[nickparker]

Have you seen General Fusion?

They spin a large ball of molten lead using pumps and shoot hot plasma into the vortex that forms in the middle, then strike the walls of the lead chamber with carefully timed steam pistons to make an implosion pressure wave not entirely unlike Fat Man’s design, except reusable and for fusion.

The fusion heat and radiation gets absorbed by the lead and they run a heat exchanger on the pump loop for power generation. They also think they can breed fuel with some lithium in the lead.

I love their plan aesthetically but I’m not qualified to judge how feasible it is. It’s very satisfyingly physical though.

[pfdietz]

I was withholding judgment on General Fusion. They had a concept that, at least in principle, would allow engineering limits on the power density at the first wall to be evaded (since the first wall would be thick liquid metal, not a solid that could sustain damage.)

But they gave up on their original "acoustic" compression scheme, and now will compress the plasma via subsonic motion of the liquid metal. This scheme involves a solid conductor going down the middle of the chamber. It will be exposed to orders of magnitude higher radiation flux than the first wall of mainstream fusion concepts, as well as pulsed loads from magnetic fields up to 100T (which correspond to pressures far higher than the chamber of a gun, and higher than the deepest point in the oceans.) Getting this conductor to survive even one shot would require heroic engineering; keeping it cooled and together as its material properties rapidly degraded would require superheroic engineering.

[lambdatronics]

I heard that the shock wave was spalling off the front few cm of the liquid metal wall as a spray of jets/droplets -- the shock reflects from the vacuum interface & where the wave interfered destructively with itself, the pressure dropped to zero, and cavitation occurred. This happens in a random & uneven fashion.

[pfdietz]

That sounds very plausible. Do you have a reference I could cite?

[lambdatronics]

Here it is: https://generalfusion.com/wp-content/uploads/2016/08/Suponit...

Edit: Here's a second one that's similar (but behind a paywall). http://www.sciencedirect.com/science/article/pii/S0045793013...

The preprint is here: https://arxiv.org/abs/1310.6010v2

Edit 2: Here's a better one with video frames from the experiment showing the jets. https://generalfusion.com/wp-content/uploads/2016/08/Richard...

[pfdietz]

Cool, thanks!

[nickparker]

Can you point me to where they say the central cones are a change in their hypothetical production design?

I was under the impression it's just there for diagnostics in the sub-scale prototype.

[pfdietz]

The abandonment of the acoustic compression scheme is explained in this poster.

https://generalfusion.com/wp-content/uploads/2018/07/ICPP_20...

The move to a spherical tokamak is explained in page 9 of these slides.

https://arpa-e.energy.gov/sites/default/files/11_LABERGE.pdf

The slower compression of the non-acoustic scheme requires better confinement than the spheromak provided.

[nickparker]

Hmm yeah that does sound like a permanent change in the second link doesn't it? Too bad, I was really rooting for their plan.

[pfdietz]

The Turchi mentioned there was associated with Los Alamos, where the LINUS concept was investigated. General Fusion seems to be moving in that direction, so this slide from Turchi at Compact Fusion Systems could be of interest. It has a focus on practical engineering and cost issues that I find encouraging.

https://nucleus.iaea.org/sites/fusionportal/Shared%20Documen...

[ColanR]

Do you know if they've achieved fusion yet? I remember reading about them in PopSci way back in the day, and wondered how they were doing.

[teachrdan]

Do you end up with a huge ball of radioactive lead at the end of the day? From what little I know of fusion, it seems you still end up with irradiated lead or other shielding.

(And yes, I know that coal releases radiation, and that the installation of solar panels and wind turbines is, currently, a dangerous job, etc.)

[nickparker]

Huh, I suppose you do! I had a naive short-circuit in my head that "neutron flux is bad because it destroys your your materials. If your material is molten lead anyway who cares?"

I also didn't know lead has such a low neutron absorption cross section.

For what it's worth lead with extra neutrons seems a lot less scary than eg uranium fission waste. Stable isotopes 206, 207, and 208 represent 98% by abundance, 208 has the lowest cross section, and 209 has a 3 hour half-life into Bismuth-209 which is nearly stable (2e19 year half-life). So it seems almost all of your neutron captures just make other stable lead isotopes or briefly-terrifying 209 that's totally safe after a couple days. You only get real scary stuff if the trace amounts of undecayed 209 manage a second capture.

Edit: I should add they want to mix lithium into the lead to absorb neutrons and regenerate fuel

[pfdietz]

Lead has a low neutron absorption cross section, but it has a fairly hefty inelastic scattering cross section for fusion neutrons (for fission neutrons, which are less energetic, it just elastically scatters them, with very little moderation). It also causes some (n,2n) reactions with fusion neutrons, which would be nice to make up for tritium losses.

[the8472]

What about 204? If that produces 205 you'd have some long-lived waste.

[grogers]

10 million year half life is long enough that it's not very dangerous anymore.