| HN Mirror

The claims that it's going to be wonderful don't get subject to this sort of scrutiny, but whatever...

There are multiple reasons to think fusion can't win. The simplest is that fusion is a thermal power technology. Heat is produced, which heats a working fluid, which drives turbines, which makes power. ALL thermal power technologies are struggling now, particularly "external combustion" ones that transmit the heat into the working fluid through heat exchangers or boilers. Nuclear fission, coal, geothermal, solar thermal: all of them are having trouble competing. Only combustion turbines are doing ok (internal combustion, avoiding expensive heat transfer stages). Even if the fusion heat source were free, a fusion power plant would not be competitive. And the expensive non-nuclear part is mature technology in which not a lot of improvement can be expected.

But beyond that, there's good reason to think fusion would be more expensive than the other thermal power sources. Compare the volumetric thermal power density of ITER vs. a PWR primary reactor vessel: ITER is worse by about a factor of 400 (0.05 MW/m^3 vs. 20 MW/m^3). Smaller concepts, like ARC or Lockheed's are better (about 0.5 MW/m^3) but still far inferior to fission. A fusion reactor would be far larger, and far more complex, than a fission reactor. Fuel is not a large part of the cost of fission power, btw.

This failure to be competitive is not an accident. It follows from the square-cube law: a fusion reactor must transmit its output through the surface of the reactor vessel, while fission and coal can transfer heat to the working fluid through the surface of thin fuel rods or boiler tubes. This generic handicap, which is independent of anything to do with plasma physics, has been known for nearly four decades, if not longer.

There are other showstoppers (materials, reliability/maintainability, tritium breeding) but those two are enough.

The only hope fusion has, and it's a thin one, is advanced fuels that would allow direct conversion, skipping the thermal stage entirely. But all advanced fuel concepts will still produce a large fraction of energy in photons, which will strike surfaces and be thermalized. And they either depend on 3He, which is science fictional in its sources (moon mining?!), or H-11B, which is likely impossibly difficult at the plasma physics level (and 2000x less reactive than DT, best case.) And even advanced "aneutronic" fuels will leave the reactor too radioactive for hands-on maintenance, due to unavoidable side reactions. Given how large and complex a fusion reactor would be, that is also a showstopper.