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Fusion product mass

  1. Aug 26, 2013 #1
    Hi I thought about a question , say we have a sphere into which D D or D T fusion takes place , now that forms the end product which differs according to the reactants used but the thing that interests me here is that the fusion end product is always heavier than each of the products that made it , so if this would take place in a closed container wouldn't the fusion end products tend to accumulate at the lower parts of the sphere while the unfused elements be higher due to gravity because the elements that underwent fusion are heavier?
  2. jcsd
  3. Aug 26, 2013 #2


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    The fusion end products are lighter than the reactants used. That's why energy is released in nuclear fusion a la [itex]E=mc^{2}[/itex].
    That being said, the Helium atom is heavier than either Deuterium atom that went into making it, it's just not heavier than both of them put together.
    If liquid helium were more dense than liquid deuterium, then it would sink in a pool of deuterium. However, liquid deuterium is actually slightly more dense than liquid helium (thank you Wikipedia) since as a liquid, it exists as diatomic molecules like hydrogen.

    What kind of closed container are we talking about here?
    The only ways I know of getting (maybe) fusion would be a tokamak reactor, which squeezes and accelerates Deuterium Plasma until they start fusing with each other, and the other is inertial confinement fusion where massively powerful lasers converge on a millimeter size ball of frozen deuterium, and squeezes and heats it until fusion takes place. Both of these I think take place on time scales too small for buoyancy to be a big factor, though.

    An interesting side question to ask would be.. where is the Helium in the Sun?

    Hope this helps:)
  4. Aug 26, 2013 #3
    Actually, density differences in ICF give rise to a serious issue: the Rayleigh-Taylor instability. Note that I'm not talking about the differences in density between deuterium/tritium and helium ash.

    Rayleigh-Taylor can arise in magnetic confinement as well, but is less of a concern most of the time.

    More to the point about your question: ignoring the relative densities of the fusion fuel and products, fusion doesn't happen in experiment in stationary environments like you describe. Gravity (and other forces) can play a role in tokamak particle transport, but by and large, the fuel (20-30 keV) and products (3.5 MeV for alpha) all have pretty large velocities, so there are many more effects to consider than just settling out due to gravity.

    Stars use gravitational confinement for their fusion reactions. As jfizzix points out, that's where I'd look to if you want to think more about your question.
  5. Aug 26, 2013 #4
    Yes indeed that's what I said and know that the end product is heavier than each of the products that made it bot not both of them together but that situation doesn't interest me because if they would be both together they would have been fused and hence become heavier than each of the separate products.
    Why liquids? Ionized gas would be closer , so ignoring (if we can) all the high energy involved , heat etc. Would the helium leftovers after the fusion would have gone for a considerable time tend to accumulate at the lower parts of any reactor ?

    Ok I will look up the sun and it's helium deposition.
  6. Aug 26, 2013 #5
    I don't think you can really ignore all the high energy involved. Speaking of magnetic confinement schemes, one of the first things you learn about is single particle motions and drifts. Gravitational drifts are one of the examples and mass does enter the equation. However, one of the goals of many magnetic confinement schemes is to provide enough rotational transform for these drifts to cancel out. In other words, the specific topology and configuration of the magnetic fields play a huge role in determining what the particles do. In even simpler words, a tokamak is not just a sphere container and gravitation is not part of the confining forces.

    Guiding center - Wikipedia, the free encyclopedia
    Guiding center - Wikipedia, the free encyclopedia

    Mind you, even though gravitational drifts can occur, there are a lot of other drifts due to the specific geometries of your confinement scheme that also occur.

    Stars are different. They do use gravitational confinement. There are lots of other processes that go on in stars, and I'm sure a stellar astrophysicist could tell you a lot more, but I think you do start to get elemental separation in a star as it evolves, with heavier elements residing in the core.

    Stellar nucleosynthesis - Wikipedia, the free encyclopedia
    Red giant - Wikipedia, the free encyclopedia
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