1. General Statement
Work to investigate the possibilities of utilizing the atomic energy of uranium for military purposes has been in progress since 1939, and a stage has now been reached when it seems desirable to report progress.
We should like to emphasize at the beginning of this report that we entered the project with more skepticism than belief, though we felt it was a matter which had to be investigated. As we proceeded we became more and more convinced that release of atomic energy on a large scale is possible and that conditions can be chosen which would make it a very powerful weapon of war. We have now reached the conclusion that it will be possible to make an effective uranium bomb which, containing some 25 lb of active material, would be equivalent as regards destructive effect to 1,800 tons of T.N.T. and would also release large quantities of radioactive substance, which would make places near to where the bomb exploded dangerous to human life for a long period. The bomb would be composed of an active constituent (referred to in what follows as 235U) present to the extent of about a part in 140 in ordinary Uranium. Owing to the very small difference in properties (other than explosive) between this substance and the rest of the Uranium, its extraction is a matter of great difficulty and a plant to produce 2–4 lb (1 kg) per day (or 3 bombs per month) is estimated to cost approximately ₤5,000,000, of which sum a considerable proportion would be spent on engineering, requiring labour of the same highly skilled character as is needed for making turbines.
In spite of this very large expenditure we consider that the destructive effect, both material and moral, is so great that every effort should be made to produce bombs of this kind. As regards the time required, Imperial Chemical Industries after consultation with Dr. Guy of Metropolitan-Vickers, estimate that the material for the first bomb could be ready by the end of 1943. This of course assumes that no major difficulty of an entirely unforeseen character arises. Dr. Ferguson of Woolwich estimates that the time required to work out the method of producing high velocities required for fusing (see paragraph 3) is 1–2 months. As this could be done concurrently with the production of the material no further delay is to be anticipated on this score. Even if the war should end before the bombs are ready the effort would not be wasted, except in the unlikely event of complete disarmament, since no nation would care to risk being caught without a weapon of such decisive possibilities.
We know that Germany has taken a great deal of trouble to secure supplies of the substance known as heavy water. In the earlier stages we thought that this substance might be of great importance for our work. It appears in fact that its usefulness in the release of atomic energy is limited to processes which are not likely to be of immediate war value, but the Germans may by now have realized this, and it may be mentioned that the lines on which we are now working are such as would be likely to suggest themselves to any capable physicist.
By far the largest supplies of Uranium are in Canada and the Belgian Congo, and since it has been actively looked for because of the radium which accompanies it, it is unlikely that any considerable quantities exist which are unknown except possibly in unexplored regions.
2. Principle Involved
This type of bomb is possible because of the enormous store of energy resident in atoms and because of the special properties of the active constituent of uranium. The explosion is very different in its mechanism from the ordinary chemical explosion, for it can occur only if the quantity of 235U is greater than a certain critical amount. Quantities of the material less than the critical amount are quite stable. Such quantities are therefore perfectly safe and this is a point which we wish to emphasize. On the other hand, if the amount of material exceeds the critical value it is unstable and a reaction will develop and multiply itself with enormous rapidity, resulting in an explosion of unprecedented violence. Thus all that is necessary to detonate the bomb is to bring together two pieces of the active material each less than the critical size but which when in contact form a mass exceeding it.
3. Method of Fusing
In order to achieve the greatest efficiency in an explosion of this type, it is necessary to bring the two halves together at high velocity and it is proposed to do this by firing them together with charges of ordinary explosive in a form of double gun.
The weight of this gun will of course greatly exceed the weight of the bomb itself, but should not be more than 1 ton, and it would certainly be within the carrying capacity of a modern bomber. It is suggested that the bomb (contained in the gun) should be dropped by parachute and the gun should be fired by means of a percussion device when it hits the ground. The time of drop can be made long enough to allow the aeroplane to escape from the danger zone, and as this is very large, great accuracy of aim is not required.
