Making a fission bomb, given a sufficient quantity of nearly pure uranium-235, is pretty trivial. Obtaining a sufficient quantity of nearly pure U-235 is decidedly non-trivial. Even if you managed to get hold of some reactor fuel, that’s still only about 5% U-235 (compared to the non-fissionable U-238), and you need more like 90% to build a bomb. And believe me, international intelligence agencies keep a real close eye on the high-tech companies that are capable of producing the components you’d need to build the equipment to purify your uranium to that level …
I think this is a gross oversimplification and underestimately of the complexity. It is true that, using published, nonclassified information, you could conceptually design a nuclear device, in terms of what general shape and amount of material would achieve a certain yield. In practice, there are a lot of details, not to mention methods of weapon material refinement and fabrication, that are beyond the knowledge of any scientific and engineering generalists. It’s one thing to gin up a few calculations or even run a hydrocode simulation of a detonation, but it’s quite another to put together a working device which requires several hundred components to fit together
Consider this analogy; one can easily obtain all the information necessary to fabricate, say, a Tucker automobile (only 50 odd in existance). Working from pictures and drawings, you could certainly sketch up something that looks like a Tucker. You could put together requirements and numbers on the size of engine, placement of headlights, gearing ratios, whatever. But to build it, you have to manufacture the frame, body work, engine, transmission, interior fittings, et cetera…either all out of commercial off the shelf parts OR by custom fabrications, from scratch. If you are a single individual, even an engineer, what is the likelyhood that you are really going to put together a working car, much less one that would fool Tuckerfan?
I’m not saying it couldn’t be done–a handful of experienced designers with access to modern machine tools and a sizable bank balance, plus fissible and (for a boosted device, fusible) material could do it, and in fact, have done so in several second-tier nuclear powers around the globe–but these resources are almost certainly beyond the means of what we traditionally think of as a terrorist group. Ground up weapon design, even by experienced designers, requires some amount of testing and simulation, and is hardly worth doing for a single device. For terrorists, a far bigger bang for the buck is to attach infrastructure or hijack aircraft, as has been aptly demonstrated; in short, to cause terror rather than strictly damage. But if they managed to purchase or steal a portable nuke from one of the less secured arsenals of the world…:eek:
Stranger
That should be:
…but it’s quite another to put together a working device which requires several hundred components to fit together and function with microsecond timing.
A simple gun type spherical critical mass device like Fat Boy is more primative, of course, but also enormous in terms of required critical mass and associated mechanisms, and terribly inefficient. A more efficient design uses just a shell of uranimum and various methods to reflect and concentrate neutron flux to get good utilization of material and maximum yield.
The long and short of it is, building a nuclear device is not a trivial undertaking, even for a simple weapon. Conceptually, it isn’t terribly complex, and a physics or nuc eng grad student could run a first-pass simulation on the yield of a given configuration, but designing an actual weapon is something that requires as much engineering experience as theoretical knowledge.
Stranger
I don’t think you understand this subject very well.
“A simple gun” = Little Boy bomb dropped on Hiroshima.
“spherical critical mass” = Fat Man bomb dropped on Nagasaki.
The two designs were completely different. One used uranium, in an actual naval gun, and was never tested prior to its first use. The other one used a spherical plutonium shell. That one was the subject of the first nuke test.
If I recall correctly, the target in the Little Boy was a sphere with a cylinder cut out of it. The gun fired a cylindrical projectile that fitted into the sphere, forming a spherical supercritical mass. So “gun type spherical critical mass device” is a reasonable discription, although it could be confused with an spherical implosion device.
The problem with chemical weapons is the delivery. If you want to kill 200 people use an explosive not a chemical weapon. That sect in japan used 20 kg of sarin nerve agent against an apartment building (1 year before the train attack) and “only” managed to kill 8 people (from http://www.hpac.com/microsites/hsb/blewett_hsbsup/blewett_hsbsup.htm) . One stick of dynamite in the right place could have done better.
Delivery and detonation are the big problems with FAE bombs.
In a way, FAE bombs have the same problems that chemical or biological weapons do, if the wind changes, you’ll miss your target or disperse the agent enough to make it ineffective.
FAE bombs depend on having little to no wind in order for the explosive agent to disperse properly, but once the agent disperses, you have a small window to ignite the agent before the concentration gets too low to ignite.
