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Poor man's nuclear energy?

  1. Jan 21, 2009 #1
    I know what fission is and what fusion is. Who needs em? I figure that most of the energy content of particles emitting from naturally radioactive materials is lost to distant locales. Am I right? If you took a sizable chunk of radioactive mineral, like uranium, whether refined or not, whether enriched or not, if that chunk were encased in a layer of lead then wouldn't it get warm after a bit? And wouldn't it stay warm for nearly forever? Please: could I use an arrangement like that to heat my cabin?
     
  2. jcsd
  3. Jan 21, 2009 #2
    Sure, and you can get electricity too (like deep space probes).

    But what's wrong with making even more efficient use of these minerals, as a reactor does?
     
  4. Jan 21, 2009 #3

    mgb_phys

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    The energy output form uranium from natural alpha radioactivity is tiny ( a couple of miCi/kg) Even for Plutonium you only get 10W/kg from spontaneous fission.
     
  5. Jan 21, 2009 #4
    10W/kg is humble alright, but it sounds like a start -- IF it can be contained/harnessed as described. You're referring to refined plutonium, yes? (not the ore). I can't figure out what MiCi stands for. You said "alpha" and I think you mean electrons, right? Is there more for the lead to absorb than just that? Aren't there also photon emissions? How about beta radioactivity? Just the 10W?? hmmm..
     
  6. Jan 21, 2009 #5

    mgb_phys

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    Uranium on it's own emits Alpha particles - the good news is that they aren't very dangerous, they can be stopped by a sheet of paper, the bad news is that they don't have a lot of energy.
    Natural uranium ore emits about 12million alpha particles/kg/s (the milliCurie is an old unit) each particle carries around 4Mev or 6x10-13 Joules of energy
    So a total of 7.6x10-6 Joules/kg/s or 7.5Watts from 1000tons!
    Thats the reason for Rutherford's famous statement that getting energy from radiation was moonshine.

    The energy in reactors come from the much more energetic fission (splititng) of atoms, For natural uranium the rate of splitting is very low, only U235 splits an it makes up only 0.2% of natural uranium. Thats why 'weapons grade' uranium needs to be enriched to contain much more U235, or you can also use plutonium it has a higher rate of splitting - but thats trickier to get hold of.
    You can increase the rate of splitting by putting a bigger lump together (the critical mass) or adding another material (a moderator) to improve the efficency - but then you have built a reactor!

    There is a kind of reactor called a pebble bed which is almost what you suggest. Small beads of reactor fuel in a big pile (inside a container!) that simply get hot enough to boil water but not hot enough to meltdown
     
    Last edited: Jan 21, 2009
  7. Jan 21, 2009 #6

    vanesch

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    Moved to engineering (it is about a practical application, not a scientific question of nuclear physics).

    Just some notes: as to plutonium, there's no ore of it, it is man-made in nuclear reactors.

    The problem with heating something with nuclear reactions is that most of the time, by the time it gets hot, the radiation is already lethal for a long time. So you need shielding, radiation protection and all that. It was the joke of the cold fusion of Pons and Fleishmann: if they really had seen some heat in their non-shielded apparatus, they wouldn't have lived to tell the world! (after that, they "invented" the radiationless fusion, but ok....) In a nuclear power plant inside the reactor, the radiation levels are so high that you get a lethal dose in less than a millisecond.

    While it is true that alpha radiation itself is harmless outside of the body (very short range of the particles), most alpha emitting also goes with gamma emissions.
     
  8. Jan 22, 2009 #7

    Vanadium 50

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    True, but it's also true that there is a trace amount in uranium ore. It's produced by the same processes as in reactors, just on a much, much slower timetable.
     
