3 ms·

Most of the answers are missing the actual (proposed) mechanism. The energy of the reaction is mostly carried away as high-energy neutrons. So, the way to get

Most of the answers are missing the actual (proposed) mechanism. The energy of the reaction is mostly carried away as high-energy neutrons. So, the way to get energy back is to "capture" those neutrons. Since neutrons are not electrically charged, you can't use them to directly create electricity, so all you're left with is using them for heat. Unfortunately, since they are electrically neutral, they're also relatively hard to catch. You need a dense material where they will have a good chance to hit some nucleus. The proposed designs are typically some kind of liquid metal blanket being circulated around the reactor and onto a place where it can boil water to produce steam to spin a turbine. Lithium is the metal most proposed for this, since it also has the advantage that it can produce tritium when bombarded with neutrons (tritium being the super rare half of the fuel that goes into the reaction).

nerdponx 3y ago
It's always fascinating to me that, no matter how many interesting new ways to release lots of energy we develop, we are still stuck with the same method for converting it to electricity: release the energy as heat, use heat to make steam, use steam to drive generator.
- NateEag 3y ago
  Helion is planning to use induction to generate electricity from the fusion reaction's generated magnetic field, IIUC: https://www.helionenergy.com/faq/ https://www.helionenergy.com/faq/ (See "How does Helion generate electricity from fusion?" question)
  - MichaelZuo 3y ago
    Which doesn't make sense for power generation since there will always be some percentage of neutrons produced by any type of fusion reaction that can only be useful for generating steam. To entirely skip the steam cycle portion is to intentionally make a much less efficient design. For space-constrained, high-value, applications where economics don't matter that much, such as a submarine, that would make sense, but otherwise...
    - jessriedel 3y ago
      You could always add the liquid metal blanket if you want to eke out the extra 10% (or if you want to generate tritium). But it's not worth the complication in an early prototype.
    - gus_massa 3y ago
      They plan to use 2D + 3He -> 4He + 1H, so no neutrons https://en.wikipedia.org/wiki/Aneutronic_fusion#Candidate_reactions https://en.wikipedia.org/wiki/Aneutronic_fusion#Candidate_re... (I'm still not convinced of their explanations, but a fast proton may be easy to catch by the magnetic field and create the effect they want.)
      - fastneutron 3y ago
        Nitpick: You’re always going to get some parasitic D+D reactions because it’s reaction cross section is appreciably higher than D + 3He below 50 keV, and non-negligible even after the crossover point.
        skykooler 3y ago
        Doesn't D+D just yield 4He (and no neutrons) though?
        philipkglass 3y ago
        It yields 3He plus a 2.45 MeV neutron at least part of the time. See the diagram here: https://undsci.berkeley.edu/teach-resources/products-of-deuterium-fusion/ https://undsci.berkeley.edu/teach-resources/products-of-deut...
    - DennisP 3y ago
      Helion's fuel mix produces just 6% of its energy as neutron radiation, and if you harvest it you'll lose a third of that. As long as you have enough net energy, collecting that 4% might not make financial sense. With fuel costs insignificant, your cost per kWh is mainly capital cost. Let's say it's all capital just to keep it simple. I don't know how much the input energy will be but if your choice is between, say, generating net energy of 50MW without a turbine or 54MW with a turbine, then you would skip the turbine if it adds more than 8% to the capital cost. I suspect Helion has done this calculation in detail.
  - tsimionescu 3y ago
