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Questions about nuclear energy and nuclear waste.

  1. Aug 2, 2012 #1
    Hello, new here.

    I'm not sure if I've placed this in the appropriate section, but here it goes.

    I have a few questions.

    1) I know that when spent fuel rods come out of a reactor they are extremely hot and if not cooled, can meltdown. How long do they need to be cooled before they are cool enough (without outside assistance) to not meltdown? I've heard it takes 5 to 10 years but I really don't know.

    2) I've been researching nuclear waste, and frankly, it is extremely confusing. On one hand people insist this stuff is an apocalyptic nightmare for the next *insert biggest number you can think of here* but then on the other hand I've heard people say that high-level nuclear waste loses a good chunk of its toxicity after a couple centuries. At what point are they as dangerous as things like lead, mercury, etc.

    3)I know radioactive waste gives off alpha, beta, and gamma rays, but the most damaging is gamma. From what I've read (and correct me if I am wrong), alpha and beta rays need very little shielding (that 10 ft of air could even do the job). How long do the gamma rays last in comparison to the alpha and beta rays?

    And last but not least.

    4) When does nuclear waste go from being a global threat, to being a local threat?

    If anyone could answer these questions I would greatly appreciate it! :)
  2. jcsd
  3. Aug 2, 2012 #2


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    It depends on the thermal conductivity of the environment and the way the material gets processed. There is no general number, but "years" is a good concept.

    Both ;). It can be dangerous after 10000 years (depends on the stuff you want to store), but it is much worse after 1 year.

    Depends on the situation. If you have radioactive material in your body (for example by eating it), alpha particles are much more dangerous. Outside your body, alpha radiation is not really dangerous, even the outermost cells of your skin can stop them (and those are dead anyway).
    Gamma radiation often occurs in combination with the other two.
  4. Aug 2, 2012 #3
    Specifically spent fuel from a normal Nuclear power plant.
    Isn't it true though, that the higher the radioactivity the lower the half-life? I know things like plutonium last (by human perspective) forever. But what does that mean? Mercury, arsenic, and lead pretty much last forever as well.
    Hrm, I see.
    Say that you had nuclear waste stored in a dry cask and then 300 years later, the cask fell apart. Would the material within cause a global apocalypse? Or would it just render the area around the nuclear waste dangerous? Or would it be as dangerous as an abandoned factory with toxic dusts?
  5. Aug 2, 2012 #4


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    This is true. And it is the reason why the radioactivity does not follow a simple exponential law: In the first years (months, days, ...), the short-living stuff dominates, giving a huge amount of radioactivity and heat. Afterwards, you have the long-living stuff, radiating with a low intensity, but for the next 10^n years.

    Usually, for radioactive material, the radioactivity determines the toxicity. Maybe eating some mg of plutonium is not healthy in a chemical way, but the radiation will be deadly anyway. With a half-life of 24000 years (for 239Pu), it is already in the low-activity, long-living region of radioactive waste.

    No. Not even Chernobyl or Fukushima did that, and this was material directly from reactors (including all the short-living stuff). And if the site is suitable for a permanent storage, it will not cause any radioactivity outside.

    As a side-remark: The tsunami in Japan killed ~20000. For Fukushima I saw an upper estimate of ~1000, mainly caused by the evacuations, with 5-200 due to radiation itself (including estimated deaths in the future).
  6. Aug 2, 2012 #5
    So, are you saying that a lot of fears of nuclear waste are exaggerated?
    So, with those events, do you think there was a lot of ignorance that lead to the fear of global catastrophe?
  7. Aug 2, 2012 #6


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    While I did not say this yet, I think it is true. Usually correlated with a lack of any knowledge in the topic - not even the fact that basically everything on earth is radioactive and it is just a quantitative thing.

    Well, "global catastrophe" similar to H5N1 ("bird flu"), H1N1 ("swine flu"), SARS, the LHC, mayas, ...
    The first 3 actually killed people.

    An interesting thing to do: Try to find events which killed more people than traffic accidents in the corresponding year.
  8. Aug 2, 2012 #7
    I've heard that high-level radioactive waste turns to low-level radioactive faster than people think (since the highly radioactive stuff decays pretty fast).
    I guess I should clarify what I mean. At what point can nuclear waste be abandoned without infecting anything more than the area it is in? If we abandoned spent fuel right now, it would meltdown and spread clouds of radioactive dust, no? Because history dictates that sooner or later, it will be abandoned and forgotten.
  9. Aug 2, 2012 #8

    jim hardy

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    there comes a point , less than the span of a man's working career where it can be put in a cask with ventilation and air cooled by convection. Some plants are starting that now.

    there's stuff in it that is valuable and could be extracted now but political climate is unfavorable for that. So we'll leave it for future generations.

    there's a lot of hype

    yet the subject is not to be taken lightly.

    Like any other human undertaking we need to do it well.

    you might find this site interesting


    i hope you educate yourself and become a captain of industry who does it well. My generation kinda fumbled this one ....
  10. Aug 2, 2012 #9
    Interesting. I know dry casks are very strong (stronger than many of us can imagine). Aren't cooling pools held in concrete structures?
    Let's just play the game of what if.
    What if future generations never use it? Will it just sit there for thousands of years? Will it be known as the "cursed" land by the locals? I suppose what I fear, is that with nuclear power, we don't anticipate history. The natural rise and fall of civilizations. Perhaps I am wrong.

    I don't know. I've learned more about nuclear energy in the past week than in my entire life.

