts reoriented toward the single purpose of building a large industrial plant and putting it in operation in the shortest possible time.
Since the plant was to require a tremendous amount of electric power, it was decided to locate it in the Tennessee Valley. Stone & Webster was selected to design and build it. General Electric, Westinghouse, and Allis-Chalmers were the major suppliers of equipment. The Tennessee Eastman Corporation was picked to operate the plant.
These companies established offices at the University of California Radiation Laboratory early in 1943. Their scientists and engineers worked in the closest conjunction with the laboratory’s physicists, chemists, engineers, and shop technicians to translate data, procedures, techniques, and equipment into a practical functioning plant design.
Tests of the mechanical and electrical equipment for the plant’s installations were carried on at the Radiation Laboratory simultaneously with the construction of the plant buildings. The results of these tests suggested many modifications of equipment to give smoother plant operation and increased output.
Building the plant involved problems of construction and design never encountered before, since it is the first and only one of its kind in the world, and there was no time even to construct a small plant that could serve as I a model. It has 270 buildings of a permanent nature. Its peak operating personnel totaled 24,000.
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|A few of the many clerical staff at Oak Ridge Tennessee.|
Since the electrified atoms to be separated must travel in a very high vacuum, high-speed vacuum pumps such
as never existed before had to be created. After much research, Distillation Products Company developed pumps that produce and maintain extremely low atmospheric pressures. No vacuum pumps operating at such high speeds and such low pressures were in use at the time in any other process.
Great difficulties also had to be overcome in designing extremely delicate control equipment for high-voltage current. Rectifier units had to be designed capable of supplying a certain amperage at a very high voltage. These requirements are far above those encountered in radio broadcasting and similar high-voltage power applications.
In the process for separating the uranium atoms the requirements limit to approximately 0.04 per cent of the mean voltage the maximum permissible variation in the value of high voltage supplied to the apparatus. Such precise regulation of high amounts of power at high voltages, to a load that intermittently acts as a short circuit, had never before been attained.
Because of the great scarcity of copper, and because time was more precious than gold, 27,680,000 pounds of silver, worth $400,000,000, were borrowed from the Treasury Department for use as winding coils and bus-bars for the multitudinous magnets. The solid-silver winding coils have a total length of more than 900 miles.
Silver is as good a conductor of electricity as copper and is not harmed by the passage of current.
The silver will be returned to the Treasury when conditions warrant. Meantime this great plant for producing the material for the atomic bomb is, among other things, also a branch office of the Treasury.
All the research involving the electromagnetic method for concentrating U.235 was carried out under Government contract at the University of California under the direction of Professor Lawrence. At the peak of the research, in August 1943, Dr. Lawrence was assisted by a staff of 1,266, including 465 laboratory and research workers, and 365 employed in plant operation.
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The gaseous diffusion plant for separating U.235 is shaped in the form of a U and covers an area of several million square feet. The method is based on the principle governing the diffusion of gases known as Graham’s law, as elaborated on by Lord Rayleigh in 1896.
According to Graham’s law, the rates of diffusion of different gases through a porous medium are, under similar conditions, inversely proportional to the square roots of the molecular weights of the gases.
For example, if one gas, A, has a molecular weight of 9, and another gas, B, has a molecular weight of 16, the rate of diffusion of gas A through a porous medium, as compared with the rate of diffusion of gas B, will be in the ratio of 4 volumes of gas A (the lighter gas) to 3 volumes of gas B.
When a mixture of gas A and gas B is allowed to diffuse through a suitable porous medium, under ideal conditions, the ratio of gas A to gas B, in the portion first passing through the medium, will thus be 1.33 times greater than the original ratio of A to B.
By subjecting the portion first diffused to the same process, a gas mixture in which the ratio of A to B is increased by a second factor of 1.33 can be obtained. In fact, the process can be repeated at will, finally achieving any desired ratio of A to B.
In a practical plant, however, the separation factor in this particular example will not reach the ideal value of 1.33, but may go as high as 1.2. As an example of such a plant, let us assume that the ratio of gas A to gas B is 1 to 50, and we want to change it to 1,000 to 1. Assuming that we obtain an increase in ratio of 1.2 of A to B at each stage, we would require a plant in which the diffusion process is repeated sixty times.
