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Advanced devices made with no knowledge

  1. Jan 6, 2016 #1
    Let's imagine following:
    For one reason or the other life on Earth evolved much faster and 2 or 3 billions years ago we got equivalent of mid 18 century civilization.
    At this time U-235 content of total U in ore would be greater than 20%.
    Say that engineers of this world were processing that ore and some clever guy have worked out how to make metal or pure oxide out of it.
    Let's assume that product have proven useful for something so large scale production have started.
    Soon after weird accidents in factories have begun resulting in some "evil flashes of light" after which all factory workers were dying by horrible death within days or weeks.
    Priests and monks of the time have started to call supernatural powers to help to protect workers and some clever witch have noticed that souls of processed ore hate souls of water and if forced together they start some evil action and peoples are dying.
    After that work in manufactures have become safer.
    Soon after someone else have noticed that when processed ore in large quantity is taken together the evil flash happens agaim, there is a mild explosion, stuff smelts around.
    Poor guy said what he has seen and died soon after.
    Clever wizard who have redrecords of all this mishaps got good idea and decided to run experiment.
    He noticed that bringing enough of material together is causing weird and awful effects so he decided to accumulate s lot in dungeon under castle tower (once evil flash was noticed and all workers around died he ordered to stop piling it up.
    Then he ordered to suspend as much as possible on the top of tower (again evil flashes and death of workers indicated the limit).
    Then he ordered to drop suspended platform into dungeon.
    Massive explosion resulted and castle together with surroundings have ceased to exist.
    Hey presto, a very crude nuclear weapon was invented.
    After some tweaking it have been proven to work once assembled under enemy castle by several horse driven carriages with contents dropped down purpose dug well.
    No understanding of science was needed.
    What do you think about such scenario?
     
    Last edited: Jan 6, 2016
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  3. Jan 6, 2016 #2

    mfb

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    A mid 18th century civilization didn't have castles as useful objects, and chemistry and the scientific method were advanced enough to investigate this process in a systematic way.

    I don't think you would get flashes of light. As production goes up, you should reach dangerous radiation levels long before you approach criticality, and radiation sickness would indicate that the processed material is dangerous. While the mechanism would be unknown, the result is sufficiently close to chemical toxic effects to make cause and effect clear. You need a strong motivation for this society to keep production up.
    Larger amounts would heat up significantly. That could be investigated (and chemical detection of fission products would give a hint what is happening), and the nonlinear relation between the amount of uranium and heat production would be very interesting. You can extrapolate that, and get the idea of a bomb. It is not sufficient to let some clump of uranium fall onto another one, you need explosives to combine them as fast as possible, otherwise the yield is negligible (just enough to separate the two parts again). Uranium for nuclear weapons gets enriched to ~90%. I'm not sure if a bomb with just 20% would work at all, but it would certainly be much weaker, and harder to get to criticality. Without understanding the process, you also don't have a neutron source, so you have to rely on a random neutron - the bombs, even if they would work at all, would have a completely random yield between nearly nothing and a full explosion.
     
  4. Jan 6, 2016 #3
    OK, but still few kt of explosive power would be delivered by dropping a ton of 20%U235 into a skip with more.
    Neutrons would be there in every kg of natural uranium is about 20 of them.
    Drop from a tower to dungeon would deliver considerable velocity and some crude acceleration based on gun powder could be provided after few experiments.
    Additionally huge mass dropped would work as a tamper.
    20% U235 would be enough at huge expense of quantity needed.
     
  5. Jan 6, 2016 #4

    mfb

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    It would not.

    The problem with neutrons is the timing. Criticality increases over time, has a peak and then decreases again, you want the starting neutron at exactly the right time. A millisecond too early or too late and the bomb doesn't explode properly. Uranium-238 emits more neutrons than U-235, so you need an even faster combination. Way beyond the speed of sound, probably several kilometers per second with just 20% U-235, so forget every gravity-based mechanism. That needs modern high-performance explosives or even something like a light-gas gun.
     
