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Is it possible for nuclear waste to somehow naturally cause a nuclear explosion? (with no human input, other than dumping the nuclear waste)

The yield of the explosion would preferably be enough to cause at least some damage. For a little context, the nuclear waste would be underwater.

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    $\begingroup$ Even a nuclear bomb is unlikely to explode "naturally" without a trigger, so nuclear wastes? let see if someone here is able to make that work $\endgroup$ – Nyashes Apr 21 '16 at 19:09
  • $\begingroup$ Setting is important. I can imagine a couple of laboratory or production scenarios before or during the Manhattan project causing serious problems, but that was before we understood what was going on (mostly anyway.) Some deaths and serious injuries actually did happen, but you won't get that set up out of waste disposal in the modern day. $\endgroup$ – Sean Boddy Apr 21 '16 at 19:21
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No. To cause a nuclear explosion you need a chain reaction. A chain reaction requires a high enough density of fissionable material so that any one reaction is statistically likely to cause more than one additional reaction.

Unfortunately, for most nuclear materials, that is virtually impossible. It's almost as though nature doesn't want it to happen. Very few materials can undergo a chain reaction in the first place. Those that can, like Uranium, typically needs to be intentionally enriched to an extra high density of fissionable atoms before it can be used in a bomb. Worse, as you bring two pieces of fissionable material together to create a critical mass, the energy caused as they come together has a tendency to force the two pieces apart (such as by blowing them apart with vaporized material). If this occurs, only a tremendously small portion of the fissionable material actually does anything like an explosion. Most of it just heats up a whole lot.

In nuclear bombs, we not only have a lot of explosives to push the fissionable material together to create a critical mass, it is very carefully shaped explosives with extraordinarily tight initiation timings that makes sure a large portion of the mass undergoes a chain reaction. The odds of this happening are so slim that it would be simpler to simply round down and call it "none."

The best hope I think you'd have would be to use the heat of the nuclear pile to superheat a small pocket of water and have a steam explosion. Mythbusters did it to a water heater once or twice. It's no nuclear explosion, but it's at least good for TV ratings!

EDIT: Reviewing additional content related to this answer, I came across a fascinating Wikipedia page on Prompt Criticality. In any nuclear system, "supercritical mass" is defined to be the mass at which point the nuclear material will undergo a chain reaction. It is dependent on many things such as density, shape, and even nearby neutron reflectors. What I just learned from the page on Prompt Criticality is that there are two definitions of supercritical: prompt-criticality and delayed-criticality. Prompt-criticality is what matters for bombs. This is when the "prompt-neutrons" emitted directly from the fission event are likely enough to cause another event that a chain reaction occurs within milliseconds. For nuclear reactors, and your nuclear waste pile, delayed-criticality is more of an issue. Many of the remains of uranium fission are, themselves, unstable, and emit many more neutrons over the course of minutes. For a nuclear reactor, or nuclear waste pile, if the pile is delayed-critical, it will eventually melt down if you don't do something about it. However, because it isn't prompt-critical, it will not cause a nuclear explosion.

Who would have thought nuclear science was this complicated!

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    $\begingroup$ Nature actually constructed a natural nuclear reactor about two billion years ago when the natural concentration of U235 was higher than today. We do not have enough evidence left to know what scale of geysers or eruptions were being generated. $\endgroup$ – nigel222 Apr 21 '16 at 20:30
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    $\begingroup$ That was nuclear energy, not nuclear explosions. $\endgroup$ – Loren Pechtel Apr 21 '16 at 21:00
  • $\begingroup$ I'm pretty sure "Unfo" should be replaced with "Fo". ​ ​ $\endgroup$ – user3576 Apr 22 '16 at 7:14
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At worst, you might get a nuclear fizzle when natural processes slowly concentrate fissile materials into a critical mass. This would be a genuine nuclear explosion, but not what most people expect when they talk about nuclear weapons.

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The answer is a solid "No".