    Helion is planning to use a different fusion reaction, one where the bulk of the energy will be coming out as charged particles, not neutrons. However, D+T fusion is the only type of fusion that we have been able to sustain for any significant amount of time with reasonable energy inputs. What Helion is planning to do is completely unexplored and requires some major scientific advances.
- jessriedel 3y ago
  The reason just that this is a simple process for which a steam turbine can achieves 90% of the thermodynamic optimum. To my knowledge, the only reason people consider alternatives is to reduce capital costs. You're still capped by thermodynamics though.
  - nerdponx 3y ago
    That's what I was wondering: if heat -> steam -> turbine is close enough to the theoretical upper limit and you get diminishing returns by other means. Still, it seems very indirect. Like generating solar power by using a parabolic mirror to heat water instead of photovoltaic panels... but of course I just found an example of doing that too: https://en.wikipedia.org/wiki/Parabolic_trough https://en.wikipedia.org/wiki/Parabolic_trough
    - Iulioh 3y ago
      There are multiple solutions of that sort but all failed due to high maintenance costs Usually the liquid is some sort of oil that csn reach higher temperature and then the oil is used to heat water Another design uses hundreds of mirror centered around a high tower with molten salts as the heat medium The really good part of this design is that the molted salts can produce energy for 3-4h after the sun is set.
- sambapa 3y ago
  It's because we're in local maximum - steam turbines are just so developed
- tsimionescu 3y ago
  Well, we do have some other strategies. Hydro and wind just turn the turbine mechanically, they don't heat up water to make steam. And photovoltaics create electricity directly using an effect that won Einstein the Nobel prize and began the age of quantum mechanics, so that's about as advanced as they come.
- rocqua 3y ago
  Steam turbines are actually quite a bad method, because pure heat is a difficult energy source. It's just also a very available energy source. The 90% efficiency quoted in the comments is 90% of theoretical maximum efficiency. This theoretical maximum is about 50% for the best systems.
quickthrower2 3y ago
What is happening in a star to emit useful radiation? I feel I should know that, I probably did but forgot.
- tsimionescu 3y ago
  Stars rely on gravitation to confine the plasma instead of magnetic fields, and they can reach the energy densities needed for other fusion reactions, typically D+D, which produces less neutrons. Also, there are hundreds of thousands of kilometers between the center of the sun and its outermost layers - neutrons produced in the core will have plenty of time to be absorbed by something else on their way out.
stouset 3y ago
What eventually happens to the excess neutron “waste”? Surely there’s some negative consequence to producing an enormous number of neutrons, no?
- tsimionescu 3y ago
  All the neutrons that don't get caught by the blanket will usually be caught by the support structure of the reactor, damaging it and making it radioactive. This puts a cap on the maximum lifetime of this type of fusion reactor until significant parts of the structure need to be dismantled and replaced, and stored as highly radioactive waste. As far as I know, current estimates suggest something like a decade - one of the biggest problems with the economics of fusion power plants.
lambdatronics 3y ago
Just to amplify on your point about lithium: the tritium production function is critical. Every fusion neutron needs to produce more than one tritium atom on average, so that the reactor is sustainable (there are inevitable losses & tritium also decays radioactively) or even making excess tritium (to bootstrap other reactors). This is challenging b/c even in the best case each neutron can produce maybe 2 tritium atoms, so there's not much margin. The lithium needs to comprise most of the material surrounding the plasma, limiting the fraction that's available for other functions (structural supports, heat shielding, cooling, plasma control & heating systems, sensors, etc).

