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I think this boils down to a question of how much fundamental understanding is needed to build a very basic nuclear reactor to boil water.

If fissile material was readily available to an otherwise primitive steam-age civilization, could they use it as a fuel source without having a deeper understanding of atomic physics? Is simply understanding that when you place these special rocks in close proximity they get hot, enough to make a steam locomotive?

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    $\begingroup$ Steam-age is hardly primitive. There is a ton of math, chemistry, engineering, and industrial capacity needed to be able to make and use steam engines. worldbuilding.stackexchange.com/q/98363/83464 $\endgroup$
    – Zags
    May 30, 2023 at 12:33
  • $\begingroup$ A discussion has broken out: how(!) is the fissile material made available? Alien intervention, dumping pure and weapons grade materials on them? Or is it a limitation of the question that they must only rely on themselves and possible natural occurrences of fissile materials? $\endgroup$
    – MichaelK
    May 31, 2023 at 14:30
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    $\begingroup$ In Brunner's "The Crucible of Time", aliens accidentally make a reactor by piling up a bunch of heavy rock to make a dam. Possible on a young planet, unlikely on Earth, where too much U235 has decayed. $\endgroup$
    – John Doty
    May 31, 2023 at 19:20
  • $\begingroup$ Why not? That is basically how we did it. Of course, without a few theoreticians to correlate the data and do the math most of the experimenters will rapidly die from causes ranging from radiation poisoning to surprise explosions to chemical poisoning...just like many of our early experimenters. $\endgroup$
    – user535733
    Jun 1, 2023 at 19:59
  • $\begingroup$ Once you've mastered the steam engine, any heat source will do to power it (ignoring the health effects of radiation, anyway :) ) $\endgroup$ Jun 1, 2023 at 20:19

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I think it matters how the fissile material is "lying around". In our world, the two limiting factors are the mining and extraction of uranium, and the enrichment of the fissile isotope. Of these, the second is substantially harder (requiring gas centrifuges and other fairly 20th century technology) - the former is feasible with only late 18th century refining - pure uranium salt was isolated in 1789.

As pointed out in the comments, enrichment is not actually necessary for modern reactors - the Chicago pile 1 ran on natural uranium. However, it seems unlikely that anyone would realise the potential to generate atomic energy without a firm theoretical motivation, which would not be possible without an understanding of atomic science. I postulate that the only way to see fission happen without understanding it fundamentally is by having your natural uranium be "highly enriched" by our standards.

Suppose that your uranium occurs naturally in a highly enriched isotope mix. I see the timeline going as follows:

  1. (formation of the planet) Any particularly rich veins of uranium ore will react with themselves, boiling off any water that leaches into them. By the time humans arrive, any uranium to be found will not be present in concentrations that are easily fissile, so you will need ordinary fire to get your industrial revolution started.
  2. (48CE-) Pitchblende is used as a yellow / green coloring agent in glass. The property of pitchblende to warm up when wetted is viewed as a curiosity, but few have access to large enough chunks to use it for fission.
  3. (1700's) Uranium can occur in the same kinds of deposits that produce copper, tin, tungsten and molybdenum. (Modern day Australian uranium is a byproduct of copper extraction) Small scale preindustrial mining would likely not have smelted sufficient quantities to notice anything fission-related, but in industrial volumes, workers notice that the heavy slag at the bottom of the tin crucible remains warm long after the metal has been removed, curiously exploding in a puff of steam when lowered into water. Scientists are baffled.
  4. (1789) First isolation of uranium salts. This is already plenty pure enough to use in a primitive reactor.
  5. (1830) Attempts to use 'tinslag' for heat lead to crude reactors. Curiously, under some circumstances, water seems to cause more heat to be released.
  6. (1845) First model of Stephenson's Eternal Engine, Prometheus, is released to wide acclaim. No longer is our nation's power beholden to the greed of coal-mining barons!

The Manhattan Project was limited by three things - rarity of uranium ore, poor understanding of the fundamental physics, and the difficulty of enriching the uranium. Much of the scientific work that went on at Los Alamos was in fact pure trial and error - empirical measurements of cross sections and critical masses, etc. If your uranium is both naturally enriched and abundant, you short-circuit that. Aside from this, early atomic piles did not strictly speaking require any technology that could not (in principle) have been made in the late 1800s.

