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Is terrestrial fusion even desirable, I understand it will not use the same "clean" reaction as stellar fusion (deuterium-tritium or D-D vs ordinary hydrogen).

Is terrestrial fusion even desirable, I understand it will not use the same "clean" reaction as stellar fusion (deuterium-tritium or D-D vs ordinary hydrogen). Can someone with more knowledge comment on whether this report is accurate? https://thebulletin.org/2017/04/fusion-reactors-not-what-theyre-cracked-up-to-be/ https://thebulletin.org/2017/04/fusion-reactors-not-what-the... To me it seems to annul the entire enterprise (other than as a pure research project) if true?

ajross 8y ago
Stellar fusion isn't really clean in the way you imagine; there are lots of reactions in the star core that produce free neutrons, they just get reabsorbed by something else because they can't realistically exit the system like they can in a tiny reactor. Nor is it realistically achievable, anyway. Fusing products out of bare protons just isn't productive enough at achievable temperatures.
- pfdietz 8y ago
  In a star like the Sun there are very few free neutrons produced. There are non-exploding stars that produce some neutrons, but they are AGB stars where the core gets hot enough for (alpha,n) reactions to occur.
  - ajross 8y ago
    Hm... maybe I'm misremembering, but I swear I saw an analysis that showed neutron loss from a small (i.e. reactor scale) blob of solar core plasma would be comparable to what you get from existing power reactions. It's true that almost all the energy production in the sun happens from reactions that don' t involve neutrons, but the neutrons are flying around nonetheless (and, in a reactor, would fly right out of the containment).
    - pfdietz 8y ago
      What nuclear reaction in the solar core are you imagining would produce neutrons?
      - ajross 8y ago
        Something endothermic? Are you really asserting that no such reaction exists? They clearly do, the question is whether the cross sections are high enough. The list of reactions you find in typical explainers are just the ones involved in energy production. And I'm no expert, so I'm not going to be able hand you the evidence here. But like I said I swear I've read this analysis somewhere, and something about your tone tells me maybe you are misunderstanding the point.
        pfdietz 8y ago
        The most likely would be 13C(alpha,n)16O, but the rate of this would be very low at the temperature of the solar core. You might also point to (alpha,n) reactions on Li, Be, or B. But these should be slower than (p,alpha) reactions on these elements, which destroy them at temperatures well below (by a factor of ~3 or more) that of the center of the sun. Let's see how far I can push your argument. There will be some 7Li produced by a side branch of the PP cycle (although it is quickly destroyed). Also, 4He produced by the various fusion reactions will initially have energies in the MeV range (~1000 mean thermal energy in the core), so it's possible (alpha,n) reactions could occur from them as they slow down.

ajross 8y ago

Stellar fusion isn't really clean in the way you imagine; there are lots of reactions in the star core that produce free neutrons, they just get reabsorbed by something else because they can't realistically exit the system like they can in a tiny reactor. Nor is it realistically achievable, anyway. Fusing products out of bare protons just isn't productive enough at achievable temperatures.

pfdietz 8y ago
In a star like the Sun there are very few free neutrons produced. There are non-exploding stars that produce some neutrons, but they are AGB stars where the core gets hot enough for (alpha,n) reactions to occur.
- ajross 8y ago
  Hm... maybe I'm misremembering, but I swear I saw an analysis that showed neutron loss from a small (i.e. reactor scale) blob of solar core plasma would be comparable to what you get from existing power reactions. It's true that almost all the energy production in the sun happens from reactions that don' t involve neutrons, but the neutrons are flying around nonetheless (and, in a reactor, would fly right out of the containment).
  - pfdietz 8y ago
    What nuclear reaction in the solar core are you imagining would produce neutrons?
    - ajross 8y ago
      Something endothermic? Are you really asserting that no such reaction exists? They clearly do, the question is whether the cross sections are high enough. The list of reactions you find in typical explainers are just the ones involved in energy production. And I'm no expert, so I'm not going to be able hand you the evidence here. But like I said I swear I've read this analysis somewhere, and something about your tone tells me maybe you are misunderstanding the point.
      - pfdietz 8y ago
        The most likely would be 13C(alpha,n)16O, but the rate of this would be very low at the temperature of the solar core. You might also point to (alpha,n) reactions on Li, Be, or B. But these should be slower than (p,alpha) reactions on these elements, which destroy them at temperatures well below (by a factor of ~3 or more) that of the center of the sun. Let's see how far I can push your argument. There will be some 7Li produced by a side branch of the PP cycle (although it is quickly destroyed). Also, 4He produced by the various fusion reactions will initially have energies in the MeV range (~1000 mean thermal energy in the core), so it's possible (alpha,n) reactions could occur from them as they slow down.