3 ms·

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 i

by 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
      > A larger, more complex object will be more expensive than a smaller, simpler object. That assumes the larger device will be more complex when it’s often simpler to make something larger than it is to scale it down. At the extreme end the sun is vastly simpler than any reactor design we are considering, those are complex because we need to operate on a smaller scale. Hell we could have built a combined fusion/fission reactor in the 1960’s using Hydrogen bombs and a large enough underground body of water to absorb the energy. Would need to be huge, but not that complicated.
      - pfdietz 2y ago
        I'm comparing fusion reactors to fission reactors there. The complexity of a fusion reactor is obviously much higher than that of a fission reactor. So is the size, for a given power output.
        Retric 2y ago
        You mentioned “Fusion reactors also have strong diseconomies of scale” so that’s what I am referring to. A fusion reactor 2x larger with the same energy flow per m2 of wall could be cheaper per kWh than a more compact reactor. The exact details matter a great deal and not every design works at every scale. Anyway, a fission reactor’s complexity is mostly outside the device. Things like multiply redundant cooling systems, backups for your backup generators, fuel processing, etc that are completely unnecessary or simpler on a fusion reactor. So overall a large fusion power plant could easily be a simpler overall system.

by Retric 2y ago

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
    > A larger, more complex object will be more expensive than a smaller, simpler object. That assumes the larger device will be more complex when it’s often simpler to make something larger than it is to scale it down. At the extreme end the sun is vastly simpler than any reactor design we are considering, those are complex because we need to operate on a smaller scale. Hell we could have built a combined fusion/fission reactor in the 1960’s using Hydrogen bombs and a large enough underground body of water to absorb the energy. Would need to be huge, but not that complicated.
    - pfdietz 2y ago
      I'm comparing fusion reactors to fission reactors there. The complexity of a fusion reactor is obviously much higher than that of a fission reactor. So is the size, for a given power output.
      - Retric 2y ago
        You mentioned “Fusion reactors also have strong diseconomies of scale” so that’s what I am referring to. A fusion reactor 2x larger with the same energy flow per m2 of wall could be cheaper per kWh than a more compact reactor. The exact details matter a great deal and not every design works at every scale. Anyway, a fission reactor’s complexity is mostly outside the device. Things like multiply redundant cooling systems, backups for your backup generators, fuel processing, etc that are completely unnecessary or simpler on a fusion reactor. So overall a large fusion power plant could easily be a simpler overall system.