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Radiation of nuclear weapon questions

  1. Aug 17, 2015 #1
    I am trying to grasp the basics of Nuclear weapons and how they work, I understand the fission and fusion principles, but im struggling with a basic question...Radiation. im by no means a physicist nor an educated chap so please go easy on me.

    If it takes a tiny amount of Plutonium or Uranium to create such a powerfull weapon, chain reaction, how come the radiation from that weapon is so strong and longlasting.
    when millions of tons lay in the earth, why is that not as potent and deadly......?
     
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  3. Aug 17, 2015 #2

    Bystander

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    Google "half-life;" compare your result to a search for "fission product half-lives."
     
  4. Aug 17, 2015 #3
    Uranium has to be highly 'enriched' to be used as bomb material, which is a complex time consuming process.
    Most of the Uranium (238) is discarded, what is used is a concentrate of the much rarer and more 'explosive' (fissionable) U235.
    Plutonium is not a naturally occurring substance, it has to be made in special reactors.
    Naturally occurring Uranium (mostly 238) is radioactive, but only mildly so, and the ore is not pure Uranium, it's mixed with other elements which need to be removed.
    I don't have the details to hand, but I think I once read that several tons of Uranium ore need to be processed in order to extract a few kg of U235
     
    Last edited: Aug 17, 2015
  5. Aug 17, 2015 #4

    mathman

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    Nuclear weapon energy results from nuclear fission (chain reaction) or nuclear fusion, not from radioactive decay. To sustain a chain reaction, you need a high concentration (and enough quantity) of fissionable material.
     
  6. Aug 17, 2015 #5

    bcrowell

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    U235 exists naturally in the earth. Plutonium does not.

    It is not really true that the radiation from a nuclear explosion or reactor waste is long-lasting -- at least, not compared to the 235U, which has a half-life of almost a billion years. One of the reasons why natural 235U in the earth doesn't cause any detectable harm is that its half-life is so long. Because of the long half-life, its rate of decay is extremely low. You can hold a small nuclear reactor fuel rod in your bare palm, and not much will happen to you.

    Once a fission chain reaction occurs, you're dealing with a witch's brew of lighter isotopes, some of which have short half-lives and some of which have longer ones. The very short-lived ones have half-lives of seconds or minutes, so they don't present any long-term threat. The ones with the longest half-lives have low decay rates, so they also don't do much harm. The harmful ones are the ones with intermediate half-lives of weeks to years.

    After waiting, say, a decade for the relatively short-lived half-lives in nuclear reactor waste to die down, we actually could process it into a fine powder or a solution or suspension, and then simply take it out on boats and mix it as uniformly as possible into the world's oceans. The volume of the oceans is vast, so the resulting levels of radiation exposure turn out to be too low to harm marine life or humans. However, this technique is not politically possible for emotional reasons.

    A secondary reason why fallout and commercial waste are more harmful than the naturally occurring fissionables, at equal concentration, is that the naturally occurring stuff mostly emits alpha particles, which are blocked by any tiny amount of material. Unless you ingest an alpha source, or get it in an open cut or something, it can't hurt you. The fission products, however, can emit a lot of betas or gammas, which are far more penetrating. Also, a nuclear bomb or a disaster like Chernobyl disperses material into the air and water, where it can easily be ingested.

    To keep things in perspective, Chernobyl is a very healthy wildlife habitat now that the humans are gone, and zero people died from radiation exposure at Fukushima (although many died from the tsunami).
     
    Last edited: Aug 17, 2015
  7. Aug 17, 2015 #6

    SteamKing

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    The amount of fissionable material in a nuclear device is only relatively tiny. It can be as much as several kilograms, depending on the design of the weapon.

    Upon detonation, most of the damage caused by a nuclear weapon is not due to nuclear radiation, except in certain devices called enhanced radiation weapons. In more common devices, the extreme amounts of heat produced by the weapon cause rapid heating of the air surrounding the explosion, which in turn crates a powerful shock wave, which can destroy many types of structures and buildings. There is also a thermal pulse generated by the explosion, which can cause combustible materials to ignite and start fires, which lead to more destruction. The thermal pulse from the initial explosion can also cause severe burns to those persons who happen to be outside when the blast takes place.

    Depending on the altitude at which the device explodes, differing amounts of radiation reaching the ground will irradiate the rock and soil in the area, causing this material in turn to become radioactive to varying degrees. This would not be so bad, but the mushroom cloud created by the blast has a tendency to sweep rock, soil, and other radioactive debris over a wide area, which often extends many kilometers from the location of the blast, depending on the direction of the prevailing winds. Dispersal of the radioactive fallout, as this is called, leads to more people being exposed to radiation from the blast, and since it's airborne, the people downwind usually breathe this radioactive material in, where there is little to no protection from the damaging effects of radiation on sensitive tissues and organs.

    Because the fallout is widely dispersed over a large area, often many thousands of square kilometers, it is impossible to clean up and decontaminate the fallout from the soil, so it must stay in the environment until radioactive decay has eliminated this material, which can take thousands of years in some cases.
     
  8. Aug 17, 2015 #7

    SteamKing

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    U235 occurs at a concentration of 0.7% in natural uranium. Typical reactor fuel may be concentrated to a level of 3-4% to be effective, but the U235 in nuclear weapons must be concentrated to 90% or greater to produce a chain reaction leading to a nuclear detonation.

    https://en.wikipedia.org/wiki/Enriched_uranium
     
  9. Aug 18, 2015 #8
    Even after a decade or two the spent reactor fuel is nowhere near safe to be handled in anything other than a custom built reprocessing facility.

    Spent reactor fuel is millions of times more reactive when it comes out the reactor compared to when it goes in. The spent reactor fuel from reactors from the forties and fifties is still too radioactive to be removed from temporary storage pools without enormous expense. It is simple untrue to suggest that that it is safe after a decade.

    Your point about the habitat of Chernobyl can also be disputed. Wildlife is thriving due to the lack of humans and despite the radiation, but we don't have a clear picture of how the radiation affects the wildlife.

    Finally, your claim that 'zero people died from radiation exposure at Fukushima' ignores the fact that many people will develop potentially lethal cancers due to the ingestion of alpha emitting particles.
     
  10. Aug 18, 2015 #9

    bcrowell

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    I wasn't referring to spent fuel. Here's a photo of someone handling unspent fuel (I'm guessing for a small research reactor) with nothing but gloves: http://www.sciencephoto.com/media/342537/view . The alphas won't get through your skin. Presumably the gloves are either to protect from betas emitted from near the surface of the pellets, or to cut down on the chances of getting material rubbed off onto your hands.

    Right. I didn't say that.

    For a lengthier discussion of this, with some references, see section 26.4 in this book: http://www.lightandmatter.com/lm/

    More information here: http://skeptics.stackexchange.com/q...r-from-the-fukushima-daiichi-nuclear-accident

    I didn't make any claim about future deaths. I would be interested to know what you mean by "many," and what your source of information is. The number, if nonzero, will probably be much too small to be detected epidemiologically.
     
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