
Why is nuclear fusion so hard? - panic
http://blog.sigfpe.com/2018/12/why-is-nuclear-fusion-so-hard.html
======
mannykannot
This is somewhat tangential to the specific issue here, but I was surprised by
this:

At the center of the Sun, fusion power is estimated by models to be about
276.5 watts/m3. Despite its intense temperature, the peak power generating
density of the core overall is similar to an active compost heap, and is lower
than the power density produced by the metabolism of an adult human.[1]

So practical fusion power requires producing much higher power densities than
are found in the Sun.

[1]
[https://en.wikipedia.org/wiki/Solar_core#Energy_conversion](https://en.wikipedia.org/wiki/Solar_core#Energy_conversion)

~~~
thomasahle
It continues like this though:

> The low power outputs occurring inside the fusion core of the Sun may also
> be surprising, considering the large power which might be predicted by a
> simple application of the Stefan–Boltzmann law for temperatures of 10 to 15
> million kelvins. However, layers of the Sun are radiating to outer layers
> only slightly lower in temperature, and it is this difference in radiation
> powers between layers which determines net power generation and transfer in
> the solar core.

So if a m^3 of sun core was moved to a powerplant it would generate a lot more
energy.

~~~
pfdietz
> So if a m^3 of sun core was moved to a powerplant it would generate a lot
> more energy.

Yes, for a tiny fraction of a second as it exploded. Its pressure would far
beyond what any material could withstand (and magnetic confinement does that
change that, as the outward pressure has to eventually be carried by the
magnet support structure).

~~~
pfdietz
Just to expand on that: the kinetic energy of the particles in 1 m^3 of the
material at the core of the Sun is equivalent to the yield of a 5 megaton
bomb. So when I say "exploded" I really mean it.

------
HocusLocus
QUOTE

"I took this fusion class when I was at Georgia Tech and I will never forget
it. We started studying and I go, "Man, this is really hard." Charged
particles don't want to get near each other. Bare nuclei are both charged,
positive charged, they want to avoid each other.

And my professor had a really great way of putting it. "It's like going to the
mini golf." He says, "You know how in mini golf you've got the volcano, and
the volcano's got the hole at the very top, and you've got to putt your ball
in a way that it goes all the way up the side of the volcano, and 'phwep!'
falls in the hole." He goes, "OK. That's like fusion. The ball is like a
nucleus, and the volcano is the scattering effect.

So any time you want to have a nucleus go to another nucleus, it scatters; it
rolls up the mountain and it rolls down the side, it rolls over here, over
there... and only when you just perfectly get it on the right angle does it go
in the volcano." Now, the problem with fusion, he goes, "You can't steer the
ball, you have to have enough temperature so that it can make it all the way
up the side of the volcano and fall in, and then you have to have enough balls
because you can't steer them there at the mini golf park", that's density,
"and then because they're flying all over the place, you've got to make sure
that there's a fence around the mini golf park so that they don't get away."
That's confinement.

He said, "Those are your three things: density, temperature, and confinement,
to make fusion happen."

I said, "Dude, that's really hard!" So, I came up with another analogy, "So, I
guess fission would be like the mini golf park except now the volcano was
flush, the hole was about this big around, the balls are going slow, and every
time the ball goes in the hole, two more balls come out."

He goes, "Yeah, that's pretty good."

ENDQUOTE

Watch the whole thing. It's good.

[https://www.youtube.com/watch?v=lG1YjDdI_c8](https://www.youtube.com/watch?v=lG1YjDdI_c8)

~~~
selimthegrim
What point in the video is this quote?

~~~
HocusLocus
[https://www.youtube.com/watch?v=lG1YjDdI_c8&t=71m26s](https://www.youtube.com/watch?v=lG1YjDdI_c8&t=71m26s)

------
lisper
> Some kinds of instability are slow enough that we can control them. For
> example bicycles are unstable, but many of us eventually learn to ride them.

Actually, bicycles are only unstable when they are moving slowly (or stopped),
and most people never learn how to stabilize one in this unstable regime. It
can be done, but it's very, very hard.

