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Comment by lake_vincent | Hacker News Reader

Yes, exactly! There are two problems here:

1. What's the most efficient way to boil water?

2. How do we generate electricity on a population scale without having to boil water?

Question 1 is, apparently, much more tractable. We are still pretty much building the world's most sophisticated tea kettle.

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There are fusion companies that are trying to make aneutronic fusion work. This form of fusion doesn't involve boiling water.

https://en.wikipedia.org/wiki/Aneutronic_fusion

Helion Energy is the best known.

https://en.wikipedia.org/wiki/Helion_Energy

HB11 is another one

https://hb11.energy/

Helion might conceivably make something useful someday.

The others will just spend investors' money until it dries up. Helion too, most likely.

Well if I'm going to throw money at a pointless endeavour, I could think of quite a few worse than this.

Pointlessness is.

If you want to maximize pointlessness, Energy Vault is right there. But defrauding investors doesn't seem like a great choice to enable.

Maybe it would be better to do what might help avert global civilization collapse, thermonuclear war, and mass starvation?

Investing in practical energy-storage tech factory build-out, solar farms co-sited on reservoirs and pastures, cost-effective hydrogen and ammonia synthesis tech are all overwhelmingly better uses, and can all be equally as cool.

HB11 strikes me a nonstarter. Their proposed direct conversion mechanism is obvious nonsense, and the putative "avalanche" mechanism they want to use to make the nuclear reactions go has been savagely critiqued in the literature.

I think there is a zeroth problem here, which is how to get the heat from the plasma to the water. You have a fusion reactor making hot plasma that also needs to have water circulating in the right way. From what I gleaned from some fusion presentations the solution to this is itself an engineering challenge.

Fusion plants also need to absorb the neutrons coming out of the reactor and secure themselves a supply of tritium for their reaction. The most common solution is to do all three with a blanket of molten lithium that can absorb neutrons and heat from the reaction, transmute into tritium, and go through a heat exchanger with water to heat it up.

Lithium fission also releases a nice bit of energy.

Lithium fission would be into Hydrogen and Helium - definitely not releasing energy. Do you mean that Lithium fusion with a proton or neutron releases energy?

No I do mean fission. Li-6 fission releases 4.8 MeV. Li-7 fission consumes 2.466 MeV.

https://en.wikipedia.org/wiki/Tritium#Production

There's a lot of lithium being mined for batteries. Just centrifuge out Li-6 or isolate it with some other clever means. Not much is needed for fusion fuel compared to industrial scale consumers of lithium and it's a healthy boost to heat output.

Molten lithium would interfere with your magnetic containment. You need a diamagnetic like LiH.

Extracting tritium from a thousand tons of radioactivated lithium hydroxide at PPB concentration is left as an exercise for the ambitious reader.

Good luck.

(Lithium hydride, not hydroxide.)

Other neutron absorbers are contemplated, but extracting tritium from them on PPB concentration is no easier.

"Fusion Reactor First Wall Cooling" https://www.youtube.com/watch?v=bHJyoqDO0zw talks about the steady-state heat transfer out of a proposed tokamak design. Around 1:09:23 they show "1.5 GW" deposited as "0.3 GW from radiative photons -> surface heating" plus "1.2 GW from 14.1 MeV neutrons -> volumetric heating".

Designing the first wall and the volumetric blanket are, indeed, engineering challenges.

1.2 GW worth of neutrons is rather a lot. Neutrons cause damage to materials (e.g. disrupting the microstructure) and also make elements radioactive through neutron capture (activation products). It's a pretty complicated issue really.

Usually we spell that "intractable".

Sometimes people know enough to see things as merely complicated.

They usually can't adequately explain why in a single forum comment.

Generally somebody should have explained somewhere. But it is not considered very interesting, because nobody seiously expects to ever need to solve it.

Sometimes, it is explained somewhere and it doesn't matter, no one is listening.

This topic is not one I'm personally familiar with, so possibly making an ass of myself, but (shrug).

Indeed. As I mentioned upthread, the limits on power/area at the first wall of the reactor lead to DT fusion reactors having horrible volumetric power density. Their "figure of demerit" vs. fission is roughly (radius of plasma)/(radius of a fuel rod).

As a Brit, I wholeheartedly approve.

The power grid is just a way to remotely boil a kettle.

Ironically all but one of our UK reactors are carbon dioxide cooled although to be fair that carbon dioxide is then used to boil water.

>1. What's the most efficient way to boil water?

Doesn't need to be the most efficient. Just more efficient per energy transferred to the water than all the other options.

I don't follow. What you said seems to me the definition of the most efficient. Or at least.. it is what I assumed they meant.

Most efficient can mean theoretically optimal efficiency or more efficient than other options. I think the OP was using the first definition.

Local maximum vs global maximum

Most efficient conpared to what we have already, but there may still be more efficient ways

Doesn't matter if you waste half the energy if it's still a cheaper way to boil water than the next best thing.

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