When our scientists and engineers first considered the possibility of separating U.235 from U.238 by the gaseous diffusion method, they were confronted with a host of obstacles that at first seemed insurmountable.
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|K-25 gaseous diffusion plant is about one mile long.|
Applying our example to the case of uranium will illustrate the magnitude of the separation problem. Since uranium itself is not a gas, some gaseous compound of uranium had to be used. The only uranium compound, at that time known that could be converted into a suitable stable gas was uranium hexafluoride, a combination of one atom of uranium and six atoms of fluorine, which would corrode practically anything with which it comes in contact.
And not only is this gas highly reactive, but it is actually a solid at room temperature and atmospheric pressure.
For these reasons a study of other gaseous compounds of uranium was urgently undertaken. As insurance against failure in this search for alternative gases, it was necessary to continue work on uranium hexafluoride, particularly on divising methods for producing and circulating this gas.
It was realized from the beginning that a plant for the concentration of U.235 by the gaseous diffusion method had to be of enormous dimensions, regardless of whether uranium hexafluoride or some other type of uranium gas was used.
This can be illustrated by taking uranium hexafluoride as an example, though it would apply to other uranium gases as well.
The molecular weight of uranium 238 hexafluoride is 352, whereas the uranium-fluorine gas containing six atoms of fluorine and one atom of uranium 235 has a molecular weight of 349. Since, according to Graham’s law, the rate of diffusion of the gas containing U.235, as compared with the gas containing U.238, would be inversely proportional to the square roots of their molecular weights—that is, in the ratio of the square root of 352 (18.76) to the square root of 349 (18.68)—the increase of the concentration of the U.235 hexafluoride would be by a factor of only 1.0043. Under actual operating conditions this value is even smaller.
This is, indeed, a very small enrichment factor. Hence, to bring it up to the desired level, it became necessary to design and construct a gigantic cascade in which the gas to be separated is made to pass through thousands of successive stages, each stage enriching the proportion of the U.235 gas over the preceding stage, the enriched mixture passing on to the next stage, where it is further enriched. No such plant for separation of gases had ever been designed or even conceived.
One of the principal problems that had to be solved before the plant could be built involved the development of a suitable porous medium, or barrier, through which the uranium-gas mixture had to be diffused in a manner to allow a greater proportion of U.235 to pass through than of U.238.
It had been established that the pores of the barrier through which a gas mixture is diffused must be considerably smaller than the average distance a gas molecule travels before it collides with another gas molecule, a distance known as the “mean free path.”
At atmospheric pressure the mean free path of a molecule is of the order of a ten-thousandth of a millimeter, or a tenth of a micron.
To ensure true diffusive flow of the gas, the diameter of the myriad holes in the barrier must be less than one tenth of the mean free path—that is, about one hundredth of a micron, or about four ten-millionths of an inch.
Such a barrier must have billions of holes of this size or smaller. Furthermore, these holes must not enlarge or plug up as the result of direct corrosion or dust coming from corrosion elsewhere in the system. The barrier must be able to withstand the pressure “head” of one atmosphere. It also had to be of a type that could be manufactured in large quantities and with uniform quality.
It was further realized that thousands of powerful pumps would be needed and thousands of kilowatts to operate them.
Also, that the whole circulating system would have to be made vacuum-tight and leak-proof, requirements presenting problems of a magnitude never faced before.
A new industry had to be developed to manufacture the porous barrier.
To satisfy the demands for power, a huge powerhouse was constructed, the largest initial single installation of its kind ever built.
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|Powerhouse for the K-25 gaseous diffusion plant.|
The scientific research work on the diffusion process was initiated by Professor Dunning, and was carried on in a large building in upper Manhattan, under a contract between Columbia University and the Office of Scientific Research and Development (OSRD), until May 1, 1943, when the work was taken over by the Manhattan Engineer District.
In 1942 the M. W. Kellogg Company was chosen to plan the large-scale plant. For these purposes that company created a special subsidiary, the Kellex Corporation, and placed P. C. Keith in charge of it. The Kellex Corporation not only planned and procured materials for the large-scale plant, but also carried on research and development in its Jersey City laboratories and with the Columbia group.
The plant was constructed by the J. A. Jones Construction Company, of Charlotte, North Carolina.
In January 1943 Carbide and Carbon Chemicals Corporation was selected as the operator of the plant. Its engineers soon began to play a large role not only in the planning and construction but also in the research work.