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Nuclear waste to Electricity?

  1. Jan 22, 2008 #1
    Sorry if this is a stupid question or in the wrong forum, but these have been burning questions and I Googled for the first forum for this kind of area. Here it is:
    Is there possible ways of converting the radioactive energy of nuclear waste into useful energy such as electricity? How small could such a device be and how much energy could it produce? Theoretically could this be used to power things such as power plants or electric vehicles?
     
  2. jcsd
  3. Jan 22, 2008 #2

    mjsd

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    mmm... interesting. not sure whether the radiation can be easily harnessed or whether it passes the cost vs benefit analysis?
     
  4. Jan 22, 2008 #3

    Astronuc

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    At the moment there is no practical way. Most nuclear 'waste' is in the forum of spent nuclear fuel. In a once-through nuclear fuel cycle, there is a great loss of potentially useful uranium and transuranics, which is why reprocessing/recycling is encouraged by some. In recycling, the fissile isotopes are recycled by into nuclear fuel, while the fission products are calcined, vitrified and placed in permanent storage.



    Some isotopes could be segregated and used in radioisotopic thermal generators, but those are used for small scale power generation and are not practical for large scale generation.

    Another possibility would be to isolate beta emitters and use them in a direct conversion system, but I don't believe a practical scheme has been devised, otherwise we would already be doing that.

    The power depends on the quantity of radionuclide(s) and the decay constant/decay rate. Since ionizing radiation is harmful to living tissue, it is impractical to make power systems for vehicles because a lot of energy would go into moving the mass of shielding. Furthermore, there are safety issues associated with accidents involving radioactive materials.

    There is a fundamental requirement of minimizing or preventing radioisotopes from entering the biosphere, i.e. environment.
     
  5. Jan 22, 2008 #4
    Oh, I forgot to add. I meant as in mainly harnessing the radioactive part.
     
  6. Jan 23, 2008 #5

    Astronuc

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    What does one mean by harnessing the radioactive part?

    Spent fuel is mostly radioactive, but there would be non-radioactive U-238, which could be separated.

    Ionizing radiation is just charged particles ( beta (-) and alpha (+)) and photons (gamma (0)) from the nucleus and X-rays from intraction of the nuclear radiation with the atomic electrons.

    Either one uses a system of charge separation/collection (direct conversion) or the thermal energy generated by the radiation as the radiation slows down in matter. In the case of thermal energy, the temperatures are somewhat limited, and thermal-to-mechanical conversion efficiency of the system is a constraint. Also, in a thermal system, the radioactive materials have to be confined in order to prevent their dispersion in whatever working fluid is used in the power conversion system.

    Radioactive decay is a continuous process, and it cannot be turned off, and large sources of radioactivity require significant shielding.
     
  7. Jan 23, 2008 #6
    Well, just doing a simple back of the book calculation, and using Cs-137 (a big component of SNF), it takes about 250 Curies just to create about 1watt, assuming a 100% conversion from the 662 keV photon to usable electricity. That means to power a typical 1000 MW power plant, you need 250,000 MCi of Cs-137. At West Valley, NY, where they reprocessed SNF, there is currently only about 4.6 MCi. So, one reason I suspect it probably isn't even researched is due to the fact that you need way too much radioactive material to make it useful.
     
  8. Jan 23, 2008 #7
    Bu could say a standard Nuclear plant use the waste as a boost?
     
  9. Jan 23, 2008 #8
    They sort of do already. In a fission reaction, there is about 170 MeV of instantaneous energy deposited in the reactor per fission event. In PWR/BWR reactors, cooling the reactor provides the steam that is diverted to a turbine which generates the electricity. There is about 18 or so MeV in the delayed energy from decay product and neutron capture gammas which also is deposited in the reactor, so in a sense the decay products are adding a little extra power to the reactor. This is why when the reactor is shut down and there are no fissions, there is still some of this decay heat which needs to be removed from the reactor. However, after some time, the SNF acts like a poison to the reactor and the neutrons are absorbed into fission fragments, which reduces the effectiveness of the fuel (meaning fewer fissions, meaning less MeV deposited in the reactor). So, basically, a little bit of byproduct material is good, but too much is bad.
     
  10. Jan 23, 2008 #9
    I mean as in after the waste is removed, the heat and whatever from it is still harvested in a separate system.
     
  11. Jan 23, 2008 #10

    russ_watters

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    I'm not sure the others were entirely clear, so lets try it another way: Your instinct is correct in that if the waste is still pretty radioactive, it has a lot of energy left to be used. But to use it, the radioactive isotopes must be separated from the stable ones in a reprocessing plant. There are also special types of reactors capable of utilizing the fuel differently to extend its life. According to the Wik article on reprocessing, these techniques can extend the usefulness of a given quantity of fuel by 60x. Put another way, we currently use less than 2% of the usable energy in our nuclear fuel.
     
  12. Jan 23, 2008 #11
    well that sucks.
     
  13. Jan 23, 2008 #12

    Astronuc

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    Actually, we get about 3-4% of stored nuclear energy of nuclear fuel in typical LWR (I think a lot of public sources are dated). Some plants may push 5%. That leaves a lot to be recycled.

    The spent fuel has a very low power density, and the thermal energy from spent fuel would not provide much of a boost to the power generation system.
     
  14. Jan 24, 2008 #13
    A friend of my used to talk about a liquid core reactor that he’d worked on. The fuel was in a uranium solution and circulated through the critical-reactivity region, the primary heat exchanger and through a filter that removed the spend Cs, and Sr. The Cs and Sr stayed in the reaction vessel, since it doesn’t take up much room, and it’s decay heat was included in the power plant production heat. I’m probably not remembering it correctly.
     
  15. Jan 24, 2008 #14
    I recall several studies on accelerator driven transmutation of nuclear waste with net excess of power being generated. Here's one I could find from LANL in 1993.

    ABSTRACT: "A medium energy, high current proton beam strikes a heavy metal target, producing a high flux of spallation neutrons. These neutrons are moderated to near-thermal energies in a blanket surrounding the target. Materials to be transmuted flow through the blanket region where they are fissioned or transmuted to stable nuclides. Stable or short-lived nuclides are separated while the long-lived radioactive species are returned to the blanket. For most applications the fission energy produced is much greater than that required to power the accelerator and can be directed to the commercial power grid."

    http://www.fas.org/sgp/othergov/doe/lanl/lib-www/la-pubs/00411602.pdf

    Best, Ed
     
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