In short, I believe that an abundance of dilute, enriched uranium could push reactors forward 50 years, perhaps more, but an advanced industrial mining complex is still needed to extract and purify useful quantities of raw material. Reactors can be extremely primitive indeed - one even formed by accident. On a human scale, a sufficiently large block of U-235 with a control rod will function as a reactor - but the process of getting to a safe design will be, erm, messy.

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    $\begingroup$ So they would need a bunch of demon cores from a preexisting civilization and trial and error from that. $\endgroup$ May 30, 2023 at 10:53
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    $\begingroup$ @user3819867 having previous civilization pre-make the uranium cores would've just bypassed any sort of limitation. Why not just have a finished reaction made in a large underground vault and opened at a particular time? Oh wait, that's done in the Fallout universe. $\endgroup$
    – Nelson
    May 30, 2023 at 13:10
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    $\begingroup$ @Nelson "The Manhattan Project was limited by three things - rarity of uranium ore, poor understanding of the fundamental physics [not needed as OP explicitly said trial and error], and the difficulty of enriching the uranium." - The only way to have enriched uranium in a habitable environment seems to be heritage. It would still be interesting to see them trial and error a steam engine or an "eternal fire" out of a radioactive core. The idea is simple: water goes in, water boils, water steams, core activity decreases. Not sure they could use it without e.g. roads though. $\endgroup$ May 30, 2023 at 13:36
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    $\begingroup$ Isotope separation was needed for weapons - the Chicago Pile and the Hanford reactors ran on natural (unenriched) uranium products. $\endgroup$
    – Jon Custer
    May 30, 2023 at 19:21
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    $\begingroup$ "The Manhattan Project was limited by three things...". The Manhattan Project depended on having some very bright guys, Including Enrico Fermi, John von Neumann, Robert Oppenheimer, and Leslie Groves, who figured out how to manage the immense team (99% of whom intentionally had no idea what they were building). They weren't just messing around trying stuff. Without these guys I don't see how it could have succeeded. $\endgroup$ May 31, 2023 at 1:05
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If the time is 2 billion years ago, yes

If we assume an Earth-like world, where life evolved about three times as fast, then they need no technical understanding at all.

...because then natural nuclear reactors existed.

enter image description here

One of the reactor zones at Oklo, Gabon, Africa (image credit: US DoE)

When the Earth came to be, the isotope composition of natural Uranium was — roughly — the same as that of present day nuclear fuel, after enriching.

This then means that what was needed to get a chain reaction going was...

  • A concentration of uranium ore in a compact volume
  • Water, as a moderator

...and this is exactly what happened at what is present day Oklo, Gabon.

So, 2 billion years ago, for tens of thousands of years, these natural reactor zones operated in cycles. Water would flow through the ore, moderate the chain reaction, cause the zone to heat up, make water to boil off, the chain reaction stops, the zone cools down, and start over, in about 3 hour cycles.

Assuming a primitive culture existed back then, they would be able to see this happen with their own eyes.

The issue would be, that this would not be very healthy for them.

But, yes, they would be able to see that if you pile that very particular kind of dense and heavy rock together in large enough a pile, and soak it in water, then it heats up.

...and then they get sick and possibly die, without knowing why.

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    $\begingroup$ In Space by Stephen Baxter, a similar naturally occurring nuclear reactor is used by a civilisation in Africa to heat water to steam for the purposes of powering a certain amount of machinery at an early Industrial Revolution level of complexity. They get around the problem of the radiation being dangerous by making it a criminal punishment for someone to be sent to work in the reactor, and the guards need only compel them to remain there until the physical signs of the radiation poisoning appear, as once that happens they're shunned by everyone else and have nowhere else to go. $\endgroup$ May 30, 2023 at 16:35
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    $\begingroup$ Already +1. Even with this it would not be easy to find this thing. It was in sandstone, which allowed the water to enter. The maximum power over a roughly-30-foot-wide disc was about 10 kW, about what 9 big toasters produce. And even that only 1/2 hour in 10 hours. Evidence indicates that the fission products moved about the width of a sand grain, so you would not get dosed even if you were sitting on the rock as little as 20 feet above this thing. No geyser, no indication of anything strange on the surface at all. $\endgroup$
    – Boba Fit
    May 30, 2023 at 19:03
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    $\begingroup$ But there is a possibility. Suppose folks are quarrying the sandstone for construction. And the uranium deposits spoil the quality, so they wind up on the slag heap beside the dig. Then they get rained on and heat up enough to produce some steam. That could easily get noticed. $\endgroup$
    – Boba Fit
    May 30, 2023 at 19:06
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    $\begingroup$ "Before we approach the god-stove, we must don the ceremonial grey metal garments so that the gods will not strike us down with a plague... we lost a lot of priests until we realized we were not dressing properly in this holy metal that also makes our water taste sweet." $\endgroup$ Jun 1, 2023 at 12:52
  • $\begingroup$ How fast would they "get sick and possibly die"? Because being a pre-modern tanner was quite unhealthy too. Same with early modern hatters $\endgroup$
    – T.E.D.
    Jun 1, 2023 at 18:44
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Yes and No.

