JF Ptak Science Books Post 280
(Note: both the MAUD and Frisch-Peierls documents are reprinted in the continued reading section below)
As stated in the first paragraph of what once was a supremely-guarded secret, the British MAUD Committee (Britain's initial steps towards the construction of an atomic weapon) :"Work(ed) 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..." There was quite allot of this work done between 1937 and 1939 (and even deep into 1940 the American physics journal Physical Review was still publishing papers they probably shouldn't have on this subject), and the MAUD committee sought to get a handle on it all.
As stated in the first paragraph of what once was a supremely-guarded secret, the British MAUD Committee (Britain's initial response to constructing an atomic weapon): "(w)ork(ed) 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..." There was quite allot of this work done between 1937 and 1939 (and even deep into 1940 the American physics journal Physical Review was still publishing papers they probably shouldn't have in the area). Then, coming out of UK's Birmingham in February 1940, Otto Frisch and Rudolf Peierls, considered the possibility of fast fission in uranium-235. They estimated the likely effects of the bomb, possible methods of assembly and made estimates on how feasible it was to separate the uranium-235.
The opening paragraphs of the 1940 Frisch-Peierls Memorandum. The text reads:
Strictly Confidential. Memorandum on the properties of a radioactive “super-bomb”.The attached detailed report concerns the possibility of constructing a “super-bomb” which utilizes the energy stored in atomic nuclei as a source of energy. The energy liberated in the explosion of such a super-bomb is about the same as that produced by the explosion of 1000 tons of dynamite. This energy is liberated in a small volume, in which it will, for an instant, produce a temperature comparable to that in the interior of the sun. The blast from such an explosion would destroy life in a wide area. The size of this area is difficult to estimate, but it will probably cover the centre of a big city.
In addition, some part of the energy set free by the bomb goes to produce radioactive substances, and these will emit very powerful and dangerous radiations. The effect of these radiations is greatest immediately after the explosion, but it decays only gradually and even for days after the explosion any person entering the affected area will be killed.
Some of this radioactivity will be carried along with the wind and will spread the contamination; several miles downwind this may kill people.
Their work was summarized in what was to become known as the Friesch-Peierls memorandum (see below for a WORDLE/word map of the report), and sent on to Marcus Oliphant, who passed the document on to Henry Tizard, chairman of the Committee on the Scientific Survey of Air Defence. The Maud Committee--molded into existence by Tizard and consisting of Sir George Paget Thomson, Chairman Marcus Oliphant, Patrick Blackett, James Chadwick, Philip Moon and John Cockcroft.--first met on 10 April 1940 to consider Britain's actions regarding the mass of possibilities swirling around the "uranium problem", and
The MAUD committee approved two reports (See below for the World word map of the report below) on 15 July 1941 and disbanded; the first report was on 'Use of Uranium for a Bomb', giving a technical description of the bomb with specs and cost estimates, concluding that the bomb was indeed feasible. The other was on 'Use of Uranium as a Source of Power' and discussed the implementation of the bomb as well as the peaceful uses for what we would call a reactor. It was a penetrating discussion.
Meanwhile, in the U.S., Arthur Compton and the National Academy of Sciences (prompted and urged by folks like E.O. Lawrence and Vannevar Bush) studied and then, on 17 May 1941, issued a report on nuclear energy, incorporating the MAUD findings (which had been received in the U.S. by March 1941) discussing the possibilities of a bomb though without entering into any detail about actually producing one.
The Brits evidently felt that the U.S. needed to take a much more advanced and urgent look at the issue than what was being allowed. The Americans were still not in the war (and Britain had been fighting for 1.5 years by that point) and were still 7 months out from Pearl Harbor. Pursuing and building the bomb was a gargantuan undertaking unlike almost anything else, and the fire had still not been lighted that would ignite this fuse. The bomb also could be built probably nowhere else on earth given the manpower and raw material and electricity and knowledge demanded by the project--at the end of the day, the Nazis, I think, just didn't have the raw stuff for the project and were still removed from the theoretical aspects. Britain of course couldn't afford the project what with actually fighting the war. Japan just wasn't in the realm of possibility. It had to be the U.S.
And so it came to pass that Sir Marcus 'Mark' Laurence Elwin Oliphant, a 42 year old Australian (who died only recently, in 2000), came to the US in to do a very hard sell on the Americans for bomb production. Oliphant, who with the rest of the nuclear establishment of Britain was puzzled by the inaction of the U.S. on MAUD meet with Lyman Briggs, his American counterpart to whom the report had been sent. Oliphant recalled:
"the minutes and reports had been sent to Lyman Briggs, who was the Director of the Uranium Committee, and we were puzzled to receive virtually no comment. I called on Briggs in Washington, only to find out that this inarticulate and unimpressive man had put the reports in his safe and had not shown them to members of his committee. I was amazed and distressed." I should say so
Oliphant moved up the ladder and met with his friends EO Lawrence, James Conant and Enrico Fermi about his "problem", which was evidently instantly digested. It was from this point that Lawrence got together with Compton and Bush (who at that time was chair of the National Defense Research Committee), and created the vaster and more encompassing Office of Scientific Research and Development--perhaps one of the smartest American moves of the entire war. (I should also point out that it was also under the leadership and guidance of Vannevar Bush that this organization functioned so well and produced the amount of work that it did, which as really astonishing. Bush in my my is one of the truly great unsung heroes of the war.) It was from the OSRD that the Manhattan Engineering District was born after Pearl Harbor. And then everything else, um, happened.
