How early can some kind of nuclear reactor resonably be invented or stumbled upon?

Question

The physics of this world work practically the same as our own. All these nuclear reactors need to do is make a somewhat usable amount of heat for power generation, possibly ship use. They have to be somewhat practical, so not to big or not too likely to melt down. Development in the fields relevant to nuclear reactors happens at a roughly similar pace to our own world. The people involved are still humans so they will die to the same amount of radiation a normal human would. If people get cancer too much from it they will be forced to stop by the government.

If all the reactor can do is make radioisotopes that are hot enough to be usable in RTGs that's also good enough.

I don't know if this is a stupid question i don't really know. I can't provide much in the way of specifics because i don't know much of anything about the field. I have a vague guess it wouldn't be too far before we did in real life, but again i don't know much about the field.

Answer 1

As pointed out in the comments by John Dallman, the Oklo natural fission reactors were active about 1.7-1.8 billion years ago, when there was a high enough concentration of primordial Uranium-235 (~3% instead of the modern 0.72%) for a light-water reactor to appear spontaneously thanks to a quirk of local geology. Since that time, radioactive decay of U235 has ensured that the situation won't arise again, because there's simply not a high enough concentration of natural fissiles left anywhere.

1.7 billion years ago, there weren't even any multicellular organisms to appreciate this feat of natural engineering. Now, if you could handwave evolution happening somewhat faster on your alternate world than happened in the real world, for whatever reason, then you could have enough U235 still available to form natural reactors. This might actually make it possible to have nuclear reactors built with steam-age technology. The death toll of such an enterprise is a detail for your story-writing, but it seems likely to be high given the history of engineering with radioactive materials (eg. the radium girls).

Given the science and engineering required, you probably couldn't push your nukes back further than the equivalent of the real world industrial age. An alternate-timeline year of 1850 might be a not-totally-unreasonable point, though I'm more or less picking something out of the air here. Although nuclear waste and the resultant pollution is likely to be a significant problem in such a setting, it is pretty hard to make a nuclear weapon from such minimally enriched uranium, and isotope separation is likely to remain an issue for your society for some considerable time.

Charles Stross had nuclear trains fuelled with unenriched uranium in his book Singularity Sky, where the world in question was quite young when it was settled by humans who had been transported from Earth. The transportees already had advanced technology by our standards, and an understanding of nuclear physics and engineering. This might not be the kind of handwave you were after, but it could be adequate.

Answer 2

4.5 billion years ago or never, depending on how you frame the question.

Silver is estimated to constitute a bit less than 0.1 parts per million of Earth's crust by mass, or about $10^{-7}$ earth's-crust masses per silver-on-earth mass.

Earth's crust is about $10^{-2}$ of Earth's mass. There's probably a lot more silver than what's in the Earth's crust, but who cares about an orders of magnitude here or there when you're doing napkin math?

244Pu has a half-life of 80 million years.

The Earth (and the rest of the solar system) is about 4.5 billion years old. To get the ratio of how much of a substance you started with to how much you have now, you raise 2 to the power of how many half lives there have been between then and now. 2 to the power of 4.5 billion over 80 million is a bit less than $10^{17}$ silver-on-earth masses of 244Pu.

So, to get an amount of 244Pu equal to the amount of Silver currently on Earth, you need to start with about $10^{17} \times 10^{-7}\times 10^{-2} = 10^{8}$ earth masses, or about 300 solar masses of 244Pu.

Hundreds of stars worth of 244Pu is guaranteed to have natural fission reactors all over the place, wherever gravity collects matter together into clumps bigger than critical mass. I don't know how it would evolve. My guess would be that you'd get a black hole with heavy element planets, but really I couldn't say. I'm 100% sure, however, that there would never be any humans, unless they came in a space ship trying to figure out how all these heavy elements got here in violation of everything they knew about astronomy and nuclear physics.

Answer 3

I will take the guess you are asking the minimal technological requirements that a human society would need to develop a nuclear reactor.

But first of all, you'll need the theory.

Building a functional nuclear reactor is an incredibly difficult task and you have no chance to discover it 'by accident'. Even natural occurences would be explained by magic or other unknown forces if your civ encounter some and they would be unable to reproduce and exploit it.

Let's retrace the nuclear fission history to identify the key events you could make happen earlier.

1 - Matter is made of atoms.

Requirements - just a brain and freedom of thought.

The atomistic theory itself dates back from the ancient greeks with Democritus. Even though, its conception of matter as atom-based was more philosophical than practical.

2 - Atoms are chemical elements

Requirements - complex glass apparatus to contain, burn, distill and observe natural extracts. Precision scales to measure material weights before/after reactions. Writing materials to take notes. One genius to understand how are atomic elements agenced.

Decomposing the matter in base elements is a fundation of both occidental and oriental alchemies, and they also brought the idea of transmutation - turning an element into another. You'll have to wait the brink of the French Revolution with Lavoisier's experiments to reconect with the idea of a particulate matter, and the fortunate intuition of Mendeleiev in the 1860's to arrange the elements in the periodic table. This new organisation brought the idea of yet unknown, missing elements that researchers around the world started to hunt frantically.

3 - Some elements are radioactive

Requirements - Photographic paper or phosphorous screen. Even finer glassmaking techniques and vacuum pumps to build these screens.

Among these elements hunters, Sklodowska-Curie and Becquerel discovered the radioactive properties of uranium and other isotopes at the brink of the XIXth century. At this time, radioactive material where considered as harmless phosphorescent materials and started to be used as a paint on watches or to dye fancy glass.

4 - Radioactivity can generate tremendous amounts of energy

Requirements - Same as above, more experiments and Albert Einstein.

The characterisation of particles and especially the neutron by Chadwick took 30 more years. Szilard suggested soon to use the neutron products of radioactive decay to trigger more reactions and get a huge energy from it, building on Einstein's most famous equation e=mc².

Formulation of the chain reaction theory could be considered in your setting as the rationale for building a nuclear reactor.

Then, you need a huge amount of material and human resources to locate, mine, extract, refine, assemble and make work even a tiny nuclear reactor. Note also that nuclear reactor where developed first as a way to generate the plutonium required for atom bombs, and without this urgent requirement petrol and charcoal were already fulfilling much of the population's needs in energy.