It takes a tremendous amount of energy to cause a nuclear explosion. That's why nuclear bombs contain a serious amount of explosives.

However that material will decay and release a lot of radiation.

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    $\begingroup$ You can get a U-235 boom without them. Take two appropriate hunks of U-235 and slam them together and you have a fair chance of taking a city with you. $\endgroup$ – Loren Pechtel Apr 21 '16 at 21:02
  • $\begingroup$ @LorenPechtel Actually, you don't. If you do that, you will certainly irradiate yourself enough to join the ranks of the walking dead, but a disappointingly small amount of material will actually undergo fission. Two two blocks will actually drive themselves apart, generating a great deal of heat, but they wont achieve a chain reaction. $\endgroup$ – Cort Ammon Apr 21 '16 at 21:46
  • $\begingroup$ @CortAmmon - depends on how you look at things. Without some chain reaction, you would not have enough radiation to hurt you. Fat man only had about 70 spontaneous fissions per second for the 65 kg of heavily enriched U-235 it used. When the bullet was 9.8 distance, the chain reaction kicked in, reaching full on explosion in about 1 microsecond during which the bullet travelling at 300 meters per second moved 0.3 mm. So, you could get a fizzle bomb - the operative word is still bomb. Such a device is still clearly not formed from nuclear waste unless defined extremely generously. $\endgroup$ – Gary Walker Apr 23 '16 at 1:24
  • $\begingroup$ @GaryWalker True. However, you will note that you had to get the bullet traveling 300m/s, which is not exactly a walk in the park. Certainly far from just slamming two pieces together. $\endgroup$ – Cort Ammon Apr 24 '16 at 4:23
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Yes for sufficiently small values of explosion. It has happened. Nothing like a nuclear bomb, though. Nothing even as big as the Chernobyl accident.

Specifically, the former USSR was dumping radioactive waste in a hole in the ground. What they did not realize was that the clay minerals in the ground selectively absorbed Plutonium. It got more and more concentrated until it reached criticality. The chain reaction continued until it had released enough energy to turn a fair bit of groundwater into steam, at which point the dump exploded like an over pressured boiler, scattering radioactive waste to the winds.

The first reaction in the West was disbelief, when the story reached us via a defector. Then proof was found in the radioactive bodies of migrating birds. Then panic because the same thing was happening in some of our dumps! Cue frantic digging and safer waste disposal before we had our own even larger mess to deal with.

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  • $\begingroup$ Are you referencing to the following accident? en.m.wikipedia.org/wiki/Kyshtym_disaster $\endgroup$ – Sundae Apr 21 '16 at 22:58
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    $\begingroup$ The reference explicitly stated that this was not a nuclear explosion. In fact it was a chemical explosion. It's listed as a nuclear accident because the chemical explosion threw up a lot of radioactive debris. $\endgroup$ – Jim2B Apr 22 '16 at 4:38
  • $\begingroup$ Chernobyl was not a nuclear explosion, it was a steam explosion $\endgroup$ – evilscary Apr 22 '16 at 7:48
  • $\begingroup$ Where do you think the energy to make the steam came from? Chernobyl was a nuclear criticality excursion. The fission chain reaction got out of control until it self-limited by blowing the reactor apart. $\endgroup$ – nigel222 Apr 22 '16 at 8:30
  • $\begingroup$ I'm sure I read about a lesser disaster than Kyshtym that was driven by nuclear criticality, and the desperate (and successful) digging up and re-disposing of Western waste dumps that followed. What I read might not have been accurate. $\endgroup$ – nigel222 Apr 22 '16 at 8:35
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The answer is surprisingly yes.

  1. Get nuclear bomb.
  2. Install red button on nuclear bomb.
  3. Put bomb in nuclear waste pile.

Now, when you dump your nuclear waste, it will hit the red button, causing a nuclear explosion. Of course, the nuclear waste wouldn't explode (that's just silly), but it will have caused the explosion.

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