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In a nutshell, what are the pre-requisites for having sailing ships fire nuclear cannonballs? The setting is entirely alternate-universe, so the available material can be a lot more naturally "primed" for military use, but not so much as to create a high likelihood of a nuclear natural disaster.

Also, a running theme of the entire setting is that (conventional) explosive devices were invented very early (by cavepeople, essentially) and have ever since been culturally shrouded in occult/"alchemical" theories, so the preferable path for developing tactical nukes would be an accidental success amid a mass of pseudo-science. Eventually, nuclear weapons would be banned around the time steampower is common, so they never get as far as strategic nukes.

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    $\begingroup$ You're going to have to strain your world quite a bit. Making a nuke turns out to be a remarkably difficult activity. It only took hundreds of the best Physics minds in the world years to make one with modern technology. How much are you willing to fudge the nuclear physics to make it easier to do, both from a "how hard is it to make a nuke explode" perspective and a "how hard is it to manufacture." $\endgroup$ – Cort Ammon - Reinstate Monica Apr 10 '15 at 15:27
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    $\begingroup$ In particular, "critical mass," or as it is better phrased, "critical density," is a big deal in nuclear physics, and its a hard one to overcome by simply having more "naturally primed" materials handy. Some of the rules of physics may need to be bent to overcome this. $\endgroup$ – Cort Ammon - Reinstate Monica Apr 10 '15 at 15:29
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    $\begingroup$ Do you mean a nuclear cannonball as in Nuclear Artillery? Or simply a shell that spreads radiation like a dirty bomb? The second would be a lot simpler, as you could make a normal cannon shell out of uranium full of explosive. A gun style nuke would be easier than a implosion style nuke, assuming you know the theory and have sufficiently enriched material. $\endgroup$ – AndyD273 Apr 10 '15 at 15:29
  • $\begingroup$ @CortAmmon Tweaking physics is not a fundamental problem; I'd just prefer to keep the concepts familiar (i.e. no obscure new "rays", chemical element types, etc.) and stay consistent. $\endgroup$ – Nikolay Ershov Apr 10 '15 at 15:33
  • $\begingroup$ @AndyD273 I'm afraid it's got to be nuclear artillery. $\endgroup$ – Nikolay Ershov Apr 10 '15 at 15:35

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Uranium that got straight from supernova can have more than 50% of U-235, after several billion years it goes through half-decay many times and you get you ordinary 0.7% uranium. So if an asteroid formed from supernova matter and then came from interstellar space and fell on Earth you would have natural enriched uranium.

Critical mass for 50% U-235 without reflector is 170 kg, so two 150 kg pieces thrown at each other would make the boom.

They should move fast enough to join before chain reaction would start. 300 kg would emit a neutron on average about every 6 msec. Smoothbore gun with black powder propellant produced speeds up to 450 m/s, so insertion would take about 0.1 msec - fast enough to make it reliable.

It efficiency can be determined by Serber Efficiency Equation - it would be 5-7 times worse than for Little Boy. But it has 5x amount of uranium so the yield would be the same or little less - 10-15 kilotons.

You can make it much more effective with beryllium reflector, but I don't think 18th century science would guess it.

So uranium would be mined for usage in paints. Then someone would notice that big blocks of it are always warm. Attempts to get bigger blocks would result in pulsed chain reaction(see Godiva device), death of experimenters and small boom. Eventually someone would fling uranium with a gun and get the big boom and that's when bomb making would start.

The bomb would consist from several ton bronze mortar that would fling one 150 kg piece of uranium into other piece. Most likely it would be used in a kamikaze brander to blow up entire enemy fleet.

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As AndyD273 pointed out, there were two early designs for nuclear explosives. Fat Man was an implosion weapon, Little Boy was a gun-type weapon. Both rely on relatively sophisticated technology to bring the nuclear material into a supercritical mass.

