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What is a possible event${^*}$ that would cause a spike in the speed of $^{235}$U fission and reduce its average concentration in ore worldwide?

As I understand the physics of the process, it is impossible to alter the fission speed itself, but if there is a way to have a thermal neutron source in the ore volume or some kind of neutron moderator for neutrons which are emitted by $^{235}$U itself that will lead to chain reaction and then depletion, like in Oklo.

The problem I am struggling with is to come up with an event that will look not completely made up.

${^*}$ The event should not be magic or alien-related.

P.S. This is my first question here if I am doing something wrong I will be happy to do it right.

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  • $\begingroup$ Would this need to be natural (a star explodes showering the planet with unobtanium) even if unlikely or could it be deliberate (somebody synthesizes unobtanium and deploys it worldwide)? $\endgroup$
    – VLAZ
    Jan 7, 2020 at 15:18
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    $\begingroup$ Some sort of "false vacuum collapse" event could happen which would make W bosons more likely to decay radioactive isotopes. This wouldn't have to be universal, could happen in a finite region of space. But it would apply to almost all radioactive isotopes (not just U235), and it would make them intensely radioactive for a short period of time. Ore would kill everyone anywhere close to it, and concentrated/refined uranium might become spontaneously supercritical. Boom. $\endgroup$
    – John O
    Jan 7, 2020 at 15:19
  • $\begingroup$ @JohnO, can you expand your answer, because it sounds like what I need $\endgroup$
    – FrogOfJuly
    Jan 7, 2020 at 15:55
  • $\begingroup$ Other than the existence of intent, high-energy neutron bursts from supernova or alien weaponry will have identical outcomes. $\endgroup$
    – Carl Witthoft
    Jan 7, 2020 at 16:19
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    $\begingroup$ Any such event will have very serious besides reducing the concentration of U235... $\endgroup$
    – AlexP
    Jan 7, 2020 at 17:11

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I agree that a possible solution would be to add in a source of high-energy ambient neutrons. Cosmic rays are a possible neutron source, at least at high enough altitudes and assuming the uranium does not have adequate shielding. A strong spike in cosmic rays could then provide such a neutron source; in turn, as one of the major sources of Galactic cosmic rays are supernovae and supernova remnants (for cosmic rays below $\sim10^{15}$ eV), it's possible that one or more supernovae nearby could produce enough neutrons to raise the rate of uranium fission enough to be detected.

Supernovae, of course, can be dangerous when close enough to Earth. Fortunately for us, that critical distance is on the order of about ten light-years. However, this means that the cosmic ray flux would be lower than if the supernova was closer, so it's something of a tradeoff. That said, for an event to cause an increase in fission worldwide, rather than just in one spot, I suspect it would likely have to be astronomical in nature, meaning you'd need some sort of energetic phenomenon. This of course means that you'll be dealing with the same sort of issue with many of the answers you'll get.

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    $\begingroup$ of course a neutron burst powerful enough will also sterilized the planet. $\endgroup$
    – John
    Jan 7, 2020 at 20:24
  • $\begingroup$ @John I would expect so. I think we're looking for a middle ground of sorts - enough of a spike to be noticeable, but not enough that the damage associated with such a supernova would be severe. $\endgroup$
    – HDE 226868 ♦
    Jan 7, 2020 at 23:07
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    $\begingroup$ the problem is noticeable in this case means penetrating hundreds of meters of rock, which Im not even sure your given scenario could accomplish. $\endgroup$
    – John
    Jan 8, 2020 at 4:20
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When does this reduction have to happen?

If it can happen in the distant past, it could be explained just by changing rates of radioactive decay. Right now, we think radioactive decay always happens at the same rate, but there have been some weird carbon dating results that may support the idea that radioactive decay rates could be variable. The current consensus is that these anomalies are just statistics problems, not a new physical phenomenon. But it wouldn't take too much suspension of disbelief to say this actually is a new phenomenon.

If radioactive decay rates can change, maybe they were higher for a short period in the distant past. This would mean less uranium would be around now.

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  • $\begingroup$ I like the idea, especially since it's been explored experimentally in, of all places, Oklo. arxiv.org/abs/hep-ph/0205340 is a good reference about the impact of variations in the fine structure constant on cross sections and resonances, if you're looking to include one. (Of course, those measurements have been used to argue that any variations in $\alpha$ have been small or null, but it still explores how cross sections would change if variations had been non-zero.) $\endgroup$
    – HDE 226868 ♦
    Jan 7, 2020 at 20:14
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    $\begingroup$ so still magic just magic in the past. $\endgroup$
    – John
    Jan 7, 2020 at 20:23
  • $\begingroup$ It is preferable for the event to be much less stretched in time than the existence of human civilization. So actually it is not important when it happens but it should be a spike on time scale of humanity. $\endgroup$
    – FrogOfJuly
    Jan 7, 2020 at 22:16
  • $\begingroup$ @Kirill It could have happened any time before the invention of radioisotopic dating. I mean, how are the Romans going to notice Uranium is decaying 10% faster than usual? $\endgroup$
    – Ryan_L
    Jan 7, 2020 at 22:19
  • $\begingroup$ @Ryan_L, it' true. But if radioactive decay rate is changing, it changes very slow on a scale of human civilization and nobody will notice. Except if changes were larger in the distant past and then just vanished, but then again, it is unclear why. $\endgroup$
    – FrogOfJuly
    Jan 7, 2020 at 22:36
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The Oklo Formation is believed to have been a naturally formed fission reactor, moderated by ground water levels over millions of years. When water levels were up, the ore body became critical and heated the land (and emitted radiation, of course). When ground water dropped, the criticality was lost and the ore body cooled. This apparently occurred several times, and was deduced based on the ore body being significantly depleted in U235 relative to its content of U238.

For this to happen worldwide, however, would require a very different mechanism, because there are (as far as I'm aware) no other similar high concentration ore bodies that could spontaneously become critical.

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Thinking of the parallel-universe concept implemented in Stephenson's "Anathem," perhaps some multiverse-catastrophe has caused two "Earths" to swap universes. The marginal differences in cosmological constants changes the half-life of U-235 in the new universe.

Of course, you'd have to MacGuffin away all the other things that don't change as a result of the change in cosmo-constants' values.

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No known process can make it happen world wide. It is possible that pockets of the process might happen.

There have been suggestions of an external neutron source. To produce fission you will need low energy neutrons, not high energy. U235 wants neutrons in the thermal range to produce significant fission.

The problem with external neutron sources is shielding. Uranium is currently mined at least 100's of meters deep in some locations. So any external neutron source that could produce significant fission would not be able to do it uniformly. 100 meters of rock is going to be a quite good neutron shield, preventing the deeply buried stuff from being affected.

Also note that, unless you have a situation with the Uranium quite closely and densely packed, you need (nearly) one neutron per Uranium fission. Without that close packing, the neutrons produced by fission are nearly all wasted.

Changing physical constants is currently at the "speculative" end of science. We don't know if it is possible, though several unification theories indicate it should be. The problem with changing physics constants is that it might affect the thresholds for critical mass and such. But to get it to remove U235 from existing ore bodies is pretty much impossible. Ore comes in a wide variety of concentrations. And anything over 1% is probably a useful commercial product. To get 1% ore to start to fission would require such grotesque changes that it is quite likely many other things would start to fission as well. Thorium for example, which is far more plentiful than Uranium. The result would be a bright flash, and no more Earth.

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