5 ms·

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, whe

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.
    - pfdietz 2y ago
      It seems like you are engaging in magical thinking. A larger, more complex object will be more expensive than a smaller, simpler object. We can quibble on the exact ratio, but making your heat source 40x larger has to have an effect. One consideration that implies accelerating cost increases is reliability. A fusion reactor will have many Criticality 1 features (like welds that, if they fail, leak coolant into the vacuum vessel). The larger the reactor, the more reliable each such component must be. Reliability is expensive.

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.
  - pfdietz 2y ago
    It seems like you are engaging in magical thinking. A larger, more complex object will be more expensive than a smaller, simpler object. We can quibble on the exact ratio, but making your heat source 40x larger has to have an effect. One consideration that implies accelerating cost increases is reliability. A fusion reactor will have many Criticality 1 features (like welds that, if they fail, leak coolant into the vacuum vessel). The larger the reactor, the more reliable each such component must be. Reliability is expensive.