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Inspired by this question:

For several excellent reasons, a 500-year-old nuclear warhead is not going to produce an actual nuclear explosion. But that doesn't mean it's not dangerous anymore.

Suppose a group of postapocalyptic villagers finds a nuclear bomb. Maybe, on the way to the apocalypse, it fell out of a plane and didn't explode, and 500 years later a farmer digs it up. Maybe they are trying to salvage usable scrap metal from the ruins of a military base, and what was the secure storage facility for nukes is now just another abandoned building. Doesn't really matter.

However they got it, they have a nuclear bomb and they don't know what it is. How much danger are they in if they tinker with it? How much danger are they in if they just take it home and put it on display as a relic of the Old People?

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    $\begingroup$ Depends on the half-life of the isotopes used to build the bomb and on the relative toxicity of the products produced as those isotopes decay. $\endgroup$ – Carl Witthoft Dec 10 '18 at 20:03
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    $\begingroup$ ...and the original design yield matters -- i.e. the amount of fissile material involved. $\endgroup$ – Carl Witthoft Dec 10 '18 at 20:07
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    $\begingroup$ There's a Star Trek TNG Episode about this. Of course the villagers there have an (amnesic, but whatever) Data to solve the problem. ;-) $\endgroup$ – Karl Dec 10 '18 at 20:24
  • $\begingroup$ Fission bomb or fusion bomb? Answers and comments currently go in all directions because you are not saying. Also: anything like corrosion assumed, or is the hull undamaged? $\endgroup$ – Jan Doggen Dec 12 '18 at 15:04
  • $\begingroup$ @Karl It's the wonderful episode by the name of "Thine Own Self". en.wikipedia.org/wiki/Thine_Own_Self Data is both responsible for exposing them to the radioactive material in the first place, but also (spoiler alert!!) saves the day in the end. $\endgroup$ – YetiCGN Dec 13 '18 at 13:27
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The crucial question is what they do with it. As a museum piece, it's not really dangerous. If they open it up, bad things can happen.

This answer assumes that we're talking about a two-stage thermonuclear device. This has a couple of main components: a primary fission charge, a secondary fission and fusion charge, an interstage, and a tamper. The primary fission charge goes off first, compressing the secondary fission charge, which then further heats and then compresses the fusion charge. The interstage and tamper ensure that this whole delicate operation goes off exactly as planned - the timing and geometry have to be just so for it to work.

The tamper is the critical part from a long-term safety perspective:

For the secondary to be imploded by the hot, radiation-induced plasma surrounding it, it must remain cool for the first microsecond, i.e., it must be encased in a massive radiation (heat) shield. The shield's massiveness allows it to double as a tamper, adding momentum and duration to the implosion.

Essentially, in addition to its other jobs, the tamper acts as a huge radiation shield. Though it contains the actual blast for only a crucial millisecond, it can contain the natural decay of the bomb components with ease. It helps that unlike a boosted fusion device (which uses short-lived but highly energetic tritium), this bomb can use stable lithium deuteride fusion fuel. Fission fuels are generally speaking relatively stable over the long term. So as long as you keep the bomb in its original packaging, so to speak, it should be quite safe.

However, if you rip it open and start tinkering with its guts, bad things can happen. Plutonium, in particular, has been linked to problems when radioisotope thermoelectric generators using it have been salvaged and then opened by damage or tampering. Per WP,

The alpha radiation emitted by either isotope [of plutonium] will not penetrate the skin, but it can irradiate internal organs if plutonium is inhaled or ingested. Particularly at risk is the skeleton, the surface of which is likely to absorb the isotope, and the liver, where the isotope will collect and become concentrated.

You should not eat nuclear bomb parts.

If you're opening the bomb up, though, chemical toxicity is a major threat. The tamper is composed of depleted uranium (U-238) which, although not a major radiological hazard, is direly toxic and a fire hazard. (In addition to being flammable, it's brittle, and the resulting dust has a charming habit of spontaneously igniting.)

There's also the interstage, which is composed of... well, nobody in the public domain really knows. But according to DoD documents, it's also toxic. Lithium deuteride, not to be left out, reacts violently with water to create caustic lithium hydroxide, and is highly flammable to boot.

So the upshot is: as long as you don't touch the bloody thing you should be safe from the radioactive materials inside. If you don't know what you're doing and open it up, it'll be a race between the various nasty, nasty things inside to see what does you in first. (My money's on the lithium fire. Those things are tough to put out if you're not expecting them.)