  • You need the math to calculate the necessary shapes and forces. An 18th century civilization would have to invent that, first.
  • Then the nuclear material has to be assembed into just the right shape. We're talking about radioactive and toxic materials, so handling it is a bad idea. The precision is probably beyond the capabilities of instrument makers, too. Historical cannonballs were a couple of percent smaller than the diameter of their gun, both to ease loading and because a better fit was beyond the technology of the time.
  • Then the nuclear pit has to be surrounded by explosives to smash it into the right shape. Those explosives have to be stable, burn in a highly predictable pattern, and powerful enough to do the job. They have to be triggered in just the right sequence by electronic switches, otherwise you might get a fizzle.

Consider how long places like North Korea India had to work on their nuclear weapons, even after they had the fissile material.

A dirty bomb would be lower tech, but it would be hard to use it with 18th century technology without danger to your own troops.

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  • $\begingroup$ An implosion weapon would be super complicated, and fairly hard to build without fizzle. A gun weapon is much much simpler, but requires a lot of material for a fairly low yield. Basically a sub-critical mass is pushed into another sub-critical mass with force, creating a critical mass. If the world has high grade uranium naturally and abundantly, reducing the amount of refining that is needed, then you could make one without a lot of technology. $\endgroup$ – AndyD273 Apr 10 '15 at 16:12
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    $\begingroup$ I'd like to re-emphasize just how precise the triggering explosions have to be. One viable way to destroy a live nuclear warhead without it going off is to set off explosives near it so that sympathetic detonation will trigger its internal explosives in the wrong order. That's just not going to be achievable. $\endgroup$ – Nathan Tuggy Apr 10 '15 at 16:43
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    $\begingroup$ @NathanTuggy Just to point out that is only true of implosion type weapons. If the force of the explosion is off just a little the fissile material will squirt out the side of the bomb like pinching a grape. A gun style bomb does not have that problem. There is only one explosive charge which is used to push one slightly sub-critical mass into another slightly sub-critical mass, combining them into a super critical mass. The reason they weren't used more is because they had low efficiency for the material used. $\endgroup$ – AndyD273 Apr 10 '15 at 18:42
  • $\begingroup$ @Nathan Tuggy: The explosions don't have to be that precise to get a crude weapon. As a thought experiment, what happens if you pick up two sub-critical masses and slap them together? The real problem with building nuclear weapons is getting a critical mass of fissile material in the first place. That's what the Manhattan Project devoted most of its effort to. $\endgroup$ – jamesqf Apr 10 '15 at 18:44
  • $\begingroup$ @jamesqf: Hmm. I guess you can get a little something out of it, but managing to get anything even as energetic as a chemical explosive is still nontrivial. (If you slap two subcritical masses together, mostly what happens is they fission enough, while you're slapping, to reduce the mass below what you need for a sustained prompt supercritical reaction.) $\endgroup$ – Nathan Tuggy Apr 10 '15 at 19:32
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A riff on @AndyD273's answer:

Pitchblende was only known in the middle ages from one place in what's now the Czech Republic. But 1% uranium Oxide yellow glass was found in Ancient Rome over 2000 years ago.

Isolating Uranium from the ore is not simple.

Some organisms can concentrate Uranium. Citrobacker can encrust themselves with uranyl phosphate crystals. Some lichens concentrate uranium, and some fungi cause plants to concentrate uranium.

Precipitating the metal involves extreme alkali solutions, which makes me think of dye making. What if… wode dye craftsmen found another plant (or lichen) that produced a vivid yellow dye, and cultivated a source of uranyl phosphate. Their use of extreme alkali solutions, and skill transfer with those also working with lye and nitrates for gunpowder manufacture leads to purposeful experimentation and careful note taking of anything odd, because of lucrative history in that area.

So they stumble onto "dark gold", which is noticed as being as heavy as gold; heavier than lead and like nothing else except gold. Given the association with yellow colored dye, alchemists jump on it thinking they are close to making actual gold.

The next problem is in recognizing what it can do. Before photography or real chemistry, how could they tell that Uranium was doing something?

