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Uranium occurs principally in deposits of pitchblends found in Canada, the Belgian Congo, and Czechoslovakia and also as carnotite and autunite in the western United States. Other deposits of uranium are found at numerous points throughout the world. The production at the start of the war was reported as not far from a thousand tons of uranium content per year. Perhaps the prin
cipal difficulty which bears directly on the problem of control is that uranium is derived not only from uranium ores. In many cases, uranium is derived, or might be derived, as a by-product from the ores of other metals, mainly vanadium, and also by means of retreatment of the new and old mine wastes, mill tailings, and the wastes and slags of the chemical and metallurgical plants, etc. Thorium occurs principally in monazite sands in India, Brazil, the Dutch East Indies, Australia, and elsewhere.
It is to be expected that the search for new deposits of uranium and thorium and the introduction of extraction methods for low grade deposits will greatly increase the potential supplies.
Processes for the Separation of U-235
There are two principal processes by which concentrated nuclear fuel can be produced from uranium: by the separation of the isotope U-235, and by the burning of the U-235 content of natural uranium to produce plutonium.
In the separation process, the first stage is chemical purification and the preparation of special uranium compounds, among them gaseous uranium hexafluoride.
Separation may be effected in several ways: In the gaseous diffusion method, the uranium compound in a gaseous state is forced through porous barriers. The U-235 isotope, being very slightly lighter, can get through the barrier somewhat more rapidly than U-238. By a large number of repetitions of the process it is possible to secure material which is considerably enriched in U-235. This is the process reportedly used in the very large plant at Oak Ridge, Tennessee. A related process depends on the difference in rate at which the two isotopes can move through a liquid layer between a heated wall and an adjacent cool one. This process is called thermal diffusion and is also reported to have been used on a moderate scale at Oak Ridge. The third process, reportedly used on a large scale, is electromagnetic separation. Intense beams consisting of molecules of a gaseous compound are projected into a magnetic field which bends Engineering and Mining Journal, Sept. 1945, vol. 146, p. 80.
their paths. The molecules containing the lighter uranium atoms, that is U-235, turn more sharply than the heavier molecules containing U-238, with the result that the isotopes are separated. There is an important difference which distinguishes this process from the others: the electro-magnetic method can yield substantially complete separation of the isotopes as compared with a very gradual enrichment by the other processes, but does it only at the price of limited production. These methods require either many stages in the process of gradual enrichment (as in the diffusion methods) or many units operating in parallel to provide significant quantities (as in the electro-magnetic method).
The output of the isotope separation plant consists of uranium compounds enriched in U-235 content to a degree determined by their intended use. That is to say, the nuclear fuel is now less dilute and, after appropriate processing, is ready for use in industrial reactors or for the production of bombs.
Processes for the Production of Plutonium
Another process for the production of a pure nuclear fuel is to make plutonium from uranium. This involves a series of operations for the careful purification of the incoming uranium, the partial conversion of U-238 to plutonium in a primary reactor, the extraction and decontamination of the plutonium, and chemical and metallurgical processing to put it into usable form for reactors or bombs.
The chemical purification of the uranium compounds as they come from the refinery differs from usual industrial processes because of the extreme purity required. The many impurities which would absorb neutrons and thus quench the chain reaction of the reactor must be removed. The quantity of materials to be handled, and the rigid purification required (several impurities may not exceed one part in a million), combine to make the operation a difficult one. Other materials (known as "moderators") are required in the construction of the reactor, and these have also to be of extreme purity. An alternative to the production of very pure graphite or beryllium for this purpose is the production of heavy water, a difficult and large-scale operation in view of the quantities needed.
The primary reactor is a very large structure containing slugs of unenriched uranium metal interspersed through a moderator material such as graphite. In the reactor a chain reaction is set up, which consumes some of the U-235 and produces excess neu
trons. Some of these neutrons are absorbed by the U-238 and convert it to plutonium-239.
The uranium slugs, after use in the reactor, contain in addition to the unchanged uranium, plutonium, and a variety of radioactive elements formed as by-products of the chain reaction. The separation of plutonium, while reasonably straightforward chemically, is a highly specialized operation because the low initial concentration of plutonium poses special problems, because the entire process must be handled by remote control to avoid danger from radiation, and because it is difficult to dispose of the radioactive by-products present with the plutonium and the uranium.
The plutonium compounds from the extraction plant are converted into metallic plutonium in a chemical and metallurgical plant. The plutonium metal from this plant is ready for use either in an industrial reactor or for the production of bombs.
