Comment by Symmetry
7 years ago
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.
Yes, the arguments about the relative complexity of fusion reactor were well taken and they present serious challenges, unlike the power density issue.
But at the same time fission reactors are very expensive. The reactors themselves cost something on the order of $10 billion and the cost of the fuel and steel that go into the reactor is a very small fraction of that. And as far as I can tell the reason for that price is that the building is built to very exacting standards. It has to be built to those because even a reactor that's been shut down is putting out roughly 10% of the thermal power it was generating when it was active due to secondary decay, which fades away over time. But that means that the cooling system cannot ever fail, which means things get very expensive. And that's the same reason why instead of using the $.10 off the shelf screws you get off the shelf NASA uses special $100 aerospace grade screws.
The paper talks about safety as if it can be separated from price but in a world where we care about making unsafe things safe the two are inextricably linked. If a cooling failure results in a meltdown that will make the systems very expensive. If a fusion containment failure just stops output and causes excess wear on the inner lining, which has be replaced periodically anyways, then that's a different issue.
Which factor, over-engineering for safety or design complexity, will dominate, I don't know.
> unlike the power density issue.
I reject that assertion. The argument you tried to give against that power density argument (using magnet advances) just showed you didn't understand the point Lidsky was making.
> And as far as I can tell the reason for that price is that the building is built to very exacting standards.
Fusion reactors will also have to be built to very exacting standards -- not because of safety concerns, but because any malfunction in the radioactive, hands-off part of the plant will be economically disastrous. Lidsky goes into this point as well.