Building an implosion device is so difficult that it is almost certainly beyond the range of any terrorist organization, but assuming that they could get the special nuclear materials they need, how hard would it be for a non-state entity to build a gun-type weapon? How much material would they need? Could something like the device featured in the Fourth Protocol be built?
Thanks for your help,
Rob
Positing that you already have the materials (notably, enriched uranium–unavoidable isotopes of plutonium (242) fission to quickly), buiding a gun-type bomb requires no more precise a fit than a car engine. The design of such a weapon is simple in concept and not too much more difficult in execution, with the critical factor being the configuration of the core such that the reaction doesn’t fizzle (blow itself apart before a significant amount of fissions occur) because of the relatively slow speed of the “bullet” and the rapid progress of the reaction (on the order of microseconds). They can, with the right design and highly enriched uranium, also be very compact–a couple of meters long and very narrow–compared to a similarly primitive explosive containment device.
Such devices, however, are inherently dangerous. Unlike explosive containment, which requires precise detonation and control to get a reaction, the gun-type device merely needs the core to enter the housing (or two hemispheres to come together, or however you’ve designed it), and so you are dependent upon mechanical safeties and interlocks to prevent this. There’s also the danger that, if submerged in water, the individual pieces could undergo a supercritical reaction, though the resultant explosion would probably be a small fizzle. And because they depend upon a slow, unaccelerated fission reaction, they are extremely wasteful, extracting only a percent or two of total energy available from the material.
Here’s the Wikipedia article on gun-type fission weapons.
By the way, while an implosion fission weapon might be beyond your average terrorist group, it is not beyond the means of a dedicated organization which has recruited any of the large number of Russian, East German, Pakastani, or other scientists and engineers familiar with nuclear weapon design. Aside from the difficulty of obtaining the special materials, simulating, designing, and building the weapon is a relatively modest undertaking, and most of the non-nuclear components can be had off the shelf in any developed nation, including explosives (high grade demolition explosives will be more than adequate). It’s also possible, in theory, to build a fusion weapon that doesn’t require a fission trigger, and some nuclear engineers believe that this has already been done. Should such technology ever find its way into the mainstream, constructing thermonuclear devices may be the work of competent technicians rather than teams of highly trained scientists and engineers.
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For this reason, the US funds several make-work research projects which involve the appropriate areas of expertise, to try to keep such scientists “off the street”, as it were. Of course, some good physics does get accomplished along the way, as well, but from the government’s point of view, that’s just icing on the cake. For instance, much the same technology that’s involved in implosion-type nukes can also be used to generate extremely strong magnetic fields (albeit for only a very short time).
Are the only two reasonable fission designs “create criticality with a precision implosion” and “combine chunks to assemble a critical mass”? Are there feasible designs involving moving neutron absorbers or reflectors around? Spallation?
Neutron generator? Some sort of electrically active moderator?
Really? Not saying you are fibbing…I am just genuinely curious (note to NSA: No, I am not trying to build a nuke). But seriously, I cannot imagine conventional explosives could produce the pressures and temperatures necessary to start a fusion reaction in a bomb. I mean, there is a reason they wrap a fission device around a fusion device. I assumed nuclear weapon designers would have happily used regular explosives to achieve fusion if they could.
Just a total WAG but you might not have to use conventional explosives. Some sort of an accelerator or laser might be capable of starting a fusion reaction maybe?
Well, it’s been quite a few years, but the information I had at the time I studied such things, the non fission fusion type reaction was one of two things, very small, or very large. In other words, you can probably build a laboratory model that will produce a fusion explosion about as strong as a hand grenade, for just a few million dollars a shot. It might have some level of reusability for the remote portions of the laboratory. It will also have an energy budget that requires a nearby power plant with only a few other customers.
The large model will give you the multiple megaton results you want. The thing is; it requires a building sized collection of very heavy equipment. It’s great for nuking your own home town, but a bit unwieldy against other nations, or even distant neighborhoods. Tre real problem is not starting a fusion reaction. The problem is keeping a fusion reaction going for long enough. However much the science of miniaturization has progressed, the problem of fizzles (your bomb blows apart before it is done blowing up.) is even more pronounced for a non nuke trigger methodology. Concentrating enough fusable plasma is tough enough, keeping it there for five or six microseconds is a major engineering hurdle.
Now, if you could put it all in a garage sized building with a three foot thick wall of tungsten, you might have something. Once again, though, your electric bill is gonna be the talk of the town.
Tris
There is speculation–as far as I know, totally unfounded–that the main reason the United States has permitted its abilities to enrich and process weapons-grade material to laps is that it no longer requires fission triggers for the next generation of thermonuclear weapons, and that such weapons can be far smaller and more refined in yield than existing multistage devices. There are certainly posibilities for creating conditions to initiate a brief nuclear fusion reaction without a fission reaction via high-energy x-ray laser containment or magnetic pulse pinch, but while it might be possible to build the actual equipment small enough to be portable (something smaller than the Sandia Z machine anyway), the power source driving it would have to be massive, or the reaction driving it (in the case of something like an excimer laser) would have to be highly energetic. I find it unlikely that such a technology exists today, but it is conceivable that in the future a pure fusion weapon, requiring no enriched heavy radioactive isotopes, will be possible.
There are also claims–again, wholly unfounded in scientific literature–about something called “red mercury”, which is gererally claimed to be a highly energetic ballotechnic paraexplosive. While ballotechnic substances due exist, none are known to have any output within several orders of magnitude of the specific energy that would be required for initiating fusion. Red mercury is generally regarded as crackpotism among nuclear scientists, despite its boosterism from Samual Cohen, inventor of the American neutron bomb.
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