The MAUD Report, 1941
Report by MAUD Committee on the Use of Uranium for a Bomb
OUTLINE OF PRESENT KNOWLEDGE
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 -U) 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 95,000,000 pounds, 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
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 is 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
, 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 -U 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 I 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
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 1-1/8 to 1-1/4 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 tip 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
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
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 25U, and for this purpose it seems desirable that certain members of the committee should visit the
. We are informed that such a visit would be welcomed by the members of the
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
should be continued and extended especially in the region of experimental work.
Frisch-Peierls Memorandum, March 1940
On the Construction of a "Super-bomb" based on a Nuclear Chain Reaction in Uranium
The possible construction of "super-bombs" based on a nuclear chain reaction in uranium has been discussed a great deal and arguments have been brought forward which seemed to exclude this possibility. We wish here to point out and discuss a possibility which seems to have been overlooked in these earlier discussions.
Uranium consists essentially of two isotopes, 238U (99.3%) and 235U (0.7%). If a uranium nucleus is hit by a neutron, three processes are possible: (1) scattering, whereby the neutron changes directions and if its energy is above 0.1 MeV, loses energy; (2) capture, when the neutron is taken up by the nucleus; and (3) fission, i.e. the nucleus breaks up into two nuclei of comparable size, with the liberation of an energy of about 200 MeV.
The possibility of chain reaction is given by the fact that neutrons are emitted in the fission and that the number of these neutrons per fission is greater than 1. The most probable value for this figure seems to be 2.3, from two independent determinations.
However, it has been shown that even in a large block of ordinary uranium no chain reaction would take place since too many neutrons would be slowed down by inelastic scattering into the energy region where they are strongly absorbed by 238U.
Several people have tried to make chain reactions possible by mixing the uranium with water, which reduces the energy of the neutrons still further and thereby increases their efficiency again. It seems fairly certain however that even then it is impossible to sustain a chain reaction.
In any case, no arrangement containing hydrogen and based on the action of slow neutrons could act as an effective super-bomb, because the reaction would be too slow. The time required to slow down a neutron is about 10-5 sec and the average time loss before a neutron hits a uranium nucleus is even 10-4. In the reaction, the number of neutrons would increase exponentially, like et/τ where τ would be at least 10-4 sec. When the temperature reaches several thousand degrees the container of the bomb will break and within 10-4 sec the uranium would have expanded sufficiently to let the neutrons escape and so to stop the reaction. The energy liberated would, therefore, be only a few times the energy required to break the container, i.e. of the same order of magnitude as with ordinary high explosives.
Bohr has put forward strong arguments for the suggestion that the fission observed with slow neutrons is to be ascribed to the rare isotope 235U, and that this isotope has, on the whole, a much greater fission probability than the common isotope 238U. Effective methods for the separation of isotopes have been developed recently, of which the method of thermal diffusion is simple enough to permit separation on a fairly large scale.
This permits, in principle, the use of nearly pure 235U in such a bomb, a possibility which apparently has not so far been seriously considered. We have discussed this possibility and come to the conclusion that a moderate amount of 235U would indeed constitute an extremely efficient explosive.
The behavior of 235U under bombardment with fast neutrons is not experimentally, but from rather simple theoretical arguments it can be concluded that almost every collision produces fission and that neutrons of any energy are effective. Therefore it is not necessary to add hydrogen, and the reaction, depending on the action of fast neutrons, develops with very great rapidity so that a considerable part of the total energy is liberated before the reaction gets stopped on account of the expansion of the material.
The critical radius γo- i.e. the radius of sphere in which the surplus of neutrons created by the fission is just equal to the loss of neutrons by escape through the surface-is, for a material with a given composition, in a fixed ration to the mean free path of neutrons, and this in turn is inversely proportional to the density . It therefore pays to bring the material into the densest possible form, i.e. the metallic state, probably sintered or hammered. If we assume for 235, no appreciable scattering, and 2.3 neutrons emitted per fission, then the critical radius is found to be 0.8 time the mean free path. In the metallic state (density 15), and assuming a fission cross-section of 10-23 cm2, the mean free path would be 2.6 cm and γo would be 2.1 cm, corresponding to a mass of 600 grams. A sphere of metallic 235U of a radius greater than γo would be explosive, and one might think of about 1 kg as suitable size for a bomb.
The speed of the reaction is easy to estimate. The neutrons emitted in the fission have velocities of about 10-9 cm/sec and they have to travel 2.6 cm before hitting a uranium nucleus. For a sphere well above the critical size the loss through neutron escape would be small, so we may assume that each neutron after a life of 2.6 x 10-9 sec, produces fission, giving birth to two neutrons. In the expression et/τ for the increase of neutron density with time, it would be about 4 x 10-9 sec, very much shorter than in the case of a chain reaction depending on slow neutrons.
