Comment by hga | Hacker News Reader

Comment by hga | Hacker News Reader

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Indeed, although it needs to be pointed out that in theory, it's very possible 1st generation fusion reactors will be economically unviable, since the neutrons they produce will steadily transmute them, that is, turn their atoms into different and generally radioactive elements, decaying and then destroying their ability to function, leaving piles of radioactive junk.

The interest in Helium-3 is due to the fact that it can be anutronically fused (although I've read there's doubts about being able to effectively extract energy from such a reactor). So Helium-3, and strip mining the moon!!! has been thought about for a long time.

Heck, it's part of the background for one of the major anime franchises that started in 1979, Mobile Suit Gundam. In that, through magic physics, they have "refrigerator sized" fusion reactors, but they must be fueled by Helium-3 because the magic physics isn't neutron proof (little is). So the "Jupiter Energy Fleet" plays a role modeled on the Arabs/OPEC of the '70s, pulling the strings behind most of the conflicts of the Universal Century timeline. (Paptimus Scirocco is an Arab as viewed by a group of Japanese animators in 1985, his hair style is much less crazy if you know that.)

I mention this to point out how long it's been generally known we're likely to need a lot of Helium-3 for fusion reactors, i.e. entering popular culture three decades ago.

Can't we produce enough Helium-3 to build and test reactors like this?

If Wikipedia is to be believed, we would need a substantial amount of Helium-3 to make a working reactor. At the moment, the US produces a mere 8kg of Helium-3 per year(and that's 60,000 litres!).

Apparently a litre can sell for as much as $2000, so even if the Chinese brought it back from the moon (I really wonder how, if 8KG take 60,000 litres of space, are they planning to bring back 20 tonnes??) they would still make profit.

It is extremely unlikely they could profit, because if they raise supply price would drop like a rock. I doubt there's much demand for Helium-3 predicated on prices lower by even two orders of magnitude to support a raise in supply, and that's probably being pretty generous in terms of how cheaply it could be supplied. Unless Helium-3 can actually be used to perform real fusion in the real world, in which case the seller would have an opportunity to capture the suddenly immensely greater value of Helium-3, there's no reason to go get until the fusion systems are running in the real world.

From what I understand, there is enough demand even without using it for fusion - in medical scanners and such. But yeah, I am not completely convinced you could make money by mining it from the moon.

Assuming China does want to go get it [1], profit may not be the motive. It might just be a resource grab. Uneconomical in the short run, but with a vague expectation that the resource will skyrocket in value years from now. I doubt China is interested in resale, so much as stockpiling.

[1] I say "assuming...," because this article is basically just speculating.

There's huge demand for liquid Helium... but that's Helium-4. It has its own supply issues coming in the future, but at the moment is relatively plentiful, and even when we run out we probably won't be willing to go to the Moon to get the tons upon tons of Helium we use. See the graph here: http://en.wikipedia.org/wiki/Helium#Applications showing the breakdown of the 47 million cubic meters of Helium used in the US in 2013. Helium-3 demand is much smaller, since, well, it has to be.

It's not quite on topic, but whynot... here's my analysis of what it would take to create all the helium we use via fusion: http://www.jerf.org/iri/post/2922

I imagine something involving balloons. I guess if you have the tech to strip mine the moon, you might have a shot at liquefying it, but that's what? 4 degrees K? Either case seems challenging.

At least you have a nice near-absolute zero heatsink for the lunar night.

Not all that "near": CMB is 2.75K. You'd also have to shield the radiators from the lunar regolith, which is quite a bit warmer, and from the Sun when it's in the sky...

You can bring it back as a liquid. (Classical, not superfluid)

Boil-off would be a problem, however. (If you want to get fancy, you bring back a He4 tank and a refrigerator, keeping the He3 liquid by boiling He4 into space)

Liquid He4 has a density on the order of 125g/L at boiling point at 1atm. Don't know about He3, but even at 50g/L, that's ~20L/kg. Not nearly as bad.

There's no reason to test a reactor like this now. The H2 + He3 reaction is about an order of magnitude harder than H2 + H3, and nobody can do H2 + H3 yet.

To be precise, nobody has done breakeven fusion with any fuel. Achieving sub-breakeven fusion is quite easy with small fusor devices, (http://en.wikipedia.org/wiki/Fusor) though that won't tell you much that's useful for scaling larger reactors, besides maybe testing first-surface materials. (http://en.wikipedia.org/wiki/Plasma-facing_material)

I'm confused where the electrons go in he3 + he3 -> he4 + 2p fusion. Anyone know?

From reading http://en.wikipedia.org/wiki/Helium-3#Fusion_reactions, the 2p are two protons, and not positrons, as valarauca1 is saying.

So, 4 protons and 2 neutrons go in, 4 protons and 2 neutrons come out.

But charge still needs to be conserved -- I presume that the spare electrons just aren't shown in the reaction; if they were bound to the protons, then we'd see 2H rather than 2p.

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