  9. Jan 22, 2009 #8
    Thank you for satisfying my curiosity
     
  10. Jan 22, 2009 #9
    There are things like what you describe, harnessing energy directly from radioisotopes; these are used in exotic places where refueling is not an option. However, these are rare and very expensive: isotopes radioactive enough to have meaningful power output must have very short half-lives, so none of these occur naturally on earth. (Most isotopes are created in astrophysical nuclear reactions, either in stars or (for the heavier ones) in supernovae. The nuclei on earth come from these primordial sources and are billions of years old; so the only significant radioisotopes here are those with billion-year half lives - Th-232, U-235, U-238, and K-40, and their daughter products.) Thus only synthetic radioisotopes are usable, and these can only feasibly be created in nuclear reactors (or perhaps particle accelerator neutron sources). So radioisotope power is really just a way of storing energy created in nuclear reactors.

    One way is just like what you describe - collecting the decay heat of radiation, perhaps using it to generate electricity in a heat engine. Radioisotope thermoelectric generators (RTGs) are used in some exotic applications where refueling is not an option - pacemakers (long-lived surgical implants), remote naviagation beacons (common in the former USSR), and space probes to the outer solar system (too far from the sun for solar panels).

    Wikipedia: Radioisotope thermoelectric generators

    Plutonium-238 glowing incandescently from its own decay heat:
    http://img340.imageshack.us/img340/7905/773pxradioisotopethermoxl8.jpg [Broken]

    RTG-powered pacemaker with plutonium:
    miscel8.jpg

    2.5 Ci * 46 MeV/decay = 680 milliwatts

    Oak Ridge: Plutonium Powered Pacemaker (1974)

    Another method is to use beta emitters (nuclei which emit high-energy electrons when they decay) to excite phosphors (chemicals which emit light when excited by energetic electrons or photons). This a light that never goes out. Tritium (hydrogen-3) is common today in battery-less emergency signs.

    Wikipedia: Self-powered lighting

    EPA: Discarded Tritium Exit Signs

    Tritium in a phosphor-lined vial on a keychain: $29.00
    http://www.unitednuclear.com/trasergreen300.jpg [Broken]
     
    Last edited by a moderator: May 3, 2017
  11. Jan 22, 2009 #10

    Morbius

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    truhaht,

    I'm afraid you are wrong. Most radioactive decay reactions only give you a few MeV [ million electron
    volts ] of energy.

    The fission reaction gives you about 200 MeV of energy from a nucleus that is about 200 amu in
    mass - so you get about 1 MeV / amu of energy per unit mass of fuel.

    D-T fusion gives you 17.6 MeV of energy from a reactants that have a mass of 5 amu - so you
    get about 3 MeV / amu of energy per unit mass of fuel for D-T fusion.

    You get a LOT more energy from fission and fusion.

    Dr. Gregory Greenman
    Physicist
     
  12. Jan 22, 2009 #11
    And fission and fusion converts only a tiny proportion of material to energy...1 or 2% in fission. So a lot of "debris" is left....
    I like the concept of harnessing the cosmological constant inherent in every piece of space....it's called dark energy or negative pressure....antigravity....but that's not likely in our life times.
     
  13. Jan 22, 2009 #12

    mheslep

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    Careful there. No 'material' nuclear particles are converted to energy in fission or fusion reactions. The total binding energy of the nucleus/nuclei changes in those nuclear reactions; that binding energy has an equivalent relativistic mass. There's probably a PF sticky FAQ on this somewhere that states the case more completely than I am able.
     
  14. Jan 23, 2009 #13

    vanesch

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    Actually, I wonder, for thermo-electric generators powered by Pu-238, does one do an isotopic separation on the plutonium to get pure Pu-238 ?
     
  15. Jan 23, 2009 #14
    Good question. If Pu-238 is produced from Np-237(n,gamma) (Np-238 beta decays), and it seems that Pu-238 has a much higher (n,gamma) cross section than Np-237 in the thermal spectrum (looking it up on NNDC), then it would make sense that "Pu-238" created in light-water thermal reactors would actually be mostly Pu-239. But I've never heard of hot-cell isotopic separation - uranium enrichment is of course very low-radiation stuff. IIRC Kirk Sorensen claimed that it's next to impossible, in the context of proliferation and the thorium fuel cycle, where short-lived U-232 contaminates fissile U-233.

