I think the gist of this argument is the same. You could build a device to accomplish the same thing as explosives. No one has mentioned a railgun yet or hydrogen gas gun or other form of high velocity driver.
However, if you have the resources to get ahold of the weapons grade fissionables, you probably can come up with some explosives.
Does anyone really think that anyone with the resources to get enough uranium or plutonium for a bomb is going to learn anything useful (that he/she doesn’t already know or can quickly find out with a little research) from any comments made here?
I took this question as: Is it possible to make a nuclear weapon that sprays, wipes, leaks, or turn into air some amount of radioactive material?
That would be a similar to a dirty bomb, usually considered to disperse nuclear material through an explosion. You could spray material out of an airplane over an area, and the material doesn’t have to be particularly dangerous, just detectable, and it could cause a panic nearly as devasting as a nuclear weapon exploding save for the physical damage.
I take the question as:
Is it possible to build a nuclear weapon in a steampunk environment?
(That is, you imagine you can acquire or create weapons-grade fissile material and maybe some imaginary innovations, materials or energy sources, but you don’t have access to electronics or high explosives.)
I wonder if something pneumatic could possibly be fast enough…
I would also point out that Joules Verne imagined a big enough explosive charge to launch two men and a capsule to the moon. Maybe those guys could do it…
I doubt there is anything we discuss that a competent physicist could not work out from first principles very quickly. Discussions about the basic physics and engineering are all over the internet. Certainly some of the stuff used to be classified. Even the basic outline of how an implosion device was built was. The difference between the principles and a weapon that is actually viable is huge. The engineering is insane. Even if you do somehow manage to acquire the weapon’s grade materials, and throw massive resources at the task, there is no guarantee that you will get it right. We are discussing things at the level of the XKCD up-goer-five versus a working moon mission.
Personally I like to know the basics, as it guides understanding of the geo-political issues. Understanding these allows one to understand the nature of the various treaties, and potential new nuclear technologies much better. One thing that is underlined; terrorist use of a nuclear weapon is essentially infeasible. Terrorism would, as noted above, be looking at dirty bombs that contaminate rather than fission.
I am surprised that nobody has noted the steam in steam punk. Heat water in a cylinder with a piston secured by a shearable pin. This allows you to store heat delivered over an extended time. If that alone doesn’t give enough speed you can use it to drive a hydrogen gun.
Oh, sure it is. It’d be expensive to build but it’s quite doable.
Even if the terrorists steal weapons grade fissionables from somewhere? Apparently, for a period of years after the fall of the Soviet Union, there were factories that had piles of the stuff stacked up in them, and very little security.
Making weapons grade uranium/plutonium may be incredibly difficult, but making an unsafe bomb (aka one that doesn’t have the protection from single point detonations) that inefficiently creates a 10 kiloton or so yield…?
I mean, I dunno, maybe it’s as hard as you say. But they got it working in 1945, and surely there were some serious mistakes they made due to limited understanding and technology.
Benchmark for current attempts is probably the North Koreans. Their 2006 test was almost certainly a fizzle. Their second test managed about 10kt.
So a nation state still has this much trouble. Now they are insane, but their scientists are not stupid, and they have the advantages of compute power unthinkable not that long ago. Sure, there might be one weird trick that the Manhattan project didn’t know about. Things like that do happen. But even with the publicly accessible material that does seem plausible, the more you look into it, the more amazing it becomes. As has been noted, getting a fission reaction is easy. Getting one that does anything more than blow up your house is very much harder, and for a wide range of really evil problems. The current discussions on a simple gun assembly weapon won’t actually work usefully for other reasons. And these are much harder to engineer right.
Are we certain the North Koreans had sufficient quantities of the good stuff (highly pure U-235 or plutonium) ?
I mean, you do have a point. The Tom Clancy story, they had a laid off soviet weapon’s designer who had access to exact schematics and techniques for how to do it. I think in the story, it was implied that he may have even copied blueprints from work (since he had some tritium hidden in his basement he stole from work)
Maybe the terrorists would require a resource like that to get even a weak yield.
But, what about military warheads going missing? There’s tens of thousands of them in the world, stored in many separate locations. It seems unlikely that the security around every last bomb is airtight, and every last bomb has a high end PAL electronics package that is resistant to tampering.
Here’s what I think. I think if there were a team of COMPETENT terrorists, with a billion dollars in funding, several hundred die hard operatives with graduate degrees, and they had several years, they could find a way to do it.
I’ll assume uranium, I very much doubt they have the resources or expertise to build a plutonium bomb. (Gun assembly won’t work, Pu needs an implosion type.) Of course we don’t know. Exactly how they obtain it is another question.
