Comment by willis936
7 years ago
I’m still only a few pages in. I don’t have the chance to read it all right now, but I will revisit this tomorrow morning. Here are my early thoughts. Disclaimer: I am an advocate of fusion and am moving into the field (professional electrical engineering support of an academic project).
We need to walk before we can run. D+T fusion is viable. Hull maintenance is a solvable problem. Economic studies come after we can walk. Similarly, actually realizable designs that can reach Lawson performance necessary for p+B11 won’t come before designs that can reach D+T are made.
There are compelling narratives around funding, but ultimately it seems they are simultaneously overly optimistic about our technical advancement and overly jaded about future prospects.
It’s important to note that there is no difference in a reactor design capable of different fusion fuels, other than its Lawson performance. You can fuse D+T just the same as p+B11 if your device is capable of p+B11. There is not magical jump in performance we can make. So why skip over D+T if we need to go through it anyway? If D+T is found to truly not be economically viable (which sounds inherently flawed, since prices go down as technology becomes more advanced) then we could still pursue confinement devices in the hopes of reaching more advanced fuels.
So I’m a few pages further and I fear I will not be finishing this paper. I am surprised it was published and is still referenced. Where are the citations? Where is the analysis? You can’t just say “fusion reactors need to be ten times bigger than fission reactors”. It’s frankly embarrassing to MIT that they would allow egregious, unfounded claims to be published. Peer review exists to shred such unsupported statements.
This was a semi-popular article. You can find more technical analyses in the literature.
The specific points made in the article have stood the test of time. Lidsky said the power density of a DT fusion reactor would be at least an order of magnitude worse than that of a fission reactor. And if you look at existing reactors and concepts, this is true. Compare to a commercial PWR, in which (thermal power)/(volume of reactor vessel) is about 20 MW/m^3. An order of magnitude worse than that would be 2 MW/m^3. The (thermal power)/(reactor volume) for ITER is about 0.05 MW/m^3. For ARC and Lockheed's concepts, about 0.5 MW/m^3.
Your outrage should not be directed at this article (although such outrage is sadly understandable, if this article is telling you things you don't want to hear.) Instead, it should be directed at the fusion community as a whole, which has downplayed these critiques and glad-handed the issues raised while marching confidently into a dead end.
The paper made many good points but I found the power density argument by far the weakest. Yes a fusion reactor will necessarily weight more than a fission reactor of the same output and if we assume a constant price per pound across fission and fusion reactors this means it will be more expensive. But the reactor itself is only a small fraction of the weight of a fission plant. And of course we have every reason to think that the cost per pound of reactors across different reactor types will be entirely different.
I see this argument a lot in discussions around rockets where some people assume that if one rocket weighs less than another per pound delivered to space it must necessarily be cheaper when in fact the effort required to make it lighter almost always means it's more expensive. I think the rise of SpaceX has finally put an end to that argument, at least.
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> The (thermal power)/(reactor volume) for ITER is about 0.05 MW/m^3.
https://www.f4e.europa.eu/understandingfusion/iterdevice.asp...
500/800 = 0.625 (plasma volume, not reactor volume)
In an interview with the Omega Tau podcast a while back, it was mentioned by ITER personnel that the DEMO plant that would be the successor to ITER was probably going to be designed as a 2000 MW reactor. Since the plasma is in shape of a torus, that volume increases pretty quickly with the size of the confinement vessel.
Also, per unit mass of fuel, magnetic confinement fusion is still posited to be thousands of times more efficient than fission power.
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My outrage is from making scientific conclusions before the science has been done. A hard justification of power density limitations And it’s economic meaning is necessary before you can make a blanket statement like “D+T magnetic confinement will never be economically viable”. The price of electricity is always changing, and public opinion is shifting towards the idea that we have been grossly underpaying for our electricity.
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Not to mention D+T has 2500 times higher power density than p+B11.
> So why skip over D+T if we need to go through it anyway?
Because much of what you'd end up doing to "do" D+T will be worthless for H+11B. Tokamaks, for example, are useless for H+11B.
Tokamaks may not even be viable for steady state D+T. The bootstrap current effect still needs to be explored further. Electrostatic confinement and inertial confinement are the only known viable options for p+B11. Electrostatic has serious issues related to conduction losses, but perhaps if they were large enough scaling laws may make them viable. It’s not well explored. Inertial confinement is getting a lot of funding and shows serious promise. However, inertial confinement is not a steady state design and its pulsed nature makes it difficult to make a power plant out of. No such engineering cliffs exist for tokamaks and stellarators. The performance just needs to keep marching towards success. Then the discussion of economics can begin in earnest.
Electrostatic confinement is not known to be viable for H-11B. Indeed, Rider showed it probably isn't viable.
For inertial confinement, I understand compression would have to reach densities of as much as 10^6 g/cc for H-11B to be viable.