~~~
adrianmonk
Now, what would be great is if that analogy could be stretched further. It
would pretty wonderful if someone found a way to do fusion where, like a bike,
it's stable once it gets going.

~~~
lisper
Well, that is actually possible. That's what happens in stars and hydrogen
bombs. The problem, of course, is that "getting going" for fusion means
getting very big. The reason fusion energy is hard is precisely because to be
practical and safe we have to "ride slowly".

------
dukoid
If you understand German, trust me and listen to this podcast about
Wendelstein 7-X O:) :
[https://alternativlos.org/36/](https://alternativlos.org/36/)

~~~
sschueller
This is a really excellent podcast. Very informative.

------
aurizon
Here is a thought experiment( no cats harmed or wetted). Show a cat how the
toilet flushes, then try to flush the cat. You have two hands, the cat has 4
paws, with sharp claws as well as teeth. This will give you an idea....

Stars have gravity and mass- we do not, so we must try and compress the
plasma, heat it AND stops the paws from grabbing the exterior. In truth
magnetic confinement is a poor way to constrain a plasma that wants to leave
at high speed = quenches the reaction. There have been many avenues tried and
they are making incremental advances, year by year. They say we are 5-10
yearsaway? The ITER Tokamak is well advanced.
[https://www.iter.org/mach/Tokamak](https://www.iter.org/mach/Tokamak) New
approaches are being tried that might leap past the Tokamak.

~~~
mdpopescu
Controlled fusion has always been 5 to 10 years away, just as we've always
been at war with Eurasia.

~~~
BurnGpuBurn
I remember distinctly reading a popular science magazine article when I was
young (it had that great plasma picture with all the plasma arcs fizzing over
the floor of a giant lab) that stated that we could have fusion power plants
in 25 to 50 years. That was 30 years ago.

~~~
mdpopescu
It seems I was indeed wrong about the deadline but not about the "forever"
part, see [1] and [2].

[1] [https://www.newstatesman.com/sci-
tech/2014/11/forever-20-yea...](https://www.newstatesman.com/sci-
tech/2014/11/forever-20-years-away-will-we-ever-have-working-nuclear-fusion-
reactor)

[2] [http://blogs.discovermagazine.com/crux/2016/03/23/nuclear-
fu...](http://blogs.discovermagazine.com/crux/2016/03/23/nuclear-fusion-
reactor-research/)

------
sudhirj
Try making a compressed, spherical ball of water using your two hands.

------
finnh
“Confining a plasma is like trying to keep water in an inverted cup by blowing
on the water with fans.”

That is fantastic analogizing right there.

------
Animats
Plasma instability is just awful. See [1]. I thought he was going to go on
about some approach to active stabilization of plasmas. But no. There's been
work done on that, but it's been years. Anyone following that?

Then there's the first wall problem.[2] Fusion generates heat and neutrons.
Lots of neutrons, which break atoms apart. Finding something which will stand
up to that in an experimental machine has been tough. Finding something which
will stand up to that in a long term production environment is really tough.
In a fission reactor, you can use water to slow and stop the neutrons, and you
just get some tritium as a byproduct. A fusion reactor's first wall faces a
vacuum, so that's out.

[1]
[https://en.wikipedia.org/wiki/Plasma_stability](https://en.wikipedia.org/wiki/Plasma_stability)

[2] [https://en.wikipedia.org/wiki/Plasma-
facing_material](https://en.wikipedia.org/wiki/Plasma-facing_material)

~~~
cjslep
Yep, studied Nuclear Engineering at NC State, didn't stick around for the grad
program. My friend did, for plasma physics, and is now working at ITER. Plasma
confinement is still improving.

------
yummybear
This a totally ignorant question - can you add “something” to the plasma to
achieve the “jello like” analogy to water?

~~~
lisper
That's actually a very reasonable question. The answer is: no, but explaining
why is non-trivial. The short version is that at 10 million degrees, matter
behaves so differently than it does at room temperature that none of your
everyday intuitions apply.

------
Creatccount
Would zero gravity be helpful in order to make it easier to get fusion
working?