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    $\begingroup$ RTG's are a different case. They use the precious Pu238, which has a rapid decay rate of 87 years hl -- desirable for RTGs but anathema to bombs. A spontaneous decay at the wrong time will cause a fizzle. Fizzles are bad because then the enemy can collect the plutonium dust and send it back to you postpaid! Bomb plutonium has slow decay, 24,000 years hl. So 1/300 the radiation. $\endgroup$ – Harper Dec 10 '18 at 20:07
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    $\begingroup$ Solid pieces of depleted uranium are not a fire hazard, and not very poisonous. And why mill it into dust? Last thing a post-apocalypse blacksmith would do with a solid piece of metal, I think. It is also not brittle, but a relatively soft, ductile material. Some people make bullets from it. $\endgroup$ – Karl Dec 10 '18 at 20:34
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    $\begingroup$ Plutonium is also incredibly toxic, quite aside from its radiological effects. $\endgroup$ – jdunlop Dec 10 '18 at 20:51
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    $\begingroup$ "You should not eat nuclear bomb parts." +1 for this advice. This probably saved my life! $\endgroup$ – Zaibis Dec 11 '18 at 13:10
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    $\begingroup$ +1 "You should not eat nuclear bomb parts". Probably one of the best quotes from Wordbuilding. $\endgroup$ – Pere Dec 11 '18 at 14:59
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You have three sources of risk:

  • radiological. This is probably negligible, because after 500 years anything with a half-life shorter than 50 years is gone. The shielding on the other hand is pretty stable.
  • explosive. Nuclear warheads have an explosive primer, containing a sizeable quantity of explosive compounds. Some of these might have become inert, some others might have become dangerously unstable. This might have transformed the warhead in a "dirty bomb".
  • chemical. In addition to toxic waste from the primer, plutonium is highly toxic (as well as carcinogenic). Depending on the device's nature (fission or fusion), it may contain other substances that are poisonous, flammable, or both (e.g. lithium deuteride for a thermonuclear design).

239Pu decay chain

The plutonium in the warhead will slowly decay along the following chain: enter image description here Since the half-life of 235U is way longer than 239Pu's, we will have mostly alpha emissions and a negligible beta-minus decay from protoactinium.

There is also a more speculative risk (the design of the weapon ought to prevent it, but you never know). Plutonium in nuclear weapons isn't pure plutonium, but rather gallium-stabilized delta-phase plutonium, which has much better characteristics from an engineering point of view. The priming explosion squeezes it in the critical alpha-phase. But it is possible that the same effect can be achieved by aging ("δ phase Pu–Ga is still thermodynamically unstable, so there are concerns about its aging behavior" says Wikipedia), or by "cooking" it at temperatures above 475 °C.

In other words, there might be significant chances for an ill-advised attempt to melt and maybe re-cast the mystery metal to, at minimum, cause poisonous fumes to be released; or, at worst, to trigger a "fizzle melt", which would probably be more than enough to kill everyone in a radius of several meters or more, and possibly contaminate the whole area.

An ancient (and, incidentally, sentient) nuclear bomb appears in Arsen Darnay's The Karma Affair (1978). I seem to remember it being intentionally detonated by letting it fall from a very high tower.

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  • $\begingroup$ What about short-lived isotopes produced by uranium and plutonium decay? Those would keep getting replenished, but maybe there wouldn't be enough of them at any one time to matter. $\endgroup$ – zwol Dec 10 '18 at 18:16
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    $\begingroup$ @zwol there aren't any significant short-lived isotopes produced by Pu239 in our time frame. There will be some thorium activity but I would not expect too much at all. $\endgroup$ – LSerni Dec 10 '18 at 18:31
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    $\begingroup$ @zwol, on a human timescale, U-235 is stable. Sure, there's a huge cascade of short-lived isotopes in the decay series between Pa-231 and Pb-207, but the slow decay of U-235 means they won't be present in significant amounts. $\endgroup$ – Mark Dec 10 '18 at 21:00
  • $\begingroup$ Isn't thallium an extremely nasty poison if nothing else? $\endgroup$ – rackandboneman Dec 10 '18 at 22:34
  • $\begingroup$ @rackandboneman thallium, yes. But plutonium is alloyed with gallium, which is a different metal (even if it's in the boron group, same as gallium). $\endgroup$ – LSerni Dec 10 '18 at 23:15
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Oh my...

This has happened before. It wasn't a bomb, though.

It was 1987. A hospital in a Brazillian town used caesium-137, which is radioactive, in a radiotherapy machine. The hospital building was abandoned with the equipment in it.

Some thieves scavenged the caesium containing equipment. They pried their salvage open and found an eerily beautiful blue-glowing dust inside...

The thieves took the dust home, and showed it off to their friends and family. People were amazed by the dust and exposed themselves to it in various different ways. One of the thieves used the dust to paint a cross on his abdomen. The other gave some to his six-years-old daughter, who used it as glitter and even swallowed some.