Fluorescence was reported as far back as 1560. Many fluorescent materials exist in minerals and organics, and without a source of UV light nobody noticed the effect. Some fluorescent material, or a mixture of florescent emitter and a UV emitter, might give off visible light when subjected to ionizing radiation.

What if dyers used Umbrelliferon as a cloth whitener? Now that process uses alchohol not alkali, but treated cloth might glow in the dark when exposed to the dark gold.

The underlying problem is that Uranium isn’t that spectacular without making an atomic pile or having some atomic theory that was discovered because of radioactivity. The build up of daughter elements makes it more radioactive and toxic as it ages. Once radioactivity is “seen” through fluorescence, the quest might be undertaken to isolate the radioactive part, and harvest what seems to be produced over time. Without chemistry, that seems like a haphazard endeavor. But in our fictional world, practical pre-chemistry might be more advanced as an art, with the rich dye-makers’ history and now culture of experimentation.

This alternate history of early discovery of radioactivity doesn’t end up with practical weapons though. Although I like Heinlein’s idea of using toxic radioactive dust (written when it was thought that a chain reaction was not possible), a toxic radioactive powder would be very expensive and impractical for this pre-technical society to use as a weapon.

The same industry, on the other hand, would have produced explosives a lot better than gunpowder, and nitrates would be weaponized, rather than radium. Radio-isotopes would be used for instruments and maps that can be read below decks, but not bombs.


footnote: For more on wode dye and alkali solutions, see Tony Robbins Worst Jobs in History. I suppose it would be the Anglo-Saxon episode. You might find it on Youtube.

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Critical mass will be your worst enemy.

You can't just scale a nuke down to any size you please, because you need enough material to build a high enough neutron density to start the reaction. Too small, and you just cant make it go boom, no matter how hard you try.

There are some tricks, like neutron reflectors, that you can pull, but I really don't think those are going to show up in 18th century cannon balls, so we're stuck doing it by pure mass.

The critical mass depends on the density of the fissile material. 20% enriched Uranium won't go critical until you hit around 400kg, which is a lot for a cannon. Fortunately, we're willing to handwave a little on the geology. Lets assume there's just slugs of uranium-235 lying around, unadulterated by all that pesky U-238 that the US spent so many millions figuring out how to separate out. From my leaked top secret nuclear test data (and by that, I mean Wikipedia's page on Critical Mass...), we see that a pure slug of U-235 has a critical mass of 15kg. Given the density of uranium, that's about 4.5 inch ball (spheres are the most efficient form we'll get).

That is reasonably on par with a cannonball. However, that's going to be troublesome once you finish putting it together. You're going to want a gun-style nuke (implosion style will be beyond the capabilities of the 18th century due to timing issues), so you're going to need two half-balls which are slammed together to cause the nuke.

This will be an issue. A nuke, as it goes critical, obviously blows itself apart. If you don't shove the parts together hard enough, you end up blowing the two halves apart before they finish reacting, causing a "fizzle." Now you just have a pile of radioactive mess to clean up. Whatever mechanism you have for this, and its fuze, need to survive being shot out of a cannon. For some perspective on the explosives involved, Thin Man demanded the two halves approach eachother at 3000 feet-per-second or faster. Any less would cause a complete fizzle (no nuclear effects). This is going to call for your 18th century team to invent high explosives -- gunpowder isn't going to cut it.

Finally, there's range to be concerned about. Remember, we can't make anything smaller than 15kg, so the only way to adjust the yield is by making it inefficient. From my other top secret source (okay, fine, Wikipedia's page on the Little Boy bomb), 15kg of material, perfectly detonated would yield about 40kilotons (twice that of the bomb dropped on Hirisohima). With some handwaving, that comes out to somewhere just around a 150m radius fireball (needless to say the lethal range for radiation is a lot larger). You're going to have a challenge firing this far enough to not irradiate your own crew into oblivion unless your bombs are markedly inefficient. The fine art of making a bomb explode at 1% yield but not fizzle would be quite a feat, so I would expect 18th century craftsmanship would not be up for the task.

