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In this setting nuclear material is incredibly cheap, while all hydrocarbons need to be synthesized from scratch. Additionally people have shadow magic which allows them to both easily shield against all radiation except non-relativistic atoms, and separate different isotopes. So with radiation not being a major concern, nuclear tech can really be used to its fullest potential.

Nuclear tech and medicine are half a century ahead in this world, while other areas like material science are a few decades ahead, and areas like AI are actually well behind our world.

Breeder reactors are common and very cheap given lax regulations, cheap resources and undead/bound spirit labor. So there is incredible flexibility with regards to what radioisotopes can be used in various applications. This also means some isotope like californium will definitely be used in order to achieve a lower critical mass (with the help of beryllium oxide or another high temp neutron reflector).

People even have personal nuclear reactors: With shadow wards that in response to high enough radiation levels or another trigger can automatically spread magical darkness at relativistic speeds to absorb radiation, preventing meltdowns. Indeed in some contexts such as nukes (used very extensively for excavation/mining) or even reactors if their wards are extremely safe from damage people can get away with using mechanisms with concentrations of radioisotopes that should instantly go supercritical without magical darkness absorbing neutrons.

With people living in an Antarctic environment, waste heat is actually a plus, and people can afford to be profoundly wasteful because they live on an infinite flat plane that people aren't concerned about polluting so resources very cheap (it feels normal downwards acceleration but while it has otherwise realistic physics it has no true gravity).

The Engine

This style of nuclear engine would be massively altered/simplified from something like the ramjet used in Project Pluto: After all spraying out large amounts of radiation can be greatly ameliorated with shadow magic, and people in this world don't mind a bit of fallout anyway as they all have radiation resistance on par with those radiotrophic fungi found in Chernobyl so open reactor designs are common.

I was imagining these engines being used very extensively as a propulsion source, compact generator, leaf blower to clear away snow, and for many other purposes. Without radiation being a concern it seems like one should be able to build this style of engine to be at least as small as a the W54 warhead used in the Davy Crockett nuclear rocket launcher.

For use as a generator the engine could be used at low power and or with something to bleed off enough speed/temperature from the exhaust for a turbine. With the engine exhaust of the engine going through a shadow barrier to block radiation and separate radioisotopes from the rest of the air stream first, before the hot air then goes to drive a small turbine. The desired intent of being this wasteful is in order to make a generator that would fit in a backpack and outperform comparably sized internal combustion based ones by leagues. Given the wastefulness of this culture, if the turbine needs to be replaced every day that's not a dealbreaker.

Its size could also make the engine usable for some truly unique applications, like powered wing-suits that can actually take off of the ground.

Is a tiny nuclear engine like this actually feasible however, given extremely good radiation shielding? and if so what kind of things would it be capable of?

My particular setting may have magic with a limited scope, but in principle you should be able to replace "magical darkness" with Clarktech radiation shielding and things would be the same. Also I don't know the extent to which incredible radiation shielding would actually be required to make the engine itself work.

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    $\begingroup$ Size is less a problem than mass. You're not going to get nuclear leafblowers, because no one wants a 40kg leafblower. (That's just the mass of the uranium you'd need, not the mass of the necessary mechanical bits to turn heat into useful work.) The same problem, writ large, is true for wingsuits. Project PLUTO (would have) worked because everyone was fine with the missile itself getting incredibly, physically, hot, not just the radiation issues, and because the missile was disposable. $\endgroup$
    – jdunlop
    May 16, 2023 at 22:39
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    $\begingroup$ (To be clear, 40kg of uranium is only about 2l of volume, so the space for your fuel in your nuclear leafblower is totally feasible, if a little bulky, but lugging all that around is wildly impractical.) $\endgroup$
    – jdunlop
    May 16, 2023 at 22:41
  • $\begingroup$ @jdunlop Thanks for your feedback. Yeah a nuclear leafblower would definitely need to be vehicle mounted. Do you think using a heat pump or thermoelectric device combined with super-insulating material would be able to keep the user of a nuclear wingsuit from burning themselves? The magical darkness can absorb IR just like any other radiation, but it's not going to stop heat being conducted through material. $\endgroup$ May 16, 2023 at 22:47
  • $\begingroup$ you may want to look up nuclear batteries. $\endgroup$
    – John
    May 16, 2023 at 23:11
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    $\begingroup$ You mention breeder reactors; I wonder if there's some exotic short-lived isotope with a much lower (and more portable) critical mass that could make your scheme feasible. Or perhaps these reactors don't use nuclear chain reactions at all; their isotopes just decay fast enough all on their own that they produce enough heat to spin a gas turbine. Like an RTG, but with a turbine instead of a thermopile. Downside: There'd be no way to turn such a thing off. No nuclear chain reaction means no control rods. Also, the core would probably melt if you cut off the airflow through it. $\endgroup$ May 18, 2023 at 2:54

