How Britain got the bomb
William G. Penney, the father of the British atom bomb, spent most of 1944 and 1945 at Los Alamos helping the United States build the first atomic bomb. He formed part of the British Mission, an elite team of Los Alamos British scientists and emigres who contributed to the development, testing and use of atomic weapons. Penney, sent to Los Alamos as a specialist on ocean waves, soon found his gifts were readily apparent and he was made one of the five members of the Los Alamos "brain trust". The group made key decisions in the direction of the weapons program, putting him in the company of Robert Oppenheimer, John Von Neumann, "Deke" Parsons and Norman Ramsey. On 27 April 1945 Penney became one of only two representatives from Los Alamos (and the only Briton) to be part of the ten man Target Committee responsible for drawing up a list of prospective Japanese atomic bomb sites. Penney travelled to Tinian Island in the Pacific to be on hand for planning and briefing the atomic bombing missions. Penney actually witnessed the bombing of Nagasaki, flying in an observation plane accompanying the attack. Afterwards he conducted damage surveys of the ruined city.
Penney returned to Imperial College immediately after the war, but accepted an appointment to head up the Armament Research Department (ARD) on 1 January 1946. On 8 January 1947 the secret GEN.163 Cabinet committee of six Ministers (headed by PM Attlee) decided to proceed with a British effort to acquire atomic weapons. Penney did not receive word of this decision until May 1947 when he was finally asked by Lord Portal to lead the British effort. The decision was not disclosed publicly in any respect until 12 May 1948, when an oblique reference was made to atomic weapon development in British parliamentary discussions.
In June 1947 Penney began assembling a team to work on the bomb. One of his first steps was to prepare a document describing the features of the U.S. plutonium implosion bomb, breaking down the development tasks required to replicate it, and identifying outstanding questions that required further research. The report, completed on 1 July, was entitled "Plutonium Weapon - General Description" (U.K, Public Record Office File AVIA 65/1163, "Implosion") and gave the British atomic weapons program a preliminary design description roughly equivalent in terms of detail to the description provided the Soviets by Klaus Fuchs.
The Penney Report
The Penney Report was declassified and made physically available at an unknown date, thought to be in the late 1990s, under the Public Records Act (now amended by the Freedom of Information Act which came into force in January 2005).
However file (covering the years 1947-1953) was withdrawn from public access during 2002, possibly due to political sensitivities after the September 2001 terrorist attacks in New York. It was not scheduled for access again until 2014.
The actual legal status of the file remains as a public record. Its access condition has been changed to "Retained by Department under Section 3.4" (of the PRA) which means that the file has been returned to the custody of the originating department (Ministry of Supply) or its successor. This limitation of access does not constitute reimposition of a secret security marking, and no attempt appears to have been made by the U.K. government to contact people who had previously obtained photocopies copies of this file, until the document appeared on Wikileaks.
On March 19, 2008, the British Foreign and Commonwealth Office, Counter Proliferation Department wrote to Wikileaks asking that the "Fat Man" bomb diagram, which was released by Wikileaks before the full report, be removed. However, after some discussion, Wikileaks did not find the request credible and did not comply. See United Kingdom atomic weapons program: The full Penney Report (1947) for the full correspondence.
Penney's description is reportedly less detailed than "Tuck's Bible", which was written around this same time by James Tuck, another member of the British Mission to Los Alamos who was deeply involved in the design and development of the implosion bomb and wrote appendix 'M' to the Penney Report which compares the British and United States weapons programs. Tuck's Bible has never been made public.
A brief analysis of the .Fat Man. diagram
1. Neutron Initiator
Polonium is a well known alpha radiation emitter. Alpha radiation is He atoms stripped of electrons and accelerated towards c. When polonium crushed onto beryllium by explosion, reaction occurs between polonium alpha emissions and beryllium leading to Carbon-12 & 1 neutron. This, in practice, would lead to a predictable neutron flux, sufficient to set off device. Widely known that once critical mass is obtained, in order for bomb to explode, requires fission initiation by neutron generation; this will do the trick. Polonium 210 specifically well known alpha emitter. Gold/nickel foil layer around beryllium is sufficient to prevent pre-reaction prior to explosive compression due to low penetrability of alpha radiation (can.t pass through paper). This allows for long-term storage of initiator.
The Boron-10 shielding is to keep stray (eg cosmic ray generated) neutrons from pre-initiating the chain reaction.
The polonium in the initiator has a short, half-year halflife.
The inner layer of the Be sphere is etched with grooves, these will create Be jets when imploded (shaped charge effect) which mixes the Be and Po very quickly.