Scale of the Installations for Producing Nuclear Fuels
Some of the complexities in the production of Pu-239 and uranium enriched in U-235 have been indicated in the discussion of the processes. An indication of the huge scale of the operations required for bomb production has been given in published descriptions of the plants which were constructed by the United States during the war. Some data on cost and physical dimensions taken from the United States publications have been tabulated in Appendix 3. One striking aspect is the small fraction of the total cost required for the facilities for bomb fabrication, as distinct from the production of the nuclear fuels for use in the bomb.
The Production of Uranium-233
Refined thorium compounds, after careful purification, yield thorium which can be incorporated into a primary reactor for the production of U-233. Chemical separation and the final production of U-233 in pure form require processes analogous to those for plutonium.
Naturally occurring thorium is a single isotope and therefore no process of separation analogous to the separation of U-235 from uranium is involved. Since this isotope has not the property of "nuclear inflammability," a primary reactor using thorium alone would have no fuel to maintain the chain reaction. It can only be used therefore if a fuel material is added. In general, the available information indicates that processes involving thorium have been less thoroughly explored than those utilizing uranium. Nevertheless, in this report U-233 is included as one
of the nuclear fuels, even though the available information states only that it is theoretically possible to utilize U-233 in reactors and in bombs.
Atomic energy in amounts of importance for industrial activities is obtainable only from nuclear chain reactions which, like fire, are self-propagating.
Three materials are known which are useful as nuclear fuels for a self-sustaining chain reaction: only one of these (U-235) occurs in nature, constituting 0.7 percent of ordinary uranium; the other two (Pu-239 and U-233) can be produced by nuclear reactions from uranium and thorium, respectively.
Nuclear fuels may be burned at a controlled rate in a reactor, or in a run-away explosion as in a bomb.
The raw materials for atomic energy, uranium and thorium, occur in widely scattered ore deposits. Mining and processing of the ores are more or less conventional operations. Pre-war production was of the order of one thousand tons of uranium. content per year.
The partial separation of U-235 from uranium to provide a high grade nuclear fuel has reportedly been accomplished by gaseous diffusion, thermal diffusion, and electro-magnetic methods. All of the processes require many separate stages or units, and huge installations, for significant production of concentrated fuel.
Plutonium (Pu-239) is formed by a nuclear reaction from uranium, specifically, from the abundant isotope, U-238. Highly purified uranium and very pure graphite can be fabricated into a primary reactor which will maintain a chain reaction burning the U-235 fraction of the uranium, and, at the same time, utilize this reaction to produce Pu-239 from the U-238 fraction.
The production of significant quantities of Pu-239 requires very large installations comprising highly specialized chemical extraction plants in addition to the primary reactors.
Cost data for the United States atomic bomb installations indicate a comparatively minor outlay for bomb fabrication as compared with the cost of production facilities for U-235 and Pu-239.
Thorium, as a source of nuclear fuel (U-233), differs from uranium in containing no "inflammable" fraction corresponding to the U-235 in uranium. Thorium can, therefore, be used in a reactor as a source of U-233 only if nuclear fuels are also added.
Chapter 2: Utilization of Nuclear Fuels
The practical applications of atomic energy all depend upon the energy, radiations, and radioactive materials resulting from nuclear chain reactions. Three fuel materials can be obtained in practical quantities: U-235, Pu-239, and U-233. The character and scale of the equipment in which these fuels are utilized differs for different applications and can best be considered in terms of the intended uses.
The characteristic of a nuclear chain reaction which is perhaps most striking is the enormous quantity of energy released in the burning of comparatively small quantities of nuclear fuels. The consumption of a kilogram (about 2.2 pounds) per day of uranium-235 generates heat at the rate of approximately a million kilowatts. The same amount of heat could be obtained by burning about 3,000 tons of coal per day, enough to supply the power and light for a city of about a million. The use of atomic energy for the large-scale generation of electric power and for industrial heating are therefore challenging possibilities. Initially, at least, nuclear reactions will probably be used to generate heat which, by means of a heat exchanger, can provide steam for conventional turbo-generators producing electrical power. Many technical problems are involved in the use of atomic energy for power, but the development seems straightforward.
The large primary reactors which have been constructed for plutonium production generate a great deal of heat in the process, and might, by redesign, be used for power production. It seems probable that the size of reactors could be reduced by using concentrated fuel, although there are engineering limitations set by the rate at which heat can be removed from the structure, and the requirements for shielding personnel from the intense radiations. Published reports indicate that concrete walls more than five feet thick completely surround the large reactors at Hanford. Published estimates indicate that units for ships may be developed, but that smaller mobile units are unlikely on account of the bulk of the shielding required for the protection of personnel from the harmful effects of radiation.
It appears likely that reactors producing large quantities of power could be built which would not contain U-238 or thorium
* At 10 percent efficiency, the heat obtained by burning a kilogram of U-235 per day could be converted into about 100,000 kilowatts of electric power.