nerdponx 3y ago

It's always fascinating to me that, no matter how many interesting new ways to release lots of energy we develop, we are still stuck with the same method for converting it to electricity: release the energy as heat, use heat to make steam, use steam to drive generator.

NateEag 3y ago
Helion is planning to use induction to generate electricity from the fusion reaction's generated magnetic field, IIUC: https://www.helionenergy.com/faq/ https://www.helionenergy.com/faq/ (See "How does Helion generate electricity from fusion?" question)
- MichaelZuo 3y ago
  Which doesn't make sense for power generation since there will always be some percentage of neutrons produced by any type of fusion reaction that can only be useful for generating steam. To entirely skip the steam cycle portion is to intentionally make a much less efficient design. For space-constrained, high-value, applications where economics don't matter that much, such as a submarine, that would make sense, but otherwise...
  - jessriedel 3y ago
    You could always add the liquid metal blanket if you want to eke out the extra 10% (or if you want to generate tritium). But it's not worth the complication in an early prototype.
  - gus_massa 3y ago
    They plan to use 2D + 3He -> 4He + 1H, so no neutrons https://en.wikipedia.org/wiki/Aneutronic_fusion#Candidate_reactions https://en.wikipedia.org/wiki/Aneutronic_fusion#Candidate_re... (I'm still not convinced of their explanations, but a fast proton may be easy to catch by the magnetic field and create the effect they want.)
    - fastneutron 3y ago
      Nitpick: You’re always going to get some parasitic D+D reactions because it’s reaction cross section is appreciably higher than D + 3He below 50 keV, and non-negligible even after the crossover point.
      - skykooler 3y ago
        Doesn't D+D just yield 4He (and no neutrons) though?
        philipkglass 3y ago
        It yields 3He plus a 2.45 MeV neutron at least part of the time. See the diagram here: https://undsci.berkeley.edu/teach-resources/products-of-deuterium-fusion/ https://undsci.berkeley.edu/teach-resources/products-of-deut...
  - DennisP 3y ago
    Helion's fuel mix produces just 6% of its energy as neutron radiation, and if you harvest it you'll lose a third of that. As long as you have enough net energy, collecting that 4% might not make financial sense. With fuel costs insignificant, your cost per kWh is mainly capital cost. Let's say it's all capital just to keep it simple. I don't know how much the input energy will be but if your choice is between, say, generating net energy of 50MW without a turbine or 54MW with a turbine, then you would skip the turbine if it adds more than 8% to the capital cost. I suspect Helion has done this calculation in detail.
- tsimionescu 3y ago
  Helion is planning to use a different fusion reaction, one where the bulk of the energy will be coming out as charged particles, not neutrons. However, D+T fusion is the only type of fusion that we have been able to sustain for any significant amount of time with reasonable energy inputs. What Helion is planning to do is completely unexplored and requires some major scientific advances.
jessriedel 3y ago
The reason just that this is a simple process for which a steam turbine can achieves 90% of the thermodynamic optimum. To my knowledge, the only reason people consider alternatives is to reduce capital costs. You're still capped by thermodynamics though.
- nerdponx 3y ago
  That's what I was wondering: if heat -> steam -> turbine is close enough to the theoretical upper limit and you get diminishing returns by other means. Still, it seems very indirect. Like generating solar power by using a parabolic mirror to heat water instead of photovoltaic panels... but of course I just found an example of doing that too: https://en.wikipedia.org/wiki/Parabolic_trough https://en.wikipedia.org/wiki/Parabolic_trough
  - Iulioh 3y ago
    There are multiple solutions of that sort but all failed due to high maintenance costs Usually the liquid is some sort of oil that csn reach higher temperature and then the oil is used to heat water Another design uses hundreds of mirror centered around a high tower with molten salts as the heat medium The really good part of this design is that the molted salts can produce energy for 3-4h after the sun is set.
sambapa 3y ago
It's because we're in local maximum - steam turbines are just so developed
tsimionescu 3y ago
Well, we do have some other strategies. Hydro and wind just turn the turbine mechanically, they don't heat up water to make steam. And photovoltaics create electricity directly using an effect that won Einstein the Nobel prize and began the age of quantum mechanics, so that's about as advanced as they come.
rocqua 3y ago
Steam turbines are actually quite a bad method, because pure heat is a difficult energy source. It's just also a very available energy source. The 90% efficiency quoted in the comments is 90% of theoretical maximum efficiency. This theoretical maximum is about 50% for the best systems.

quickthrower2 3y ago

What is happening in a star to emit useful radiation? I feel I should know that, I probably did but forgot.

tsimionescu 3y ago
Stars rely on gravitation to confine the plasma instead of magnetic fields, and they can reach the energy densities needed for other fusion reactions, typically D+D, which produces less neutrons. Also, there are hundreds of thousands of kilometers between the center of the sun and its outermost layers - neutrons produced in the core will have plenty of time to be absorbed by something else on their way out.

stouset 3y ago

What eventually happens to the excess neutron “waste”? Surely there’s some negative consequence to producing an enormous number of neutrons, no?

tsimionescu 3y ago
All the neutrons that don't get caught by the blanket will usually be caught by the support structure of the reactor, damaging it and making it radioactive. This puts a cap on the maximum lifetime of this type of fusion reactor until significant parts of the structure need to be dismantled and replaced, and stored as highly radioactive waste. As far as I know, current estimates suggest something like a decade - one of the biggest problems with the economics of fusion power plants.

lambdatronics 3y ago

Just to amplify on your point about lithium: the tritium production function is critical. Every fusion neutron needs to produce more than one tritium atom on average, so that the reactor is sustainable (there are inevitable losses & tritium also decays radioactively) or even making excess tritium (to bootstrap other reactors). This is challenging b/c even in the best case each neutron can produce maybe 2 tritium atoms, so there's not much margin. The lithium needs to comprise most of the material surrounding the plasma, limiting the fraction that's available for other functions (structural supports, heat shielding, cooling, plasma control & heating systems, sensors, etc).