    Interesting. I wonder if such a location would be good for burying nuclear waste?
  11. Aug 2, 2012 #10

    jim hardy

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    I'm in the autumn of my years now and looking back.
    Robert Pirsig's line in Zen and Motorcycle Manitenance is sounding more and more prophetic....
    In my youth i gave this table talk
    i spent my career maintaining instruments in a nuke power plant and raised my family immediately downwind. We made a lot of cheap electricity and no piles of coal ash. Had our plant been coal it would've made enough flyash to build a Great Pyramid sized pile of the stuff every few years.
    But, there's a concrete building that houses a pool with quite a bit of spent fuel in it.
    They're starting now to build onsite long term storage buildings for casks.
    If civilization crumbles that'd be a building to avoid.
    But so will the Everglades what with all those giant pythons.

    That's a thought i'd not heard before.

    I am an idealist who'd rather process it, reduce its volume and get it off the planet.
    Has anyone figured just how many kilograms of nuclides Fukushima released?
    A tera becquerel isn't very many grams.
    But it's a challenge to separate the radioactive from the non-radioactive atoms.

    There are more practical approaches than mine though.

    You might like John McPhee's "Curve of binding energy" which is ostensibly about enrichment and proliferation, but is a delightful character study of the Manhattan physicist who miniaturized weapons , and a good introduction to this subject.
    Also Freeman Dyson's "Disturbing the Universe". He's about the last of the Manhattan scientists left and is still active, look for his articles .

    old jim
  12. Aug 2, 2012 #11
    I've heard that with nuclear power, the toxicity of its waste decays with time, but the toxic chemicals left behind from coal plants will be there for eternity (things like mercury). I wonder if today's spike in cancer has anything to do with the pollution of coal plants?

    So the plant would be a locally dangerous structure? Vs today if a nuclear plant crumbled it could poison the world many times over. These questions may seem trivial (paranoid even), but if I do not ask them, the thoughts will remain in my head.
    Interesting. This is a lot of new information to me. I've never heard of John McPhee. I only became interested in nuclear energy a few weeks ago.
  13. Aug 2, 2012 #12

    jim hardy

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    There are places in the plant one should not spend much time.
    But it's not glowing white hot like a movie set.
    Cask storage areas can be entered for doing work
    as can spent fuel pits if you stay away from the used fuel
    but the untrained shouldn't be roaming around oblivious to radiation fields.
    How would you handle that after societal collapse?

    that's an exaggeration but i get your thought.
    I assume a societal collapse would be gradual and we'd put things in a safe arrangement for long term unattended storage

    there's elements out there that fearmonger. I understand.

    The best way to handle worry is a two step process.

    1. Accept the worst possible outcome as a possibility
    2. Work your butt off to cause a better outcome

    1. If society crashed tomorrow , there would be a really bad situation with all the spent fuel we have in pools in this country. Accept that.

    2. Don't help the forces trying to crash western civilization. Don't even "Go Galt". Instead write your congressman urging him to get going on nuclear fuel waste processing and long term storage .
    Learn more about nuclear power and the people who work in it.
    Then you can decide whether it's something you want to support or oppose, and it'll be your decision not some proselytizer's...
  14. Aug 2, 2012 #13
    So the real threat would only be when the nuclear plant was vulnerable to meltdowns?
    I don't think you could. I think eventually people would consider it "cursed" and stay away.

    I know one thing that our government is trying to figure out is how to tell people in the distant future that nuclear waste is dangerous. They are even required to find a way to make it clear to hunters and gatherers should society ever degrade that much.

    I hope so. My other major interest is history, and I don't know of any society that ever collapsed over night. Even Rome took centuries to collapse (longer if you include the Byzantines). I don't anticipate society collapsing, but I think it is something we MUST keep in mind when planning for the future.
    Yes, and it is difficult to distinguish what is genuinely worth fearing and what is an exaggeration. From what I've learned, it seems that much of the fear from nuclear waste is exaggerated (although you can't treat it lightly).
    That is excellent advice. :)
  15. Aug 2, 2012 #14

    jim hardy

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    Then, and so long as there is fuel in the spent fuel pools that's too active to be air cooled.

    Used fuel in long term storage air cooled casks doesn't worry me.
    Used fuel in pools does. Water can always leak or boil away.

    New fuel is benign. We handle it with cotton gloves so as to not get fingerprints on it.

    Thanks for your kind words. And for your interest !

    old jim
  16. Aug 3, 2012 #15
    Thank you for taking time to answer my questions. I've been searching high and low trying to find the answers, and now I have them. :)
  17. Aug 6, 2012 #16


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    If "spent" rods still give off heat and can cause a meltdown why are they non usable for energy production? I would assume its just not cost effective after a certain time. But if there is still energy there why cant you still use it to produce some energy?

    Im sorry if this question is so easy, Im just a normal person with Nuclear Energy interest.
  18. Aug 7, 2012 #17


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    Right. A nuclear power plant produces something like ~2GW thermal power. If the used rods produce 1MW, the additional infrastructure to use this would be too expensive.
  19. Aug 7, 2012 #18


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    Yup, the typical spent fuel pool heat loads are on the order of 1-10 MW. Not very much compared to other station loads. You could possibly use the thermal power to run heat exchanger pumps to keep itself cooled in case of a station blackout, but there are easier ways to provide emergency power and cooling that make doing so impractical.
  20. Aug 7, 2012 #19


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    If heat is not removed (from spent fuel or any mass), the temperature would continue to rise. Spent fuel is cool under water. The heat is dissipated in a large mass of water, which itself is cooled such that the temperature is rather low such that it would be impractical to use as an energy source.

    Note that temperature (T) and enthalpy (H) are related by H = mcpT, where H and T are relative to some reference values. For a given ethalpy, as the mass in which the enthalpy (heat) is distributed goes up, the lower the temperature.
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