of the nuclear fuels, even though the available information state only that it is theoretically possible to utilize U-233 in reactors and in bombs.

Summary

Atomic energy in amounts of importance for industrial activ ties 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-233, 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 of units, and huge installations, for significant production of cor

centrated fuel.

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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.

Power

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.

from which new nuclear fuel would be produced. Such reactors would be consumers only of nuclear fuel which would have to be produced elsewhere.

Reactors using so-called "denatured" fuel material will be considered in Chapter 3.

A possible complication in the operation of atomic power plants lies in the cumulative effects of the materials left over from the chain reaction. These comprise a variety of elements, usually called fission products, some of which may absorb neutrons to such an extent that they would reduce the efficiency of the reactor, or even stop its operation. Decontamination plants, analogous to the plants for extracting the plutonium produced in primary reactors, may therefore be needed.

Information is lacking on another important aspect of the operation of atomic power plants: This is the question of whether or not enough additional fuel can be made in a power reactor to replace the original supply of nuclear fuel being consumed. If not, the world supply of nuclear fuels is measured by the amount of U-235 present in nature, extended a few-fold by such additional quantities of Pu-239 or U-233 as are generated in the consumption of U-235. On the other hand, if the regeneration of nuclear fuels can fully replace the original materials, then all of the U-238, one hundred and forty times as plentiful as U-235, and also the world's supply of thorium which is more plentiful than uranium, constitute potential nuclear fuels.

Published information indicates that the generation of electric power from atomic energy is still in the early developmental stage, with active work in progress at Oak Ridge, Tennessee. Costs may be competitive with electric power from coal, at least in some parts of the world.

Radiations

In addition to the energy generated as heat in the operation of a reactor, very intense radiations, particularly gamma rays and neutrons, are released. Such radiations from radium, large X-ray units, and cyclotrons, have been used in the past for radiation therapy. The intense radiations available from reactors should make them useful for these purposes. In industrial chemistry, new processes appear possible based on the chemical actions induced by intense and penetrating radiation. In physical research, the intense beams of radiation from reactors are already proving powerful tools.

Radioactive Isotopes

In addition to radiations, the operation of a reactor produces comparatively enormous quantities of radioactive materials, approximately a kilogram of fission products for each kilogram of nuclear fuel consumed. This material is equivalent in its effects to many thousand times the same amount of radium because of the more rapid rate at which many of its constituents disintegrate. Although the fission products consist of some thirty chemical elements of medium atomic weight, radioactive isotopes of most other elements can be produced as a result of neutron absorption. Since a great many neutrons exist inside and around an operating reactor, it is easy to use them in the preparation of substantial quantities of the desired isotopes. The availability of radioactive materials in adequate quantities should permit a renewed and very vigorous attack by tracer techniques on many research problems, notably in physiology, medicine, and the mechanism of chemical reactions. Some of the fission products may possibly replace radium in cancer treatment in the future. There is in addition the possibility of new techniques based on the selective localization in malignant tissues of chemical compounds containing radioactive elements.

Comparatively small reactors will usually be adequate, and in many cases most convenient, for applications requiring radiations and radioactive isotopes, although the latter may often be obtained as by-products from large reactors operated for other purposes. Atomic Bomb

The atomic bomb constitutes a highly specialized type of reactor in which the principal design requirement is that as much as possible of the nuclear fuel in the bomb shall be consumed in the very short time before the bomb bursts apart. Clearly, highly concentrated fuels are required to permit the most rapid combustion. Such materials will explode spontaneously as soon as the quantity of material in a single piece becomes large enough that the neutrons are effectively confined and utilized. The detonation of the bomb is then a matter of bringing together rapidly two or more pieces of fuel material which together exceed this critical size. Little more information than this has been released about the construction of atomic bombs, except that the critical size was predicted in 1941, within very wide limits, as more than

two and less than one hundred kilograms. In all probability, highly skilled personnel and specialized facilities are required for bomb production, but a large establishment does not appear to be necessary.

Estimates of the Amount of Power or the Number of Bombs Available

An estimate of the total electric power, or alternatively, of the number of bombs, which might result from the utilization of the world's production of uranium can hardly be made with any reasonable accuracy. The present production of uranium, the amount of material required for a single bomb, and the fraction of U-238 (or thorium) which can be utilized for power are all figures which are not available. However, the annual production of uranium in 1939 can be estimated at approximately one thousand tons (uranium content in ores) on the basis of data in the Engineering and Mining Journal for September 1945.

One might, as an example, assume that one thousand tons of uranium per year are available for the production of electric power. One thousand tons of natural uranium contain about seven thousand kilograms of U-235. This amount of U-235, used in power plants having an over-all efficiency of ten percent, could provide two million kilowatts of electric power for one year. If all of the uranium (and thorium) can be used, the amount of power available would be several hundred million kilowatts. In this connection, it may be of interest to quote a passage from the report of the Lilienthal Board: "We have examined in some detail the technical problems of making available heat and power on the scale of present world consumption from controlled nuclear reactors. We see no significant limitations on this development, either in the availability or in the cost of the fundamental active materials."

As an alternative example, if one were to assume that one thousand tons of natural uranium, containing approximately seven thousand kilograms of U-235, were available each year for the making of bombs, then using the limits on critical size given above as the amount of U-235 required for a bomb, the number of bombs which could be produced from all of the available uranium would be between 70 and 3,500 per year.

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* Report of the National Academy of Science, quoted in sec. 4.49 of “Atomic Energy for Military Purposes" by H. D. Smyth.

'A Report on the International Control of Atomic Energy, sec. II, chap. 3, par. 10 [Department of State publication 2498).