The Acheson-Lilienthal Report


The Problem Has Definable Boundaries

This problem of building security against catastrophic use of atomic energy is not one without boundaries. This is important. For if the fact were that tomorrow or a year hence we might reasonably expect atomic energy to be developed from clay or iron or some other common material then it is apparent that the problem of protection against the misuse of energy thus derived would be vastly more difficult. But such is not the case. The only scientific evidence worthy of regard makes it clear that in terms of security uranium is indispensable in the production of fissionable material on a scale large enough to make explosives or power. The significance of this fact for effective international control will appear.

As a first step in our work, we undertook a study, with the help of the qualified members of our group, aimed at an understanding of the well-established principles of nuclear physics upon which, among other things, the conclusion is based that uranium is indispensable as the primary source of atomic energy. These scientific principles are not familiar, but they are capable of being appreciated by laymen. Because the specific content of any system of control will be importantly influenced by the scientific principles and facts, we would emphasize the importance of an appreciation of them. For present purposes, we shall state in greatly simplified terms certain conclusions that are drawn from a full technical account of this subject.

Until 1942 the energy which man had learned to control for his useful purposes derived almost exclusively (except for water, wind, and tidal power) from chemical reactions. For practical purposes, chemical combustion was the main source of energy. This energy is the product of rearrangements of electrons in the periphery of atoms and results from the change in chemical structure which occurs in the process of combustion.

"Atomic energy," as that term is popularly used, refers to the energy that results from rearrangements in the structure of atomic nuclei of elements. There are very strong forces which hold such nuclei together and account for their stability. The nature of these forces is not adequately understood, but enough is known about their behavior, not only to make it certain that the energy of an atomic

bomb or an atomic power plant comes from the work done by these forces when the structure of atomic nuclei is rearranged, but also to explain one major fact of decisive importance: Only in reactions of very light nuclei, and in reactions of the very heaviest, has there ever been, to the best of our knowledge, any large-scale release of atomic energy. The reasons for this can be given in somewhat oversimplified form.

As to the light nuclei--The forces which hold all nuclear particles together are attractive. When lighter nuclei combine to make heavier ones, and in particular when the lightest nucleus of all, that of hydrogen, is combined with another light nucleus, these attractive forces release energy. This combination of light elements to form somewhat heavier ones occurs in the stars and of the sun; in the sun effectively what happens is that hydrogen nuclei combine to form the more stable nuclei of helium. Almost all sources of the energy used on earth come to us from the sunlight which this great atomic energy plant provides. But the conditions which make this plant possible are very special, and we do not know how to duplicate them on earth; we may very well never learn to do so. They depend on maintaining matter deep in the interior of the sun at very high temperatures--many millions of degrees. The nuclear reactions themselves provide the energy necessary to keep the matter hot; and it is kept from expanding and cooling by the enormous gravitational forces of attraction which hold the sun together and provide a sort of container in which this temperature and pressure can be maintained. For the foreseeable future the maintenance of such reactions on earth will not be possible; in the immediate future it is certainly not possible.

As to the heaviest nuclei-- Although nuclear reactions can be carried out in the laboratory for all nuclei, and although in some cases a given nuclear reaction may release energy even for nuclei of intermediate weight, the properties which make the large-scale release of such energy possible are peculiar, to the very light nuclei and to the very heaviest. And the very heaviest nuclei have a property shared by none of the other elements. These very heavy nuclei generate energy if they can be caused to split into lighter ones; this unique process is called "fission." Perhaps a dozen nuclear species are known which can be made to undergo fission; under more drastic treatment no doubt the list will be extended. But to make atomic energy takes more than the property of fission. The fission process itself must maintain itself or grow in intensity so that once it is started in a few nuclei a chain of reactions will be set up and a large part of the material will become potentially reacting. The agency which initiates this process is the neutron. In fission neutrons are emitted; and in certain nuclei bombardment by neutrons is enough to cause fission.

There are several substances for which this is true, but there is only one substance which occurs in nature with any significant abundance for which it is true--that substance is uranium. Uranium is the only natural substance that can maintain a chain reaction. It is the key to all foreseeable applications of atomic energy.

One may ask why there are so few materials which undergo fission, and why so few of these can maintain a chain reaction. The reason lies in the fact that only the heaviest nuclei are sufficiently highly charged to come apart easily, and that only the most highly charged of all are sufficiently susceptible to fission on neutron bombardment to maintain a chain reaction. It is not to be anticipated that this situation will be invalidated by further scientific discovery.

A word needs to be said about the role of thorium, which is slightly more abundant than uranium, and for which fission is also not too difficult to induce. Thorium cannot maintain a chain reaction, either itself or in combination with any other natural material than uranium. Nevertheless, it occupies an important position with regard to safeguards. The reason for this is the following: Without uranium, chain reactions are impossible, but with a fairly substantial amount of uranium to begin with and suitably large quantities of thorium a chain reaction can be established to manufacture material which is an atomic explosive and which can also be used for the maintenance of other chain reactions.

Absolute control of uranium would therefore mean adequate safeguard regarding raw materials. Yet, since any substantial leakage of uranium through the system of controls would make possible the exploitation of thorium to produce dangerous amounts of atomic explosive, provisions governing thorium should be incorporated in the system to compensate for possible margins of error in the control of uranium. The coexistence of uranium and thorium in some natural deposits makes this technically attractive.

There can be little hope of devising a successful scheme of control unless the problem can somehow be limited to the immediate future, by arrangements that have a reasonable prospect of validity for the next decade or two, and which contain sufficient flexibility to accommodate themselves to inevitably changing conditions. We believe that a system of control which disregards all materials except uranium and thorium satisfies these conditions. Indeed if a successful system of control can be commenced now, based upon these materials, and if the time should ever come when other materials lend themselves to the same activities, it should in fact be far easier to include them within the system than it will be to set up the initial control system with which we are now concerned.

Because the constituent raw materials of atomic energy can be limited to uranium and thorium, the control problem is further narrowed by the geological conditions under which uranium and thorium are found, and the fact that at present those elements have only restricted commercial significance. Although they are distributed with relative abundance throughout the world, and although it is clear that many sources beyond the known supplies will be discovered, it is apparently the view of the authorities that these elements occur in high concentrations only under very special geologic conditions. This would seem to mean that the areas which need to be surveyed, to which access must be had, and which would ultimately have to be brought under control, are relatively limited.