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There are use-cases, but not as you've envisioned them

As other answers have pointed out, without magical neutron reflectors, you're stuck with requiring either heavy things (neutron reflectors) or critical masses of fissionable elements (other heavy things).

Project PLUTO worked for flight because it was wholly disposable. Heat was not a concern - they just used materials that could survive the heat for long enough. Radiation was not a concern, because it was a doomsday weapon. Neutron-activated radiation was not a problem, because you eventually intended to blow the whole thing up.

You can't do any of that for a wingsuit; you're not going to have a human-portable critical mass to generate the necessary thrust-to-weight ratio, you can't keep the blazing-hot exhaust away from the user, and unless you can totally shroud the user in magical darkness, direct radiation damage will also probably kill them.

As discussed in comments, you're not going to get anything other than a vehicle-mounted leaf blower, either, as even using plutonium you'd need to be carrying 20kg of just the fuel, notwithstanding the moving parts of the device.

Which brings us to another problem not at all touched upon: the most dangerous part of working with most transuranics (the long-halflifed ones, at least) is not the radiation, but heavy metals poisoning. If you remove most of the containment, the risk of fuel fleas making their way into food, water, and air become a much bigger problem.

Now, the setting you're in sounds like something of a dystopia, so maybe that's fine, but having everyone have their own home nuclear reactor, Fallout-style, is going to inevitably result in many, many more dangerous material releases than our current centralized nuclear power system. This obviously gets worse if you're trying to make small ones for everyone to use.

Even with magical radiation-absorbing fields and advanced medicine, there's no easy way to flush heavy metals out of the body. Most of the measures for doing so are dangerous, largely-ineffective tools of last resort.

So ultimately, I suspect you'd see more nuclear technology being used, but a lot of the same care being taken with its casing and protection that we use now, but for chemical reasons rather than radiological ones.

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  • $\begingroup$ As a suggestion, what might make sense is using utility-scale nuclear reactors to electrolyse hydrogen, and then that hydrogen can be used directly for fuel. Mass is much less of a problem, refuelling becomes trivial. People could still have basement reactors if you want, to refuel their hydrogen cells from the comfort of home, but then you don't need to worry about lugging enormous piles (hah) of fissiles around. $\endgroup$
    – jdunlop
    May 17, 2023 at 3:26
  • $\begingroup$ Given that magical darkness can be used to easily separate isotopes, people can just use that to remove basically any toxin from their body or environment (they even use it in the form of "shadow-brooms" which gather up particulates). I wonder if you think you can make wingsuits viable if you have magical heatsinks in addition to radiation immunity. As I realized that magical darkness should allow for one to make some incredibly good thermodynamics violating heat sinks: worldbuilding.stackexchange.com/questions/246259/… $\endgroup$ May 17, 2023 at 23:00
  • $\begingroup$ @VakusDrake If you have impossible heatsinks, you don't need nuclear power; but ultimately, it's your world. Wingsuits would still not be viable because they'd have to weigh about as much as a cruise missile; energy density for nuclear applications grows geometrically with mass. If you want nuclear-powered wingsuits, you can just lie and say "magic makes it happen differently", but nuclear powered-flight has always resulted in engineers speculating about enormous planes, because that's the only way you get enough power to lift the reactor. $\endgroup$
    – jdunlop
    May 17, 2023 at 23:11
  • $\begingroup$ My bad I meant flying exosuit/small mech as somebody else mentioned how a W54 warhead's scale can't be directly applied to a device that maintains criticality for a long period of time. Using californium or some other similar isotope along with RL neutron reflectors can still get the required critical mass down, though I don't know down to ultimately how much. $\endgroup$ May 17, 2023 at 23:21
  • $\begingroup$ @VakusDrake - I was also the one who pointed out that the W54 warhead is a bad comparison because of the difference in purpose between a bomb and a reactor. And using shorter half-life isotopes gets your critical mass down, but don't make your energy density that much more spectacular because their neutron cross-section isn't that much bigger. So you'd get a lower weight, but it'd still be too little energy to get off the ground. Moreover, the smaller the reactor, the greater the marginal mass expense for the bits that use the energy. ('swhy bigger is better in nuclear power.) $\endgroup$
    – jdunlop
    May 17, 2023 at 23:29
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Note that the smallest fission reactors use neutron reflectors to decrease the mass of fissile material required to reach criticality. However, real neutron reflectors are lossy and produce a diffuse reflection, appearing at best like a translucent gray to neutrons. If your magical neutron-absorbing darkness can be turned into a high-quality magical neutron mirror, this would be of great use in making more compact fission reactors.