The poor girl died a month later from a very slow and painful death, horribly disfigured and bleeding internally, and alone in a hospital because the nurses were too afraid to come near her (they knew about radiation and had no equipment to deal with it). The kid had to be buried in a lead casket. The populace was mad at her death, but they didn't know who to blame... They were poor, uneducated people. They were also afraid her burial would pollute the cemetery with radiation.

Besides the girl, three other people died in hospital. Other 250 people had enough caesium in them to be picked up by a geiger counter, but only 20 showed any signs of radioactive poisoning, and they all survived.

You can read more about it here or listen to the BBC Witness podcast about it.

I suspect that if people open up a nuke 500 years from now, specially if they don't know what they are doing, a similar incident would happen.

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    $\begingroup$ Every time I read of that event, I feel like it's one of those helplessness horror movies where you find yourself banging on the thick glass, screaming at them not to do it, and cannot help but watch in some morbid fascination as the scene unravels in front of you. $\endgroup$ – Cort Ammon Dec 10 '18 at 19:30
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    $\begingroup$ I would note that Cs 137 is (absent a criticality accident) MUCH nastier then plutonium from a radiological perspective. It is a GAMMA emitter which means it penetrates, Pu and its U235 decay product are alpha emitters and thus no hazard providing they do not get inside the body. a reliably sub crital mass of pure Pu I would handle with washing up gloves, Cs 137 I will use a robot from behind a lot of concrete. $\endgroup$ – Dan Mills Dec 10 '18 at 19:42
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    $\begingroup$ The reason the Goiania incident was so bad was because the isotope involved, Cs-137, undergoes beta decay with gamma emission. A nuclear warhead, on the other hand, is a mix of Pu-239, U-235, and U-238, all of which are pure alpha emitters. Your skin is adequate shielding against alpha radiation, while beta requires a thin layer of metal at a minimum, and gamma requires thick sheets of lead or even thicker concrete walls. $\endgroup$ – Mark Dec 10 '18 at 21:06
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    $\begingroup$ Caesium-137 has half-life of 30 years, as opposed to 24,110 years for Pu-239. This means that bomb's content will be orders of magnitude less nastier (as long as radioactivity is the main concern). $\endgroup$ – Alexander Dec 11 '18 at 0:29
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    $\begingroup$ I downvoted this answer because it is misleading: The contents of a nuclear warhead are not nearly as unhealthy as Caesium (unless the warhead detonates, of course). $\endgroup$ – Philipp Dec 11 '18 at 14:02
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If it's a thermonuclear bomb, the lithium deuteride will likely catch fire as soon as someone breaks its sealing. They won't be able to extinguish the fire (using water, of course, instead of sand), the thing will burn out, including the chemical explosive (no, it won't explode), spreading radioactive char all over the place. The villagers die, nobody else will dare come close for ages.

(Right, solid pieces of LiH don't spontaneously ignite in dry air. So? One moist helping hand, and they have their own little Mini-Chernobyl.)

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    $\begingroup$ It's only the powdered form of lithium deuteride that spontaneously ignites. As far as I know, nuclear weapons use solid lithium deuteride, which merely tarnishes when exposed to air. $\endgroup$ – Mark Dec 11 '18 at 1:03
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    $\begingroup$ @Mark That will depend a bit on humidity though as it also reacts with water. It'll be rather like opening a lithium battery. Most of the time it'll just tarnish, but get it the slightest bit damp and you're likely to end up with a fire, and the odds will increase the more you mess with it. Of course, that assumes that it hasn't leaked enough air in and out of the casing over 500 years to render the stuff mostly inert unless you actually cut into it. $\endgroup$ – Perkins Dec 11 '18 at 18:49
  • $\begingroup$ @Perkins Why should the casing leak air? I'm pretty sure people building thermonuclear warheads have the air and water diffusivity of their materials checked. ;-) $\endgroup$ – Karl Dec 11 '18 at 19:39
  • $\begingroup$ @Karl I'm sure they do, but I'm also sure they don't design them to go unmaintained for 500 years in questionable storage circumstances, potentially after having been dropped from 50,000 feet or more. Leakage is a definite possibility. $\endgroup$ – Perkins Dec 11 '18 at 20:02
  • $\begingroup$ @Perkins Not really. I mean of course it could happen. But you don't want a warhead accidentially lost to burn up, just because it rained the day before on ground zero. $\endgroup$ – Karl Dec 11 '18 at 20:16
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It depends on how deep you bury it. Radiation can be blocked by soil: Material Thickness (inches) Lead 4 Steel 10 Concrete 24 Packed Dirt 36 Water 72 Wood 110

and detonation can also be muted by soil to the point where the Tar Bomba feels like a tremor.

