3 ms·

Yes, it does. This follows from the fact that structural mass is proportional to stored magnetic energy, just as the mass of a pressure vessel is proportional

Yes, it does. This follows from the fact that structural mass is proportional to stored magnetic energy, just as the mass of a pressure vessel is proportional to volume x pressure (for materials of a given strength). Viewed another way: if we double B, we quadruple the forces on each segment of conductor, and therefore must quadruple the strength of the supports. I explicitly said "support structure", the structure that is resisting the force of the magnets. There are other parts that scale more slowly, but I don't need to show that all components scale poorly to show that overall there are diseconomies of scale. Note that this is NOT like in a fission reactor. In a fission reactor, everything scales just about linearly with power.

Retric 2y ago
> I don't need to show that all components scale poorly to show that overall there are diseconomies of scale. Yes you do. If A represents 0.001% of the cost then increasing it by 20x doesn’t matter and the support structure is cheap. Same deal with a fission reactor there’s several diseconomies of scale for example around the length of control rods passive cooling after an accident etc, most of them just don’t matter much.
- pfdietz 2y ago
  Very well. I will simply note that 2/3rds of the mass of ARC is the support structure for the magnets. And the rest of the mass includes the blanket.
  - Retric 2y ago
    Now just look up how expensive steel is per kg vs superconductors or lithium and … ahh yea weight is a poor predictor of cost.
    - pfdietz 2y ago
      You do realize that almost all the mass of a PWR reactor is steel, right? The steel pressure vessel, steel supports for the fuel bundles. So the argument you are making there is that the mass discrepancy between fusion reactors and PWRs grossly understates the difference in cost.
      - Retric 2y ago
        Your point? The cost of the reactor is just a fraction of the total cost of a fission power plant.
        reaperman 2y ago
        In a separate line of inquiry, this is what I’ve been thinking about wrt fusion lately. Even if the “fusion” part were free, how much would the electricity cost just based on the steam generator turbine part? (More generally, the sum of all the non-fusion parts of a fusion power plant) Further, if electricity from a fusion plant were effectively free (like it didnt need turbines or something) what would large-scale desalination cost even if it did use truly free energy? This is what I’d be excited for from fusion power, because potable water scarcity is a huge upcoming/ongoing problem in some developing nations with very large populations.
        Retric 2y ago
        Nuclear fission and fusion power plants can use sea water for most of their needs no need for desalination when you just want thermal mass. Remember nuclear subs operate under the ocean. Deuterium requires processing a great deal of water, but can be moved around the world cheaply at the levels needed for fuel. As to the costs of fusion or fission power if the nuclear bits were free, it’s roughly half the cost of coal.
        reaperman 2y ago
        I meant the economic effects of using fusion as a cheap energy source to provide drinking water to ~1 billion people or so (some part of the population who live in places which currently cannot afford to provide their citizens with proper potable water). Desalination uses a ton of electricity. It's a very significant portion of the cost (~50%). But even if the energy from fusion was magically absolutely free, it would "only" reduce the cost of desalination by about 50%. This might not be enough for those who need it. It's a parallel to how with a fusion power plant, even if the reactor only cost $50,000 (some comically small amount near zero), the electricity from the overall power plant connected to the reactor might still be fairly expensive. Like - if you have access to free steam, is turbine generation actually cheaper than solar? Even assuming absolute best-case, fusion's place might not be "lowest cost source of energy" but rather "a notably cleaner and lower-cost base load" which can provide electricity when weather causes the solar and wind farms to underproduce. In 30 years in developed nations, its competition will be batteries, pumped hydro, and other energy storage farms.

Retric 2y ago

> I don't need to show that all components scale poorly to show that overall there are diseconomies of scale. Yes you do. If A represents 0.001% of the cost then increasing it by 20x doesn’t matter and the support structure is cheap. Same deal with a fission reactor there’s several diseconomies of scale for example around the length of control rods passive cooling after an accident etc, most of them just don’t matter much.

pfdietz 2y ago
Very well. I will simply note that 2/3rds of the mass of ARC is the support structure for the magnets. And the rest of the mass includes the blanket.
- Retric 2y ago
  Now just look up how expensive steel is per kg vs superconductors or lithium and … ahh yea weight is a poor predictor of cost.
  - pfdietz 2y ago
    You do realize that almost all the mass of a PWR reactor is steel, right? The steel pressure vessel, steel supports for the fuel bundles. So the argument you are making there is that the mass discrepancy between fusion reactors and PWRs grossly understates the difference in cost.
    - Retric 2y ago
      Your point? The cost of the reactor is just a fraction of the total cost of a fission power plant.
      - reaperman 2y ago
        In a separate line of inquiry, this is what I’ve been thinking about wrt fusion lately. Even if the “fusion” part were free, how much would the electricity cost just based on the steam generator turbine part? (More generally, the sum of all the non-fusion parts of a fusion power plant) Further, if electricity from a fusion plant were effectively free (like it didnt need turbines or something) what would large-scale desalination cost even if it did use truly free energy? This is what I’d be excited for from fusion power, because potable water scarcity is a huge upcoming/ongoing problem in some developing nations with very large populations.
        Retric 2y ago
        Nuclear fission and fusion power plants can use sea water for most of their needs no need for desalination when you just want thermal mass. Remember nuclear subs operate under the ocean. Deuterium requires processing a great deal of water, but can be moved around the world cheaply at the levels needed for fuel. As to the costs of fusion or fission power if the nuclear bits were free, it’s roughly half the cost of coal.
        reaperman 2y ago
        I meant the economic effects of using fusion as a cheap energy source to provide drinking water to ~1 billion people or so (some part of the population who live in places which currently cannot afford to provide their citizens with proper potable water). Desalination uses a ton of electricity. It's a very significant portion of the cost (~50%). But even if the energy from fusion was magically absolutely free, it would "only" reduce the cost of desalination by about 50%. This might not be enough for those who need it. It's a parallel to how with a fusion power plant, even if the reactor only cost $50,000 (some comically small amount near zero), the electricity from the overall power plant connected to the reactor might still be fairly expensive. Like - if you have access to free steam, is turbine generation actually cheaper than solar? Even assuming absolute best-case, fusion's place might not be "lowest cost source of energy" but rather "a notably cleaner and lower-cost base load" which can provide electricity when weather causes the solar and wind farms to underproduce. In 30 years in developed nations, its competition will be batteries, pumped hydro, and other energy storage farms.