The core idea has enough merit that it could be plausible. However just getting things hot enough to boil water is the easy part.

Doing it in such a way that you don't accidentally irradiate everyone and kill them off in horrible ways is much much harder.

And therein lies the problem with this idea. With modern physics, it took us nearly 30 years to discover that radiation was actually quite bad for you and even longer still to fully appreciate that.

But let's assume that because of the amount of Fissile material lying around, our inhabitants have evolved some resistance to radiation:

Without an understanding of what is happening - the chance of having a runaway chain reaction leading to a meltdown or nuclear explosion increases drastically

Imagine you are but a simple worker:

If one lump of magic rock is good, 2 must be better. If two lumps of magic rock is better, then 3 must be amazing. If three lumps of magic rock is a Amazing then 4 must be Nuclear detonation.

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    $\begingroup$ It's not as bad as a nuclear detonation though. Sure, a flash caused by accidental criticality will be lethal to anyone standing close by but not instantly so and it wouldn't develop into a full-on bang. At least if the two accidents documented with the so-called Demon Core are anything to go by. $\endgroup$
    – biziclop
    May 29, 2023 at 22:34
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    $\begingroup$ " then 4 must be Nuclear detonation" No, that is not how nuclear chain reactions work. Nuclear chain reactions change as the material heats up, in particular doppler broadening and — in the case of nuclear reactors — the moderator boils away. Both of these phenomena act as a self-choaking leash on a nuclear chain reaction. It takes extremely precise geometry, purity of material, and no small amount of "help" for a nuclear chain reaction to run away into a detonation. $\endgroup$
    – MichaelK
    May 29, 2023 at 22:43
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    $\begingroup$ @MichaelK Exactly. What you get is not a detonation but an...excursion. (One of my favourite euphemisms that, making it sound more like a picnic rather than a lethal flash of gamma radiation.) Mind you, it's still bad news for anyone who happened to be tending "the fire" and it's probably a wise idea to give the location of the incident a wide berth for a long time. $\endgroup$
    – biziclop
    May 29, 2023 at 22:56
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    $\begingroup$ @TheDemonLord They did separate the halves, but that was not what stopped the exponential increase in reactivity, it did that on its own as it heated up. If it was not for the choke-leash physics of it, humans would never have a chance of regulating nuclear chain reactions, because each "link" in the chain reaction happens in microseconds. So even if you had a mere, say, 0.01% increase in reactivity per generation, it would still detonate in your face before you had time to pull breath to exclaim "Oh, bugger..." $\endgroup$
    – MichaelK
    May 29, 2023 at 23:05
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    $\begingroup$ Downvoted because of the nuclear detonation statement. The rest of the answer is alright, but that misinformation makes the answer bad. $\endgroup$
    – jdunlop
    May 30, 2023 at 0:25
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Nuclear reactors are simple in principle but in practice are complex and delicate machinery, building and operating them safely requires a good understanding of the underlying processes. Given the dangers it's unlikely this understanding can be developed by pure, unassisted trial and error.

Frame challenge: Radioisotope thermal generators avoid most of these pitfalls.

While still needing extremely dangerous materials (let's not forget that fissile materials are typically also highly toxic), it is just about possible for an early civilisation that can already extract, smelt and process lead to discover that certain Deadly Rocks can be encased in lead and be used to heat stuff.

The crucial thing about RTGs is that not only are they mechanically simple (no moving parts, no special materials, just some fissile stuff surrounded by heat transferring shielding), they use fissile isotopes incapable of sustaining a chain reaction.