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  • $\begingroup$ From wikipedia: "The Little Boy was 120 inches (300 cm) in length, 28 inches (71 cm) in diameter and weighed approximately 9,700 pounds (4,400 kg)." Good luck launching something that large using 18th century technology. Best I can figure is you'd use suicide vessels, similar to fireships. $\endgroup$ – Deolater Jul 16 '15 at 20:36
  • $\begingroup$ @Deolater The idea of suicide vessels with nuclear weapons is nasty! $\endgroup$ – Cort Ammon - Reinstate Monica Jul 16 '15 at 20:53
  • $\begingroup$ I believe one of the reasons early nuclear weapons were so heavy is the fissile material was surrounded by a fairly heavy and dense neutron reflector to ensure you could get nuclear fission. Making neutron reflectors from lightweight but toxic materials like beryllium is difficult enough for 20th century technology, so I don't see that happening in this setting. $\endgroup$ – Thucydides Dec 18 '16 at 0:43
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You probably don't want nuclear bombs in the modern sense.

  1. They are sensitive to isotopes, which seems a reach for the tech level. Refining the weapons grade materials is the big issue even now.

  2. Making them explode predictably requires more precision than seems likely. It is not quite as difficult as some answers imply to simply get an explosion, but if you also want it to be safe when you do not want it to explode and not to be unnecessarily large and heavy and not to generate excessive nuclear contamination and... Well, you really want control and that requires precision.

  3. It is hard to imagine anyone accidentally noticing the conditions needed for nuclear explosion in a way that gives anyone a chance to make notes of it. When a successful experiment implies instant death of all witnesses that tends to slow down progress.

  4. As others have noted effective cannon range is too short for the ship firing the round to have a realistic chance of survival. This weapon would be very unpopular with the crews of your warships. Maybe something similar to fireships, sail towards the enemy and then kill them all, might be practical.

But, if you skip the huge fireball requirement and scale down to something your own ship might survive, even if the crews have weird diseases afterwards, something might be practical.

Basically, you want three or four different metals.

  1. A neutron source. A highly radioactive material that generates a relatively steady source of neutrons. Doesn't need to absorb neutrons and be capable of chain reaction. Better if it isn't.

  2. A fissile material. Absorbs the neutrons and produces radiation on fission, either neutrons or gamma rays. Better if this isn't capable of chain reaction either, so either gamma rays or neutrons of the wrong type. I'd guess most would produce both of the radiation types.

  3. A neutron reflector (or moderator). Something that lets you control the exposure of the fissile material to the neutrons produced by the source.

  4. A gamma reflector. Optional, but it is generally a good idea to make an effort to send more energy towards the enemy than your own ship.

A projectile would be cylindrical, maybe fired from a rifled cannon. It would have some empty space inside with neutron reflector and the fissile material at front and moving part of neutron source and reflector and gamma reflector at back. Normally the neutrons would mostly escape (some fluid moderator might be good though), but on impact the neutron source would slam into the fissile material and be fully encased in reflector and the neutrons would be much more likely to hit it. This would result in large pulse of radiation mostly reflected forward before the warhead vaporizes. And covers the enemy in hot radioactive material, which would burn the ship and kill the crew. Except the radiation pulse already did it.

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Ok, here's my brief history of the discovery of nuclear weapons.

[From the sealed archives of the Royal Alchemist][Secret]

Viewing of this document by unauthorized persons is prohibited.
If you have started reading this in error, stop now and report to the King's Executioner immediately.


It was discovered two hundred years ago, back in 1604, by the famous alchemist Baston, that certain elements had very strange properties. They were warm to the touch, and in pure enough concentrations would heat water or cause burns. It was also discovered that these elements were harmful to living tissue. Sadly this was only really discovered after Baston's death. With more caution other alchemists researched these elements.