~~~
hwillis
The particles in a fusion reactor are moving at 10 km/s. They don't
particularly care about gravity.

For components of the reactor, gravity tends to be helpful. It's much harder
to cool superconductors or generate steam if your liquids aren't flowing along
the bottom. Turbines don't work nearly as well if their inlets are sputtering
like a coffeemaker.

~~~
pfdietz
They are moving a lot faster than 10 km/s.

------
lerno
This is why I have little confidence that Tokamaks and similar will have much
success.

Instead of trying to prevent the non-linear behaviour of plasma it should be
utilized.

Look at (grossly underfinanced) attempts like focus fusion, where there is no
attempt to confine the plasma, instead self-interacting nature of the plasma
is used to focus high temperature regions to a small region where higher
temperature fusion processes can occur.

Long time plasma confinement is a dead end.

~~~
kristianp
I'm a fan of IEC, what happened to the Polywell?

[https://en.m.wikipedia.org/wiki/Inertial_electrostatic_confi...](https://en.m.wikipedia.org/wiki/Inertial_electrostatic_confinement)

------
rkagerer
This is probably another ignorant question, but why can't we "aim" and shoot
protons at each other to make them smack together without requiring all that
heat and pressure?

~~~
abecedarius
I've wondered, aren't there any fusible elements that can be solids in vacuum?
Then shoot pellets at each other at a sizable fraction of c. Way better
density than a particle beam.

For instance, lithium-6 is a stable solid metal with the right nucleon
composition and binding energy to fuse into carbon-12. I have no idea if it
actually would to a useful degree.

~~~
Retra
Solid in a vacuum at what temperature?

~~~
abecedarius
Whatever's practical for your giant pea-shooter and reaction chamber. But
lithium is a soft metal at room temperature, so this part doesn't seem too
constraining.

~~~
Retra
If you want to sustain a fusion reaction for generating power through heat,
you're not going to do it at room temperature, and there're not many materials
that will remain solid at fusion temperatures.

~~~
abecedarius
I said "shoot pellets at each other at a sizable fraction of c". The point is
to hold together before the splat, so that by high density enough nuclei
actually hit each other. This is a bang-bang kind of thing, not a continuous
reaction.

~~~
Retra
We don't have any trouble creating fusion explosions. The whole trouble of
fusion is sustaining a reaction. Fusion bombs already exist, and what you're
describing isn't that different from an Ulam-Teller device.

~~~
abecedarius
Existing fusion bombs need a fission bomb for ignition. This lower-bounds both
the bang and the unpleasant byproducts. A smaller bang can be used much more
practically. For example, 20th-century cars were also powered by explosions.
Somehow they outcompeted the previous dominant technology of continuous
reactors, steam engines.

To dismiss my half-assed idea by pointing to another, I'd bring up something
more comparable, like inertial-confinement fusion.

------
dTal
Maybe we just haven't tried hard enough.

While researching a comment for another thread I discovered an interesting
fact: the US has now spent approximately as much money on fusion research
_ever_ (since 1955) as it did on the Manhattan project, _alone_. Perhaps we
shouldn't be so surprised that progress is so slow.

------
dangjc
Does fusion still have much of a role now that renewables are increasingly
competetive? We can pretty much already harness infinite energy for all our
needs from an already working fusion generator our planet happens to orbit
around.

~~~
ethbro
> now that renewables are increasingly competetive...

... with legacy energy sources.

When talking about fusion / nuclear vs renewables, think in terms of max
energy produced per site, not cost per unit.

Similar to total thrust vs specific impulse.

The largest respective power generation stations by source -- Three Gorges Dam
/ Itaipu Dam (Hydro, 22,500 MW capacity, ~100 TWh/yr), Kashiwazaki-Kariwa
(Nuclear, 7,965 MW capacity, 60 TWh/yr, currently suspended for earthquake-
proofing), Tengger Desert Solar Park (Solar, 1,547 MW capacity, ? TWh/yr),
Alta Wind Energy Center (Onshore Wind, 1,547 MW capacity, 2.68 TWh/yr, Gansu
not included due to utilization issues), Walney Wind Farm (Offshore Wind,
~1,000 MW capacity, >1.3 TWh/yr?).