2. Diagram
Roughly to scale. No easy feat in days prior to computerized drafting tools. Measurements located on table in top left roughly match drawing scale. Note archaic units (lbs): physicists after .50s probably would have used SI units, regardless of country. Also note quality of arcs (Fast HE/Slow HE) indicates is drawn by professional draftsman.
3. High Explosives & Miznay/Schardin effect (e.g. shaped charge)
Miznay/Schardin effect will work in this design, in all likelihood, though the additional layer of HE after the first layer of lenses is a surprise. Are the lenses strong enough to compress the second layer of HE? In any event, there.s enough explosive in there to cause the Miznay/Schardin effect, and enough aluminum to convincingly crush the core.
The outer layer of slow + fast explosives is used to create a number of converging planar shock wavefronts. The inner layer of solid HE is not compressed, but is initiated fairly uniformly by the many planar wavefronts hitting it. The uniformity of initiation is important to the compression of the core.
Also note the squiggly lines indicating compression.
Note also the .possibilities table. in the bottom left. This indicates several possibilities as to how much explosive is necessary, indicating that the design is not yet fixed.
4. Weaponization.
The weapon has a removable core, or at least a serviceable one, as evidenced by felt layers. This is necessary to allow the bomb to be disassembled.
5. Assessment.
This diagram is not really a secret to foreign intelligence services; nobody is going to be surprised by this design, just by the fact that it.s appeared in public. Open sources have speculated on these matters for a long time (see nuclear weapons design article in Wikipedia), and this just confirms that they were right.
This is a crude, but effective, plutonium based design. Devices that are orders of magnitude more efficient are possible. A disclosure of, for example, the plans of the W-88 or a Russian equivalent, would be far more threatening, as there are actually real secrets involved there not known to all the NWS (the Big-5 + India, Pakistan, Israel, North Korea) or Virtual NWS (Germany, Japan, Sweden, South Korea, Canada, Ukraine, Taiwan, Italy, Spain.to name a few) intelligence agencies. After 1949 or so, disclosure of this would not have been a real threat to U.S. national security.
The real problem about building one of these designs is the rarity (at least outside of NWS nuclear facilities) of plutonium and polonium, as well as the ability to fabricate sophisticated high explosives to exacting specifications. We.re not talking about IEDs here. To build a nuclear weapon requires a state.
Text version of AVIA file 65, "Implosion", by Williom G. Penney, 1 July 1947
Plutonium Weapon - General Description.
Introduction.
The following general description of the plutonium weapon has been compiled with the object of anticipating difficulties in experimentation, design, and manufacture, so that the progress of development may run concurrently.
Of necessity, the description can only give an overall picture and does not profess scientific or technical detail.
Components Of The Weapon.
2. The components may be divided into seven separate assemblies consisting
* (a) The imploder system.
* (b) The plutonium core.
* (c) The initiator.
* (d) The casing of the explosive assembly.
* (e) The detonator firing mechanism.
* (f) The proximity fusing device.
* (g) The ballistic outer casing.
Although the imploder system is shewn above as one assembly, it is, in fact, a multiple assembly having the following components.
* (a) The detonators.
* b) The outer composite H.E. shell.
* (c) The inner homogeneous H.E. shell.
* (d) The aluminium inner liner.
* (e) The boron 10 shield.
* (f) The uranium 238 tamper.
In addition to those components, there is also a thin felt washer between the homogeneous explosive and the aluminium liner in order to take up manufacturing inaccuracies.
3. The main object of this somewhat complicated imploder system is to ensure that the detonation waves initiated by the detonators arrive at the main plutonium core as one concentric converging shack waves without'jets '.
4. A sectional schematic drawing approximately to scale is attached and each component is briefly described in the appendices listed below:
Appendix 'A '. The Detonator System.
Appendix 'B '. The Outer Composite H.E. Shell.
Appendix 'C '. The Inner H. E. Shell.
Appendix 'D '. The Aluminium Liner.
Appendix 'E ' The Boron 10 Shield.
Appendix 'F '. The Uranium 238 Liner.
Appendix 'G '. The Plutonium Core.
Appendix 'H ' The Initiator.
Appendix 'I ' The Casing and Explosive Assembly.
Appendix 'J ' Firing Mechanism and Proximity Fusing Device
Appendix 'K ' The Ballistic Outer Casing.
Appendix 'L ' The Arming Plug.
Appendix 'M ' Tabulated Notes prepared by Dr. Tuck.