It was discovered that lead could somehow block these effects while other metals couldn't. Then one day, a researcher named Jeon who was trying to understand the element's ability to heat water stacked two large and fairly pure samples together. The lab had to be abandoned, but it did open up new avenues of research: How much material was needed to cause the reaction? Was just touching the elements together enough? Would putting them together forcefully make any difference?

Luckily after Jeon's accident it was decided by the alchemy guild that copies of all research notes had to be kept off site in a secure location, since no records could be recovered from the crater.

At this point the king stepped in to stop general research into the material. This was partly because several hundred acres of prime wilderness had been leveled, partly because he was wary of the alchemist guild having that kind of power and what it might mean for his rule, but mostly because the ongoing tension between himself and the king of Goan to the south, and the ability to dispose of Goan's troops the next time they marched on his kingdom.

And so his most trusted state alchemists carefully continued the research into element weapons.

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  • $\begingroup$ So why didn't it happen that way? It did happen, once Curie isolated radioactive elements, but nobody did so that far back. The didn't know about elements or real chemestry: Newton wasted a lot of time at it but chemestry could not be invented until more scientific principles and methodology existed, and Galleleo-Newton was the beginning of that. $\endgroup$ – JDługosz Apr 11 '15 at 2:40
  • $\begingroup$ @JDługosz This is an alternate history/alternate dimension where they had explosives in the stone age. Stuff happened differently. This dimensions version of Curie happened a bit earlier. They don't have the theory or knowledge of elements, But they do have a bunch of alchemists that are trying to do silly things like turn lead into gold. By chance they discover radiation, without understanding anything about it, and then through a series of events over a long time evolve it into something much more destructive. It's a silly little story, but I thought it answered the question. $\endgroup$ – AndyD273 Apr 11 '15 at 5:23
  • $\begingroup$ See my elaboration on that as another Answer. $\endgroup$ – JDługosz Apr 11 '15 at 6:01
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Say the approach outlined by AndyD273 in his answer works. The Royal Alchemists have, though careful experimentation and the judicious expenditure of expendable apprentices, build their bomb which really explodes instead of fizzling out. (A fizzle would be much more likely than a full yield explosion.)

  • Will the bomb survive being fired from 18th century cannon? The first nuclear weapons were bombs, not shells. Even ordinary explosive shells only really worked in the 19th century.
  • Does the range of the cannon make any practical difference? If it is mounted on a ship, friend and foe will be caught in the same fireball.
  • What about the fuse? Can it be made to explode on impact (or even better in the air)?
  • Just how big is the bomb, compared to typical 18th century cannon? Little Boy might have fitted into the tsar cannon, but not into a practical cannon.

Instead of cannon, use black powder rockets.

Much easier to simply abandon one of those expendable alchemists' apprentices on a raft to trigger the weapon by hand.

(I'm feeling sorry that I sound so negative about your story, but you'd better stick to "because it is magic, don't ask" instead of scientific explanations.)

