
The Trouble with Fusion (1983) [pdf] - akakievich
http://orcutt.net/weblog/wp-content/uploads/2015/08/The-Trouble-With-Fusion_MIT_Tech_Review_1983.pdf
======
willis936
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.

~~~
willis936
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.

~~~
pfdietz
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.

~~~
Symmetry
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.

~~~
pfdietz
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.

~~~
Symmetry
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.

~~~
pfdietz
> 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.

------
pfdietz
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://pdfs.semanticscholar.org/fce7/a35629488ec030d983025a44dc00d38d0f68.pdf)

[https://dspace.mit.edu/handle/1721.1/11412](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/items/item_2131865/component/file_2...](https://pure.mpg.de/rest/items/item_2131865/component/file_2131864/content)

~~~
Symmetry
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.

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

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

~~~
pfdietz
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.

------
maxharris
Commonwealth Fusion Systems was formed out of the work presented here:
[https://www.youtube.com/watch?v=KkpqA8yG9T4](https://www.youtube.com/watch?v=KkpqA8yG9T4)

A lot has changed since 1983. The superconducting materials that CFS depends
on weren't even discovered until the late 80s, and it took a very long time
for that to become practical. Fast-forward to today and you can buy this stuff
on Alibaba!

~~~
pfdietz
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.

~~~
pietjepuk88
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.

~~~
pfdietz
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](https://arxiv.org/pdf/1409.3540.pdf)

For PWRs:

[https://ocw.mit.edu/courses/nuclear-
engineering/22-06-engine...](https://ocw.mit.edu/courses/nuclear-
engineering/22-06-engineering-of-nuclear-systems-fall-2010/lectures-and-
readings/MIT22_06F10_lec06a.pdf)

(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.)

------
bsder
We know what the issue is--fusion never got a useful amount of funding.

Now, it is entirely possible that even given an enormous amount of funding--
fusion might still not work. However, engineers with lots of money are
remarkably clever and effective beasts (see: radio, semiconductors, plastics,
steel).

Fusion has been funded at "Fusion Never" levels for almost 40 years while we
subsidize every other significant energy source to the tune of billions or
trillions of dollars every single year. (For example, how much money got
poured into the technology that became "fracking"?)

~~~
pfdietz
No, that's not what the issue is. That's an effect, not a cause. The cause is
that fusion turned out to be less promising than had been thought, and that
led to budgets being tight. Lidsky's devastating critique was part of that
(tokamaks not being as good as early hopes implied was another.)

~~~
bsder
> The cause is that fusion turned out to be less promising than had been
> thought

So did a whole lot of chemical rocketry. Instead of whining about it--we spent
a lot of money on engineering, we made the Saturn V, and we went to the moon
anyway.

The issue is that fusion only has one end point--providing energy. Researching
chemical rocketry made better weapons--so we funded the snot out of it. The
DOE spent billions on the Unconventional Gas Research Programs and lined a lot
of pockets.

Funding isn't guaranteed to make progress, but lack of funding practically
guarantees lack of progress.

~~~
pfdietz
Not a good analogy. Chemical rockets are the only real way to get to space. So
if you make them better, even incrementally, you have a win. It also helped
that launchers were very far away from fundamental economic limits on their
performance. Expendable launchers, unlike power plants, are expended.

But fusion is competing against a plethora of other approaches to production
of energy that actually work, and are being used, and are arguably superior.

Fusion also uses components that are mature due to their use in these other
approaches. DT fusion, which Lidsky is addressing, will produce its energy as
heat. This heat has to be turned into power using turbines and generators, a
mature technology. And it's a mature technology that's a major part of the
cost of coal and nuclear power plants, and is a big reason why those power
plants are no longer competitive.

(This echoes an argument from the mid 20th century, when it was pointed out
that fission power would, at best, be only slightly less expensive than power
from coal, due to all the common elements the power plants shared. And it
turned out nuclear fission was more expensive than that, as one could not do
the nuclear parts too cheaply. DT fusion reactors promise to be much larger
and more complex than fission reactors, for the fundamental reasons Lidsky and
others gave, so one can reasonably expect their economics to fail even more.)