4. Probable Effect
The best estimate of the kind of damage likely to be produced by the explosion of 1,800 tons of T.N.T. is afforded by the great explosion at Halifax N.S. in 1917. The following account is from the History of Explosives. “The ship contained 450,000 lb. of T.N.T., 122,960 lb. of guncotton, and 4,661,794 lb. of picric acid wet and dry, making a total of 5,234,754 lb. The zone of the explosion extended for about 3/4 mile in every direction and in this zone the destruction was almost complete. Severe structural damage extended generally for a radius of 11/8 to 11/4 miles, and in one direction up to 1-3/4 miles from the origin. Missiles were projected to 3–4 miles, window glass broken up to 10 miles generally, and in one instance up to 61 miles.” In considering this description it is to be remembered that part of the explosives cargo was situated below water level and part above.
5. Preparation of Material and Cost
We have considered in great detail the possible methods of extracting the 235U from ordinary uranium and have made a number of experiments. The scheme which we recommend is described in Part 11 of this report and in greater detail in Appendix IV. It involves essentially the gaseous diffusion of a compound of uranium through gauzes of very fine mesh.
In the estimates of size and cost which accompany this report, we have only assumed types of gauze which are at present in existence. It is probable that a comparatively small amount of development would enable gauzes of smaller mesh to be made and this would allow the construction of a somewhat smaller and consequently cheaper separation plant for the same output.
Although the cost per lb. of this explosive is so great it compares very favourably with ordinary explosives when reckoned in terms of energy released and damage done. It is, in fact considerably cheaper, but the points which we regard as of overwhelming importance are the concentrated destruction which it would produce, the large moral effect, and the saving in air effort the use of this substance would allow, as compared with bombing with ordinary explosives.
One outstanding difficulty of the scheme is that the main principle cannot be tested on a small scale. Even to produce a bomb of the minimum critical size would involve a great expenditure of time and money. We are however convinced that the principle is correct, and whilst there is still some uncertainty as to the critical size it is most unlikely that the best estimate we can make is so far in error as to invalidate the general conclusions. We feel that the present evidence is sufficient to justify the scheme being strongly pressed.
As regards the manufacture of the 235U we have gone nearly as far as we can on a laboratory scale. The principle of the method is certain, and the application does not appear unduly difficult as a piece of chemical engineering. The need to work on a larger scale is now very apparent and we are beginning to have difficulty in finding the necessary scientific personnel. Further, if the weapon is to be available in say two years from now, it is necessary to start plans for the erection of a factory, though no really large expenditure will be needed till the 20-stage model has been tested. It is also important to begin training men who can ultimately act as supervisors of the manufacture. There are a number of auxiliary pieces of apparatus to be developed, such as those for measuring the concentration of the 235U. In addition, work on a fairly large scale is needed to develop the chemical side for the production in bulk of uranium hexafluoride, the gaseous compound we propose to use.
It will be seen from the foregoing that a stage in the work has now been reached at which it is important that a decision should be made as to whether the work is to be continued on the increasing scale which would be necessary if we are to hope for it as an effective weapon for this war. Any considerable delay now would retard by an equivalent amount the date by which the weapon could come into effect.
7. Action in U.S.
We are informed that while the Americans are working on the uranium problem the bulk of their effort has been directed to the production of energy, as discussed in our report on uranium as a source of power, rather than to the production of a bomb. We are in fact cooperating with the United States to the extent of exchanging information, and they have undertaken one or two pieces of laboratory work for us. We feel that it is important and desirable that development work should proceed on both sides of the Atlantic irrespective of where it may be finally decided to locate the plant for separating the 235U, and for this purpose it seems desirable that certain members of the committee should visit the United States. We are informed that such a visit would be welcomed by the members of the United States committees which are dealing with this matter.
8. Conclusions and Recommendations
(i) The committee considers that the scheme for a uranium bomb is practicable and likely to lead to decisive results in the war.
(ii) It recommends that this work be continued on the highest priority and on the increasing scale necessary to obtain the weapon in the shortest possible time.
(iii) That the present collaboration with America should be continued and extended especially in the region of experimental work.