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Comment by akeck | Hacker News Reader

Probably just "plasma breakeven" Sabine talks about the issue.

[1] https://backreaction.blogspot.com/2021/10/how-close-is-nucle...

Sabine actually is completely misleading and misconstrues a bunch of facts.

None of these plants are even attempting to have real energy breakeven and spend a ton of energy supplying experiments and unrelated support equipment. They don't even have a method of capturing energy as that's not the point as it would make it harder to test the physics. Additionally these plants have high amounts of "startup energy consumption" that is also factored in to the energy usage but would be amortized out over a long run. Trying to use the absolute power consumption of the experiment as if that's where the state of the art is at for true energy break even is completely wrong.

Plasma breakeven is all anyone is really working on. Once you have plasma breakeven you have a self-sustaining heater basically, which then can be used to create energy. The point of an "ignited plasma" is that it's self-sustaining and just pumps out heat, even if most of the energy is used to keep the reaction going.

I think her meaning is pretty clear and correct. As much as plasma breakeven may be the entire goal of ITER it's absolutely setting them up for a badly missed public expectation. The day they declare net positive output, the world will ask when we can start building infrastructure and the answer will be "30 more years" and then they'll get their funding yanked forever.

>As much as plasma breakeven may be the entire goal of ITER

Who gave you that impression? They were lying. The goal of ITER has always been to study burning plasmas and experiment with solutions to problems that a reactor-grade MCF machine faces.

ITER isn't even possible to create an economic nuclear reactor out of because it's too big. The sheer size of a ITER-sized reactor doesn't get us to economical reactors. ITER is a science experiment, not a commercial reactor design. High-field strength high temperature superconductor based allows much smaller sizes than ITER, but ITER was designed with the technology that was available in the late 1990s.

I think your statement "Once you have plasma breakeven you have a self-sustaining heater basically" is false. According to Wikipedia [1] - if I interpret it correctly - the fusion energy gain factor from plasma must be 5 (!) to have a self-sustaining heater:

"Most fusion reactions release at least some of their energy in a form that cannot be captured within the plasma, so a system at Q = 1 will cool without external heating. With typical fuels, self-heating in fusion reactors is not expected to match the external sources until at least Q = 5"

[1] https://en.wikipedia.org/wiki/Fusion_energy_gain_factor

Fusion begets fusion. ITER plans to have high-intensity, relatively short Q=10 shots. If the plasma heats itself then it doesn't need much heating. This sudden focus on Q is clearly the result of one vocal non-expert not understanding the field and everyone listening to them like they have something valuable to teach.

I oversimplified in that statement, you need more than a factor of 1 because of heat losses to the environment yes. However 5 is not much different than 1. We've gone from 0.0001 only a few years ago to close to 1 now.

And btw, you really want more than 5, 10 or 20 ideally, but again, that's not too hard as compared to how far we've come and new reactors will be beyond that soon.

> Plasma breakeven is all anyone is really working on. Once you have plasma breakeven you have a self-sustaining heater basically, which then can be used to create energy. The point of an "ignited plasma" is that it's self-sustaining and just pumps out heat, even if most of the energy is used to keep the reaction going.

This is dead wrong. First of all, the experiment described here is ICF, in which you have to constantly re-heat new pellets of fuel. Even for MCF, you have to spend inordinate amounts of energy just containing the million kelvins plasma with few kelvin superconducting magnets, and to constantly deliver new D+T into the plasma.

If containment fails at any time for any amount of time, your reactor is instantly obliterated.

Not to mention, your source of heat only heats up by about half of the energy - the other half is radiated away as hard to capture neutrons, which are almost entirely a waste product.

I have no idea why you think that ignited plasma is enough to maintain an energy-producing reactor.

Edit: million kelvins should have been billion kelvins...

> the other half is radiated away as hard to capture neutrons, which are almost entirely a waste product

I thought the neutrons were supposed to take away the heat, to be absorbed in layers of water?

Reading more about this, it seems that one of the ideas is indeed to capture the neutrons in a liquid lithium blanket, that would then produce both heat and tritium, and using that heat, that is outside the magnetic confinement, to connect to a turbine.

Unfortunately, I believe that the area of actually capturing the energy of the fusion reaction is almost entirely unstudied yet in practice.

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