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Can fission be coupled to he-3 or H3 to reduce waste?

  1. Dec 2, 2009 #1
    Can fission be coupled to fusion he-3 or H3 ?

    The goal is to reduce long-lived by products of uranium fission for a given amount of energy (heat) released.

    Could uranium fission, releasing neutrons, be coupled to some kind of "neutron" fusion reaction such as H3 (tritium) or He-3 or lithium or boron or other light weight elements, or even some kind of "proton fusion" reaction with deuterium or helium-3 or lithium.

    The idea is to split as little uranium as possible, and shift the energy production to the lighter elements for a more short-lived half-life.
     
    Last edited: Dec 2, 2009
  2. jcsd
  3. Dec 3, 2009 #2
    I believe neutron capture in Li is used to produce T (tritium). This has nothing to do with the fission products though - you are only using the neutron from the fission process. The fission fragments will be produced whether a T is created from the process or not.

    I have heard a proposition of the fission products being removed from used nuclear fuel and then "burned" by being placed around a core and taking advantage of the leaking neutrons, but I can't remember how well they study said it would work.
     
  4. Dec 27, 2009 #3
    I believe that Li-6 can be split with thermal or near thermal neutrons while Li-7 requires higher energy neutrons to split, but I'm assuming that the question was relating to the fission of heavy isotopes.

    There have been suggestions for coupled fussion-fission reactors. Plasma fusion reactors do not contain the neutrons produced in the fusion reaction. A hybrid system would surround the fusion reactor with fisile or fertile fission fuel. The high energy neutrons from the fusion reaction could then produce a fission reaction in the fission fuel. Since the neutrons from fusion are high energy they could even be used to split much of the nuclear waste produced by traditional fission plants into shorted lived isotopes. This would also allow for higher burnup since criticality of the fission fuel does not need to be maintained.

    To the best of my knowledge such a system has never been tested. My guess is that plasma fusion is simply isn't developed enough yet to make such a system practicle.
     
  5. Dec 27, 2009 #4

    Astronuc

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    Re: Can fission be coupled to fusion he-3 or H3 ?

    No - not really.

    He-3 absorbs a neutron forming He-4, although I say an article that stipulated He-3 + n -> T + p, which makes no sense because T decays to He-3. On the other hand, if the energy is high enough, there could be an (n,p) reaction.

    Fission is pretty much confined to fissile isotopes, but U-233 (in Th-232) is an alternative and there are approaches to 'burn' the transuranics.
     
  6. Jan 4, 2010 #5
    Re: Can fission be coupled to fusion he-3 or H3 ?

    What about an isotope of lithium and boron, which with neutron capture, from splitting uranium, releases energy and forms a more stable isotope or element?
     
  7. Jan 4, 2010 #6

    Astronuc

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    Re: Can fission be coupled to fusion he-3 or H3 ?

    For homework, please write out the possible nuclear reactions for Li and B isotopes, and note the energy release. If one uses U-235/U-238 in the system, one will end up with long-lived transuranics. Using Th-232/U-233 reduces the long-lived.

    One has to look at the enrichment requirements from the standpoint of desired exposure and criticality requirements. Depending on the fuel matrix, exposure determines the fission products and transuranic buildup.

    One has to look at the long-term consequences of consuming Li and B, as well as Th or U.
     
  8. Jan 5, 2010 #7
    This is not possible.

    The recoverable energy from fission is a massive 190 MeV/fission. The majority of this is due to the slowing of fission fragments, whose kinetic energy is converted to heat, through local slowing down in the fuel pin. Additional contributions include neutron thermalization and gamma-ray interactions.

    Now, the reactions you're referring to generate products with far less kinetic energies. Consider the following Q-values.

    B-10(n,α)Li-7 has a Q-value of 2.3 MeV/reaction (94% yield to excited state)
    Li-6(n,α)H-3 has a Q-value of 4.78 MeV/reaction

    In order to utilize this material in lieu of the fissile material, you would have to alter the fuel pin in several ways, all of which cause problems.

