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Nuclear fission chain reactions occur when a nucleus breaks apart, producing at least one neutron, which then causes nearby nuclei to fission, a multiplying process when the average number of produced neutrons per fission is large enough (U235 has >2 even when capture is included).

If fissions never produced enough neutrons for this to happen (for example, a slight modification to the laws of physics), then we would not have nuclear bombs or reactors (natural or otherwise) and nuclear physicists would get different results for their detailed experiments.

But would there be noticeable differences other than these?

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    $\begingroup$ You may wish to post this on Worldbuilding instead. What-if questions like this are typically seen as not mainstream enough, and your question is rather broad. However, on Worldbuilding, this would be spot on, and you'd get quite a few useful answers. $\endgroup$ – Cort Ammon Oct 15 '19 at 4:35
  • $\begingroup$ Thanks for this info - very useful. I will try that. $\endgroup$ – abby yorker Oct 15 '19 at 21:41
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You are asking a science fiction question, not a physics question, and in science fiction one can imagine anything. The only answer one might give connected to physics is that the mathematical models fitting the microcosm of elementary particles that leads to the understanding of nuclear physics , by its form , will always give the result of the periodic table of elements, i.e. some stable some unstable that would lead to nuclear reactions.

enter image description here

A diagram by the Joint Institute for Nuclear Research showing the measured (boxed) and predicted (shaded) half-lives of superheavy nuclides, ordered by number of protons and neutrons. The expected location of the island of stability around Z = 112 is circled

As you can see from the island of stability there are many more unstable elements than stable, so even with some variation of the constants entering the standard model ,the mathematics would lead to a similar plot. The underlying reason is the mathematics of quantum mechanics with its postulates, which your question is really asking: if there were no quantum mechanics would we have the world as we see it? The answer is "no". Classical theories cannot build up consistent models of the world, from elementary particles to the cosmos.

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  • $\begingroup$ The question was indeed a science fiction question but one I hope would have a scientific answer. The plot above (thanks - informative) indicates many elements with various degrees of stability but, in the whole plot, only one isotope exists that is both amenable to fission chain reactions and is observed in nature in any appreciable abundance. I would guess that the latter property would be required for there to be any significant implications for the macroscopic world. $\endgroup$ – abby yorker Oct 15 '19 at 21:24
  • $\begingroup$ Regarding QM: Practically modification to the theory would produce macroscopic consequences so I, too, would rule these out. Similar with the standard model - it is difficult to imagine changes in, say ,QCD that would change only an obscure fission cross section. So the question is something of a "magic" what if. What if the laws of physics, in all their glory, did not allow fission of U235 (and its chain reaction properties). Would our lives (prior to 1938) have been changed in any way(aside from natural reactors)? $\endgroup$ – abby yorker Oct 15 '19 at 21:34
  • $\begingroup$ As I said this should be asked in a science fiction site. What if or planet had very little accessible uranium because of the cosmological spot it is found? (more probable to happen, and accepting physics as is, as a hypothesis) $\endgroup$ – anna v Oct 16 '19 at 4:13
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First I will answer the question in the title. You are asking about nuclear chain reactions.

Now if nuclear chain reactions were not possible, then the proton proton chain reaction would not be possible either, that is one of the two known sets of chain reactions by which stars transform hydrogen into helium, like our Sun.

The proton–proton chain reaction is one of two known sets of nuclear fusion reactions by which stars convert hydrogen to helium. It dominates in stars with masses less than or equal to that of the Sun,[1] whereas the CNO cycle, the other known reaction, is suggested by theoretical models to dominate in stars with masses greater than about 1.3 times that of the Sun.[2]

Although often called the "proton–proton chain reaction", it is not a chain reaction in the normal sense of the word (at least not branch I – in branches II and III, helium, which is the product, also serves as a catalyst). It does not produce particles that go on to induce the reaction to continue (such as neutrons given off during fission). In fact, the rate is self-limiting because the heat produced tends toward reducing the density. It is however a chain (like a decay chain) and a reaction, or more accurately a branched chain of reactions starting with two protons coming together and yielding deuterium.

https://en.wikipedia.org/wiki/Proton%E2%80%93proton_chain_reaction

Now you are asking about nuclear fission.

When an atom undergoes nuclear fission, a few neutrons (the exact number depends on uncontrollable and unmeasurable factors; the expected number depends on several factors, usually between 2.5 and 3.0) are ejected from the reaction. These free neutrons will then interact with the surrounding medium, and if more fissile fuel is present, some may be absorbed and cause more fissions. Thus, the cycle repeats to give a reaction that is self-sustaining.

You are saying that the laws of physics would change so, that the fission itself would not happen. This could mean too that decay would not be possible, and molecules chemical reactions would not be the same (and so the matter around us would not be the same). We would not have electricity (currently based on these power plants), and because we would need power plants based on cole (and other pollutive sources) the environmental pollution would be much higher.

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  • $\begingroup$ Sorry for the confusion - the question was about removing fission chain reactions only, either by reducing the neutrons or by eliminating fission itself. And I was wondering if there are processes other than bombs and reactors whose absence or modification would be important to the macroscopic world. $\endgroup$ – abby yorker Oct 15 '19 at 21:00
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    $\begingroup$ why the downvote? $\endgroup$ – Árpád Szendrei Oct 15 '19 at 21:16
  • $\begingroup$ I didn't downvote. Actually, I just upvoted to cancel it. $\endgroup$ – abby yorker Oct 15 '19 at 21:36
  • $\begingroup$ @abbyyorker thank you! $\endgroup$ – Árpád Szendrei Oct 16 '19 at 0:27

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