5. Although production of plutonium cores must be on a limited scale in the first instance, consideration must be given to the total number of bombs likely to be required as this may affect manufacturing processes of components i.e. a limited number hand-made or in sufficient number to justify special presses, moulds, etc.
Appendix 'A '
The Detonator System
Initiation Of Detonation.
There are 32 points of initiation of detonation around the surface of the H.E. outer shell. Each detonator is in fact a twin system to ensure against failure.
DESCRIPTION. Each detonator is of the "fuse-bridge" type, the wire bridge being imbedded in a small quantity of PETN (Pentelite) and having a tetryl booster.
STANDARD OF ACCURACY The whole bridge system from bridge to booster must be accurate to within 0.2 microseconds to ensure subsequent concentricity of the detonation wave.
MANUFACTURING PROBLEMS. The main manufacturing problems are the consistency of the pentelite and tetryl as well as the characteristics of the bridge to ensure this high order of timing accuracy.
DISCUSSION.
When facilities are complete, C.S.A.R. will be able to undertake all research, experimentation, design and production.
QUESTIONS AS AT 1.7.194.
Nil, pending further progress with preliminary research.
Appendix 'B '
The Outer Composite H.E. Shell.
OBJECT.
A detonation wave initiated in a H.E. progresses spherically outwards but as it is essential for the implosion wave to arrive as a convergent sphere, some mechanical mean is required to convert the former type into the latter, and this is the main function of the outer H.E. shell. DESCRIPTION. The shell consists of 20 hexagonal and 12 pentagonal uncased H.E. lenses, each being a truncated pyramid about 8" high, and having an external radius of about 27". Each lens comprises a composite filling, the outer being 60/40 RDX/TNT having a relatively high rate of detonation, and the inner being BARITOL having a slower rate of detonation. Other explosives may be used as a result of experimentation. The size and shape of the inner cavity, which contains the Baronal (sic Baritol?), Is governed by the relative rates of detonation as it is in this assembly that the conversion mentioned in paragraph 1 above is effected.
Pockets to accommodate the detonators are provided on the outer surface of the sphere, their relative positions being determined by the need for symmetrical initiation.
Design And Manufacturing Problems.
* (a) Developing the technique of producing consistent RDX/TNT, and Baronal, or other similar explosives.
* (b) Accurate determination of rates of detonation of the intended explosives.
* (c) Size and shape of the cavity containing the slower explosive.
* (d) the method of pressing the fast explosive in the first instance, and then the slow explosive into the cavity to obtain consistency of detonation and avoidance of jet action.
* (e) Accurate shaping of the sections to ensure face-contact.
ASSOCIATED PROBLEMS.
* (a) Variations in temperature during storage or carriage affect the density of explosive and thus vary the rate of detonation.
* (b) Design of containers for transport so that the sections do not become chipped, flaked, cracked or distorted.
DISCUSSION.
When facilities are complete, C.S.A.R. will be able to undertake all research, experimentation, design and production.
QUESTIONS AS AT 1.7.1947
Nil, pending further research.
Appendix 'C '
The Inner H.E. Shell.
OBJECT.
To produce the initial implosive effect on to the main core.
As the detonation wave is initiated over the whole outer surface of this component through the medium of the outer shell, it will travel through the inner shell as a convergent wave.
DESCRIPTION.
The shell is composed of segments approximately 9" thick of RDX/TNT.
DESIGN AND MANUFACTURING PROBLEMS.
As with the outer shell, consistency throughout this component is essential as is the flush fitting of all faces.
Note. On the inner face of this component is a felt lining approximately 0.15" thick; the object is to take up manufacturing irregularities but as it is a minor component, a separate appendix is not justified.
DISCUSSION.
When facilities are complete, C.S.A.R. will be able to undertake all research, experimentation, design and production.
QUESTIONS AS AT 1.7.194.
Nil, pending further research.
Appendix 'D'.
The Aluminium Liner.
OBJECT.
The main object of this liner is to smooth out any irregularities or jet proclivities in the convergent detonation wave.
DESCRIPTION.
2. It is a hollow sphere about 4½" thick.
DESIGN AND MANUFACTURING PROBLEMS.
3. The manufacture of this component is relatively simple. It will probably be made in two hemispheres screwed together but the faces at the joints must be flush.
DISCUSSION.
C.S.A.R. would be able to produce this component within his resources, but, in order to relieve his workshops of unnecessary work, it may be advisable to put this component to the trade.
QUESTIONS AS AT 1.7.194.7.