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  • $\begingroup$ A black powder rocket is possibly not a bad idea. There has been one nuclear artillery shell tested. It had a 15 kiloton blast, which has a lethal radius of about 1.3 kilometers. The Atomic Cannon was a really really big gun. I don't know if it could be ship mounted on anything smaller than a destroyer... It had a range of about 10,000 meters. $\endgroup$ – AndyD273 Apr 10 '15 at 19:35
  • $\begingroup$ It's perfectly okay to be negative, actually I'm not that enthusiastic about the idea myself. I'd actually prefer it to be something difficult to achieve and hence limited-use and far from a military game-changer. If it's still too much of a stretch I may have to do with something entirely made-up. $\endgroup$ – Nikolay Ershov Apr 10 '15 at 20:08
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    $\begingroup$ @AndyD273, there were plenty of different nuclear shells. American 155mm, 203mm, the early 280mm, and the Soviets had their own designs. It should be no problem to mount them on a smallish gunboat, like the 19th century Rendel gunboats, but first you have to have the cannon. There are centuries between an 11" smoothbore firing round short and the 280mm Atomic Cannon. $\endgroup$ – o.m. Apr 11 '15 at 6:29
  • $\begingroup$ @o.m. I know that. But only one was fired, at least by the US. And if explosives were discovered that early, then why couldn't other advances be discovered earlier, like rifling, etc. $\endgroup$ – AndyD273 Apr 11 '15 at 17:06
  • $\begingroup$ Firing delicate high explosive shells in the late 1800's led to the development of "Dynamite Guns", essentially huge air cannon. The The Zalinski dynamite gun is perhaps the best known, but a more portable "field" version using a black powder charge to drive the air compressor was also designed and fielded (The Sims-Dudley dynamite gun). infogalactic.com/info/Dynamite_gun $\endgroup$ – Thucydides Dec 18 '16 at 0:50
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Since you seem to be open to messing with reality a bit I can think of two possible tweaks that would permit nuclear cannonballs:

1) Make U-238 capable of sustaining a bomb-type reaction. (Reality: The energy of the neutrons it releases during fission is too high to cause fission in the next atom struck. A moderator is needed to slow them down, this also slows it down below possible bomb use.) This permits a uranium gun design like Little Boy. While O.M.'s precision argument is very relevant to an implosion design it doesn't apply to a gun design. You can considerably over-engineer the masses so the weapon is well above critical mass when assembled--it need not be perfect. Likewise, the assembly isn't that critical--if you simply pick up the two halves in your hands and slam them together it's got roughly a 50% chance of functioning properly.

2) Substantially reduce the neutron emission rate of plutonium. This would mean you could use a gun type device to set off a Pu-239 bomb.

You do have the Davy Crockett problem here, though. The Davy Crockett was basically a nuclear bazooka round that was the butt of jokes because the lethal range of the warhead exceeded the flight range.

For the army this was acceptable--you dig a foxhole and everyone gets in it while the weapon is in flight. Since the threat is direct radiation (for baby nukes the radiation is the longest-ranged threat) it doesn't take much depth to provide a nearly perfect shield (remember, the radiation can't turn corners, the bomb is low in the sky meaning it has to go through a lot more dirt than the depth of the foxhole and every foot of soil cuts the dose tenfold.)

However, you are talking a naval environment. Last I knew you couldn't dig a foxhole in water. Your nuclear cannon is basically 100% lethal to the crew of the ship that fires it.

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  • $\begingroup$ Foxholes, specifically, don't do very well in water, but the concept of their use in this context works just fine there. Meter for meter, sea water shields better than dry soil. Build your ship with a fairly high ratio of ship mass to crew size, and sacrifice a little bit of sailing speed by disrupting hydrodynamic hull shape, and you should be able to easily stick a compartment in the underwater portion of the hull that is positioned such as to provide basically the same benefit via sea water as the foxholes provide via dirt. $\endgroup$ – Matthew Najmon Apr 11 '15 at 14:27
  • $\begingroup$ Or you could just build the ship to be basically semi-submersible. Before firing it, flood some tanks, and the ship drops down til there's just a slight bit peaking up out of the waves. $\endgroup$ – Matthew Najmon Apr 11 '15 at 14:31
  • $\begingroup$ @MatthewNajmon The flight time of the cannonball will be too short to reach safety--you'll have to be below the waterline when you pull the trigger. Good luck aiming that gun remotely with 18th century tech! (Firing could be done--you pull a spring-loaded rope to pull the ignition source to the cannon.) $\endgroup$ – Loren Pechtel Apr 11 '15 at 18:03
  • $\begingroup$ You get all but one guy (the gunner) already into the shielded compartment before you fire. That guy has a direct vertical shaft down to a padded surface. He lines up the shot, then jumps, holding the string. That minimizes the delay between aiming and firing (so they have less chance to move, hence better accuracy), while still giving the gunner just enough time to get into the shielded hold before detonation $\endgroup$ – Matthew Najmon Apr 12 '15 at 15:31
  • $\begingroup$ This pillow shaft would, of course, be a later improvement to provide such accuracy benefits. Initially, shots would simply be fired a couple seconds after they were aimed (the time it takes the gunner to rush down a ladder), and the weapon would mostly be used on things with slow enough, or at least predictable enough, movement for leading of shots to be sufficient to deal with the delay $\endgroup$ – Matthew Najmon Apr 12 '15 at 15:33
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I believe dirty bombs is the way: poison their cities.

