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A stupid question about nuclear disposal

  1. May 27, 2007 #1
    It's one basic question about nuclear disposal that I have never understood.

    Nuclear "fluel" like Uran, etc comes from nature - right ?

    The way nuclear fuel is produced is by increasing the consentration of nuclear material - right ?

    As the nuclear fuel is used and energy is taken out should'nt the amount of redioactive energy be less than it was at the starting point ?

    While burning radioactive fuel, and producing energy from this, should'nt the amount of radioactivity in the world be less and not greater ?

    Why is is not possible in some way to reverse the radiactive fuel back to the original materiel after it has been used ?

    Why is it like that radioactive stuff that has been used for energy production is very dangerous, while beeing in the nature without having been used for energy production, it is not dangerous.

    When producing nuclear energy, you are taking something out of the nature, radioactive fuel that is concidered to be not so dangerous, and when you "burn it" and the energy and the radioactive content is less, then it is more dangerous.

    How can this be ?
  2. jcsd
  3. May 28, 2007 #2


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    Uh huh.
    Uh huh.

    Uh huh. We normally want some specific isotope, as more favorable than another.
    Yeah, that's why the nucleus is ripping apart.

    And at this point I have another question! If the force that keeps quarks and neutrons and protons together gets stronger with distance, then how can the nucleus be ripped apart at all?

    I'll guess entropy. It's kind of hard to burn a match, and then put all the molecules that eventually turned into smoke, and all the energy released as heat and light, and the deformations in the match and undo the chemical reactions, back to getting the match you started with before the reactions took place.

    Because we concentrated it?
  4. May 28, 2007 #3


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    Staff: Mentor

    Many good questions. One could also ask - why are some isotopes radioactive and others not?

    Firstly, why are radioactive materials bad/dangerous? Well, it has to do with the ionizing radiation emitted when a nucleus decays. Alpha (nucleus of He), beta (high energy electron, and similar positron, which is a positive electron), and gamma (high energy photon) can damage cells by destroying molecules by disrupting atomic bonds. That is why radioactive materials are controlled, i.e. isolated from the environment at an appropriate distance and/or behind appropriate shielding. I should point out that alpha decay is restricted to the heavy elements, with Bi effectively being the lightest element to have an isotope undergoing alpha-decay (although Pb-210 may on very rare occasions undergo alpha decay). http://www.nndc.bnl.gov/chart/ (use Zoom 1 to see details of individual nuclei)

    It is true that uranium is mined from nature and processed and concentrated to use in nuclear reactors for energy generation. By mining and processing, we put uranium in a more chemically reactive form that would make it easier for the U to get into the environment without additional barriers such as metal cladding and various storage or containment systems.

    As to why we create more reactivity with fission, one must realize that the fission process creates two nuclei from each fission, and we call these two new nuclei fission products. Most fission products are radioactive and will emit beta particles (or positrons) and gamma rays, by which the nucleus decays to a lower energy state. When beta (or positron) particles are emitted, the nuclear charge (Z) changes, and so therefore does the nature of the nucleus (element). Gamma decay simply reduces the nuclear energy, without changing the nuclear species.

    Radioactivity, by that I mean rate of decay, depends on the half-life of the nuclide. The longer the half-live, the lesser the rate of decay, and therefore for a given number of atoms, the lower the level of radioactivity. The two prinicipal isotopes of U have long half-lives: U-235 (700 million yrs) and U-238 (4.47 billion yrs), so they have very low activity. Fission products have half-lives on the order of fractions of seconds to millions of years, so some are highly radioactive while others are much less so. The good news is that the shorter the half-life, the faster the decay rate, so the radioactivity decreases with time.