  6. Jan 6, 2016 #5
    I can agree in general regarding timing issues with one exception:
    In gun type designs, as opposed to implosion driven ones, too early is certainly bad, but too late is not.
    Once all in clump, there is condition known as *full insertion* and whenever neutron comes, it would start full power show.
    You might be overlooking a relatively huge tamper effect associated with dropping large mass, in range of ton or more.
    That could assist inertial confinement and buy extra 50 nanoseconds or so for reaction to proceed.
    OK, you would get maybe 0.5% conversion.
    OK, 0.5% out of 5 tons of 20% U235 together with some fast fission of U238 is still something....
    I do not insist, that such bomb would work.
    Just a thought experiment and look on some pros and cons.
    NB.
    Little boy had insertion speed in range of 200 - 500m/s.
    Free drop from 40m would deliver something like 25m/s.
    Now, little boy consumed about 10-20% of fissile material, here we are aiming at fraction of %...
     
  7. Jan 6, 2016 #6

    mfb

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    It won't stay in one clump, especially with a design similar to Little Boy. Anyway, without enrichment "too early" is the more pressing issue.
    You are overestimating it, and the nonexistence of nuclear weapons without explosives shows that.
    Where does that number come from?
    And 90% enrichment in the core. You have a factor of ~5 more neutron production there, a naive scaling would need 1 to 2.5 km/s to get the same failure probability of ~15%.
    And your nuclear reaction will happen right at the point of first criticality, which means the components fly apart long before significant amounts of uranium fissioned.
    Don't get me wrong, it is still a bomb, but where is the point if the same amount of conventional explosives gives a better result?
     
  8. Jan 6, 2016 #7
    What about "drop on bottom of a pit" design?

    Agreed.

    No, it doesn't show that.
    All what it show is that 40m long designs would be cumbersome to handle.
    For certain, enola gay would fail to lift one.


    Speculation.
    OK, let's settle wilth only 0.05% conversion out of 5 tones of 20% U235.
    Still 0.5 kg would go off with low kt range....

    What means "failure"?
    For little boy 5% conversion would be disappointing, 1% a great failure and 0.1% a catastrophic failure.
    Here we could live with even 0.05% conversion and still get low kt range, which peoples from mid 18 century would consider a remarkable success upon assessment of effects.
    I agree that fission would work promptly upon reaching criticality.

    Purpose of my thought experiment is to assess, if peoples with no relevant knowledge but keen observers and who found themselves in favorable circumstances could produce low kt range nuke with no understanding at all what they are really doing.

    Btw, have red long time ago somewhere ( www.nuclearweaponarchive.org ?) that for 99% U235 gravitational drop from 10 meters would give approx. 50% chance of full insertion.

    Of course I do not insist that it is true as accurate properties of fissile materials are classified, but it seems plausible.
     
  9. Jan 6, 2016 #8

    mfb

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    What is the structural integrity of gas?
    Speculation again. Get a reference, or get a reliable simulation, but stop making up random numbers.
    99% enrichment is a completely different material.
     
  10. Jan 6, 2016 #9
    Nuclear reactors occurred naturally far in the past. They have been found. Probably it caused the moderating water to boil. I dunno if it was dangerous. I guess it would be if you drank the water.

    I think it's a clever idea. You definitely could get a nuclear reaction easily. Maybe you could get a Little Boy type explosion if very unlucky. Good enough for SF!
     
  11. Jan 14, 2016 #10
    No way.

    Dropping two masses of uranium together would not cause fission, even if the uranium was enriched.

    If it was really that easy we would have a real terror thereat on our hands on Earth.

    The best you might get is a dirty bomb, but the real value in a dirty bomb is the fear factor. Your civilization is much too green to understand what atomic energy is, even in a rudimentary way. Note, it is ignorance that tends to create fear, but a dirty bomb would be perceived as no more a threat as any other explosive. You might as well deliver a ship full of plague infested rats. The death toll would be far greater.