(This is also why the focus on thorium and SMRs to try to keep fission alive
is probably hopeless.)

("Those who cannot remember the past are condemned to repeat it")

The complaint about funding is a red herring. It has the presumption that if
funding had been available, fusion had a real chance of succeeding. But as
Lidsky points out, it didn't have a real chance of succeeding. Even if the
program had produced a reactor, no one would have wanted it.

~~~
bsder
> This heat has to be turned into power using turbines and generators, a
> mature technology. And it's a mature technology that's a major part of the
> cost of coal and nuclear power plants, and is a big reason why those power
> plants are no longer competitive.

Erm, natural gas uses turbines and generators and nobody seems to be whining
about that. It isn't the turbine driving the cost in coal (nuclear is a
different story).

First, we _still_ can't engineer a superconductor. We have barely doubled
magnetic field strength since 1970ish. And the big advance of
superconductivity in graphite with slight offsets demonstrates just how little
we know. Superconductors would have had a massive improvement in basic science
with funding (this was one of the huge losses in not funding the
Superconducting Supercollider).

The FFT (fast fourier transform) was effectively useless in the 1970s and
1980s--until Moore's Law made it not so useless. Similarly, computational
dynamics made huge advances since the 1980s--to the point where non-simple
toroids are now the standard.

Knowledge advances in a "front". If you throw money at a point (especially a
fundamental one), it drags related knowledge forward as well. Look at steel,
for example. Steel has been considered "mature and well-understood" (hah!)
practically since 1910--but there was so much money being thrown at it that it
continuously advanced for almost a century. Once steel moved forward,
architecture and construction moved forward. Then we got new applications like
cars. Then we got new tooling like heavy presses. Then we could use more
exotic materials like titanium. I can go on and on.

~~~
pfdietz
Natural gas uses combustion turbines, not steam turbines (except as a
bottoming cycle in combined cycle plants, but that produces only 1/3 of the
output of the plant).

What combustion turbines allow you to do is avoid heat exchangers, and also
operate at a temperature much higher than a steam turbine because no solid
material needs to be at the temperature of the working fluid. A simple cycle
gas turbine (without regeneration) has no heat exchangers. Heat exchangers are
expensive; transfer of heat across a solid/fluid interface is not as fast as
we'd like it to be.

------
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.

------
theothermkn
I'm a layperson like everyone else here, so I'm just going to rely on the fact
that fusion research is ongoing, well-supported by government and private
sector investment, and is an active area of research for hundreds or thousands
of intelligent and sane institutionally supported academics, as reasonable and
practical justifications for my belief that fusion is not quite as problematic
as laid out in Lidsky's dated and superseded work.

However, for a sketch of how to address the talking points of the more
strident objectors one encounters in the wild, one can perhaps turn to
[https://fire.pppl.gov/fusion_critic_response_stacey.pdf](https://fire.pppl.gov/fusion_critic_response_stacey.pdf)
for some ideas. Cheers!

~~~
DennisP
In fact, section III of that document specifically rebuts _The Trouble with
Fusion_ , based on the state of knowledge 16 years later.

~~~
pfdietz
This is all you really need to read from Stacey:

"Based on our present understanding, D-T tokamak fusion reactors project a
cost-of-electricity that is about 50% larger than the projected cost-of-
electricity from advanced light-water reactors in the middle of the next
century."

We all know what happened to the projected cost of fission reactors -- the
projections turned out to be hopelessly optimistic, because of complexity and
loss of experience. Fusion would face these problems in even worse form
(indeed, ITER's cost ballooned 4x or more past the initial projections.)

The experience with fission has enabled us to calibrate the optimism bias in
these projections, with damning results.

Simply being competitive with fission is no longer good enough for fusion to
succeed. It has to be significantly better than fission.

------
ncmncm
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 neutron flux, such as those pursuing pB reactions. I read of another
where neutrons are emitted in a place some distance from where the expensive
machine parts are.

