Comment by willis936
7 years ago
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.
Fission reactor cores and reactor vessels are made of fuel and steel. They are very simple compared to fusion reactors. Fission reactors can therefore be expected to be much cheaper than fusion reactors, even on a per-pound or per-volume basis. They can also be expected to be much more reliable.
(A fission reactor vessel will also last the lifetime of the power plant. A DT fusion reactor, not so much.)
(The average density of ITER is about the same as the average density of an unfueled PWR reactor vessel, btw.)
The analogy with rockets would be for those advocating air-breathing launchers. Make the launcher much more complex, just so one can save on LOX. Make the reactor much more complex, just so one can not pay for uranium. But LOX is cheap, and fuel is not a major cost driver for fission power.
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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.
Using plasma volume is clearly not appropriate, since the point of the comparison is to compare cost of the equipment, by comparing the volume and size of the equipment between the two approaches (and ignoring that fusion equipment is much more complex and sophisticated than that in a fission reactor.)
The fuel comparison is silly, since fuel is not a major cost driver for either approach. The misconception that fuel costs matter is one of the errors that leads people to think fusion is desirable.
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.
You're just passive-aggressively rejecting his argument on facile grounds, that this specific popular exposition didn't include the details you demand. It's sad you don't have enough self awareness to see what you're doing here.
If you want more detail of this sort of analysis, look at the Pfirsch & Schmitter paper from 1987 I linked to below. Lidsky also had a paper in the technical literature going into more details, although I don't have that reference. The basic argument as given by Lidsky in TTWF is not difficult to understand though, if you allow yourself to try to understand it.
In any case, the experience since then confirms what he was saying. Why don't you show me a fusion reactor concept with a power density approaching that of a fission reactor? Someone, somewhere must have been able to produce such a schematic if Lidsky's analysis was wrong, yes? Understand Lidsky's point is independent of limits due to plasma physics.
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