And then realize that hydro & wind are both location-limited. And solar has a
large footprint: Tengger is 43km^2.

Nuclear (and eventually fusion) scales footprint much more slowly with
capacity. Kashiwazaki-Kariwa is 4.2km^2.

------
kjar
For most of my life fusion has been teased but not delivered. fission has been
pimped for decades yet cannot get private funding, or build reactors in a
decade, or provide solutions for the waste. Yet still the industries persist
with their propaganda while renewables beat them in cost and time to
operation. If I managed the funds I'd cut nuclear and redirect to wind and
solar immediately.

~~~
josefx
> If I managed the funds I'd cut nuclear and redirect to wind and solar
> immediately.

To make those a reasonable base you need to add enough storage to keep
everything working even when your current weather does not meet the energy
demand. I think setting up that storage is currently going to be the biggest
cost factor.

~~~
assblaster
When you factor in storage costs, how do solar and wind compare to
alternatives?

~~~
epistasis
Currently, solar that has 3-4 hours of output at 50% of peak primary solar
outbut, is about $35-$40/MWh in Colorado, Nevada, and I think even Indiana had
bids in that ballpark. This is between 1/5 to 1/2 the cost of nuclear,
depending on who you believe for nuclear's costs. Storing a MWh probably costs
around $80-$150 currently.

Why so little storage? It's not really needed at the scale of projects that
are being planned, and storage gets cheaper every year.

So let's say we could start building a nuclear plant and get it done in 10
years (which is highly speculative, but let's give nuclear the benefit of the
doubt). What are the costs going to be 10 years from now? Or more importantly,
what are costs going to be 30 years from now? Because the storage lasts 10-15
years, so you get to replace it with cheaper tech at that point, whereas with
nuclear you've locked in your costs for the next 60 years.

This is why no sane entity wants to build nuclear, unless they are
ideologically motivated. There are some nuclear startups that may make nuclear
cheaper, and less risky to build, but they are still startups without
functioning prototypes or manufacturing facilities. By the time they start
production, the utility will be ready to replace any batteries that get
deployed on the grid today, so one may as well start deploying wind/solar +
storage. And add more time of use rate plans so that energy is used
efficiently.

------
schwarze_pest
Wrong title. Fusion is easy, plasma physics isn't.

~~~
pfdietz
Fusion is likely to be a nonstarter even if the plasma physics issues are
completely solved. It has fundamental engineering and cost problems.

------
qrbLPHiKpiux
It's hard now because we don't know how to do it well. In the future, we'll
learn, then it will be easy. History tells us this.

------
robertsteinhaus
Mankind is currently in possession of a practical fusion technology.

It might be worthwhile to remember that Ivy-Mike fission-fusion technology
worked the very first time it was tried in 1952. Mike technology was the basis
of the first thermonuclear weapons in the US arsenal. Adapting Mike technology
to be pure hybrid DT-DD fusion opens up many new applications in economical
power generation.

In 60 years, no other fusion technology (Magnetic Confinement or Inertial
Confinement) has ever produced any net energy (more energy out of the fusion
reaction than it takes to get the fusion plasma to fusion conditions).

In 60 years, all existing MCF and ICF fusion systems have never worked (in the
sense that they have not produced more energy from fusion than it took to get
the fusion plasma to fusion conditions).

Mike fission-fusion technology worked the first time it was tried and produced
huge amounts of net energy (and has never failed).

Rather than placing our faith in scaling laws while we build ever larger and
more expensive Magnetic Confinement fusion experiments (tokamaks and
stellarators) while trying to achieve break even energy generation - why not
go back to the field and adapt technology that has never failed to finally
find success in fusion?

~~~
pfdietz
They looked at this. It's grossly uneconomical as a power source.

"In a 1975 review of the various Plowshares efforts, the Gulf University
Research Consortium (GURC) considered the economics of the PACER concept. They
demonstrated that the cost of the nuclear explosives would be the equivalent
of fuelling a conventional light-water reactor with uranium fuel at a price of
$328 per pound."