5. If only limited numbers are required, they could be turned out from the solid, but, if otherwise, pressings or moulds will be necessary. What should be the policy in this respect?
Appendix 'E '
Boron 10 Liner.
OBJECT.
To prevent "rogue" neutrons from outside sources entering the core and initiator assemblies,
DESCRIPTION.
2. Consists of a hollow sphere having a thickness of approximately 0.125".
DESIGN AND MANUFACTURING PROBLEMS.
3. Nil.
DISCUSSION.
In view of the limited numbers required, it may be unnecessary to go to the expense of making dies for this pressing; therefore, hand manufacture may be preferable.
5. It may be advisable to give this work to the trade.
QUESTIONS AS AT 1.7.l947.
6. Opinion seems to be divided on the necessity for this component; therefore, is further research required to establish the need.
7. If found necessary, are any special measures required in connection with boron chemistry, extraction and manufacture?
Appendix 'F '.
The Uranium 238 Liner.
OBJECT.
The object of this liner is fourfold.
* (a) To convert the detonation shock wave into an impulse.
* (b) To smooth out any remaining irregularities in the wave.
* (c) To act as a reflector of neutrons during fission.
* (d) To act as a "container" to the plutonium during fission and thus prevent premature disruption of the plutonium core.
DESCRIPTION.
2. A hollow sphere having a thickness of approximately 2½".
DESIGN AND MANUFACTURING PROBLEMS.
3. In all probability this shell would be made in two hemispheres and no manufacturing difficulties are anticipated except possibly the means of fastening them together.
DISCUSSION.
4. C.S.A.R. could undertake the manufacture of this component.
QUESTIONS AS AT 1.7.1947.
5. According to the numbers required, should the liner be handmade or will pressings be necessary?
Appendix 'G '
The Plutonium Core.
OBJECT.
The main fissile material.
DESCRIPTION.
2. The core consists of a hollow sphere approximately 2" thick of a plutonium/gallium alloy. The proportion of gallium is of the order of 3 atoms per cent.
DESIGN AND MANUFACTURING PROBLEMS.
3. Very little is known concerning plutonium chemistry in this country nor the machining or processing of the element.
4. It is believed that the method of manufacture of the hollow hemispheres was done by hot pressing; but the method of finishing the inner faces and bolting the hemispheres together is uncertain.
5. Appropriate precautions against radioactivity will have to be taken throughout manufacture.
Associated Problems.
6. Suitable containers for storage and transport.
DISCUSSION.
7. No facilities exist for the handling or fashioning of plutonium; nor has a technique been developed in this country.
8. The question arises where a plutonium workshop should be erected. The alternatives are at Springfield, Harwell, or within C.S.A.R. 's organisation. There are advantages or disadvantages on each of these alternatives, but, whichever be selected, early consideration must be given to the design and erection of the plant and to getting the appropriate nucleus of the staff considering the problem.
9. The team engaged on manufacture of this item must have developed their processes to a high order of perfection by the time plutonium is available in adequate quantity; therefore, early consideration must be given to this problem.
QUESTIONS AS AT l.7.l947.
10. What facilities are required for research into and final production of this component.
11. Where should they be located.
12. Have we the competent staff within our resources or will it be necessary.
Appendix 'H '
The Initiator.
OBJECT.
To ensure the release of sufficient neutrons to initiate fission, by the admixture of Beryllium and Polonium.
2. The initiator consists of two main components.
3. The outer component is a hollow beryllium sphere 1 cm. diameter having the inner face serrated by four-sided 60o pyramids.
4. Inside this cone is another sphere of beryllium which is centred by means of radial pins projecting internally from the outer shell.
5. Both the inner serrated surface of the outer shell and the surface of the inner sphere are coated with nickel or gold, or possibly both. On top of the nickel deposit of the inner sphere a film of polonium is deposited. Thus, the nickel deposit acts as an "insulator" to prevent neutron reaction between the beryllium and polonium until the appropriate time.
6. The serrations on the inner surface convert the shock wave into a multitude of jet actions which thus ensure complete shattering and mixture of the two elements, thus causing neutron emission.
DESIGN AND MANUFACTURING PROBLEMS.
7. The main difficulty is our lack of knowledge of polonium chemistry and considerable research will no doubt be required on this aspect of the project.
8. As with the other assemblies, the method of the manufacture of the beryllium outer shell and bolting the two hemispheres together requires development.
DISCUSSION.
9. The design and manufacture of this component also calls for techniques new to this country. It is estimated that a nucleus staff of one engineer and. a chemist will require a year deliberating on the problem before they could start to tackle it.