The problem is that it is a bit like a soufflé. Any tiny part not right and you get a fizzle. There isn’t a graceful degradation of yield. Still, even a few hundred tons equivalent would level a city block, so would be a handy terror weapon. Reminds me of a sub-plot in the Illuminati trilogy.
Best Bond story ever was Thunderball, and I think in part because the core idea was just too plausible. Such a good story they just kept on making it into movies.
Nukes don’t store well, and most need the triggers renewing, so a weapon that is a few years old won’t work. Eventually the fissile material needs to be reprocessed too. This is part of the reason the US was so keen to build the massive super-computer systems (ASCI Red and Blue initially) to allow them to understand what happens as weapons age without actually setting them off. The continual testing the US did was mostly taking weapons out of service and making sure they still worked, and exactly how well they did. But no doubt, starting with one that works, or did work, is a big leap.
Of course, we only need one Brit in a tux to stop them in their tracks. In reality it would be very hard to keep their efforts under wraps, and even harder to convince anyone who had control over a billion dollars that the most cost effective way of spreading terror or whatever agenda they had was a nuke. Most big terrorist groups exist on clandestine funding from countries or wealthy groups that perceive value in supporting their activities. I suspect no-one would feel comfortable converting such a group into a member of the nuke club. Even the most insane axis of evil nation. But ball park difficulty and funds, I would agree. Sounds sort of plausible. But we are back to North Korea as a benchmark. Both of will and insanity. Even they realise that it is the possession, and not the use, that provides the power.
If we look at the members, or past members, of the nuke club, not all were huge. India, Pakistan, Israel, South Africa, SPECTRE. Clearly it is possible to get a reasonable way once you have the will. Which is why nukes do remain a worry.
How about using a LOT of rubber bands?
The biggest one was the assumption that a plutonium gun-type weapon would be feasible. Originally, they knew enough to be very confident that a U-235 gun-type weapon would work, and spent a lot of time and effort on the “Thin-Man” gun-type plutonium bomb before figuring out that a lot of their estimates were based on ultra-pure Pu-239 samples from cyclotrons, and that reactor-bred plutonium would be contaminated with enough Pu-240 to make gun-assembly infeasible due to the spontaneous fission rate and neutron emission.
That’s why they ended up doing Little Boy as a gun-type weapon and Fat Man as an implosion weapon- they barely scraped up just enough uranium of just enough enrichment to be assured of a big enough explosion. Had they used something like 20% more or enriched it 20% higher in U-235, they’d have had something like twice the explosion.
It’s also why they actually tested the Trinity weapon in 1945- they didn’t know if the implosion design would actually work or not.
OP chiming in—late to the party—yes, this is, essentially, what I had in mind.
Lots of interesting material to chew on, here, so far!
You might be able to use criticality’s in a fashion that compresses an enriched core properly.
Also, gasses compress easier than solid metal, so maybe a gas that has a fissionable element? Though that would probably require very high enrichment and a very large device.
Layering of neutron reflectors, moderators, absorbers, and various levels of enrichment might be able to be arranged in a such a fashion that it implodes a fair bit before it explodes.
You could go all Kim Jong and build a nuclear reactor purposely designed to go more critical as it heats up (ala Chernobyl but taken to the max.) Surround with a metric butt load of cobalt, iodine and a few other nasties. Fire a enriched core into the center of the mess. Not transportable but should piss off a few billion humans pretty well.
OP: I’ve got a little experience on this one. And I’ll keep it short (for once).
No. Not without wasting “bang,” and wasting your time.
I’d weigh in further, but the thread’s pretty much been hashed out for potential options and results. 
Tripler
Oh, let the me and TriPolar confusion begin. 
Yeah and we go back to the crux of the problem. The guys who have advanced degrees. They are not going to want to work for Mr Wannablowupacityforshitsandgiggles.They are going to find high paying jobs which get them fat bonuses and beach houses.
You could get one or a fews guy with such a background who is motivated enough to do this. Not a thousand. And you need many more than that to get working bomb. People often don’t appreciate the massive scientific, engineering and industrial infrastructure that is needed for a programme; to get ONE successful explosion. In short, as North Korea has shown (and Romania, Yugoslavia, Egypt etc showed earlier), the resources of a nation state are not nearly enough.
The trick to a steampunk nuclear weapon would completely be the enrichment of the uranium.
I don’t know how steampunky calutrons are, so I’m guessing centrifuge-based enrichment would pretty much be the only way to go, and I’m not at all convinced that any known 19th century technology would be able to handle building such a precision and high-speed instrument.
Plus, I’m pretty sure that 19th century chemical technology could produce uranium hexaflouride gas either.
That said, with some finger waving to get the HEU, a gun-type weapon is totally doable with genuine 1880s era smokeless powder, I’d think.