What about ITER's plans makes it sustainable?

[Terrified Virus]

Why would the ITER nuclear fusion reactor be sustainable? What exactly is in their plan which could lead to more energy being produced than is consumed - and what makes it different from all the other (failed) previous attempts at sustainable nuclear fusion?

 

[phyzguy]

Simply put - it's bigger. As you scale these things up, energy is generated in the volume of the plasma, and energy is lost through the surface area. Since the volume scales as the size cubed and the surface area scales as the size squared, as it gets larger the ratio of energy generated to energy lost increases.

 

[Terrified Virus]

That's actually amazingly interesting - the notion that energy is lost proportional to a square but energy gain is proportional to a cube. Could you provide me with information to read up? - I am deeply interested but don't know where to begin researching this.

So this would mean if we could, essentially, somehow build a huge Tokamak the size of a small nation, it would have a huge pay off in the distant future?

 

[mfb]

It is an example of the cube-square law. For the same reason, larger persons tend to sweat more and babies need warmer clothing - heat production occurs in the volume but heat loss at the surface.
And while it is not related to heat, it is also the reason all the huge fictional creatures/robots would not work and break down immediately: mass scales with the volume, but strength only with the cross section.

You cannot scale that up as much as you want - tokamaks need powerful magnetic fields, and producing large and strong magnetic fields is very expensive. There are also issues with turbulence and plasma instabilities that don't allow arbitrary scaling.

 

[phyzguy]

That's actually amazingly interesting - the notion that energy is lost proportional to a square but energy gain is proportional to a cube. Could you provide me with information to read up? - I am deeply interested but don't know where to begin researching this.

This is old, but the attached presentation is one I downloaded from the internet and shows some of the details. Slide 22 shows the size of ITER compared to past machines.

So this would mean if we could, essentially, somehow build a huge Tokamak the size of a small nation, it would have a huge pay off in the distant future?

Sure. But generating energy is all about cost. If we covered the Sahara with solar cells, we would have more energy than we can use. But the cost is huge.

 

[mheslep]

The scale up of magnetic confinement fusion, suggested by the square-cube law, is limited more directly than indicated by plasma instabilities and magnet costs.

Experience indicates a price limit of nuclear power has already been realized via existing nuclear fission power plants in the US and Europe, with their existing power densities. The nuclear fission power price suggests the far inferior power density offered by ITER is already too small (i.e. the volume is much too large for its power production) relevant to fission. A nuclear fission PWR has radius perhaps 2.5 M with thermal power 3 GW. ITER by contrast has radius 6-7 M, thermal power 500 MW.

Granted ITER is an experiment, but since the ITER size rationale is the square-cube law, there is no path to reduce the size, i.e. increase the power density of a practical prototype. Live by square-cube and die by it, as did the steam engine.

This observation was first made four decades ago by MIT nuclear physics professor Lidsky. The point was not to toss aside fusion research, but to refocus it away from straight magnetic confinement to some path that could actually be useful, that is, useful that to those beside magnetic confinement researchers and international consortia.

 

[phyzguy]

mheslep: I agree completely. I wasn't suggesting ITER is on a path to a viable energy source, I was just trying to answer the OP's question. As you said, if we want it to be viable, we need to increase the power density. There is some encouraging work along these lines, especially looking at spherical Tokamaks with high B fields. If high-temperature superconductors can increase the allowable magnetic fields, which seems possible, this helps the scaling a lot, since the economics improves at a high power of |B| (I think |B|^4 or |B|^5).

 

[mfb]

We are not running out of space on Earth yet. The costs are the only downside of a larger machine.

 

[mheslep]

Costs are the only relevant issue for a useful fusion power machine. I'm not interested in size other that it directly drives cost. The world already has the technology for safe power, already has affordable power. If fusion is not on a path to accomplish these as well, then its an interesting science project and should compete accordingly with other science projects for funding.

 

[mfb]

Okay, then how does it help to compare volume (or power density) to fission power plants? Is there a universal cost per volume that is the same for all power plants?
And even if there would: scaling up increases power more than linear with volume.

 

[mheslep]

... Is there a universal cost per volume that is the same for all power plants?

There's a power plant or factory minimum cost that goes up with mass and volume regardless of the content. There will be, for instance, a minimum cost per square foot and cubit foot of foundation whether or not a fusion of fission plant rests on top. Existing thermal power plants are cooled by water, fission and probably fusion, so they typically require water front property; there's minimum cost per sq ft for water front property. If fission plants are now at the limit of what's considered affordable, then a fusion plant ten times the size for the same power is likely to cost more than ten times as much a fission plant - a non starter.

And even if there would: scaling up increases power more than linear with volume.