    -Considering that B-10 and Li-6 are commonly used as proportional counter gases for neutron counting, it would be difficult to stuff it in fuel pins.
    -During fuel fabrication, all fuel pins are already at the highest possible mass density, leaving room only for fission product gases (so as to minimize pin bowing). There's no room left!
    -The most important fuel property in design is thermal conductivity. Inserting alien material into a fresh fuel pin may have drastic effects on it with which simple models may be unable to resolve (e.g. Molecular Dynamics may be required...)
    -These materials' interaction cross sections, despite being fairly significant at thermal energies, will always compete with that of the fissile material, poisons (Xe-135), and actinides.
    -Perhaps most importantly is the fact that these absorptions do not generate a neutron. So in order to maintain a sustainable reaction, more fissile material would be needed...
     
  9. Jan 5, 2010 #8

    Astronuc

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    Actually B-10 is used in nuclear fuel pins as burnable absorber. It is coated as ZrB2 (Zr diboride) on circumferential surface of UO2 pellets. Otherwise is used in Pyrex or Alumina-B4C in stainless steel rods, which would be inserted in the guide tube locations. Boron burns out without the residual that gadolinia leaves.

    Li hydroxide is used as a buffering agent with boric acid for soluable or chemical shim in PWRs. It contributes little to the overall power. LiOH is typically in the low ppm range, while boric acid is used in the 1200-2200 ppm range and decreasing to zero at EOC.
     
  10. Jan 7, 2010 #9

    vanesch

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    This is a very important point. There are not so many neutrons "to spare" in a (thermal) reactor, so "spend your neutrons well". The best you can get is something like 1 "spare" neutron per fission neutron, and then, as sourceless pointed out, the energy you can win with that extra neutron on light elements is very small as compared to the fission energy you get from your fission neutron. Neutrons are "expensive" in a reactor.

    In fact, the opposite is rather planned: use a neutron source (like a fusion plasma, or a spallation source) to induce fission to get energy.
     
  11. Jan 9, 2010 #10
    Thanks.

    From this discussion I infer that long-lived transuranic waste that antinuke leftists crow about, could, with future technology, be re-processed and re-used or burned up.
     
  12. Jan 10, 2010 #11

    Astronuc

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    For long-lived actinides, actually transuranics, there is a plan to use fast reactor as an actinide burner.

    Some interesting concepts

    The TMSR as Actinide Burner and Thorium Breeder
    hal.archives-ouvertes.fr/docs/00/14/36/80/PDF/LPSC_07-37.pdf

    Minor Actinide Burner Reactor and Influence of Transmutation on Fuel Cycle Facilities
    http://www.iaea.org/OurWork/ST/NE/inisnkm/nkm/aws/fnss/fulltext/26050881.pdf

    Some useful information -

    Comparison Calculations for an Accelerator-Driven Minor Actinide Burner
    www.nea.fr/html/science/docs/2001/nsc-doc2001-13.pdf [Broken]
     
    Last edited by a moderator: May 4, 2017
  13. Jan 10, 2010 #12

    vanesch

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    Ah, I misunderstood maybe your original post. I thought the question was: could we couple a (classical) fission reactor to something else, where we use the neutrons from the reactor to "burn" other, light-element stuff so as to give us a lot of energy with that, supposing that this would give you the bulk of the energy, and hence that one needs much less classical reactor fuel, and hence has much less classical reactor waste including transuranics.

    As has been demonstrated here, the potential energy gain from a neutron outside the reactor on light elements, is only a small fraction of the energy this neutron could give you with a fission reaction, so it won't be possible to have the bulk of the energy production "outside" the reactor.

    However, if the question was: "how to get rid of transuranics?", then yes, there are experimental techniques as astronuc indicates. The "simplest" is probably simply a fast reactor, but people also look at sub-critical spallation-source driven reactors as actinide burners.

    However, two questions/problems are to be posed with this kind of thing:

    1) it will probably not be possible to burn ALL transuranics to the last atom, so some waste will always remain. One can however diminish greatly the volume this way. But some final disposal of transuranic waste will in any case have to be considered.

    2) is it necessary ? Does the extra risk of accidental release of this stuff into the biosphere increase or decrease ? By this, I mean: what is the (tiny) risk that these transuranics will get accidentally in the biosphere after geological disposal and how does that (tiny) risk compare to the extra risk we introduce by these extra manipulations/transport etc... to burn them in such a device ?
     
  14. Jan 11, 2010 #13
    Oh you were right, that was my original question since
    H1 + neutron = dueterium + energy release, with splitting uranium as source of neutrons.
    Deuterium and tritium are either nonradioactive or short lived half life. The idea is to cause as much neutron-proton fusion energy release so as to produce as little uranium waste as possible.

    Burning up transuranics is also desirable.
     
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