2 ms·

At the end of the day Fusion has been treated as a very expensive science project not some critical technology we really need to get working. Thus delays which

by Retric 2y ago

At the end of the day Fusion has been treated as a very expensive science project not some critical technology we really need to get working. Thus delays which have nothing to do with feasibility or even cost just politics. Joint European Torus (JET) built in 1983! eventually hit a plasma heating Q of 0.67. Scaling that up isn’t some big leap of faith. ITER was started as an agreement between Regan in Gorbachev in 1985, but didn’t begin construction until 2013! based on a significantly scaled down and very conservative design from 2001, with an expected completion date of 2025-2030. It’s terrible, but under funded multinational projects don’t move quickly. That’s why nothing got done with fusion, you need experiments to make progress running computer simulator and toy machines isn’t enough. Further, you can start site prep and building the facilities well before a design is finished. So much of these delays were completely political in nature not technical, we could start site prep for DEMO today but expect site selection and prep to add a few years of completely avoidable delay to that project as well. Of course being able to build a working device is only part of the story, it also needs to make economic sense. But that’s a different matter from why things have taken so long.

pfdietz 2y ago
I agree that fusion has been treated as a science project, but I take a different implication from that. The importance of the science has been overstated, when more mundane engineering considerations are likely more important. There's this pervasive idea that once the science is done, that once the plasma is contained well, we're golden and fusion will be a sure thing. This is very far from the truth. Lawrence Lidsky pointed out back in the 1980s that there are serious arguments against DT fusion, even if you assume the physics isn't a problem at all. Just assuming you have a magic black box that can make DT go and you still need to capture the neutrons, and this simple sounding engineering problem makes DT fusion unattractive compared to alternatives, with volumetric power density at least an order of magnitude worse than fission reactors. Subsequent experience bears this out. Even supposedly compact designs like ARC will still be a factor of 40 worse than PWRs by this metric. Note that better superconductors than the HTSs in ARC would not help, since the mass of ARC is dominated by the structural material needed to keep the magnets from flying apart. Any stronger magnetic field is ruled out by practical considerations of strength of this structure. Maybe the limits can be relaxed somewhat if the reactor has thick liquid lithium as the first wall, so power/area can be increased (the liquid would be exposed directly to vacuum; at sufficiently modest temperature the vapor pressure of liquid lithium can be very low). Zap uses that approach, but notably Zap doesn't use external magnets (superconducting or otherwise); the magnetic field is generated by current flowing in the plasma. Zap doesn't cover all 4 pi steradians around the plasma with flowing metal though, so neutron load on exposed components may still limit their power density.
- Retric 2y ago
  Volumetric energy density is largely meaningless when the inside is a low density plasma. It’s not like you need to pay for that volume of steel or something it’s basically free. What matters is the cost of the walls which mostly scales with surface area not internal volume.
  - pfdietz 2y ago
    > It’s not like you need to pay for that volume of steel or something it’s basically free. I'm talking about the volume of the reactor, not the plasma, and no, it's not "basically free". Very far from it! The size and cost of the reactor is the big issue. In a DT fusion reactor, you're basically (from an economic point of view) burning the reactor, not burning the fuel. The reactor is the consumable that drives the cost of produced energy. The cost of the reactor is a function of its size and complexity. A fusion is reactor is both much more complex than, and much larger than, a fission reactor of the same thermal output. So, if you're using both as heat sources, the fusion reactor will not be able to compete with a fission reactor. Fusion reactors also have strong diseconomies of scale, since they are limited by power through the first wall, but (contrary to your assertion) cost scales more with volume. So, you want to make a DT fusion reactor as small as possible. However, size is limited from below by the need to absorb neutrons in the blanket, and this distance is set by nuclear cross sections. These considerations are what drives the desire the use fusion directly to produce electrical energy, not produce heat, so the apples-to-apples comparison with fission can be evaded. For this reason, I consider Helion the least dubious of the fusion enterprises.
    - Retric 2y ago
      > I'm talking about the volume of the reactor Again, the reactor’s physical structure and cost doesn’t scale 1:1 with the internal volume, so it’s a meaningless metric. As to your assumptions, a smaller device may be cheaper or more expensive depending on what it takes to make it smaller. It’s a complex machine you can’t just price things by weight or something. Really the argument is like complaining about fission reactors because all that water is heavy what matters is cost not vague proxies for cost.

by Retric 2y ago

pfdietz 2y ago

I agree that fusion has been treated as a science project, but I take a different implication from that. The importance of the science has been overstated, when more mundane engineering considerations are likely more important. There's this pervasive idea that once the science is done, that once the plasma is contained well, we're golden and fusion will be a sure thing. This is very far from the truth. Lawrence Lidsky pointed out back in the 1980s that there are serious arguments against DT fusion, even if you assume the physics isn't a problem at all. Just assuming you have a magic black box that can make DT go and you still need to capture the neutrons, and this simple sounding engineering problem makes DT fusion unattractive compared to alternatives, with volumetric power density at least an order of magnitude worse than fission reactors. Subsequent experience bears this out. Even supposedly compact designs like ARC will still be a factor of 40 worse than PWRs by this metric. Note that better superconductors than the HTSs in ARC would not help, since the mass of ARC is dominated by the structural material needed to keep the magnets from flying apart. Any stronger magnetic field is ruled out by practical considerations of strength of this structure. Maybe the limits can be relaxed somewhat if the reactor has thick liquid lithium as the first wall, so power/area can be increased (the liquid would be exposed directly to vacuum; at sufficiently modest temperature the vapor pressure of liquid lithium can be very low). Zap uses that approach, but notably Zap doesn't use external magnets (superconducting or otherwise); the magnetic field is generated by current flowing in the plasma. Zap doesn't cover all 4 pi steradians around the plasma with flowing metal though, so neutron load on exposed components may still limit their power density.

Retric 2y ago
Volumetric energy density is largely meaningless when the inside is a low density plasma. It’s not like you need to pay for that volume of steel or something it’s basically free. What matters is the cost of the walls which mostly scales with surface area not internal volume.
- pfdietz 2y ago
  > It’s not like you need to pay for that volume of steel or something it’s basically free. I'm talking about the volume of the reactor, not the plasma, and no, it's not "basically free". Very far from it! The size and cost of the reactor is the big issue. In a DT fusion reactor, you're basically (from an economic point of view) burning the reactor, not burning the fuel. The reactor is the consumable that drives the cost of produced energy. The cost of the reactor is a function of its size and complexity. A fusion is reactor is both much more complex than, and much larger than, a fission reactor of the same thermal output. So, if you're using both as heat sources, the fusion reactor will not be able to compete with a fission reactor. Fusion reactors also have strong diseconomies of scale, since they are limited by power through the first wall, but (contrary to your assertion) cost scales more with volume. So, you want to make a DT fusion reactor as small as possible. However, size is limited from below by the need to absorb neutrons in the blanket, and this distance is set by nuclear cross sections. These considerations are what drives the desire the use fusion directly to produce electrical energy, not produce heat, so the apples-to-apples comparison with fission can be evaded. For this reason, I consider Helion the least dubious of the fusion enterprises.
  - Retric 2y ago
    > I'm talking about the volume of the reactor Again, the reactor’s physical structure and cost doesn’t scale 1:1 with the internal volume, so it’s a meaningless metric. As to your assumptions, a smaller device may be cheaper or more expensive depending on what it takes to make it smaller. It’s a complex machine you can’t just price things by weight or something. Really the argument is like complaining about fission reactors because all that water is heavy what matters is cost not vague proxies for cost.