While purifying the uranium and building a gun-type bomb with it would be within reach of their technology (it's really not all that difficult), inventing it without knowing nuclear physics is completely impossible. The methods of purifying uranium are quite counter-intuitive; you won't find them through blind luck of random experimentation. The actual mechanism of the bomb, while not very complex, requires very high precision of the parts; you won't get it right if you don't know what you're doing.

You can get the natural uranium relatively easily and you can poison people with it, but extracting the highly-radioactive isotopes would be out of reach of anyone, who doesn't already know what exactly needs to be done (e.g. expose a liquid uranium compound to increased gravity: put it in a container and spin it around in a centrifuge fast enough that the layers with heavier isotope separate; it's not a salad spinner scale of force. More of a continuous operation trebuchet!)

Now if you still aren't satisfied with dirty bombs and need the nukes, you will need a bit of handwavium: a manual book falling through a hole in time. It should explain all the processes necessary (using modern technology), effects and caveats. The 18th century scientists would be then able to re-create the technology using their resources, of course at a drastically higher costs, both in money and in human lives.

Of course getting a kiloton out of the constructed bomb would be a huge success, and considering the costs, its direct military advantage would be so small redirecting the money towards the classic army would be more profitable, still - the psychological effect of the detonation might win the war and repay the expenditures.

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  • $\begingroup$ That's what Heinlein wrote about forecasting the Cold War, before it was known that fission chain reaction was possible. He postulsted radioactive dust as a weaponnwith no defense. $\endgroup$ – JDługosz Apr 11 '15 at 1:30
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OK, back to first principles. You have a setting which has reached approximately 18th century technology overall, but it is advanced in the field of explosives. The explosives might be cloaked in alchemical gibberish, but let's avoid actual magic if we can.

The goal is to develop a weapon with a effect, but difficult to build (so they're not found all over the place) and ultimately banned by common agreement between all civilized people.

Naval Combat

  • How about heated shot or the later Martin's shell? Iron armor of reasonable thickness and quality is probably beyond 18th century metalworkers, so all ships are wooden and quite flammable.
  • Alternatively, early explosive shells.

Ground Combat

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This might work using a grab bag of technologies and a few handwaves.

Starting with the device itself, a fission "gun" bomb is by far the easiest to construct. Many posters and commenters have discussed the difficulty of developing a nuclear theory so people know that nuclear reactions are possible at all, so I'm going to pass that part over and just assume that this was discovered by accident or serendipitously.

In order for the "Gun" type bomb to be useful as a seagoing weapon, it needs to be as light and simple as possible, so a lot of devices we find on modern weapons will be missing. The most obvious one will be a neutron reflector, but fortunately, water can serve, so the warhead needs to be detonated underwater. For a weapon fired against other ships, or potentially into the harbour guarded by a fortress, a nuclear explosion in the water will provide pretty much all the mayhem you need.

enter image description here

Swordfish nuclear test

As noted, a conventional cannon isn't going to provide either the range you need, and potentially the shell will be either wreaked by the explosion or be detonated inside the barrel, both obviously bad for you. In the late 1800's, early high explosive shells filled with dynamite also had the same problem, so giant compressed air weapons called "Dynamite guns" were developed. For a sailing ship, having a large steam powered air compressor may be problematic, so the smaller alternative Sims-Dudley dynamite gun would be preferable. Using a black powder charge to drive the compressor piston, this is somewhat possible using the technology of the day (after all, the Girandoni air rifle was fielded in 1779). Since nuclear weapons are area weapons, so long as the device falls in the water near the target, you should be able to ensure critical damage.