If you can store and release neutrons, that opens up even more possibilities, like subcritical "batteries" running off neutrons kept in cold storage by that shadow magic instead of sustaining a chain reaction.

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  • $\begingroup$ Unfortunately the magical darkness will only absorb radiation, so you do still need to make use of neutron reflectors. Though the magical darkness does let you do stuff like run reactors in configurations which ought to be supercritical but are kept critical by controlling the strength of magical darkness shielding the components. Though you should still be able to make a nuclear reactor that can be carried by hand using a turbine connected to one of my proposed nuclear engines, or a nuclear stirling engine if you only need reasonable amounts of power. $\endgroup$ May 17, 2023 at 0:04
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    $\begingroup$ @VakusDrake - this comment raises an important question: does the magical darkness only absorb electromagnetic radiation? $\endgroup$
    – jdunlop
    May 17, 2023 at 0:39
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    $\begingroup$ It's stated to be able to absorb neutrons. Which does raise the question of whether it can absorb atomic matter...what would make this shadow magic treat a neutron and a hydrogen atom differently? You might be better off running your turbines off ambient atmosphere and a shadow-powered infinite vacuum. $\endgroup$ May 17, 2023 at 0:48
  • $\begingroup$ @ChristopherJamesHuff I was imagining that it can absorb relativistic nuclei, but not atoms with electrons (though I may revise that to include any relativistic particle if atoms are a really major radiation source). Also absorbing large masses of material so as to run a turbine would be way too expensive to do with shadow magic even if you could affect atoms. $\endgroup$ May 17, 2023 at 1:05
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    $\begingroup$ Thermal neutrons are the same mass and overall charge as hydrogen atoms, and quite non-relativistic (a couple km/s, corresponding to a temperature of around room temperature). Even fast neutrons are only a few percent of c. $\endgroup$ May 17, 2023 at 1:18
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No matter what you do - Time is always a finite resource. And to make something like a Turbine requires a lot of time and precision engineering.

The overall premise of there being small, portable nuclear engines that are shielded by Magic though is pretty sound - it's just if you want them to be disposable - then with a Turbine, that's not so much a possibility.

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    $\begingroup$ I just brought up that idea of disposable turbines because it occurred to me that if you're willing to be really inefficient, then for a stationary application you might be able to get away something far cruder and less precisely engineered than any examples of turbines I can think of in RL. Don't know whether doing things quicker and dirtier would actually help, but I figured I'd bring up the possibility. $\endgroup$ May 16, 2023 at 22:06
  • $\begingroup$ I added one of the most extreme use case which would be useful if feasible: Do you think this would be viable for making a nuclear versions of those jet powered wing-suits which could actually take off of the ground? $\endgroup$ May 16, 2023 at 22:17

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