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Fusion for power?

  1. Aug 29, 2007 #1
    Fusion for power??

    I am guessing that the Coulomb Barrier is the major problem??? What exactly is necessary to breach the Barrier, as far as the amount of energy, heat, velocity of proton, etc, etc. If the Barrier COULD be breached without destroying the city at the same time, what other problems are there? Converting the MeV inside the system into voltage outside of the system, maybe?? Keeping the reaction going for sufficient time periods? Interesting stuff!
     
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  3. Aug 29, 2007 #2

    Astronuc

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    Indirectly.

    The problem is that to get fusion going, particularly for more favorable aneutronic reactions, high plasma temperatures are required. The limitation is actually on 1) plasma pressure, which is limited by the strongest superconducting magnets we can build and by the strength of the supporting structures, and 2) energy losses which increase with temperature.

    Plasma pressure and energy losses are also a function of plasma ion/electron density.

    Plasma pressure ~ nkT where n is the particle (ion/electron) density, T is temperature, and k = Boltzmann's constant.

    The energy losses are more complicated because there are different mechanisms, e.g. cyclotron radiation, brehstrahlung, recombination, etc. each of which is affected differently as a function of temperature, particle density, and Z (nuclear charge). Cyclotron radiation is due primarily to electrons.
     
  4. Aug 29, 2007 #3

    berkeman

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    BTW, here's a good discussion thread on the status of fusion power generation from a little while back here on the PF:

    https://www.physicsforums.com/showthread.php?t=162537



    .
     
  5. Aug 29, 2007 #4
    Very fast question I didn't wanna waste a whole new thread on---

    what is the proper term for an element in it's "pure" isotope--the isotope when the neutrons and protons are equal in number--it's natural and most common (I'd think) state?
     
  6. Aug 29, 2007 #5
    No such thing.

    You're thinking of all radioactive states as some kind of abomination, for you to contrast with "stable" states (like black and white.. that approach leads to outlawing nuclear industry and stem cell research, or permitting slave trade :wink:). Instead think of all possible nuclei as being "equal" and radioactive, although some have such short half-lives we don't even see them, and some have such long half-lives you might never see one decay, and others lie anywhere in between (like shades of grey). Yes, the number of protons in a nucleus determines the number of electrons it attracts, and hence the way it behaves chemically (the element). And it's natural that nuclei which don't last as long aren't seen as often, but no reason there there couldn't be several (practically) equal-stable-ness isotopes of a single element, none could claim to be the more "pure" element (though the term you asked for is likely either "(most) stable isotope" or "most (naturally) abundant isotope").. and incidentally, isotopes are generally more stable if they have more neutrons than protons (more glue per repulsive charge).
     
    Last edited: Aug 29, 2007
  7. Aug 30, 2007 #6
    Just a comment, this is true for heavier elements. Lighter elements (for example carbon) like N=Z more.
     
  8. Aug 30, 2007 #7

    arivero

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    I'd bet for cross-section as a problem too. But yep, we try H + H instead of C+C, so Coulomb seems to be an important issue.
     
  9. Aug 30, 2007 #8

    arivero

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    I can not think of a name now. It is only natural if isospin is preserved, and electromagnetic charge breaks isospin. But within isospin symmetry, all the nuclei in the series from Z=1, N=k to Z=k, N=1 are in the same footing.

    The nucleus with the number of neutrons and protons exchanged is usually called a "Mirror nucleus" of the original one.

    The line of the most stable nuclei along all the NZ plot is called the "stability line". Perhaps you are asking for this. As other have pointed out, the stability line only coincides with the diagonal N=Z for lowest values of N,Z.
     
  10. Aug 30, 2007 #9

    Astronuc

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    Ar-36 (Z=N=18) is the heaviest stable nuclide for which Z=N, although Ca-40 (Z=N=20) might be considered stable since it's half-life is > 3 x 1021 yrs. Heavier elements have N > Z for stability much for the reason indicated by cesiumfrog - that the neutrons dilute the coulomb repulsion while providing the nuclear binding forces.

    One can review the properties of nuclides at -
    http://www.nndc.bnl.gov/chart/ - use Zoom 1 to see the details.
     
  11. Aug 30, 2007 #10
    jeez--just thought there might be some cute little name is all--didn't mean to get into who is radioactive and who objects to being 'labeled" radioactive and what "normal" means, or anything like that. I just thought the might be a name for the isotopes like H1, He4, Li6, C12, etc, etc, etc. But, interesting info anyways---thanx.
     
  12. Aug 30, 2007 #11

    mgb_phys

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    Other than stable, I don't think there is a prticular name.
    For larger elements they are still stable when there is a small surplus of neutrons.
     
