4 ms·

There is absolutely no reason to think this is true. The sun has the approximate energy density of a dung hill. The idea that nature will cooperate and make bre

There is absolutely no reason to think this is true. The sun has the approximate energy density of a dung hill. The idea that nature will cooperate and make break even possible at convenient human scales is ... optimistic. There's basically zero evidence for it.

DennisP 7y ago
I'm sorry but that's ridiculous. The sun uses a different fusion reaction. There is plenty of evidence that with other fuels, net power from fusion is feasible on Earth. In particular, the D-T reaction is especially easy, and tokamaks have well-known scaling laws. The JET reactor in the UK is the only reactor using tritium and has produced 67% of its input power. In 1999 the JT-60 in Japan achieved results with D-D fuel that would have hit breakeven with D-T. With a larger reactor, stronger magnetic fields, and D-T fuel, net power is the expected result. (Not to mention, we've already achieved net power at human scales with thermonuclear bombs, which have far more energy density than a dung hill.)
- jacknews 7y ago
  indeed but D-T is not at all "clean" fusion "like the sun", as is being claimed for terrestrial fusion.
  - DennisP 7y ago
    It's not all that bad though. The troublesome waste from fission is the end products of the nuclear reactions. With D-T fusion that's just helium. You do have high-energy neutron radiation, but most of that you're capturing to breed tritium fuel from lithium. Some of it makes reactor parts radioactive, but that's not a major waste problem; according to presentations I've seen from MIT fusion scientists, they'd stop being significantly radioactive after a few decades.
    - jacknews 7y ago
      in practice it will not be so neat. I suspect it will be as polluting as fission, perhaps worse, as the contamination will be in the superstructure, not the waste. And less efficient, and even more expensive.
      - DennisP 7y ago
        Whether it will be neat depends on the reactor design. MIT's for example 3D-prints the inner core and replaces it annually; it's surrounded by molten salt with a lot of lithium, and everything around that hinges so it can be opened up easily. Regardless, the waste won't need long-term containment. The economics of fusion reactors will also depend on the design. Some look economically feasible, others not so much. But it's clear that fusion is possible, contrary to the comment above.
        jacknews 7y ago
        possible, absolutely. but for sure no panacea.
- scottlocklin 7y ago
  OK dude; why do you believe in Fusion? Faith?
  - DennisP 7y ago
    "OK dude" is not a rebuttal. There are arguments to be made about fusion's practicality, but your claim that it's not achievable at all is simply inaccurate. Fusion researchers do it all the time. In fact, for about a thousand dollars in parts you can do it yourself: https://en.wikipedia.org/wiki/Fusor https://en.wikipedia.org/wiki/Fusor The hard part is getting more power out than you put in, under controlled conditions. Here's an overview: https://en.wikipedia.org/wiki/Fusion_power https://en.wikipedia.org/wiki/Fusion_power Probably the most reliable route to practical fusion is outlined in this presentation by the head of MIT's fusion program: https://www.youtube.com/watch?v=KkpqA8yG9T4 https://www.youtube.com/watch?v=KkpqA8yG9T4
    - DennisP 7y ago
      Too late to edit: just read your above comment again and I see you're just saying breakeven isn't doable. So skip that first wiki link, I think you're not disputing that fusion is doable at a net loss. The links on fusion power and MIT's design are still relevant though.
      - scottlocklin 7y ago
        Thanks for eventually reading more carefully. FWIIW I've worked in this field, have written popular articles on Farnsworth and his Fusors, have written popular articles on present day fusion efforts, and will not be convinced by MIT press releases any more than I was the LLNL press releases or anyone else's until they actually start making measurable progress, which nobody has ... in a long time. The most likely useful efforts are at Tri-Alpha, not MIT, and they're about as likely to pan out as I am to find 100lbs of gold outside my front door this evening.
    - deleted 7y ago
      [deleted]
dTal 7y ago
The average volumetric thermal energy density of the sun is a red herring. The vast majority of the mass of the Sun isn't fusing at any given moment; most of the gas contributes only by applying gravitational pressure, a conspicuously inefficient method for achieving fusion pressures, volumetrically speaking - particularly when the gas in question is hydrogen, the lightest element that can exist.