You can tell if a fusion process is serious by whether they have an answer to
the neutron problem. Tokamak doesn't.

~~~
credit_guy
High neutron flux is useful not only for weapons but for civilian purposes
too, for example for "burning" nuclear waste, or for fusion-fission hybrid
reactors. I wonder why the Department of Energy does not invest more in this
area.

~~~
pfdietz
Fusion-fission hybrids combine the worst features of both. There is no user
"pull" for the concept. If you want power with fission, just build a fission
reactor; that's going to be simpler, cheaper, and altogether more sensible. If
you want to dispose of waste, just seal it in dry casks and wait a century or
three before deciding what to do with it. That will also be much simpler and
(due to nonzero interest rates) cheaper.

~~~
credit_guy
> Fusion-fission hybrids combine the worst features of both.

Maybe, but maybe not.

Worst features for fusion reactors: 1. they don't exit now and they won't
exist for the next 50 years; 2. they produce lots of neutrons, which make the
surroundings radioactive

Worst features for fission reactors: 3. they can go Chernobyl, 4. they produce
long-living radioactive waste, 5. they are horribly expensive 6. proliferation
concerns

How do these things look for a fusion-fission hybrid:

1\. fusion reactors don't exist. Well, they do exist but they are well below
the breakeven point. For a hybrid, the fusion part has (a very) negative
energy balance, but it's more than made up for by the fission part, so being
above breakeven is not a concern. The technology to manufacture the fusion
part of a hybrid exists today (and has existed for decades)

2\. fusion reactors produce lots of neutrons. For a hybrid, this is actually
the point of the fusion half

3\. fission reactors can go Chernobyl. This is so because the current fission
reactors are powered by a chain reaction. This chain reaction threads the very
fine line between subcritical and supercritical, in other words a classical
fission reactor sits in a very narrow region between a bomb and a fizzle. The
fission reactor in a hybrid gets its neutrons from its fusion partner, not via
a chain reaction. The beauty of not having a chain reaction is that you can't
have a supercritical chain reaction, or a Chernobyl event

4\. fission reactors produce long-lived nuclear waste. I agree with you that
this is not the big deal that's made up to be by environmental groups, but the
fact that you can burn it via a fusion-fission hybrid is a nice bonus point

5\. fission reactors are expensive. this is fundamentally a consequence of 3,
that they present the danger of going boom. And as long as the fission
reactors get their energy from a chain reaction, this danger exists. If you
have a design that cannot go supercritical because it does not rely on a chain
reaction, this is going to be inherently passively safe.

6\. proliferation concerns. Here I simply have no idea how fussion-fission
hybrids compare with classical fission reactors. That's why I mentioned the
Department of Energy. If they develop and run these new reactors, then
proliferation concerns become moot.

Besides all these points, the fusion-fission hybrids have another advantage:
they can burn U-238 [1], which makes up 99% of the uranium on Earth. This
means not only you have more fuel available, but you don't have to go through
the stupendously expensive process of enrichment. Or it can burn Thorium-232,
which is 3 times more abundant than uranium. In other words, not only the
construction costs would be much lower, but the operation costs too.

Oh, and here's another advantage. Because classical fission reactors are based
on a chain reaction that has to be very narrowly confined between
supercritical and subcritical, at any given point only a very tiny fraction of
the fuel is burning. Nuclear advocates don't like to dwell on that, but they
like to point to the flip side of this coin, that the fuel lasts for a very
long time (years). However, if you could burn the fuel faster, you can get the
same power from a smaller reactor. We could be talking a factor of 100. Since
construction costs don't scale linearly with size, a reactor that's 100 times
smaller could easily be 1000 or 10000 times cheaper. And we could end up being
able to send gigawatt-size reactors to Mars, rather than the kilowatt-size
currently envisioned by NASA [2]

[1]
[https://en.wikipedia.org/wiki/Nuclear_fusion%E2%80%93fission...](https://en.wikipedia.org/wiki/Nuclear_fusion%E2%80%93fission_hybrid)

[2]
[https://en.wikipedia.org/wiki/Kilopower](https://en.wikipedia.org/wiki/Kilopower)

------
mjevans
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 also use in space)

~~~
carapace
"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](https://arxiv.org/abs/1510.01788)