    Let's say a given isotope has a half-life of 1 second. In 10 half-lives, the number of nuclei is reduced by a factor of 210 or about 1000. In 20 half-lives, the number of nuclei is reduced by a factor of 220 or about 1 million, and in 30 half-lives it is about 1 billion, and so on. That's good for short half-lived nuclei. However, some nuclei have half-lives on the order of 100's or 1000's of years, so in order to keep them out of the environment, they must be isolated in an inert containment systems for 10's or 100's thousands of years - and that is a key issue at the moment for the Yucca mountain repository. How do we guarantee the isolation for thousands of years - since man made structures are only hundreds or a few thousand years old. Well - we put the isolation system in a geologic formation that has been stable for the last million years, and surround by a man-made system that itself is relatively stable/inert.
    Last edited: May 28, 2007
  5. May 30, 2007 #4
    Thanks a lot !

    Do I understand it right if I read/understand the answer like this.

    "It is true that the vaste from a nuclear reactor is more dangerous than the Uranium from nature that is was made from. Due to changes in material on nuclear level the vaste from a reactor will be more dangerous than the Uranium fuel that were mined out from nature".

    Is this a correct understanding ?
  6. Jun 4, 2007 #5


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    Staff: Mentor

    Up to a certain point in time, yes - the waste, or rather spent fueln from a reactor is more dangerous than uranium in a deposit in the ground. The spent fuel is more concentrated source of radioactivity, particlularly while operating or shortly after discharge, and is chemcially less stable.

    That is why the fuel operates in a reactor enclosed in a high pressure system (primary system including pressure vessel), which is enclose in a reinforced concrete structure.
    Last edited: Jun 4, 2007
  7. Jun 4, 2007 #6
    Okay, so let me see if I have this straight. It's better to have something that gives off a lot of "poison" in a short space of time, because you can concentrate and collect that poison and keep it out of harms way. But what about something that leaks this poison very slowly, surely that's safer? because even though it lives longer, whilst it is alive it can only exert some maximum damage which pales into insignifance compared to the potential damage of the shorter lived, more intense species.

    The hydrogeology is perhaps the single most important factor here. Lots of clay (impermeable and good at soaking stuff up), few fractures, and preferably low recharge and a low water table. It's the fear that this stuff wil get into the water system which I believe to be the amjor concern for the environmentalists.
  8. Jun 4, 2007 #7


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    Staff: Mentor

    No one said anything about 'better'. Spent fuel is what it is - there is no getting around the fact that fissions produce radionuclides of varying half-lives. However some decay very rapidly to stable (inert) nuclides, while others take hours, days, weeks, months, years, decades, centuries. But one needs to look at the set (population) of nuclides to see how it changes with time.

    High level waste and spent fuel are stored in canisters, which are then supposedly place in geologically stable formations, which means ostensibly that water has not flowed through the formation, nor has there been any lava or movement of earth for 10's or 100's of millenia.

    There was a concept of Synroc (synthetic rock) developed a few decades ago. I am not sure where that is now.
  9. Jun 5, 2007 #8

    Oops there is a radioctive waste product on the loose with an official half time of 5730 years. Some environmental hostile entity has decided to produce it in the atmosphere on a 24/7 basis:

    14N + n => 14C + p

    The ratio of 14C to 12C is about 1.3*10-12. The amount of carbon in the atmosphere is about 750GTC or 750 * 1012 kg. http://www.undeerc.org/PCOR/sequestration/cycle.asp

    hence there is about 1000 kg highly radioactive waste in the air.

    edit: but wait:

    Organic carbon is coming from CO2 in the air, loaded with radioactivity, so with the same ratio that's about 2500kg highly radioactive waste just around where-ever you are. :surprised

    So, how much highly radioactive waste must be stored for so long?
    Last edited: Jun 5, 2007
  10. Jun 5, 2007 #9


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    Staff: Mentor

    This source - http://dels.nas.edu/dels/rpt_briefs/going_the_distance_final.pdf - reports that 55,000 MT are in the US, and probably another 30,000 MT will be generated, even without new plants. That is about 1/4 of the world's inventory. C-14 is not considered highly radioactive, especially when 1 in a trillion C atoms is C-14.
  11. Jun 6, 2007 #10
    As we health physicists like to say, the solution to pollution is dilution!:biggrin:
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