enter image description here

Sims-Dudley dynamite gun

Lastly it was noted that being too near a nuclear explosion isn't very good for the crew of your ship, either. Wooden hulls don't provide much radiation protection, and an airburst would also send a huge thermal and radiation pulse, most likely setting the rigging and the rest of the ship on fire as well. Once again, the Captain and crew might have a few reservations about this project. However, having a semi submersible ship is entirely possible in this time period (Leonardo da Vinci had drawn plans for workable submarines and paddlewheel boats as far back as the 1490's, so depending on the availability of genius inventors, you could move you timeline around considerably). Robert Fulton was such a genius, and he built and demonstrated the first truly practical submarine, the Nautilus, between 1793 and 1800.

enter image description here

Robert Fulton's Nautilus

Fulton had drawn up plans for a much larger follow up boat, capable of carrying a crew of six and supplies for 20 days at sea.

enter image description here

Fulton's improved design

The combination of a hand cranked submarine, a compressed air cannon and using a nuclear device more like a mortar or howitzer than a cannon (and handwaving the nuclear weapon itself) seems to overcome many of the objections of this idea. It is still only on the very edge of practical, but given the immense power of nuclear reactions, it seems that using single weapons against very high value targets is going to be both more practical and more realistic than having a Line of Battleship unload a 70 gun broadside of nuclear shells. Even if using nuclear weapons is totally impractical or impossible, the combination of a submarine and an air cannon provides the means to carry out raids against high value targets.

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1. Weapon of mass destruction

Assumes radioactivity was discovered in 18th century and people have understood the relationship between mass and energy, they will definitely exploit the power locked inside the atoms.

A single neutron can set off a chain reaction provided the density of neutrons in the region of space is high. I'm talking about highly enriched uranium which is made of at least 90% of uranium isotopes (U-235), natural uranium ore extracted from earth has less than 1% of these isotopes on average. A lengthy and tedious process is required to enrich the uranium, I personally favor gaseous diffusion technique, the process convert the uranium ore into vapor and introduce chemical for purification then spin it really fast so that the heavier particles will flung against the wall via uranium centrifuge. Repeats this multiple times so that we have a weapon grade uranium, didn't I told you it is not going to be easy. Once you have achieved this target of density of U-235, designing a reliable remote controlled firing mechanism to set of a series of chain reaction will look much easier. There you go the power of atom at your finger tip.

Verdict

  • Difficulty: High
  • Destruction: Devastatingly High
  • Feasibility: Low

2. One shot one kill

You may ask what about the byproducts from the abovementioned diffusion technique, don't discard it you can recycle these into depleted uranium shell or bullet which is many folds denser than conventional projectiles. Now you can pieces your enemies from afar.

Verdict

  • Difficulty: Low
  • Destruction: 1/1(see sub title)
  • Feasibility: Moderate

3. Shields up

The byproducts can also be used to improve armor rating of military transportation and infrastructure due to it high density. No worry the radiation emitted is too little to pose a threat of human health.

Verdict

  • Difficulty: Low
  • Destruction: Negligible
  • Feasibility: High

4. Health screening

On the battlefield things will look nasty and harsh especially when soldier suffered broken arms or legs. The doctor can apply radiography to peek into the affected body parts without the needs to cut it open and risk infection. They only need a X-Ray generator and a photographic films and a willing wounded.

5. Verdict

  • Difficulty: Low
  • Destruction: Very Low
  • Feasibility: High

6. Note

The list can go on forever but for economical solution I pretty sure there are many alternatives available for the 18th century folks, unless your nation wants to build pressure on your enemy this is your chance.

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I would say a small, fizzle-based dirty bomb could work, provided they realize that the nuclear reaction causes illness in everyone who happen to be near it.

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