  13. Aug 30, 2007 #12

    Morbius

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    Astronuc,

    Just to add to what Astronuc has stated, it is also instructive to look at the Semi-Empirical Mass Formula
    which is an empirical fit to the binding energy:

    http://en.wikipedia.org/wiki/Semi-empirical_mass_formula

    Both neutrons and protons add to the binding energy via the "volume" term - the first term proportional to
    A - which is the sum of the number of protons and number of neutrons.

    However, the protons also bring in an "unbinding" energy due to their mutual repulsion. That's the
    third term which is negative, so it is "unbinding" and is proportional to Z(Z-1). The more protons
    are added this "unbinding" energy is going becoming larger roughly with the SQUARE of Z.

    If you add a new proton to a nucleus with Z protons - then there will be Z/2 new pairs of proton-proton
    repulsions. That's where the Z(Z-1) term comes from. For each of Z protons; there are (Z-1) other
    protons to repel. This double counts the number of pairs - so you divide by 2; but that is absorbed in
    the leading coefficient.

    So for large atomic number nuclei - large Z - the addition of protons leads to a proportionately larger
    increase in "unbinding" energy. Adding more neutrons doesn't have that effect; so neutrons work like
    "glue".

    Dr. Gregory Greenman
    Physicist
     
  14. Aug 30, 2007 #13
    OK, cool. Perhaps in my mind I will use a term like the "standard" isotope of Whatever. Thanx all.
     
  15. Aug 30, 2007 #14

    Astronuc

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    It's not so much that there is a standard isotope of a particular element, but rather certain isotopes are more abundant than others.

    Some elements have only one stable isotope, e.g. Be, F, Na, Al, P.

    Ruthenium (Ru) has 7 stable isotopes, Mo and Pd have 6, although some consider Mo-100 stable, which would be a 7th stable isotope.



    This is cool - http://en.wikipedia.org/wiki/Isotope_table_(complete)
     
  16. Aug 30, 2007 #15

    berkeman

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  17. Aug 30, 2007 #16
    Last edited: Aug 30, 2007
  18. Aug 31, 2007 #17

    Morbius

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    berkeman,

    Actually it's NOT the Periodic Table. The Periodic Table graphically shows the Elements - the building
    blocks for the science of Chemistry. However, in the Periodic Table, there is a SINGLE entry for any
    given Element. There's only one entry for Tungsten, for example. That's because all isotopes of a
    given Element behave identically in their chemical properties.

    The table Astronuc posted is a Table of the Nuclides. It gives an entry for each isotope of each element.

    Although different isotopes of the same Element are chemically identical; their nuclear properties are
    NOT identical. Deuterium [ D2 or H2 ] , the heavy form of Hydrogen has a lower propensity to absorb
    neutrons than its chemical twin "Protium" or ordinary light Hydrogen. That's why "heavy water" has
    different nuclear properties than "light water".

    In summary, the Table of the Nuclides is more than just an alternate presentation of the Periodic Table.

    It contains MORE information.

    Dr. Gregory Greenman
    Physicist
     
  19. Aug 31, 2007 #18
    Exactly, Dr. I highly recommend checking out the interactive Table of Nuclides I mentioned above--link provided. It is awesome! And I agree with Dr. about the usefulness of the Periodic Table--great for Chemistry, but of limited value in our applications.
     
  20. Aug 31, 2007 #19
    Question. Note that for the isotope chart:
    http://en.wikipedia.org/wiki/Isotope_table_(complete)

    we find no stable isotopes at n= 19, 21, 35, 39, 45, 61, 89, 115, 123 (for n from 0-126)
    and no stable istopes at z = 43 and 61 (for z from 1-82)

    Does anyone know theory that helps explain these patterns:confused:--seems strange that so many gaps in stability exist in the building of atoms. How are these gaps explained by shell model and/or collective model (or any other model of atomic nuclei) ?
     
  21. Aug 31, 2007 #20

    Astronuc

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    Tc (Z=43) and Pm (Z=61) are interesting cases.

    Another part of the puzzle is the stability of even Z and the lack of stability of odd Z.

    Z=39, 41, 45, 49, 53, 55, 57, 59, 63, 65, 67, 69 all have only one stable isotope, where as the neighoring even Z elements have multiple stable isotopes.

    This might help somewhat -
    Nuclear mass and stability - http://book.nc.chalmers.se/KAPITEL/CH03NY3.PDF
    Nuclear structure - http://book.nc.chalmers.se/KAPITEL/CH11NY3.PDF

    From
    http://book.nc.chalmers.se/KAPITEL/CONTENT.HTM
     
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