    :confused:

    2z8vrjd.jpg
     
    Last edited: Jan 23, 2009
  16. Jan 23, 2009 #15

    vanesch

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    Ah, that's it. First, Np-237 is extracted from nuclear spend fuel, and then separately re-irradiated in a neutron flux.
    You can work with low production fractions, and then the production rate of Pu-238 (proportional to the density of Np-237) is larger than the production rate of Pu-239 (proportional to the density of Pu-238), like they do in production reactors for Pu-239 (with U-238 this time). If you only convert, say, 1% of the Np-237 into Pu-238 before separation, you get pretty pure Pu-238.
     
  17. Jan 23, 2009 #16
    And now our enemies have the recipe they've longed for? :uhh:
     
  18. Jan 25, 2009 #17
    Oh, so obvious! :redface:

    Relax. Everyone knows how make plutonium - it's no use if you don't already have a working nuclear reactor (no other source of neutrons is powerful enough). Also, a subtle point: Pu-239 is the fissile isotope usable in weapons, whereas Pu-238 is a non-fissile isotope used in RTGs as a heat source.
     
    Last edited: Jan 25, 2009
  19. Jan 25, 2009 #18

    Astronuc

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    http://consolidationeis.doe.gov/background.html [Broken]
     
    Last edited by a moderator: May 3, 2017
  20. Jan 25, 2009 #19

    Morbius

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    truhaht,

    It's a pretty OBVIOUS thing to do. Additionally, Pu-238 production doesn't give one the recipe for
    a bomb. One needs fissile Pu-239 for a nuclear weapon - not radioactive Pu-238.

    In fact, Pu-238 is something you want to LEAVE OUT OF a nuclear weapon - the Pu-238 doesn't
    help in the fission reaction - and its radioactivity and heat just complicate matters.

    Dr. Gregory Greenman
    Physicist
     
  21. Jan 25, 2009 #20

    vanesch

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    I understand why one wants to leave out Pu-238, but nevertheless, the k_inf is more than 2.5 in its own fission spectrum. It is not fissile in a thermal spectrum, but it is in a fast spectrum.
     
  22. Jan 25, 2009 #21

    Morbius

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    vanesch,

    Actually, Pu-238 IS fissile. At room temperature 0.025 eV;
    Pu-238 has a (n,fission) microscopic cross section of about 15 barns.

    That is in comparison to about 700 - 800 barns for Pu-239.

    So Pu-238 is fissile - but it isn't going to help much - and the
    negatives are overwhelming.

    A nuclide can't be not "fissile" in a thermal spectrum but fissile in
    a fast spectrum- by definition.

    The word "fissile" means that the nuclide will fission with thermal neutrons.

    If a nuclide like U-238 that will only fission with neutrons of energy above
    a certain threshold - like with fast neutrons - those nuclides are termed "fissionable".

    U-238 is NOT "fissile" - but it is "fissionable".

    Dr. Gregory Greenman
    Physicist
     
  23. Jan 26, 2009 #22

    vanesch

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    You are right concerning terminology. I should have said fissionable, not fissile.

    I just wanted to point out that a big enough lump of Pu-238 will undergo just as well a fast power excursion (if you see what I mean) as a big enough lump of Pu-239, as its k_inf value is far above 1. (which is not the case for U-238 btw).
     
  24. Jan 26, 2009 #23

    Morbius

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    vanesch,

    For high energy neutrons, the fission cross-sections for Pu-238 and Pu-239 are quite comparable.

    The problem is that you are not going to be able to get the Pu-238 to the same densities that one
    can with Pu-239.

    The problem with looking at just k-infinity is that infinite sized devices are really tough to transport
    to the target.

    Dr. Gregory Greenman
    Physicist
     
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