There is still a lot of experimentation required to do this reliably and without the heat melting the lead casing but depending on the relative abundance or scarcity of other sources of heat it could be a tempting enough option.

Especially if we consider that in a society that already has much higher mortality levels than rich modern societies, detecting longer-term harms of radiation would've been much more difficult. Just look at how long it took even as recently as the 19th century to establish that stuff like arsenic-laced wallpaper, gas lighting or coal-burning fireplaces were slowly killing you and thus weren't great to have in your home.

The main challenge with RTGs however is that while they last an awfully long time, their power density is fairly low, and they need isotopes with relatively short half lives, so there must be a natural process that continuously produces lots of them. As no such process exists on Earth, real-life RTGs rely on isotopes created in nuclear reactors, which puts us back to square one: you need to build a nuclear reactor first.

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  • $\begingroup$ "The main challenge with RTGs however is that" you die in three days after you bring back to base and use it as a camp fire. The Soviet Union's Deadly Abandoned Nuclear Generators youtube.com/watch?v=NT8-b5YEyjo - Three victims, one fatality. $\endgroup$
    – Mazura
    Jun 2, 2023 at 1:16
  • $\begingroup$ > Lia, Georgia Radiation Incident of 2001, when 3 Georgian woodsmen, accidentally stumbled upon an abandoned, highly radioactive RTG core. Which sadly killed one man and seriously injured the others. An incident that needed human radiation clean up techniques not seen since the 'bio-robots' of the 1986 Chernobyl Disaster. $\endgroup$
    – Mazura
    Jun 2, 2023 at 1:18
  • $\begingroup$ @Mazura Those cores are unshielded except for their cladding, if you strip that away, yeah, you're in trouble. Don't strip the cladding off RTG cores, kids. $\endgroup$
    – biziclop
    Jun 2, 2023 at 8:54
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Maybe and no.

Nuclear reactor as such is not an issue at all: they can occur spontaneously, too, as already pointed out. The issue is basically the same as with the steam in steam engines: producing steam is not a problem at all: it can occur naturally as well.

The actual problems lie in everything else but the power source.

You can revisit the question by asking if the civilization could create the same thing by burning wood, ie. obtaining the heat from combustion instead of nuclear fission.

If the answer is "no", then they cannot build the same thing with nuclear power source either. Which is probably the case.

However, if the answer is somehow, miraculously "yes", then it is somewhat plausible that they could end up using fissile nuclear material instead of combustible one, but there are still some extra issues left: eg. shielding against radiation, controlling the chain reaction (it needs way more precise control than a furnace), and handling the waste.

In my opinion, there are too many issues with the trial-and-error approach that I think it is almost implausible, but not completely impossible. Science, after all, has involved lot of trial and error, too, but they also tried to understand what was going on, and make hypotheses and test them etc. It is hard to believe that all of this could be achieved just by trial and error, ie. without something resembling science.

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For a crude nuclear reactor, you basically need three things: A fissile fuel (Uranium-235), typically in a concentration of 3%-5%, a moderator (water) to slow neutrons down to thermal energies (this increases the probability of fission due to reasons), a coolant to transfer heat away from the reactor (also water). You might also want a strong neutron absorber like boron to control the reactor, but this can also be achieved by removing fissile material.

If you assume that your world conveniently already has some naturally enriched uranium, then it's not that hard to create a reactor, and I could see it being discovered by accident. Like someone was storing a bunch of uranium ore, and it flooded (adding a moderator), resulting in a chain reaction. From there they might just start putting uranium into a big water tank and seeing what happens. In this scenario, it's actually kind of hard to have a meltdown, since boiling water will displace the moderator and slow down the chain reaction. This is known as a boiling water reactor (BWR) if you want something to google. Now it's not impossible to have an accident with a BWR.

Keep in mind though, when nuclear power gets discovered, the conversation usually goes like so: "Hey there's way more energy stored in them there glowy rocks than any of our fossil fuels", "Yeah, I bet we could make a pretty kick-ass bomb with that stuff".

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No, a primitive civilization would not be able to trial-and-error their way to a functional nuclear reactor without a deeper understanding of atomic physics. Building a basic nuclear reactor requires not only access to fissile material but also a fundamental understanding of criticality, radiation, heat transfer, and reactor dynamics. Without this knowledge, it would be extremely difficult to harness nuclear energy safely and efficiently.

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