Yes, but the same square cube law works against the structure that holds it all together. The strength of materials generally goes up with the square of diameter (N/m^2) while mass goes up with the cube (kg/m^3). The plumbing and wiring have similar problems. The structure mass snowballs (and thus cost) trying to support itself, i.e. the why-Godzilla-would-collapse law. A giant fusion D-T magnetic confinement might not reach the limits of tensile strength but I think it would exceed the limits of funding per Watt.

 

[mfb]

There will be, for instance, a minimum cost per square foot and cubit foot of foundation whether or not a fusion of fission plant rests on top.

The strength of the foundation depends on the type of power plant. The ratio of fusion volume to surface area gets better for larger power plants.

Most of the area of a power plant is not the reactor itself, but auxiliary infrastructure, which is completely different.

Fission and fusion power plants will operate at different cooling liquid temperatures, water consumption per electricity production will differ.

 

[mheslep]

The strength of the foundation depends on the type of power plant. The ratio of fusion volume to surface area gets better for larger power plants.

Most of the area of a power plant is not the reactor itself, but auxiliary infrastructure, which is completely different.

Fission and fusion power plants will operate at different cooling liquid temperatures, water consumption per electricity production will differ.

Hince the qualifier "minimum" I used for unit cost. If fusion uses water as the working fluid then it too will operate at similar temperatures as fission reactors. Gas cooling is possible at high temperatures, but it's been researched ad nauseum, tried, and nobody has made it work reliably and economically yet for fission.

 

[mfb]

Molten salts are discussed for fusion as far as I know. The details don't matter. A fusion power plant will look different from a fission power plant, just saying the same reaction chamber size will cost the same does not work, not even as order of magnitude estimate.

 

[mheslep]

Molten salts are discussed for fusion as far as I know.

To take heat off the reactor and deposit it in yet another heat transfer fluid that's suitable to spin a turbine through entropy changes.
Lidsky also commented on the difficulties for heat transfer in a fusion reactor, again problematic because the same square cube law that aids fusion power works against heat transfer.

Consider heat transfer in fission and fusion re-
actors. In today’s typical light-water reactor
(LWR), there is generated by fission in fuel
pins containing uranium. The heat is then
transferred to the coolant at the surfaces of a
relatively large number of small diameter pins.
This arrangement provides a larger surface area
to transfer heat than, say, a single large fuel cyl-
inder. Indeed, by decreasing the diameter of
the pins even further (but increasing their
number to keep the amount of uranium un-
changed), the total surface area available to
transfer heat would be further increased. Thus,
the actual heat-transfer rate through any given
square inch of surface on a fuel rod is not criti-
cal. Suffcient heat can always be removed
merely by increasing the total area.

This strategy does not work in a fusion reactor.
The heat-transfer surface is limited to the in-
side of the wall surrounding the plasma, and
the relatively small surface area of this wall
cannot be increased without further increasing
the size of the reactor. In fact, bigger reactors
need larger heat-transfer rates. Thus, the actual
heat-transfer rate per square inch must be ex-
tremely large and cannot simply be reduced by
a design change.

Suppose a fission reactor and a fusion reactor
were built with equivalent heat-transfer rates.
Knowing this, one can calculate two other
critical engineering factors: the flux of neu-
trons at the heat-transfer surface, and the
overall power density of the reactor. The neu-
tron flux should, of course, be as low as possi-
ble, because it damages the reactor structure
and makes it radioactive. And the power den-
sity should, as mentioned, be as high as possi-
ble, so that a reasonable amount of power will
be produced in a reactor of a given size.

On these counts, a comparison between cur-
rent LWR fission reactors and the somewhat
optimistic fusion designs produced by the
DOE studies yields a devastating critique of
fusion. For equal heat-transfer rates, the criti-
cal inner wall of the fusion reactor is subject to
ten times greater neutron flux than the fuel in
a fission reactor. Worse, the neutrons striking
the first wall of the fusion reactor are far more
energetic — and thus more damaging — than
those encountered by components of fission
reactors. ...

Modern combustion power plants run heat transfer plumbing inside the boiler, with plumbing diameter chosen to make the heat per unit area nearly whatever the choose. A torroidal fusion plant does have that ability.

 

[Hercuflea]

Why would the ITER nuclear fusion reactor be sustainable? What exactly is in their plan which could lead to more energy being produced than is consumed - and what makes it different from all the other (failed) previous attempts at sustainable nuclear fusion?

If you could answer this you would get a Nobel prize...

ITER is not a power plant. It is a physics experiment. The goal is to try and get 10x the amount of energy in heat out of the reactor through fusion reactions than is put in through various heating mechanisms. (However this is not the "wall-plug" efficiency).

DEMO would be the hypothetical power plant that would be built if ITER can demonstrate it is possible in theory.