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The Trouble with Fusion (1983) [pdf]

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Re: The Trouble with Fusion (1983) [pdf]

#31

Post by pfdietz »

pietjepuk88 wrote: post #26
pfdietz wrote: post #7

Earlier quoted context omitted.

Lidsky's arguments against fusion apply to the ARC design. Its overall volumetric power density is 0.5 MW/m^3, 40 times less than a PWR reactor vessel.

What defines the volume to get to that number? From what I can tell from the ARC specs the plasma volume is 141 m3, and the expected power output is 200-300 MWe, which makes it 1.5-2 MWe/m3. Do you have some numbers on a PWR reactor vessel? I was trying to look up some details on the APR-1400, but could not find any.

The volume is the volume of the reactor, including blanket, magnets, and the structure needed to support the JxB forces on the magnets, not the volume of the plasma. See table 11, page 30, in the ARC paper:

https://arxiv.org/pdf/1409.3540.pdf

For PWRs:

https://ocw.mit.edu/courses/nuclear-engineering/22-06-engine...

(take the dimensions given for the primary reactor vessel, compute the volume as a cylinder with spherical end caps, and divide that into 3400 MW(th). The result is slightly below 20MW/m^3. Note also the power density of the core itself is given as greater than 100 MW/m^3.)

Re: The Trouble with Fusion (1983) [pdf]

#32

Post by pfdietz »

Symmetry wrote: post #24
pfdietz wrote: post #2

This old critique has aged fairly well. It was too optimistic about advanced fission, and the suggestion to move to advanced fuels for fusion was mostly shot down by Lidsky's student, Todd Rider. https://pdfs.semanticscholar.org/fce7/a35629488ec030d983025a... https://dspace.mit.edu/handle/1721.1/11412 A similar critique was being made around the same time by Pfirsch and Schmitter in Europe. https://pure.mpg.de/rest/i…

There are a few parts that have been overtaken by advances. Improvements in superconductors have driven the minimum size for a fusion reactor far down, for instance.

Lidsky's argument is entirely unaffected by that advance. The power of his argument is that one can just ignore the plasma physics.

Re: The Trouble with Fusion (1983) [pdf]

#33

Post by pfdietz »

moogly wrote: post #29
pfdietz wrote: post #18

Earlier quoted context omitted.

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 r…

> 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 o…

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.

Re: The Trouble with Fusion (1983) [pdf]

#34

Post by Fronzie »

Didn't the feasibility of fusion increase with advances in magnets. I thought the much stronger magnets being available today allowed for much smaller (volume per watt generated) systems, smaller even than the Iter design.

Re: The Trouble with Fusion (1983) [pdf]

#35

Post by carapace »

mjevans wrote: post #4

The concerns and criticism make too much sense; as much as I want to believe a comic/movie density power solution (like arc reactors) can work... I think the criticisms would have gone over better if something more than the implicit: 'look for better ideas' proposal had been included as a path. It's been over 30 years; is there anything else as an idea in the field of power generation? (Preferably something we could…

"Fusion in a magnetically-shielded-grid inertial electrostatic confinement device"

> Theory for a gridded inertial electrostatic confinement (IEC) fusion system is presented that shows a net energy gain is possible if the grid is magnetically shielded from ion impact. A simplified grid geometry is studied, consisting of two negatively-biased coaxial current-carrying rings, oriented such that their opposing magnetic fields produce a spindle cusp. Our analysis indicates that better than break-even performance is possible even in a deuterium-deuterium system at bench-top scales. The proposed device has the unusual property that it can avoid both the cusp losses of traditional magnetic fusion systems and the grid losses of traditional IEC configurations.

https://arxiv.org/abs/1510.01788

Re: The Trouble with Fusion (1983) [pdf]

#36

Post by Symmetry »

pfdietz wrote: post #27
Symmetry wrote: post #23

Earlier quoted context omitted.

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…

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 d…

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.

Re: The Trouble with Fusion (1983) [pdf]

#37

Post by Symmetry »

pfdietz wrote: post #32
Symmetry wrote: post #24

Earlier quoted context omitted.

There are a few parts that have been overtaken by advances. Improvements in superconductors have driven the minimum size for a fusion reactor far down, for instance.

Lidsky's argument is entirely unaffected by that advance. The power of his argument is that one can just ignore the plasma physics.

Plasma physics tells us the relationship between the minimum workable size and the containment field strength. Change the field strength and the size changes.

Re: The Trouble with Fusion (1983) [pdf]

#38

Post by pfdietz »

Symmetry wrote: post #37
pfdietz wrote: post #32

Earlier quoted context omitted.

Lidsky's argument is entirely unaffected by that advance. The power of his argument is that one can just ignore the plasma physics.

Plasma physics tells us the relationship between the minimum workable size and the containment field strength. Change the field strength and the size changes.

Yes, and Lidsky's argument works even if one assumes arbitrarily good plasma performance. He was not basing his argument on magnetic field strength or beta limits, or even any particular device geometry. Assume 100T magnetic fields and beta=1; his argument still applies.

What may be confusing you is that very low power density designs, like ITER, may be even worse than Lidsky's bound. So they could be improved somewhat by better magnets or plasma physics tricks. But once Lidsky's bound is reached, further improvements of that sort are no help.

Re: The Trouble with Fusion (1983) [pdf]

#39

Post by pfdietz »

Symmetry wrote: post #36
pfdietz wrote: post #27

Earlier quoted context omitted.

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 d…

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 pric…

> 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.

Re: The Trouble with Fusion (1983) [pdf]

#40

Post by DennisP »

ncmncm wrote: post #15

There has never been any serious expectation of getting usable power from fusion. All the reactor designs worked on in mainstream research would destroy themselves in a short time by high-energy neutron flux. Fusion research is, instead, a jobs program for high-neutron flux physicists, to provide a pool to draw on for weapons work. There are interesting commercial projects for designs that do not suffer from high neu…

Some tokamaks do. MIT's ARC, for example, uses jointed superconducting tapes that let the reactor be opened up to replace the inner wall. That's done annually, and the wall is 3D-printed. Surrounding the inner wall is molten FLiBe salt, which acts as coolant, neutron absorber, and breeding blanket. Commonwealth Fusion is attempting to commercialize the design.
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