    You could write the story, but the less information you volunteer as far as an explanation of what this great explosion is made of, the better off your story will be. I would wouldn't even hint at it being nuclear because you will turn off some of your readers.
     
  12. Jan 15, 2016 #11
    Hornbein is correct, fission can actually occur in nature, all by itself. There's a place in Africa called Oklo where natural, self-sustaining nuclear fission has occurred in the past. It was over a billion years ago, so unfortunately there were no humans present to take advantage of it, but the output was fairly respectable - around 100kW.
    The theoretical 18th century folks postulated here could surely have contrived a way to harness at least some of that energy.
    Good luck with your story.
     
  13. Jan 15, 2016 #12

    mfb

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    Using it as power source is easier, especially if you don't have an electricity grid so you cannot use more than a few megawatts anyway. The topic was about a nuclear weapon.
     
  14. Jan 15, 2016 #13
    Natural nuclear fission is one thing, but that is not an explosive nuclear bomb, which is something else.

    I just don't that you are going to get that by simply banging two uranium rocks together.
     
  15. Jan 19, 2016 #14
    Apologies, I didn't read the OP carefully enough. I didn't mean to imply a fissionable weapon could be made by "banging rocks together." It just brought to mind the article I once read on ancient fission sources on Earth.
     
  16. Jan 19, 2016 #15
    Surely it does. It was fission that slew Daghlian and Slotin.
    However, little of that energy would be converted to mechanical.
    I hear that the yield of Slotin explosion was estimated as 300 kJ - which is 70 microtons.
    If Slotin had ran rather than tear apart the Core, what would the yield have been?
     
  17. Jan 20, 2016 #16
    I don't think you could accidentally create an atomic bomb, but a civilization using uranium or plutonium without understanding it's dangers is perfectly logical. Romans used lead for almost everything, and didn't learn until far into their empire that it was killing them. Oracles and Delphi breathed toxic hallucinogens and believed they were entering the realm of the gods. I'm sure metallurgy was a spiritual thing to neolithic humans. I don't see why anyone would though, these elements are rare for a reason: they decay and they're dense so when a planet is still liquid, they sink to the middle.
     
  18. Jan 20, 2016 #17
    No, they don´t. They are lithophiles and therefore don´t sink. Actually, as incompatible elements (strong field cations) they concentrate to surface.
     
  19. Feb 29, 2016 #18

    DHF

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    I like the idea of a civilization stumbling through the dangers of using radioactive materials without understanding the danger, as others have said though, without expert understanding of how to utilize its potential, fission explosions are highly unlikely

    What I am more concerned about however is the time scale, I think it is unnecessary to set it back 2-3 billions years. I understand you want to do this so there are greater abundances of Uranium but you would be ignoring so many fundamental facts to make the story happen that it quickly morphs into fantasy. 3 billion years ago life was just getting started and by just started I mean bacteria, that was it. Besides 3 billion years ago the Earth was a fundamentally different place and not at all friendly to complex life. For one thing at that time the atmosphere was still largely CO2, the air pressure was pretty harsh and the temperature was like a desert on steroids. Rather then ignoring all of these facts just to have a higher abundance, it would make more sense to simply have an 18th century time line that follows our own but simply have the geography of the area be rich in U-235 as you want. It is far less hand wavy to simply put the characters on top of a massive Uranium deposit then it is to mess with biological and planetary evolution.
     
  20. Mar 1, 2016 #19

    mfb

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    There is no natural process that would enrich uranium in any relevant way.
    If the timescale looks too unrealistic (we don't know how fast intelligent life could develop), let some single-celled organisms from elsewhere get transported onto that planet.
    But don't explain it, every explanation would just lead to more questions.
     
  21. Mar 2, 2016 #20
    How complex was Ediacara biota compared to Franceville biota?
     
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