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Fusion power, Q values

  1. May 16, 2015 #1
    Dear PF Forum,
    I'd like to know more about fusion power. Perhaps someone can give me idea.

    1. Is Q is inefficient, take ITER for example? Supposed Q < 1, can't the produced heat boils water and recycled back to the system to produce fusion again? After all, fusion power plant (if it can be built after all) is just a very sophisticated James Watt kettle.

    2. Can tritium be produced from deuterium?
    3. If yes, what's more difficult, to produce tritium from deuterium or lithium?
    4. Is fusion power plant really clean? If it could be build, does it really produce no radioactive material?
    If yes, what radioactive material that it generate?
    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. May 16, 2015 #2


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    You cannot heat a 100 million degree plasma with hot steam. Yes in theory Q<1 could still work, if your Carnot cycle (getting electricity out of the hot material) is more than 50% efficient and if all other losses are negligible. In real power plants, you want a larger Q.
    It is possible, but it is horribly inefficient. Lithium is used for a good reason.
    It produces radioactive materials (tritium obviously, but also other nuclei), but it is much better than fission power plants. See the wikipedia article for an overview.
  4. May 16, 2015 #3
    Thanks mfb again for your answer.
    I'm sorry, did you just say (or type) TRITIUM? But, isn't that what we need? Seawater contains only deuterium, right? At least 1/6500 of it. If fusion power plant produces tritium isn't one of the problem solved? We only need seawater.


  5. May 16, 2015 #4


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    A fusion power plant would produce tritium from lithium, extract it and then use this tritium as fuel. That still means a power plant has to handle radioactive tritium - probably several kg of it (you need about 500g per day for 1 GW electric power).
  6. May 16, 2015 #5
  7. May 16, 2015 #6


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    You don't need that much - a kilogram Li-6 per GW*day, less if you get some tritium from Li-7 as well. You can sell unused Li-7 again (some , but that is a tiny detail compared to other costs.
    It is produced in some nuclear (fission) reactors with heavy water.
  8. May 16, 2015 #7
    Thanks mfb for your answer.
    And I'm sorry if this sounds like a discussion to you.
    Yes, 1 Kg Li 6, as you say, can produce 1 GW per day, 86 TJ?
    But just how many tonnes of soil that we have to dig, right?
    And one more thing. According to what I read.
    Li6 + neutron -> He4 + tritium + 4.8 Mev, no Li7 there?

    About heavy water.
    It is D2O.
    Must it be D2O, or perhaps HDO can be used to breed tritium?
    What does 1/6500 part of sea water means? Or it doen's care D2O or HDO, but at least 1/6500 of hydrogen is deuterium?
    I think producing tritium from fission (still we can use the energy from fission) is very inefficient.

    Sorry if this sounds like a discussion to you,
    btw, do you know when this ITER will really operate?
    And after ITER, what next? DEMO?
    Will DEMO be the real economical fusion power, that we can actually buy energy from that or it's just the next experiment?

    Last edited: May 16, 2015
  9. May 16, 2015 #8


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    Coal allows to get about 3kWh/kg.
    A 20kg/200Wpeak solar cell (optimistic) is not a fuel, but over a lifetime of 20 years it gives ~200kWh/kg.
    Uranium gives about 1,200,000 kWh/kg
    Lithium leads to about 25,000,000 kWh/kg or 25 TWh/t

    Another way to see those numbers: today's lithium production is roughly 500,000 tons/year. Electricity consumption is about 25,000 TWh/year, which would need 1000 ton of lithium per year. If you want to use fusion power for heating, transportation and all other energy needs, this increases to 5000 tons per year. Still a small effect.

    What is the question?
    If you buy lithium you get a mixture of Li-6 and Li-7. You use most Li-6 and some but not all Li-7, which means a power plant gets some spare Li-7.

    In HDO, the neutron is much more likely to get absorbed by the H.
    One in 6500 atoms is deuterium and they get exchanged between different molecules quickly, so about 1 in 3250 molecules is D2O, and a really tiny fraction of molecules has two deuterium atoms at the same time (D2O).

    You certainly don't want to run a fission plant just for tritium production, but it is produced anyway, so it can be used.

    ITER aims for first plasma around 2023 I think, DT plasma 2027 or something like that (expect at least one more delay).
    DEMO... hopefully. It will still be an experiment, but with the aim to show the economic viability of power plants.
  10. May 16, 2015 #9
    Thanks, that's a very interesting figure you showed me. It's just that I don't think about that energy source. Yes, yes. Fusion is very efficient. (if it can operate in this century, after all).

    Yes, yes, how foolish I am. Of course Li/element doesn't come pure in nature. Always mixtured. Like carbon 12 and carbon 14 that we breath, or U235 and U238 for that matter.
    Is: neutron + Li7 -> Tritium + He3 + neutron, very inefficient?
    Is:neutron + Li7 -> Tritium + He3 + neutron, correct?
    I got it from http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/fusion.html
    It's 8 baryons -> 7 baryons? where is the other baryon?

    Okay, so D2O is the choice, But is there a nuclear reaction for HDO + neutron -> D2O?

    This question sounds like chemistry.
    Are you saying that in sea water (or bathtub) deterium just detaches by itself and join another H2O molecule (and hydrogen from H2O can detach by it self?)
    Statictically, HDO is very rare?

    What do you mean by this? D2O/sea water is very rare? 1/3250?
    Or D2O/HDO is very rare?

    Yeah, fusion power is the energy of the FUTURE and it's consistent. In the future, fusion is STILL energy of the future.

    Thanks for your endless effort to fullfil my curiousity.
    Last edited: May 16, 2015
  11. May 16, 2015 #10


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    There is a typo and it should be [itex]_{2}^{4}He[/itex] in the products. Baryon number: saved!
  12. May 16, 2015 #11


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    Yes, that's exactly the reaction you want to avoid - it wastes a neutron.
    Yes, in the same way other hydrogen atoms do.
    If you mix pure H2O and pure D2O, for example, after a while (typical timescale: hours) you get HDO as well.

    Out of ~6500 hydrogen atoms, one is deuterium. That gives approximately 1 water molecule of HDO for 3249 water molecules H2O => 1 deuterium atom and 1+2*3249=6499 protium atoms.
  13. May 16, 2015 #12
    Good, good. Thanks mfb
    Now, I know that our future for energy is not so bleak if not bright.
    Even if Li6 is rare, but its production is 500 thousands tons/year
    Deuterium is ... 2.2E13 ton in sea water (I've calculated from the volume of ocean * 95%, H2O concentration / 6500 / 9)
    Somehow if we can manage to produce tritium from D2O, then we can have "fish" from the ocean.
    Much less if we can use DD, or even P+P like the sun.
    But with this ITER DT is still far away, well, it might take some time for human civilization to benefit from fusion power.

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