10. In addition, plant will be required for the fabrication and this should be ordered in good time.
11. The location of this plant is also debatable, the alternatives being similar to that for the plutonium core.
QUESTIONS AS AT 1.7.1947.
12. What facilities are required for research into and final production of this component.
13. Where should they be located.
14. Have we the competent staff within our resources or will it be necessary to recruit from outside.
Important footnote.
The half life of the initiator is approximately six months; therefore, replacements will have to be continuously provided.
Appendix 'I '
The Casing For Explosive Assembly.
OBJECT.
The object or this component is to held the whole explosive and fissile assembly solidly together.
DESIGN AND MANUFACTURING PROBLEMS.
Little is known concerning the method used in previous models, but, all probability it consists of the aluminium shell about ½" thick having separate polar, caps and equatorial sections. The various sections would have to be bolted together as assembly of the bomb progresses.
3. As some mention has been made in various reports concerning the effect of temperature changes during flight, it is questionable whether internal or external lagging should be incorporated to prevent heat loss.
4. Owing to the difficulty of ensuring that the holes in this casing for the detonators coincide with the detonator sockets of the lenses, enlarged holes in the former must be provided with "floating seals" having holes of the right size, superimposed.
5. Owing to the limited number of weapons envisaged, it is questionable whether the cost of making presses for this casing would be justified and whether hand manufacture should not be undertaken in the same way that the early parabolic reflectors for radar were handmade to a high degree of accuracy. If presses or moulds have to be made, early consideration of their design will be necessary, and it might be advisable to think in terms of plastics rather than metal.
DISCUSSION.
6. As with the aluminium liner, the fabrication of the casing is a straightforward metal-working job and could probably be put to the trade.
QUESTIONS AS AT 1.7.1914.
7. According to the numbers required, should they be handmade or machine fabricated.
8. Should the job be put to the trade?
Appendix 'J '.
Firing Mechanism And Proximity Fusing Device.
Whereas the firing mechanism may be relatively simple and standard components could b.
used, the proximity fusing device will call for considerable research and experiment.</p>
2. C.S.A.R. visualises that such a fuse must have the following characteristics: .
* (a) Selective fusing between 1500 feet and 100 feet above the target.
* (b) Accuracy to within + or - 200 feet at the greater height but + or - 30 feet at the lesser.</p>
* (c) Probability of failure reduced to the minimum.
* (d) Immunity from jambing or from other interferences.
DISCUSSION.
The fulfilment of this requirement will be a major task for the Electronics Section and will finally necessitate air trials, but these might be incorporated with the normal ballistic trials if the fuses are completed in time.
4. No doubt T.R.E. could undertake the research into and design of the proximity fuse but it is for discussion whether it would not be preferable to detach personnel from that Establishment to work under the direct supervision of C.S.A.R.
QUESTIONS AS AT l.7.l947.
5. Who should undertake this research and where should it be located.
6. What is the earliest date on which it should start.
Appendix 'K '.
The Ballistic Outer Casing.
The design of this component must also wait until the general overall assembly has been decided upon, but it would also house the firing and fusing mechanisms.
DISCUSSION.
2. The design of the ballistic casing would primarily concern the bomb design section at R. A. E., but it would be preferable for personnel from that Establishment to be loaned to C.S.A.R. during the design period, which would cover the ballistic trials.
3. The manufacture of the casing should be put to the trade, and orders for 2-or-3O0 given. Some of these cases will have to be innered [crossed out: handwritten above ?inert] filled for ballistic trials whilst others will be required for fuse functioning tests.
QUESTIONS AS AT l.7.1947.
4. As a large number of these outer casings will be required for ballistic trials, is it agreed that the trade should undertake manufacture.
5. Should the design team work at R.A.E. or be attached to C.S.A.R..
6. When should work start on this component?
Appendix 'L '
The Arming Plug.
The weapon is assembled component by component, the last being the plutonium pore into which the initiator has already been inserted. It therefore follows that a passage has to be provided through which the core can be entered, the passage being finally sealed.
2. Starting from the inside of the assembly, a hole of the proper dimensions is cut in the uranium tamper, the boron shield, and the aluminium liner.
3. In the H.E. component, a section of the inner H.E. shell corresponding to the dimensions of a complete lens is also removed.
4. After the insertion of the plutonium core, these plugs are replaced, thus sealing the assembly. In all cases, f1ush fitting of the plugs in their respective sockets must be guaranteed.
QUESTIONS AS AT l.7.1947.
Nil.
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