 18 June 1985 

A 2.5 kiloton nuclear device was detonated at the bottom of a shaft 2,850 m (9,350 ft) deep at a location 60 km (37 miles) south of Nefte-yugamsk, Siberia, Russia, on 18 June 1985. The detonation was carried out in an attempt to stimulate oil production. For a comparison, the Hiroshima bomb had a yield of around 15 kilotons.

The Russians carried out 116 nuclear explosions between 1965 and 1988 in a program known as No. 7 - Nuclear Explosions for the National Economy. Known internationally as peaceful nuclear explosions (PNEs), their uses included reservoir and dam-building, mineral prospecting, increasing oil and gas production by liberating material from rocks, creating underground gas stores and putting out subterranean oil and gas fires. The United States had its own PNE program known as Project Plowshare.

http://www.guinnessworldrecords.com/world-records/deepest-nuclear-explosion-underground

enter image description here

enter image description here Russian testing grounds. The Official CTBTO CC BY 2.0

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Radioactive isotopes eventually decay, or disintegrate, to harmless materials. Some isotopes decay in hours or even minutes, but others decay very slowly. Strontium-90 and cesium-137 have half-lives of about 30 years (half the radioactivity will decay in 30 years). Plutonium-239 has a half-life of 24,000 years Most nuclear weapons these days have a reservoir of Tritium gas, which is a radioactive isotope of hydrogen. The half life of tritium is about 12 years, so the reservoir needs to be periodically replenished to maintain reliability.

Plutonium is very active, decaying at a high enough rate to actually heat itself up significantly. Little is known about the effects of long term radiation damage on plutonium metal, and when coupled with the sustained high temperature, it's conceivable that adverse changes in the crystal structure could take place.

Nuclear weapons are full of unstable material. in most modern weapons, insensitive high explosives are used, but they are exposed to decay heat and a constant stream of gamma radiation from Pu-240 impurities in the bomb's core- this can lead to expansion and cracking of the explosives, damaging them and preventing the bomb from yielding the proper amount. Also, the explosives decay over time; both of these effects necessitate replacement of the explosives.

The bomb's pit (the core) is typically plutonium; plutonium is radioactive and is constantly decaying, and as such microscopic helium bubbles form in the pit, and can affect the symmetry of the core's implosion. Also, the aforementioned decay heat can warp the metal, and radiation from decay can damage the crystalline structure of the pit- plutonium has six common allotropes, and radiation/heat can cause the metal to change allotropes. All of these effects require the pit to be reformed every once in a while.

Modern bombs use tritium boosting gas. Tritium has a half life of about 12 and a quarter years, and as such must be regularly replaced to maintain the ability to yield properly. While nuclear weapon maintenance schedules are obviously classified, those two items probably give you a ballpark estimate of a few years or so. While an unmaintained warhead might still fire, the yield could be significantly reduced. A weapon sitting at the bottom of the sea would be rendered non-functional pretty quickly, but the materials would be salvageable.

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    $\begingroup$ You repeat the information about tritium in the first and last paragraphs. Obviously a 500-year old bomb would have no tritium, so it wouldn't explode - but the OP is asking what else could go wrong (and other answers have explained that). $\endgroup$ – Martin Bonner Dec 12 '18 at 11:34
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I suspect as long as the pit is left alone there is not a problem. A lot of effort is expended in isolating the fissile material from the handlers and the thick metal walls will take eons to corrode unless accelerated by some chemical reaction. However there are the conventional explosives that when detonated compress the pit to produce the nuclear yield. Over time I imagine the conventional explosives will be come unstable and be a monumental risk if someone sneezes at the device or a mouse crawling over it farts.

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    $\begingroup$ The thing about implosion-type bombs is that they require the explosives to go off in the correct sequence to get a nuclear explosion. Early implosion bombs used symmetric setups (Fat Man, for example, had 32 detonators that had to go off simultaneously), while later ones use asymmetric setups that require the detonators to go off in a specific order. If the conventional explosives of an implosion bomb go off in the wrong order, all that happens is that chunks of plutonium get scattered around the area. $\endgroup$ – Mark Dec 13 '18 at 0:07
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The danger of a criticality accident.

The warhead would not be in a condition of exploding, but fissure material (Plutonium-239) would still be very active. If the farmer (or rather a smith) would get an idea to collect the pieces of plutonium and melt them together in a forge, he will create a localized irradiation effect.

P.S. The previous version of my answer is corrected to reflect that there is no danger of Chernobyl-size accident from a single nuclear warhead, because amount of fissile material is too small for it. A single nuclear reactor, by comparison, contains an equivalent of 100+ critical masses.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – James Dec 13 '18 at 21:53

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