freedom of design permits a larger fraction of the volume to be occupied by cooling pipes, etc. Small size is not necessarily an advantage when a great deal of energy is desired from a chain reaction. The difficulty of removing a large amount of energy rapidly from a small space is a well-known engineering problem of heat transfer. The power available from a chain reaction is limited essentially only by the rate that energy can be removed. As sources of power, therefore, large reactors have the advantage that more energy can be removed from them in a given time. Small Energy Units It seems less likely that neutron reactors will soon be used for small power plants such as automobiles. The reactor itself can be made small but the necessary shield for the very dangerous gamma rays and neutrons would necessarily be a heavy and bulky structure. The usefulness of atomic energy for the production of power will in the near future be mainly restricted to large power stations in which the thick radiations shields will not be a limiting factor. Small Chain Reactors as Research Tools If energy is not the prime concern, small chain reactions can be made by mixing separated U-235 or plutonium with water to slow down the neutrons, thereby making them more effective in producing fission. With such purified materials the capture of neutrons by impurities is not very great. Therefore a considerable loss of neutrons by leakage can be tolerated. A chain reaction can thus be maintained in a relatively small room if fissionable materials are available in a purified form. Amounts of material which are only a fraction of the amount needed for an atomic bomb can be used. These reactions are not useful for large energy release because of the difficulties of the removal of considerable quantities of energy from a small space. These small neutron reactors are very useful research tools in that they generate a copious supply of neutrons. The effects of the neutrons on various substances and living things can be studied. As a result of intense neutron irradiation, it is quite likely that some materials may change their physical properties in a useful way, e.g., become harder. The small chain reactor can also be used to produce small amounts of radioactive isotopes. In addition, many radioactive materials last such a short time that it may be very desirable to produce them on the spot in the laboratory in which they will be used. Radioactive Isotopes The radioactive fragments from the fission process may be very useful. Radioactive elements can be used in medicine, in scientific research, and in industrial processes as tracer elements. Some of these applications are described in the fourth, fifth, and sixth articles of this volume. Suggestions Made as to "Denaturing" For peacetime utilization of atomic energy, suggestions have been made as to "denaturing" of fissionable materials. If uranium-235 is used to make an atomic bomb, it must be fairly free of uranium-238 or the bomb will not operate. Material in which the amount of uranium-235 has been enriched but not sufficiently for use in a bomb could nevertheless be used in a chain reaction to generate energy or for other peaceful uses. The uranium-235 could then be said to be denatured against its efficient use in a bomb and will be available for the operation of certain chain-reacting units. There is another isotope of plutonium in addition to Pu-239 whose presence would render that material less suitable for bomb manufacture and which could be used in a similar manner to denature Pu-239. In order to separate these isotopes, that is, in order to remove the denaturing isotope, plants similar to but not as extensive as those in Oak Ridge would be necessary. It is to be specially emphasized that a system of control based solely on denaturing could not provide adequate safety against military utilization. It would be disastrous to arouse false hopes in this connection. FUTURE DEVELOPMENTS It might be well to point out in what ways the advance of knowledge can be expected to alter the atomic energy situation. It may be that there are more fissionable materials able to maintain a chain reaction. It is, however, certain that they would all be very heavy elements. It would almost certainly remain true that the primary ores would be those of thorium and uranium as there are no other naturally occurring atoms which are sufficiently heavy, except for some exceedingly rare elements. It is physically possible to release nuclear energy from the very light elements. Such a release is believed to exist in the sun and other stellar bodies. At present the possibility of using reactions of this type seems quite remote. There is no reason to expect that the methods of separating the uranium isotopes are not subject to considerable improvement. Separation may, in the future, be accomplished with much less extensive apparatus and with less power consumption. The greatest development will probably come in the applications of chain reactors and the radioactive isotopes which they produce. Only a small start has been made in applying these materials and the radiations which they emit. 2. Production and Utilization of Uranium-235 and Plutonium-239 By K. D. Nichols Brigadier General, Corps of Engineers Director of Inorganic Research, Mallinckrodt Chemical Works Introduction As far as our present information goes, it is possible to use only a few kinds of atoms as sources of atomic energy and, of these, only a small number are reported to have been studied extensively; these are atoms of the elements uranium and