4 ms·

Solar was 2 orders of magnitude out of the running around 1975 -- and now it's at grid parity. So maybe fusion is 45 years away? :P You're right that the cost

Solar was 2 orders of magnitude out of the running around 1975 -- and now it's at grid parity. So maybe fusion is 45 years away? :P You're right that the cost is going to have to come down, and the power density up, before fusion can compete. High-field superconductors are one obvious route, and using plasma configurations that make better use of the magnetic fields are another. Either one of those approaches could deliver about two orders of magnitude in power density -- and they could be combined. >The only reason we're working on it is because we had been. I think there's been an element of that in the way the federal program has been run, but claiming it's the only reason to pursue fusion is not defensible.

pfdietz 6y ago
That approach doesn't work, because reactor power density becomes limited by wall loading (neutron and/or thermal), regardless of how good the plasma physics is. It's the square cube law. At a given wall loading limit the volumetric power density is inversely proportional to the linear dimensions. And a DT fusion reactor must be meters across, due to the fixed cross section of the neutrons with wall materials. In contrast, fission fuel rods are 1 cm in diameter and are closely spaced, so the available surface area for heat transfer is much higher (neutrons in fission are also much less of a problem, since they carry a much smaller fraction of the energy output and are of much lower energy, on average.) (I think the only hope for DT fusion is something like LINUS, where the entire first wall is thick flowing liquid lithium.) > I think there's been an element of that in the way the federal program has been run, but claiming it's the only reason to pursue fusion is not defensible. What is the other reason? I cannot find any other plausible justification. This is particularly true now that fission is out of the running. Fusion used to be justified by "it won't be too much more expensive than fission, but it's safer and won't run out of uranium." That argument is now pointless.
- lambdatronics 6y ago
  >the entire first wall is thick flowing liquid lithium Yeah, I agree on the liquid walls. I think the optimum might be to have a thin Li first surface flow that is relatively cool, then a shell of SiC, and behind it a PbLi breeding blanket that can be at higher temperature. There's a trade-off because the first-surface can't be made too hot or the evaporation will pollute the plasma, but getting high thermodynamic efficiency means using higher coolant temperature. The thickness of the liquid first-surface plays into the lifetime of the SiC shell -- more shielding in front of it means longer life (so lower maintenance costs & higher reactor up-time, and less radwaste), but lower thermal conversion efficiency on average. There are problems of course -- corrosion by Li, splashing of droplets into the plasma, MHD drag & pump power requirements, incompatibility of lithium with many forms of sensors & actuators for plasma control.... (OTOH, most sensors and actuators can't tolerate radiation anyway, so fancy control techniques just don't stand a chance in a reactor anyway. We need boring, stable plasma configurations that just sit there and work.) >What is the other reason? Some people think they can make a fortune :D

pfdietz 6y ago

That approach doesn't work, because reactor power density becomes limited by wall loading (neutron and/or thermal), regardless of how good the plasma physics is. It's the square cube law. At a given wall loading limit the volumetric power density is inversely proportional to the linear dimensions. And a DT fusion reactor must be meters across, due to the fixed cross section of the neutrons with wall materials. In contrast, fission fuel rods are 1 cm in diameter and are closely spaced, so the available surface area for heat transfer is much higher (neutrons in fission are also much less of a problem, since they carry a much smaller fraction of the energy output and are of much lower energy, on average.) (I think the only hope for DT fusion is something like LINUS, where the entire first wall is thick flowing liquid lithium.) > I think there's been an element of that in the way the federal program has been run, but claiming it's the only reason to pursue fusion is not defensible. What is the other reason? I cannot find any other plausible justification. This is particularly true now that fission is out of the running. Fusion used to be justified by "it won't be too much more expensive than fission, but it's safer and won't run out of uranium." That argument is now pointless.

lambdatronics 6y ago
>the entire first wall is thick flowing liquid lithium Yeah, I agree on the liquid walls. I think the optimum might be to have a thin Li first surface flow that is relatively cool, then a shell of SiC, and behind it a PbLi breeding blanket that can be at higher temperature. There's a trade-off because the first-surface can't be made too hot or the evaporation will pollute the plasma, but getting high thermodynamic efficiency means using higher coolant temperature. The thickness of the liquid first-surface plays into the lifetime of the SiC shell -- more shielding in front of it means longer life (so lower maintenance costs & higher reactor up-time, and less radwaste), but lower thermal conversion efficiency on average. There are problems of course -- corrosion by Li, splashing of droplets into the plasma, MHD drag & pump power requirements, incompatibility of lithium with many forms of sensors & actuators for plasma control.... (OTOH, most sensors and actuators can't tolerate radiation anyway, so fancy control techniques just don't stand a chance in a reactor anyway. We need boring, stable plasma configurations that just sit there and work.) >What is the other reason? Some people think they can make a fortune :D