DennisP 7y ago

I'm sorry but that's ridiculous. The sun uses a different fusion reaction. There is plenty of evidence that with other fuels, net power from fusion is feasible on Earth. In particular, the D-T reaction is especially easy, and tokamaks have well-known scaling laws. The JET reactor in the UK is the only reactor using tritium and has produced 67% of its input power. In 1999 the JT-60 in Japan achieved results with D-D fuel that would have hit breakeven with D-T. With a larger reactor, stronger magnetic fields, and D-T fuel, net power is the expected result. (Not to mention, we've already achieved net power at human scales with thermonuclear bombs, which have far more energy density than a dung hill.)

jacknews 7y ago
indeed but D-T is not at all "clean" fusion "like the sun", as is being claimed for terrestrial fusion.
- DennisP 7y ago
  It's not all that bad though. The troublesome waste from fission is the end products of the nuclear reactions. With D-T fusion that's just helium. You do have high-energy neutron radiation, but most of that you're capturing to breed tritium fuel from lithium. Some of it makes reactor parts radioactive, but that's not a major waste problem; according to presentations I've seen from MIT fusion scientists, they'd stop being significantly radioactive after a few decades.
  - jacknews 7y ago
    in practice it will not be so neat. I suspect it will be as polluting as fission, perhaps worse, as the contamination will be in the superstructure, not the waste. And less efficient, and even more expensive.
    - DennisP 7y ago
      Whether it will be neat depends on the reactor design. MIT's for example 3D-prints the inner core and replaces it annually; it's surrounded by molten salt with a lot of lithium, and everything around that hinges so it can be opened up easily. Regardless, the waste won't need long-term containment. The economics of fusion reactors will also depend on the design. Some look economically feasible, others not so much. But it's clear that fusion is possible, contrary to the comment above.
      - jacknews 7y ago
        possible, absolutely. but for sure no panacea.
scottlocklin 7y ago
OK dude; why do you believe in Fusion? Faith?
- DennisP 7y ago
  "OK dude" is not a rebuttal. There are arguments to be made about fusion's practicality, but your claim that it's not achievable at all is simply inaccurate. Fusion researchers do it all the time. In fact, for about a thousand dollars in parts you can do it yourself: https://en.wikipedia.org/wiki/Fusor https://en.wikipedia.org/wiki/Fusor The hard part is getting more power out than you put in, under controlled conditions. Here's an overview: https://en.wikipedia.org/wiki/Fusion_power https://en.wikipedia.org/wiki/Fusion_power Probably the most reliable route to practical fusion is outlined in this presentation by the head of MIT's fusion program: https://www.youtube.com/watch?v=KkpqA8yG9T4 https://www.youtube.com/watch?v=KkpqA8yG9T4
  - DennisP 7y ago
    Too late to edit: just read your above comment again and I see you're just saying breakeven isn't doable. So skip that first wiki link, I think you're not disputing that fusion is doable at a net loss. The links on fusion power and MIT's design are still relevant though.
    - scottlocklin 7y ago
      Thanks for eventually reading more carefully. FWIIW I've worked in this field, have written popular articles on Farnsworth and his Fusors, have written popular articles on present day fusion efforts, and will not be convinced by MIT press releases any more than I was the LLNL press releases or anyone else's until they actually start making measurable progress, which nobody has ... in a long time. The most likely useful efforts are at Tri-Alpha, not MIT, and they're about as likely to pan out as I am to find 100lbs of gold outside my front door this evening.
  - deleted 7y ago
    [deleted]

dTal 7y ago

The average volumetric thermal energy density of the sun is a red herring. The vast majority of the mass of the Sun isn't fusing at any given moment; most of the gas contributes only by applying gravitational pressure, a conspicuously inefficient method for achieving fusion pressures, volumetrically speaking - particularly when the gas in question is hydrogen, the lightest element that can exist.