At the risk of getting too technical, is there anything besides lack of knowledge preventing a soulcaster from turning some rocks into a bunch of plutonium and exploding?
I know you've got some rules attached to time bubbles to avoid those going nuclear so I wouldn't be surprised if there was something or another.
Well, Soulcasting isn't fission or fusion. It's a spiritual transformation process, not a physical one, and so you don't have to worry about some of these issues. There IS historical precedent of accidentally setting off fission reactions in the cosmere using the magic, but that was a different process. Soulcasting is actually pretty safe. (Well, on a grand scale.)
You could end up irradiating yourself, though, which wouldn't be very fun.
If you know what you were doing, making plutonium or uranium on Roshar wouldn't be difficult. The problem is more a matter of knowledge, and room for scientific exploration. They're unlikely to make atom bombs for the same reason they haven't made gunpowder. Once they figure out that some substances are important, they can learn to make them with Soulcasting (assuming they have Radiants) but some substances just don't occur naturally--so discovering them in the first place is difficult, and would require more modern scientific process.
Okay, just to clarify here (since I'm not sure how up you are on early nuke designs)
A big enough chunk of uranium or plutonium will explode regardless of whether it's in a bomb or not. Early bomb designs just slammed two smaller chunks together so they'd be one big chunk.
For plutonium 'big enough' is about 35 pounds in one place - a chunk somewhere between the size of baseball and volleyball.
If I understand properly, people can soulcast from the cognitive realm into the physical, which implies once we get into a more modern stormlight setting soulcasters will make nuclear submarines look like small potatoes.
Slamming two chunks together so they became one big chunk seems an understatement, from what I remember. I'm under the impression that you had to use a great deal of explosive force to ram them together in order to set off a viable fission reaction. Doesn't it have to be compressed somewhat in order to react with itself?
I'll admit, it's been a long time since I've looked at this, but I remember glancing it over, and deciding that you'd need more than just soulcasting to get it to happen. Though it's not outside of reason that a soulcaster could learn to create super-dense plutonium. The problem is one of understanding, however.
Just like it's totally possible that we, with our current technology, could figure out some huge breakthrough in science allowing FTL or other incredible discoveries. But we don't have the understanding to pull it off yet.
In a modern setting, however, a lot of these complaints go out the window. Let's just say that this isn't the only reason a modern society that can instantly transmute one substance to another is potentially a very interesting place.
You're totally right that everyone currently uses an 'implosion' style compression design. It's a lot more bang for your buck, and you need less radioactive material to work with. They're also a lot safer, because just sitting around they're well below critical mass - without the power-boosting tricks they basically can't go off.
The old "nobody uses these anymore" designs were 'Gun-Type'. Very simple - shoot a uranium bullet into the center of a uranium ring (or vice versa). Inefficient as heck (the Hiroshima bomb only fissioned 1.4% of its uranium), but also super simple to put together.
Despite being simple to build, gun-types were also super unsafe relative to modern implosion devices (among other worries, dropping a gun-type device into the ocean could potentially set it off because of how neutrons react with water). Also, getting the timing perfect on the fissile 'bullet' was a problem, so practically speaking it could only be done with uranium.
After WWII, the only use the US ever had for gun-types was in bunker busters and nuclear artillery (because of course that was a good idea).
Darn, that post turned out longer than I expected it to.
Anyway, I'm looking forward to see you make something really cool out of a post-scarcity transmutropolis setting (especially since the liespren would be in charge of nuclear treaties), and also my roommate just pointed out all the laying out of nuclear bomb details is pointless if they could just make antimatter instead. D'oh.
This is useful information for me, but my gut says that Rosharans couldn't get this working with their current tech level. That said, the REAL issue (as you mentioned in your original question) is knowledge, not feasibility. They'd have to know how to make the right kind of Uranium or Plutonium--and would need to be able to get this across to a soulcaster in a way that works, then THEY would need to get this across to spren. Cross that hurdle, and I suppose it's not at all implausible to imagine Alethi during Dalinar's era with nukes. I suspect the right kind of fabrial could make a trigger device to match ring and bullet at the right time. Depends on how quickly it needs to be going, though.