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"solar roads for sci fi fans" Solar roads are expensive and worked out economocially nowhere, where they were build. Unless you mean roads that have roofs of s

"solar roads for sci fi fans" Solar roads are expensive and worked out economocially nowhere, where they were build. Unless you mean roads that have roofs of solar panels - that is a sound concept, because you can have standard cheap panels and replace them at will, but is not what is generally understood with solar roads. Also about fusion: "when it has all the radioactivity problems of fission" Not at all the same. What exactly are you referring to? Fission with heavy isotopes has radioactivity by design - but Fusion not really. There is some radiation with some designs, but the basic idea of fusing hydrogen into helium works without radioactivity. "massive scale challenges" And where exactly are they? Once a small reactor runs, what is the problem to build a big one? "and no reason to think that it will ever be anything except massive construction project" Because fusion bombs work since 70 years. And fusion in labs are not hard either. We can do fusion - we just cannot do it in a controlled way yet. But recent records with controlling hot plasma indicate that there is real progress. So (allmost) no one is currently betting that we will soon have fusion power, but at some point we will, if we keep on working.

RajT88 3y ago
> Unless you mean roads that have roofs of solar panels This would be really cool and may make sense with cheap/efficient enough panels. But of course, you have to solve the 11'8" problem wherever these things get installed.
epistasis 3y ago
> Solar roads are expensive and worked out economocially nowhere, where they were build. This is exactly my claim for fusion: an uneconomical idea that sounds really cool in theory. > the basic idea of fusing hydrogen into helium works without radioactivity. Where does the deuterium come from? There's going to be neutrons bombarding everything, by design, all sorts of radioactivity. It's literally at the core of the entire thing. This point is swept under the rug, but ensuring that the radioactivity is short lived will be a major design requirement of most components for the reactor. > scale challenges The scale challenge might be building a small reactor, instead of a large one. We don't know what sort of containment will work, and at what sort of physical scales. Which is why massive construction projects are a problem. We are terrible at massive scale construction, and much better at small scale manufacturing. > fusion bombs Not sure why fusion bombs would indicate we could ever do sustained and controlled fusion reactions. These are entirely different processes with almost no connection to each other. This is an error on the order of magnitude of thinking that fusion is not radioactive. And this is why fusion is solar roads for sci fi fans. People love solar roads because they don't know the particulars about solar panels and roads and why they are a bad fit for each other. People love fusion power because they don't know the specifics about fusion or power and why they are a bad fit for each other.
- ben_w 3y ago
  > Where does the deuterium come from? Sea water, it's surprisingly plentiful. What I'm not convinced by is that the neutron emissions that come from D+D fusion are in any sense "safe" or claims that that the neutrons are in a form that "can't" ever be used for development of fission weapons…
  - epistasis 3y ago
    That was a basic goof, should have been tritium there.
    - ben_w 3y ago
      Fair enough. Tritium comes from D+D reactions, which… make a lot of neutrons in the process.
gene-h 3y ago
>There is some radiation with some designs, but the basic idea of fusing hydrogen into helium works without radioactivity. Per unit energy D-T fusion, the fusion reaction most likely to be viable near term, generates more neutrons than the fission of U235. The U235 fission reaction[0] releases 3 neutrons and 202.5 MeV of energy, 8.8 MeV escapes as antineutrinos, so let's call it 193.7 MeV, so 64.5 MeV per neutron. D-T fusion generates 17.6 MeV and 1 neutron, so 17.6 MeV per neutron. So for the same amount of energy produced it makes about 3.5x more neutrons. Worse yet, in D-T fusion, 14.1 MeV of the energy released is in the kinetic energy of the neutron. Compare this to the 8.8 MeV energy of all three neutrons in a fission reaction. Neutron activation and damage are significant engineering concerns for fusion reactors. [0]https://en.wikipedia.org/wiki/Uranium-235 https://en.wikipedia.org/wiki/Uranium-235 [1]https://en.wikipedia.org/wiki/Deuterium%E2%80%93tritium_fusion https://en.wikipedia.org/wiki/Deuterium%E2%80%93tritium_fusi... https://en.wikipedia.org/wiki/Uranium-235 https://en.wikipedia.org/wiki/Uranium-235
- epistasis 3y ago
  Thank you for some much needed solid information. And then you need to use that neutron to breed new tritium from lithium-6, which is only about 2% of lithium. And since our only feasible source of tritium is a single tritium atom from that single neutron from a D-T fusion reaction that consumes a tritium, we had better be damn efficient at making sure the neutrons go only to breeding new tritium. Which is good, because then they cause less neutron radioactivity of all the rest of the reactor parts. But bad, because it's pretty unrealistic to capture all the neutrons via lithium blanket. Which means we need ultimately need an outside tritium source, external from the fusion reactions, which means another neutron source, which means, fission reactors! The whole idea of controlled terrestrial fusion being an inexpensive source of electricity is just absolutely baffling to me. As is the idea that it will somehow be a good extraterrestrial energy source, or power some sort of spaceship. Fission is far more realistic, likely to be compact, etc. Direct conversion of aneutronic fusion is complete sci-fi at the moment, we may as well be talking warp drives.
- hutzlibu 3y ago
  "Per unit energy D-T fusion, the fusion reaction most likely to be viable near term" Ok well, I see the use of tritium only as a step towards normal hydrogen as fuel in fusion reactors. Tritium as fuel is just the "easy" target and yes, it is in limited supply and causes problems. But "in theory" we could have higher temperatures and not be dependant on tritium.
dredmorbius 3y ago
"Solar Roadways[tm]" is a specific startup, and is all but certainly what was referenced upstream. The proposal and technology were obviously nonviable to anyone with the least technical knowledge from the beginning. Wikipedia: <https://en.wikipedia.org/wiki/Solar_Roadways https://en.wikipedia.org/wiki/Solar_Roadways> Phil "Thunderf00t" Mason on YouTube had a particularly brutal critique. Repetitively repetitious and occasionally repeating itself (as is his wont), but on point: <https://yewtu.be/watch?v=H901KdXgHs4 https://yewtu.be/watch?v=H901KdXgHs4> (Several follow-ups, including an on-site visit.)

