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Old 08-10-2020, 01:50 PM   #1
AlexanderHowl
 
Join Date: Feb 2016
Default Why mine Saturn for He-3

I am curious why anyone mines Saturn for He-3? D-He-3 fusion is more difficult than D-D fusion, which produces He-3 and T as byproducts (every gram of deuterium (D) fused creates 375 milligrams of helium-3 (He-3) and 375 milligrams of tritium (T)). Assuming a 40% efficiency of converting the thermal energy to electricity and a consumption of half of the electricity produced to sustain the reaction, each gram of D fused would produce around 7 MW-h of exportable electricity, meaning that it produces net electricity, helium-3, and tritium. Tritium can then be stored until it decays to helium-3 while the helium-3 and electricity are sold off.

Of course, you would need to burn 4,000 tons of deuterium per year to produce the 3,000 tons of helium-3 per year consumed in Transhuman Space (1,500 tons per year from direct production and 1,500 tons per year from tritium decay), but that would just mean an extra production of around 3 TW of electricity, which is less than 2% the system demand in Transhuman Space. While the existence of nearly 40,000 tons of tritium would be a security issue, it could be addressed by having a portion of a national military devoted to securing, transporting, and storing the tritium until it decays into helium-3.

So, why mine Saturn for helium-3? Is it that much cheaper than forging it in a reactor? A 10 GW D-D fusion power plant would produce 510 grams of helium-3 per hour (and 510 grams of tritium per hour), which could supply sufficient fuel to supply (directly or indirectly) a 75 GW D-He-3 fusion power planet, so I do not imagine that it is the price. Is it the security issues surrounding the production and storage of tritium? If so, how do they prevent the Duncanites from just using D-D fusion to achieve energy independence?
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Old 08-10-2020, 02:30 PM   #2
Fred Brackin
 
Join Date: Aug 2007
Default Re: Why mine Saturn for He-3

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Originally Posted by AlexanderHowl View Post
I am curious why anyone mines Saturn for He-3? ?
My goodness, that was a long post on the subject that just happened to not use the words "neutron" and "lithium". 80% of that generic-sounding "energy" is actually in the inconvenient form of neutrons and those neutrons have to be absorbed by lithium to recover that energy and make all that He3 and Tritium.

Then there's the intersection of "lithium" and "China". Right now in 2020 China is the overwhelming supplier of the world's lithium which is in huge demand for batteries for small electronics and could be for rechargeable electric vehicles.

You'll never fuse your way to energy independence buying lithium from the Chinese and you have to worry that if a non-terrestrial souce of lithium shows up the Chinese might control it anyway. Earth in general is probably the dominant source for De too.

So if you're a Duncanite you have to go pretty far out for energy sources that aren't already controlled by someone else directly or indirectly.
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Old 08-10-2020, 03:31 PM   #3
Anthony
 
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Default Re: Why mine Saturn for He-3

Manufacturing 3He is a bit of a pain, actually; yes, it's a byproduct of D-D fusion, but some of it will get consumed or lost, and in general the appeal of D-3He is that it's very low neutron, which is lost if you're breeding it.
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Old 08-10-2020, 03:42 PM   #4
Johnny1A.2
 
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Default Re: Why mine Saturn for He-3

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Originally Posted by Fred Brackin View Post
My goodness, that was a long post on the subject that just happened to not use the words "neutron" and "lithium". 80% of that generic-sounding "energy" is actually in the inconvenient form of neutrons and those neutrons have to be absorbed by lithium to recover that energy and make all that He3 and Tritium.

Then there's the intersection of "lithium" and "China". Right now in 2020 China is the overwhelming supplier of the world's lithium which is in huge demand for batteries for small electronics and could be for rechargeable electric vehicles.

You'll never fuse your way to energy independence buying lithium from the Chinese and you have to worry that if a non-terrestrial souce of lithium shows up the Chinese might control it anyway. Earth in general is probably the dominant source for De too.

