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Nick McGreivy's avatar

Nice article. I especially liked the history of fusion research. A couple points:

(1) As you mentioned, the energy density (energy per unit mass) of fusion is four times higher than nuclear fission. However, the power density (power per unit volume) of fusion is 1-2 orders of magnitude lower than fission. In other words, fusion is great if you want to build a thermonuclear warhead, but if you want to build a power plant, fusion's low power density is a significant drawback compared to fission. To explain in more detail: LWRs have a power density on the order of 100 MW/m^3 in the core, while D-T fusion has a (maximum) power density of 0.1 MW/m^3 times the pressure (in atmospheres) squared. ITER is expected to have a pressure of 2.6 atm (0.5 MW/m^3), while tokamak power plants would likely have a pressure of about 7 atm (5 MW/m^3). So, a D-T fusion reactor is expected to have somewhere in the ballpark of 20x lower power density than a LWR.

(2) The power density (at constant pressure) of deuterium-tritium (D-T) fusion is 2 orders of magnitude higher than the second-highest power density reaction, deuterium-helium 3 (D-3He). Other fuels (such as D-D or p-11B) are roughly another order of magnitude lower still than D-3He. Anyone attempting to build a fusion reactor with "alternative" (non-D-T) fuels will either need to create extraordinarily high pressures, build an extraordinarily large reactor, or produce extraordinarily little power. In my opinion, it's almost impossible to imagine reactions with such low power densities being used for power plants.

(3) The "success" of a fusion reactor is often discussed, as it mostly is here, in terms of achieving a triple product greater than the Lawson criterion (8 atm*s for DT). However, there are two other essential things a fusion reactor must do. First, it must be able to create tritium fuel inside the reactor. Second, it must not destroy itself. These are extremely difficult engineering problems. Part of what makes them so challenging is that, while possible solutions can be analyzed theoretically before a reactor is built, those solutions can only be tested inside a power-producing reactor. Thus, progress on these difficult problems must be made in series, not in parallel. Only once you've spent a few billion dollars on a power-producing fusion reactor can you actually test whether it "works". At that point, you might not produce as much tritium fuel as you had calculated that you would. Or, your reactor could destroy itself -- and it is quite likely to do so, either via neutron damage, heat flux damage, or in a disruption. In my opinion -- and as far as I know this is a widespread view among academics unaffiliated with fusion startups -- it is certainly possible that one of the fusion startups will "successfully build a" [power-producing] machine, but it is very unlikely that any of them will successfully build a fusion power plant. The opposing view -- and to the best of my knowledge, this is how Commonwealth Fusion Systems thinks about these challenges -- is that the public excitement created by the achievement of high-Q magnetic confinement fusion will lead to so much investment in fusion that we'll be able to afford to solve those difficult engineering challenges later on. Kicking the can down the road, if you will. In any case, I think a discussion of the engineering challenges associated with fusion power plants is essential.

(4) Using the development of the atomic bomb as an analogy: in 1940, the Frisch-Peierls memorandum performed some simple calculations suggesting that an atomic bomb was theoretically possible, and would require a few kilograms of U-235. We now have enough knowledge, at least for tokamaks (the reactor concept with by far the best performance), to calculate whether a fusion reactor can scale to a power plant. This is done in "Designing a tokamak fusion reactor—How does plasma physics fit in?" (J.P. Friedberg et al., 2015), which writes "Unfortunately, a tokamak reactor designed on the basis of standard engineering and nuclear physics constraints does not scale to a reactor." In my opinion, this is a fundamental difference between the development of nuclear fusion and nuclear fission or the atomic bomb: with fusion, our best calculations suggest that it will not work. Or more precisely, tokamaks will not work barring significant improvements in certain engineering constraints.

I think you've done a nice job of laying out the bull and bear cases for fusion. However, I think the bear case for fusion could be made much stronger with some relatively simple technical arguments. (I've thought about writing some of that up myself, now that I'm done with my PhD and have some time off.)

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Chris A.'s avatar

Thank you for this outstanding article. I teach fusion physics on a university level, and this is the best historical overview I have ever read. Keep up the great work!

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