RajT88 3y ago

> Unless you mean roads that have roofs of solar panels This would be really cool and may make sense with cheap/efficient enough panels. But of course, you have to solve the 11'8" problem wherever these things get installed.

epistasis 3y ago

> Solar roads are expensive and worked out economocially nowhere, where they were build. This is exactly my claim for fusion: an uneconomical idea that sounds really cool in theory. > the basic idea of fusing hydrogen into helium works without radioactivity. Where does the deuterium come from? There's going to be neutrons bombarding everything, by design, all sorts of radioactivity. It's literally at the core of the entire thing. This point is swept under the rug, but ensuring that the radioactivity is short lived will be a major design requirement of most components for the reactor. > scale challenges The scale challenge might be building a small reactor, instead of a large one. We don't know what sort of containment will work, and at what sort of physical scales. Which is why massive construction projects are a problem. We are terrible at massive scale construction, and much better at small scale manufacturing. > fusion bombs Not sure why fusion bombs would indicate we could ever do sustained and controlled fusion reactions. These are entirely different processes with almost no connection to each other. This is an error on the order of magnitude of thinking that fusion is not radioactive. And this is why fusion is solar roads for sci fi fans. People love solar roads because they don't know the particulars about solar panels and roads and why they are a bad fit for each other. People love fusion power because they don't know the specifics about fusion or power and why they are a bad fit for each other.

ben_w 3y ago
> Where does the deuterium come from? Sea water, it's surprisingly plentiful. What I'm not convinced by is that the neutron emissions that come from D+D fusion are in any sense "safe" or claims that that the neutrons are in a form that "can't" ever be used for development of fission weapons…
- epistasis 3y ago
  That was a basic goof, should have been tritium there.
  - ben_w 3y ago
    Fair enough. Tritium comes from D+D reactions, which… make a lot of neutrons in the process.

gene-h 3y ago

>There is some radiation with some designs, but the basic idea of fusing hydrogen into helium works without radioactivity. Per unit energy D-T fusion, the fusion reaction most likely to be viable near term, generates more neutrons than the fission of U235. The U235 fission reaction[0] releases 3 neutrons and 202.5 MeV of energy, 8.8 MeV escapes as antineutrinos, so let's call it 193.7 MeV, so 64.5 MeV per neutron. D-T fusion generates 17.6 MeV and 1 neutron, so 17.6 MeV per neutron. So for the same amount of energy produced it makes about 3.5x more neutrons. Worse yet, in D-T fusion, 14.1 MeV of the energy released is in the kinetic energy of the neutron. Compare this to the 8.8 MeV energy of all three neutrons in a fission reaction. Neutron activation and damage are significant engineering concerns for fusion reactors. [0]https://en.wikipedia.org/wiki/Uranium-235 https://en.wikipedia.org/wiki/Uranium-235 [1]https://en.wikipedia.org/wiki/Deuterium%E2%80%93tritium_fusion https://en.wikipedia.org/wiki/Deuterium%E2%80%93tritium_fusi... https://en.wikipedia.org/wiki/Uranium-235 https://en.wikipedia.org/wiki/Uranium-235

epistasis 3y ago
Thank you for some much needed solid information. And then you need to use that neutron to breed new tritium from lithium-6, which is only about 2% of lithium. And since our only feasible source of tritium is a single tritium atom from that single neutron from a D-T fusion reaction that consumes a tritium, we had better be damn efficient at making sure the neutrons go only to breeding new tritium. Which is good, because then they cause less neutron radioactivity of all the rest of the reactor parts. But bad, because it's pretty unrealistic to capture all the neutrons via lithium blanket. Which means we need ultimately need an outside tritium source, external from the fusion reactions, which means another neutron source, which means, fission reactors! The whole idea of controlled terrestrial fusion being an inexpensive source of electricity is just absolutely baffling to me. As is the idea that it will somehow be a good extraterrestrial energy source, or power some sort of spaceship. Fission is far more realistic, likely to be compact, etc. Direct conversion of aneutronic fusion is complete sci-fi at the moment, we may as well be talking warp drives.
hutzlibu 3y ago
"Per unit energy D-T fusion, the fusion reaction most likely to be viable near term" Ok well, I see the use of tritium only as a step towards normal hydrogen as fuel in fusion reactors. Tritium as fuel is just the "easy" target and yes, it is in limited supply and causes problems. But "in theory" we could have higher temperatures and not be dependant on tritium.

dredmorbius 3y ago

"Solar Roadways[tm]" is a specific startup, and is all but certainly what was referenced upstream. The proposal and technology were obviously nonviable to anyone with the least technical knowledge from the beginning. Wikipedia: <https://en.wikipedia.org/wiki/Solar_Roadways https://en.wikipedia.org/wiki/Solar_Roadways> Phil "Thunderf00t" Mason on YouTube had a particularly brutal critique. Repetitively repetitious and occasionally repeating itself (as is his wont), but on point: <https://yewtu.be/watch?v=H901KdXgHs4 https://yewtu.be/watch?v=H901KdXgHs4> (Several follow-ups, including an on-site visit.)