If the reaction proceeds until most of the uranium is used up, temperatures of the order of 1010 degrees and pressure of about 1013 atmospheres are produced. It is difficult to predict accurately the behavior of matter under there extreme conditions, and the mathematical difficulties of the problem are considerable. By a rough calculation we get the following expression for the energy liberated before the mass expands so much that the reaction is interrupted:
E = 0.2M(r2/τ2)√((r/ro)-1)
(M, total mass of uranium; r, radius of sphere; ro, critical radius; τ, time required for neutron density to multiply by a factor e). For a sphere of radius 4.2 cm (ro = 2.1 cm), M = 4700 grams, τ = 4 x 10-9 sec, we find E = 4 x 1020 ergs, which is about one-tenth of the total fission energy. For a radius of about 8 cm (m = 32 kg) the whole fission energy is liberated, according to the formula (1). For small radii the efficiency falls off even faster than indicated by formula (1) because τ goes up as r approaches ro. The energy liberated by a 5 kg bomb would be equivalent to that of several thousand tons of dynamite, while that of a 1 kg bomb, though about 500 times less, would still be formidable.
It is necessary that such a sphere should be made in two (or more) parts which are brought together first when the explosion is wanted. Once assembled, the bomb would explode within a second or less, since one neutron is sufficient to start the reaction and there are several neutrons passing through the bomb every second, from the cosmic radiation. ( Neutrons originating from the action of uranium alpha rays on light-element impurities would be negligible provided the uranium is reasonably pure.) A sphere with a radius of less than about 3 cm could be made up in two hemispheres, which are pulled together by springs and kept separated by a suitable structure which is removed at the desired moment. A larger sphere would have to be composed of more than two parts, if the parts, taken separately, are to be stable.
It is important that the assembling of the parts should be done as rapidly as possible, in order to minimize the chance of a reaction getting started at a moment when the critical conditions have only just been reached. If this happened, the reaction rate would be much slower and the energy liberation would be considerably reduced; it would, however, always be sufficient to destroy the bomb.
For the separation of the 235U, the method of thermal diffusion, developed by Clusius and others, seems to be the only one which can cope with the large amounts required. A gaseous uranium compound, for example uranium hexafluoride, is placed between two vertical surfaces which are kept at a different temperature. The light isotope tends to get more concentrated near the hot surface, where it is carried upwards by the convection current. Exchange with the current moving downwards along the cold surface produces a fractionating effect, and after some time a state of equilibrium is reached when the gas near the upper end contains markedly more of the light isotope than near the lower end.
For example, a system of two concentric tubes, of 2mm separation and 3 cm diameter, 150 cm long, would produce a difference of about 40% in the concentration of the rare isotope between its end without unduly upsetting the equilibrium.
In order to produce large amounts of highly concentrated 235U, a great number of these separating units will have to be used, being arranged in parallel as well as in series. For a daily production of 100 grams of 235U of 90% purity, we estimate that about 100,000 of these tubes would be required. This seems a large number, but it would undoubtedly be possible to design some kind of a system which would have the same effective area in a more compact and less expensive form.
In addition to the destructive effect of the explosion itself, the whole material of the bomb would be transformed into a highly radioactive stage. The energy radiated by these active substances will amount to about 20% of the energy liberated in the explosion, and the radiations would be fatal to living beings even a long time after the explosion.
The fission of uranium results in the formation of a great number of active bodies with periods between, roughly speaking, a second and a year. The resulting radiation is found to decay in such a way that the intensity is about inversely proportional to the time. Even one day after the explosion the radiation will correspond to a power expenditure of the order 1,000 kW, or to the radiation of a hundred tons of radium.
Any estimates of the effects of this radiation on human beings must be rather uncertain because it is difficult to tell what will happen to the radioactive material after the explosion. Most of it will probably be blown into the air and carried away by the wind. This cloud of radioactive material will kill everybody within a strip estimate to be several miles long. If it rained the danger would be even worse because the active material would be carried down to the ground and stick to it, and persons entering the contaminated area would be subjected to dangerous radiations even after days. If 1% of the active material sticks to the debris in the vicinity of the explosion and if the debris is spread over an area of, say, a square mile, any person entering this area would be in serious danger, even several days after the explosion.
In estimates, the lethal dose penetrating radiation was assumed to be 1,000 Roentgen; consultation of a medical specialist on X-ray treatment and perhaps further biological research may enable one to fix the danger limit more accurately. The main source of uncertainty is our lack of knowledge as to the behavior of materials in such a super-explosion, an expert on high explosives may be able to clarify some of these problems.
Effective protection is hardly possible. Houses would offer protection only at the margins of the danger zone. Deep cellar or tunnels may be comparatively safe from the effects of radiation, provided air can be supplied from an uncontaminated area (some of the active substance would be noble gases which are not stop by ordinary filters)
The irradiation is not felt until hours later when it may become too late. Therefore it would be very important to have an organization which determines the exact extent of the danger area, by means of ionization measurements, so that people can be warned from entering it.
O. R. Frisch