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If Uranium's half life is 4.5 billion years, why does it become waste

  1. Jun 17, 2011 #1
    Okay, If Uranium's half life is 4.5 billion years, why does it become waste at a reactor before it decays 4.5 billion years later.....

    Forgive me if my question is badly worded or really stupid; I am a high-school student who has just finished the topic of "Nuclear Energy" in my physics class (I REALLY ENJOYED IT) and I've become very interested in the area and I want to know a lot about it :)

    P.S. I am also sorry if I've posted in the wrong sub-forum :/
     
    Last edited by a moderator: Jun 17, 2011
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  3. Jun 17, 2011 #2

    Borek

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  4. Jun 17, 2011 #3
    In a reactor it's made to deliberately split (not decay, but similar end result) so we can gather the energy of the splitting and use it.

    In nature it becomes radioactive waste too, just a lot slower. The waste eventually decays into stable compounds making fission a clean, green, and fully renewable energy source over time.
     
  5. Jun 17, 2011 #4

    Borek

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    This is off. It is not clean as radioactive waste needs thousands of years to decay, and it is not renewable, as stable isotopes that are at the end of the decay chain can't be reused. Whether it is green depends on how you define green - if everything works OK and if we will finally decide where to safely store radioactive waste, then yes, it will be green. Unfortunately, as for now there is no decision on where to build the safe depository.
     
  6. Jun 17, 2011 #5
    They can't be reused in a reactor but they can be used to make cars and vacuum cleaners, just like recycled glass and plastic.

    (yes, it takes a very long time; my point is that radioactivity in that LONG run is greener than say styrofoam which is probably more stable and less beneficial in the wild.)
     
  7. Jun 17, 2011 #6

    Borek

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    With each iteration you are sinking deeper. Fission products - after separation and waiting till they decay - will be more expensive than gold. And styrofoam decays in the wild in years, perhaps tens of years, while some isotopes will be still dangerous even after many thousands of years. No matter how you will try, your original post can't be defended.
     
  8. Jun 17, 2011 #7
    Radioactive waste is a regulatory term, not a scientific term. If we could find a use for it, it wouldn't really be waste. One man's DU waste is another man's shielding.
     
  9. Jun 17, 2011 #8
    Why will they become so expensive where they are just waste?
     
  10. Jun 17, 2011 #9
    I will defend my post to the death! Small amounts of radiation are good for you. They stimulate the immune system and hasten evolution through accelerated mutation. I'll bet your weak styrofoam can make no such claims. So there!
     
  11. Jun 17, 2011 #10

    Morbius

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    In nature, Uranium is 0.7% U-235 and 99.3% U-238. Nuclear reactor fuel is "enriched" in U-235 so that it is about 3-4% U-235 and 96-97% U-238. The U-235 has a half-life of 705 million years to U-238's 4.5 billion. However, the reason we are interested in the U-235 is because it is "fissile" - which means it fissions or splits when hit by even slow moving neutrons.

    When U-235 fissions, it releases a great deal of energy - and it's the energy that we want. The splitting or fissioning of U-235 releases more neutrons, and we can keep the reaction going as a "chain reaction".

    However, when a U-235 splits, the remnants of the U-235 nucleus after splitting are elements such as Strontium-90 or Cesium-135, or Iodine-131... Those are the things that are radioactive, and are of concern in the waste.

    However, you are correct in wondering in the sense that 96% of what we call "nuclear waste" is U-238 that is no more radioactive than the day it was dug out of the ground. If we could "reprocess" the waste - chemically separate the U-238 from those highly radioactive "fission products", we could reduce the volume of nuclear waste by a factor of 25.

    Dr. Gregory Greenman
     
    Last edited by a moderator: Jun 18, 2011
  12. Jun 17, 2011 #11

    Drakkith

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    I think in context of a realistic situation, the majority of the radioactive waste is not reusable for society. I say that because I don't see 10,000+ years down the road as a realistic time frame for this discussion.
     
  13. Jun 18, 2011 #12
    So a reactors waste isn't exactly "waster" (i.e. used up), it's just radioactive isotopes such as Strontium-90 or Cesium-135, or Iodine-131.
     
  14. Jun 18, 2011 #13

    Drakkith

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    Lets put it this way. IF we had the time to wait until all of these isotopes decayed into stable elements, we could use them. Unfortunately this process takes thousands of years before they are safe to use.
     
  15. Jun 18, 2011 #14
    Ohhhhhhhh, so are you saying that we lets say "tire them out" and the only way to be able to use them again is when they aren't tired anymore, which is thousands of years from now.
     
  16. Jun 18, 2011 #15
    Welcome to the hottest forum on the internet of late (pun intended)

    As was explained, most of the spent fuel is not waste, in the sense of something useless. The waste part is the radioactive elements created from Uranium, through fission. Plutonium is one of the most serious concerns, as it is both waste, and more valuable than gold. By the end of a nuclear cycle, the fuel is creating 40% of the heat from the Plutonium that was created from Uranium.

    You might ask, if there is all that heat from Plutonium, why not keep using the fuel? Why reload fresh fuel if the fuel is still hot?

    That is a very good question, but not directly part of the answer.

    Same for the question, "Why not use the spent fuel rods to generate heat?". Why is it waste if it still has all that power available?

    Or, "Why not use the plutonium to run a reactor?"

    Why is considered waste if it's valuable? Or useful?
     
  17. Jun 18, 2011 #16
    I'm asking all those questions! :P

    So..... Can I have answers?
     
  18. Jun 18, 2011 #17

    Drakkith

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    Nope. When uranium decays, there is no more uranium.
    Look up Nuclear Fission here: http://en.wikipedia.org/wiki/Nuclear_fission

    Go through that article and bounce over to the Nuclear Power one along with any other ones you need to go through to get a basic understanding of the process.
     
  19. Jun 18, 2011 #18
    I think the point is not that anyone wants to use the cesium and other daughter products in the spent fuel. What is desired, is the plutonium, which is mixed in with everything else. While a thousand years from now many of the non valuable elements will be much less dangerous, the plutonium and other highly radioactive products will still be quite hot, and a valuable commodity.

    This is one major issue for "storing" spent fuel. 10,000 years from now the plutonium will still be desired. Plutonium is always worth something. Even just the ton or so produced each year by a reactor is worth a lot. So the concerns over secure storage are very valid.
     
  20. Jun 18, 2011 #19

    Drakkith

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    How is this relevant? Waste is waste. Even if we wanted to use it we couldn't because it is not safe. Whether we will or will not want to use later it is a different story.
     
  21. Jun 18, 2011 #20

    Astronuc

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    Actually, there is interest in the rare earths produced. However, the complication is one of extracting (economically as well as technically) the inert (non-radioactive) isotopes from the radioactive isotopes.

    Pu from LWRs has several isotopes from Pu-237 to Pu-242, and there are isotopes of Am and Cm as well. Some of these are more useful for fuel than others, but it is a matter of fabricating fuel economically for operating in commercial reactors.

    As for the OP - please see

    http://hyperphysics.phy-astr.gsu.edu/Hbase/nuclear/radser.html

    http://hyperphysics.phy-astr.gsu.edu/Hbase/nuclear/radact.html

    http://hyperphysics.phy-astr.gsu.edu/Hbase/nucene/fission.html
     
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