plutonium. This memorandum will be confined to a brief, elementary, and essentially nontechnical discussion of atomic energy derived from atoms of these elements. Mention is also made of the role of thorium in the production of atomic energy. Isotopes Before proceeding further with our discussion, it is necessary to say a few words about "isotopes". Many elements occur in more than one variety. These varieties are indistinguishable one from the other in almost all respects; i.e., their chemical properties are practically identical; they have almost identical physical characteristics, such as melting point, hardness, density, and so forth. However, there are differences between the innermost structures of the different varieties of the atoms which may result in profound differences in their behavior from the point of view of atomic energy. Different varieties of the same element are referred to as "isotopes"; thus we shall speak of "isotopes" of uranium and of other elements. These isotopes are conventionally designated by a number written after the name or chemical symbol of the element. Thus we refer to uranium-235 and uranium-238, or more simply to U-235 or U-238, etc. Key Substances There are two key substances now available in quantity which can be used as a starting point for the generation of atomic power. These are: Uraninum-235 (U-235) One of these key substances occurs in nature, namely, uranium–235. The other is a synthetic element made by the action of neutrons on uranium-238, which occurs in nature. Let us examine some of the characteristics of uranium-235, which is the cornerstone for the generation of atomic power. Uranium as it occurs in nature is actually a mixture of three different kinds of uranium-that is, a mixture of three isotopes of uranium, namely, U-238, U-235, and U-234. 99.3% of the total uranium content of any naturally occurring ore is U-238. About 0.7% of the uranium content is U-235; only traces of U-234 are present. U-235, even as it occurs in nature admixed with a preponderance of U-238, has the property of being able to generate neutrons, as described in the preceding article. This it does when it is put in a machine called a "pile", about which more will follow later. Uranium238 does not have this property. U-235 is the only naturally occurring substance available in quantity which will serve as a source for the generation of neutrons in any appreciable amounts. Let us see what can be done with these neutrons. U-238 plus neutrons gives plutonium-239 Thus starting with uranium-238 which occurs in nature and utilizing neutrons generated from U-235, a new synthetic key element can be prepared. It should be understood that it is really a coincidence that naturally occurring uranium contains both U-235 and U-238. This is a fortunate circumstance in some ways when it comes to the synthesis of Pu-239, since U-235 is a substance from which neutrons can be obtained and U-238 is the raw material needed to react with neutrons to make plutonium. It is also believed possible to utilize thorium in the preparation of key substances. The importance of thorium in the problems related to atomic energy is summarized in a later section.1 Peacetime vs. Military Uses of Atomic Energy Some of the peacetime uses of atomic energy may be: A. Production of power. For footnotes see bibliography on p. 35. B. Synthesis of elements. C. Source of rays, such as neutrons and gamma rays. A. Production of atomic bombs. B. Production of chemical or radioactive warfare materials. A key substance may be used to release atomic energy for peacetime purposes or may be used to provide the "explosive" for atomic bombs. Thus it becomes obvious that, to some extent at least, if one is enjoying the peacetime benefits of atomic energy derived from a key substance, he also de facto has in his possession to a greater or less extent the material and plants necessary for the production of atomic bombs. A more complete discussion of some of the features of the production of key substances is presented in the following sections. Raw Materials A. Uranium Although uranium is reported not to be a very rare material, relatively few deposits of commercial importance have been discovered up to the present time. Deposits of commercial interest from which uranium has been recovered in the past occur in the Belgian Congo, Canada, Czechoslovakia, Portugal, and the United States. In addition, small amounts of uranium have been produced in England, Madagascar, and the U.S.S.R.2 Uranium is always associated in nature with radium in the ratio of about one gram of radium to 3 long tons of uranium. The principal interest in uranium ores prior to 1941 was due to the radium content. In fact, in the 1930's, it was rumored that uranium itself was a drug on the market. In 1941, radium sold for about $25,000 per gram; uranium sold for about one to two dollars per pound in the form of commercial uranium products, such as black uranium oxide or yellow sodium uranate. The principal commercial use of uranium has been the coloring of ceramic ware." Naturally occurring uranium, as we have seen, is actually a mixture of three different kinds of uranium; namely, U-238, U-235, and U-234. The relative amount of each variety or isotope present never changes regardless of where the uranium is mined. Of the three varieties, U-235 and U-238 find application in the production of atomic energy. B. Thorium Thorium has been reported to be more abundant than uranium; it has been recovered commercially from deposits of monazite sand, the principal thorium containing ore. |