Comment by pfdietz | Hacker News Reader

Comment by pfdietz | Hacker News Reader

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This is greatly overrated.

The design of the ARC reactor in the 2014 arxiv paper produced 500 MW(th) fusion power. The FLiBe bath contained some 90 tons of beryllium. The annual world production of Be is just 220 tons. If the world's entire estimate resource of mineable Be (100,000 tonnes, according to USGS) were used to make ARC reactors, they might supply 1% of the world's primary energy demand.

The FLiBe also contains about 1/4 the 6Li produced by the entire US hydrogen bomb program. The facility that enriched that lithium used 10,000 tons of mercury and is an environmental problem even to this day because of leaks. That process is also now illegal in the US due to this pollution, and there is no facility in the world that could make that much 6Li.

Perhaps those problems could be worked around, but there's a deeper issue. The power density of the ARC reactor is around 0.5 MW/m^3. This is an order of magnitude, or more, lower than the power density of a PWR fission reactor core. The ARC reactor is also much more complex, and will require very expensive maintenance operations every couple of years (manufacturing a new pressure vessel and remotely installing it, as the innards of the reactor will be too hot for hands-on access.)

So, how could this possibly be cost competitive with fission reactors, never mind the energy sources that are currently beating fission?

It's ironic that the ARC reactor is just demonstrating the fatal flaws with DT fusion reactors, flaws that were also pointed out at MIT by Lawrence Lidsky some 35 years ago. ARC is better than ITER, but that's damning with faint praise.

At 45 min in, they mention that FLiBe is ideal, but NOT required. Any low-Z liquid would be acceptable:

https://youtu.be/KkpqA8yG9T4

They also discuss advancements in internal chamber maintenance access. That side is really an engineering problem.

No, "any low Z liquid" is not acceptable. It absolutely must contain 6Li to breed tritium. And it's quite likely that strong neutron absorbers would not be acceptable unless they also made tritium. The beryllium serves as a neutron multiplier -- the 5th neutron has the lowest binding energy of any neutron in a stable nucleus -- so without it the neutronics will suffer. DT reactor concepts already have issues breeding enough tritium to overcome inevitable losses.

Dismissing something as "an engineering problem" is typical of plasma physicists. Fission has only "engineering problems" and yet those are quite sufficient to render a technology uncompetitive.

Beryllium supply does seem like a serious issue. I've often criticized renewables for battery raw material limits and those aren't near as bad.

I think lead does the same thing for neutronics...somebody educated me once on why it's not a viable option but I forget the details.

Maybe somebody can find a better way to purify 6Li.

I think the issue with using lead is not that it would have a lower neutron multiplication, but that it wouldn't moderate the neutrons as effectively. I think this would force the reactor to be larger to keep the neutron dose on the magnets within limits.

A process based on binding of Li ions to crown ethers could probably be developed for Li enrichment.

estimate resource of mineable Be

actual limit, or economically feasible? If the latter, that may change

Economics has to come into it somehow, or else one would have to count all the beryllium in the Earth's crust (~2ppm).

As you can see from some here, people just want to believe. Fusion is the power source of sci-fi, and they’re scared of fission. As with self-driving cars, people just imagine what it could be like and that’s enough for them. Never mind that decades of R&D on fission has yielded breakthroughs in safety and efficiency, and by the time a workable fusion design is possible far more advanced fission will be possible. People want the future to be now, so they believe their way into it. Scientists want funding for research, so they sex it up for the media and mass consumption.

The truth is that politics aside, we could be using fission today to solve the problems people want fusion to solve decades from now. Granted, if aneutronic fusion becomes possible (no time soon, even experimentally with a surplus of energy) that will be a miracle. DT fusion though, is only useful for research purposes.

Most people, including most people here don’t have a working understanding of nuclear physics or the requisite engineering of a power plant. When you don’t understand the hurdles, fusion seems kind of magical. If you’re desperate for advanced space flight, fusion seems kind of magical. Even more, no one has any negative experiences with fusion, while we’ve been literally burned by fission.

It’s hard to argue against a fantasy, and hoping for fusion also let’s people ignore the hard work of using fission. The politics feel intractable in the US, the waste is manageable, but scary. Fusion isn’t real yet in that sense, so like an online romance people can project a fantasy onto it.

"Granted, if aneutronic fusion becomes possible (no time soon, even experimentally with a surplus of energy) that will be a miracle."

A group at Princeton has a concept for a FRC-based reactor burning D-3He that, through a combination of quite interesting tricks, reduces the fraction of power output in neutrons to as little as 0.5%. The design is also very small, with a power output of 1 MW. At this level of neutron output the reactor structure are lifetime components, with no replacement needed due to neutron damage. Power density is still a struggle, although the small size of the reactor helps there.

The downside (assuming the aggressive plasma physics doesn't disappoint) is where do you get the 3He.

At this point, my default vision of the future is neither fission nor fusion, but rather renewables and storage. The engineering and economic issues of these appear much more tractable. Simply extrapolating solar down its demonstrated experience curve puts the cost of PV electricity at $0.01/kWh or less when fully scaled out.

I was only aware of Helion doing D-3He, and they're in Washington. Do you have a link for the Princeton group?

https://en.wikipedia.org/wiki/Princeton_field-reversed_confi...

The small company proposing using this for a notional Pluto orbiter:

http://www.psatellite.com/

Primary patent for the reactor:

http://www.freepatentsonline.com/9767925.html

Sam Cohen’s Rotating Magnetic Field experiments have shown higher temperature and reasonably long-lived FRCs. His vision is steady operating, 3He systems, atleast at first targeting propulsion. As you dial up the Helium percentages, the neutron output goes down, though the required ion temperature goes up. Princeton Satellite Systems has several NASA programs looking at the propulsion applications.

"required ion temperature"

The scheme involves significantly non-maxwellian ion distributions, so "temperature" isn't really appropriate. In particular, 3He ions get pumped to higher energy than D ions, which helps suppress DD fusion. They claim the scheme is consistent with Rider's limits on energy circulation in non-maxwellian plasmas.

To be fair some reponsabilites also come from poor performances of fission industrie leaders. In France the next generation fission power plant EPR was so much delayed that a lot of people are getting skeptical about this tech if not totally opposed for environmental reasons (trues or romanticized).

This is a shame as I globally agree with you that fission might have a transitional role to play in the mix needed to reduce our global warming impact. Consuming less non-renewables ressources being the first line of action anyway.

Before you judge their efforts, I would watch this video:

https://youtu.be/KkpqA8yG9T4

Sounds like the only thing standing in the way is prototype funding.

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