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I Why not deuterium-proton fusion?

  1. Jun 28, 2017 #1
    I've been reading all I can google about fusion for several months now, both to write better spaceship drives and to better understand the fusion start-ups who promise to bring this planet-saving technology online far sooner than the government's (deliberately?) time- and money-wasting approaches. I'm patrticularly excited by General Physics' steampunk approach--they gobsmack plasma to fusion!--and their liquid lead-lithium wall that soaks up the high-energy neutrons produced by deuterium-tritium before they can damage the reactor vessel. By Lawrenceville Plasma Physics dense plasma focus and aneutronic proton-boron 11 cycle--if they can deliver on 5Mw reactors that will fit in a pickup truck, for $ a few hundred k, the third world can leapfrog fossil fuels. And Helion Energy's strategy--if you can't get He3, make it--is also brilliant. And that's what my question is about.

    Helion plans to fuse deuterium-deuterium (D-D) to make He3 and tritium (T), then somehow stop the reaction before the T fuses with D to make a bunch of those pesky 14.1 MeV neutrons. Burn the He3 with D to produce power (I think they'll need two reactors, a breeder to produce fuel and another to burn it, but they don't say so that I've read yet), and store the T until it decays into He3. But preventing the D-T fusing seems difficult, given that D-T ignites at ~1/3 the temperature required by D-D. Might D-p be easier?

    There are other low-Lawson criteria (if I understand the term--low tripple product required? Easy to ignite-) fusion fuels, easy enough to ignite that they fuse in protostars and brown dwarfs: protons with both isotopes of lithium, others--and D-p, deuterium-proton. D-p starts at only 10^6K, doesn't get much hotter than that--is that a problem? produces an He3 and an x-ray (I think, and maybe a neutrino?), and that's about all I can find about it online. Why does there seem to be so little interest in this reaction? Why wouldn't this be a good fusion fuel for power plants? Low energy produced? Anyone know how many MeV, total reaction chain? Or like the p-p that fires smaller stars, is D-p's reaction rate too slow? And even if it's a low-energy reaction, could it be sped up enough (if a slow reaction rate is the problem) to use it to make He3?
     
  2. jcsd
  3. Jun 28, 2017 #2
    Same question, different fuels: why not lithium-proton? If it'll burn in brown dwarfs....
     
  4. Jun 28, 2017 #3

    mfb

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    The D-p fusion rate is tiny, like everything that produces only one target nucleus and requires the emission of a photon. It needs the electromagnetic interaction, and that is weak compared to the strong interaction.
    For the same reason D-D doesn't produce He-4 in relevant amounts.

    It is not as bad as the pp reaction (which relies on the weak interaction), but still impractical.

    Li-6 + p -> He-4 + He-3 has a much lower energy yield than Li-6 + D -> 2 He-4. The latter releases 5 times the energy (22 MeV instead of 4 MeV), although there are side-reactions that lead to lower energy (and a few neutrons).
     
  5. Jul 1, 2017 #4
    "For the same reason D-D doesn't produce He-4 in relevant amounts."
    Typo? It's He3 we want to produce. I found a note earlier that says stellarators burning D-D don't burn the T or He3 they make. Their reaction rate is obviously not break-even yet--no one's is. No chance of producing enough He3 to run a reactor, even running two or three stellarators to breed it? Can you think of any reactor/reaction that might produce sufficient He3?

    Li-6 + D -> 2 He-4 does sound promising. Why is no one pursuing it? Too difficult to ignite?

    Reference https://www.physicsforums.com/threads/why-not-deuterium-proton-fusion.918797/
     
  6. Jul 1, 2017 #5

    mfb

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    D+D -> He4+gamma would release much more energy and it is without neutrons. But the reaction is very rare.
    Depends on the length of the ignition and the plasma conditions. You'll certainly get some D+T side-reactions.
    Too difficult to ignite and you'll get D-D reactions.
    So far we don't have a use for He-3 (apart from some scientific applications, e. g. to reach very low temperatures). We cannot even break even with D-T, and getting net power out of fusion reactions with He-3 is much harder.
     
  7. Jul 1, 2017 #6

    Vanadium 50

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    I know people who has spent their lives working on this - it';s a tough, tough problem. You've spent a couple months reading. You might want to reflect on that before accusing people of fraud and scientific misconduct.
     
  8. Jul 1, 2017 #7

    Vanadium 50

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    Neutron detection for national security applications. However, making a macroscopic amount via nuclear reactions is simply not practical. (Indeed, the whole point of fusion is that it doesn't take a lot of fuel or produce a lot of products)
     
  9. Jul 1, 2017 #8

    mfb

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    See the exception I mentioned. MRI scans are another application. The total demand is quite small, at least at the current price levels.
    A single fusion reactor producing He-3 would produce about 100 g to 1 kg per day (depends on the reaction of course), more than 10 times the global market today.
     
  10. Jul 18, 2017 #9
    So, if any of you were designing a spaceship drive, did not want to replace a neutron-eaten reactor core every--few months?--needed, obviously, to keep it light, and preferably compact, and wanted to avoid high-energy neutrons for safety, what more or less aneutronic fuels/reactions would you pursue? Never mind that the technology isn't available now, or that we currently don't have enough He3 to experiment with; I'm thinking best case scenario, and ~35 years in the future, and hoping some of the fusion start-ups pursuing aneutronic fusion hit paydirt.

    Both Lawrenceville Plasma Physics and Tri-Alpha Energy think they can make proton-Boron-11 work, and that's a difficult reaction. At only 8--.7?--MeV, it's not as energetic as many others. But isn't it particularly aneutronic? And amenable to direct energy conversion--no cumberson Rankine cycle generator required. Powering a VASIMR, even though it's a lower-energy reaction, might it make a decent drive? Lawrenceville is underfunded, but TriAlpha has attracted $ half a billion in private research funds, much of it from people pretty good at picking wilnners. And Helion Enegy thinks they can make D-He3 fly, and they, too, have attracted some pretty astute funding. I'm encouraged that maybe fusion isn't perennially 40 years out, and that it needn't be toxic, clunky D-T. But D-He3 will produce some D-D>D-T>Neutron reactions. I suppose that if you produced any 14.1 MeV neutrons at all, you'd need as much sheilding, same depth, as if you had a lot of high-energy neutrons to stop? If all you had to stop were 2.45 MeV neutrons, would you need less? Anyone know how much (lithium blanket) you'd need to stop "slow" neutrons?.

    Is He3-He3 less neutronic? 3He + 3He4He + 2 1p + 12.86 MeV ( https://en.wikipedia.org/wiki/Aneutronic_fusion ). I don't see any neutrons there, or any place for any to come from. And I wonder if you couldn't add B-11 at the right time, react it with those hot protons, and get another 8.7 MeV out of that one reactor pulse?
     
  11. Jul 18, 2017 #10

    mfb

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    That also means we don't know how compact suitable reactors can get, and how difficult is a better neutron shielding vs. better plasma confinement or whatever.

    Shielding is mainly a matter of intensity. Sure, higher energies are a bit worse, but mainly you have to provide enough material to avoid diffusion through it.

    Different fusion reactions run under different conditions. Throwing in some B-11 in a He-3 fusion reactor will probably cool it more than it helps, and if you can do p+B-11 in the reactor why bother with He-3?
    He-3 + fast proton from a previous fusion reaction could release a neutron, but that is probably a very rare reaction.
     
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