Point taken about the wind and small window for detonation. I believe the necessary materials are not uncommon: propane, butane, acetylene or the like and some kind of detonator. Maybe something like this would be better installed inside a building.
Regards
Testy
Yes. The Little Boy weapon design used a “critical mass” (actually supercritical) of U-235 in a spherical configuration with a hole through it. A plug of material was shot through the sphere to complete the geometry. Because the flux rate is not as critical with U-235, the slower but mechanically more simple gun mechanism could be used. As Demostylus indicated, this was a simple device that the Project scientists felt comfortable enough with that no test was required prior to it’s use over Hiroshima.
The Trinity test article and the Fat Boy used a subcritical sphere, comprised of 32 individual pieces of Pu-239 that were compressed to criticality by conventional explosive charges. Both formed, when put together, a solid sphere of material, though later designs used much more efficient convoluted shell designs, with walls as thin as .030in for the most compact devices. Because of the necessitated higher flux rate and the need for faster criticality–requiring compression in a timespan of less than 5 microseconds–the gun-type mechanism won’t work with plutonium.
More compact weapons that could be man-portable or easy shipped are far more complex than a simple sphere, whether the gun or explosive containment devices, and require extensive modeling and access to experiemental data and/or a test program to acheive assured and efficient detonation. This is well beyond the means of any small party of technical generalists. Of course, there are a number of people with the specific skills and experience required who are presumably at loose ends and could possibly be enticed into working for some resourceful group or government that wanted to develop nuclear capability; not only former USSR nuclear designers but those from other nations who have or have had development programs as well, including Israel, South Africa, India, and Pakistan. (The head of Pakistan’s nuclear weapon development is in fact under permenant house arrest for selling information to other governments, including North Korea.)
But, again, it would take more than a simple machine shop and a general knowledge of fission principles to make a weapon. Material refinement, high precision machining and the particular hazards associated with nuclear materials, an understanding of high precision explosive devices and detonators, et cetera, would be required to make something that isn’t likely to fizzle. I’d be far more concerned about someone sabotaging fuel refineries or contaminating water reservoirs or somesuch than I would about a terrorist group building and using a nuke. I’m not saying it couldn’t happen, by any stretch of the imagination, and the developed nations should continue efforts to limit nuclear proliferation, but it’s not on the top of the list of personal concerns.
Here’s a site to check out that gives some more information about the generalities. I found a few others looking around that provide a little too much information for my comfort (in terms of conforming to the posting standards) but there’s plenty of info out there.
Stranger
Little Boy’s target was a cylindrical annulus, i.e., a cylinder with a hole through its axis. The projectile was, as you say, a cylinder capable of fitting into the hole. The resulting supercritical mass was cylindrical, not spherical.
Repeat after me: Fat Man.
I’m not sure where you’re getting this stuff from, but as far as I know there were only two pieces. Each piece was hemispherical, and when assembled, they formed a hollow sphere. I’m also not sure what you mean by “subcritical”. Criticality has a number of factors, the principal of which are mass and shape/density/mass-surface area ratio. There was certainly a critical (actually supercritical) mass present both before and after firing was initiated. What changed was the shape/density/mass-surface area ratio.
Stranger, I think the 32 pieces you’re thinking of were the explosive outer shell used to compress the hemispheres of plutonium. According to Richard Rhodes’ The Making of the Atomic Bomb, getting the detonator blast right (it had to be a perfectly spherical inward travelling shockwave) was one of the most technically challenging aspects of the entire Manhattan Project, and was the most uncertain when the Trinity test was finally carried out.
Okay, it looks like we’re going to need some cites.
Re Little Boy:
This supports Desmostylus’ contention that Little Boy used a cylindrical annulus. I’m certain I’ve read in other sources that it used a spherical target - possibly Stranger has read the same thing.
Re Fat Man:
It wasn’t a hollow shell design, although it did have a small central cavity for the neutron generator. The core was formed from two hemispheres plus a plug to form an almost-solid 9cm diameter sphere with a 2.5 cm diameter central cavity. The explosive compression system did indeed consist of 32 explosive lenses.
The assembly was subcritical, and there was no critical mass of fissile material present.
"The pit and the tamper together made a marginally subcritical system. When compressed by the implosion up to 2.5 times its original density (possibly somewhat less), the pit became an assembly of some 4-5 critical masses. Before use, the bomb was safed by use of a cadmium wire in the pit."
Your quote doesn’t mean what you think it means. Where the author says “The pit and the tamper together made a marginally subcritical system”, he means that prior to the implosion, the system was only marginally subcritical. It was almost at the point of being critical. When compressed, it became critical.
Think about Little Boy. There were two subcritical mases. When the two were put together, the mass was critical. In Fat Man, there was only one mass, but it was “marginally subcritical” only by virtue of its shape, i.e., the hollow in the centre. The hollow effectively stopped a self-sustaining reaction. Compress it a bit, and the reaction becomes self sustaining. Your quote actually says that there were 4-5 critical masses present.
The quote means exactly what I think it means. 4-5 critical masses, at around 2.5 times normal density of plutonium. The explosive doesn’t just eliminate the hole, it elastically compresses the plutonium to a very high density, so the chance of a given neutron causing a plutonium fission is very much greater for a given path length. This allows a subcritical mass under normal conditions to become a highly supercritical mass when explosively compressed.
If you read the whole thing, further down it says:
"The critical mass of the uranium reflected core in the delta phase was 7.5 kg, but only 5.5 kg in the alpha phase. Any accidental detonation of the high explosive (in a fire or plane crash for example) would be certain to collapse the 6.2 kg delta phase core to the supercritical alpha phase state."
A bare plutonium sphere has a critical mass of around 11kg. With a sufficiently thick tamper, you can bring this down to 5kg. Cite:
http://www.fas.org/nuke/intro/nuke/design.htm
In Fat Man, the tamper was enough that a critical mass would have been 7.5kg of delta-phase plutonium. However, there was only 6.2kg of delta-phase plutonium in the device. There was no critical mass of material in the bomb, and the internal hole served only to contain the neutron generator, not to make the assembly subcritical. If the hole were eliminated to give a solid sphere of 6.2kg of delta-phase plutonium, the assembly still wouldn’t have become critical.
If you increase the density by changing the structure to alpha plutonium, then a solid sphere is supercritical. But that wasn’t the way the bomb achieved most of its supercriticality - 6.2/5.5 critical masses is only 1.13 critical masses. To get the equivalent of 4-5 critical masses, you need the compression caused by the explosives. There was not 4-5 times a critical mass of material in the bomb.
I think I explained somewhere above that criticality is not simply a function of mass. It’s also a function of shape and density. You also seem to understand this point. I’m not sure why you’re getting hung up on it.
I can see that you’re operating with a definition of critical mass as being “a spherical mass at STP capable of sustaining a reacion”, but I can’t see why that’s a good definition.
I guess I misunderstood you. Your comment about the hollow in the centre is factually incorrect - Fat Man would have been subcritical even if the pit were completely solid.
Because of these two comments I felt you were implying that the implosion assembly in Fat Man only eliminated the hollow in the centre extremely rapidly, leaving a x4-5 supercriticality at around STP. That’s what I was getting hung up on, because it’s a very plausible misunderstanding, but also very wrong!
There is no real meaning to the term"critical mass" - unless you also specify the geometry and density. A sphere at STP is a reasonable choice. To say that there was a supercritical mass present before firing is a bit misleading to me, although I guess not to you.
Well, I’m certainly not going to get in the middle of this pissing match, but I thought I’d chime in with a fact that I found fascinating. I was present at an interview with Philip Morrison, the Manhattan Project physicist whose car was used to transport the plutonium sphere out to the Trinity site.
He had a spare of the ring of gold foil that had been used to separate the two hemispheres. It was much smaller than I had expected. I’ve read books that said the sphere was about the size of a softball. In fact, it was smaller than a small tangerine, about 2.5 inches (6-7 cm.) Dense stuff, that plutonium.
From my understanding of the concept of critical mass, if you pile enough fissionable material together, you will generate the reaction you’re looking for. This isn’t likely from a terrorist group, however, because it’s inefficient; it requires much more of the fissionable material than a more sophisticated bomb would, and the fissionable material would be either insanely expensive to purchase from black-market sources or so difficult to make that anyone who could make it could design a better bomb than a simple pile of uranium.
A much easier to make WMD would be radioactive material wrapped around TNT. Granted there would be no fission or fusion, and the explosive power would be minimal, the radioactive material would be widely spread. Not to mention that panicking people would probably cause more damage than the bomb itself, running away from it.