So if you're a Duncanite you have to go pretty far out for energy sources that aren't already controlled by someone else directly or indirectly.
Yeah, but that only makes sense if you're a Duncanite or other space-based faction.

When you fuse deuterium with deuterium, you get tritium and/or helium-3 as reaction products, no lithium breeding required for that. One reaction chain yields tritium and free protons, the other yields helium-3 and a free neutron. Both generate comparable energy.

But you can take that tritium and he-3 and fuse it with more deuterium, thus removing whatever security issues you have with storing tritium and getting more energy out of your initial batch of fuel. If for some reason you do want to store the tritium, it'll decay to he-3 and you can still reburn it with deuterium.

For space propulsion, he-3 fusion is much preferable to d-d, because it doesn't make so many useless dangerous neutrons. It makes a much better rocket.

But for groundside use on Earth...well,, the neutrons are still a headache. But you can shield against them because mass is not so much of an issue, and you can get power from them by using them to heat a working fluid or such methods. Protons are better because you can tap them directly, but on the ground the disadvantages of the neutron emissions are not nearly as bad as they are in space, and you can use them to breed fuel from lithium if you want to (though I'm not sure it makes sense economically or logistically).

But set against the problems of the d-d reaction, on Earth, is the fact that deuterium is cheap. Relative to he-3 it's close to free. The oceans contain immense masses of the stuff. The heavy water is only a tiny fraction of the ocean mass, of course, but with 300 million cubic miles of water, a tiny fraction adds up to a whack of a lot of heavy water.

Realistically, bringing in he-3 from space to use on Earth makes no sense in the THS setting. The advantages of using he-3 are real, but compared to the fact that it's 'immensely expensive but somewhat better fuel' vs. 'dirtier, lower-grade, less convenient fuel that costs relatively nothing'...

Mining Saturn (or the Moon, or wherever) for he-3 makes good sense...for fueling space propulsion systems and space-based power plants where mass is critical. But he-3 use on Earth as a motive for space-infrastructure doesn't add up, unless there's some specific glaring reason why you have to do it that way, which isn't the case in THS.
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Old 08-10-2020, 03:51 PM   #5
Anthony
 
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Default Re: Why mine Saturn for He-3

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Originally Posted by Johnny1A.2 View Post
But you can take that tritium and he-3 and fuse it with more deuterium
You don't really have a choice on that, the reaction rate of D-T is something like a hundred times D-D so if you're fusing D-D almost all of the T gets consumed. The 3He won't get reliably consumed, though.
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Old 08-10-2020, 06:11 PM   #6
AlexanderHowl
 
Join Date: Feb 2016
Default Re: Why mine Saturn for He-3

Actually, there is some argument concerning that. While many of the models have D-T reactions occur faster than D-D, they do not bear out in the test reactors. The most successful fusion reactor on Earth is a D-D reactor in the UK (the MAST, if I remember correctly), which reaches 70% efficiency, and none of the D-T experiments reaches even half that amount except in microsecond bursts.

Even if D-T does burn faster than D-D though, it would increase the effective energy production by 60% and get rid of the security issues, so I doubt that anyone would mind. You would need to burn around 175% more deuterium (11,000 tons), meaning that you would end up producing 340% more total energy during the process, but you would be producing twice as much helium-3 for immediate consumption. Just as an aside, a cubic mile of water possesses over 58,000 tons of deuterium, so the oceans contain enough deuterium for 1.7 billion years of D-D fusion.
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Old 08-10-2020, 06:38 PM   #7
Anthony
 
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Default Re: Why mine Saturn for He-3

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Originally Posted by AlexanderHowl View Post
Actually, there is some argument concerning that. While many of the models have D-T reactions occur faster than D-D, they do not bear out in the test reactors. The most successful fusion reactor on Earth is a D-D reactor in the UK (the MAST, if I remember correctly), which reaches 70% efficiency, and none of the D-T experiments reaches even half that amount except in microsecond bursts.
JET, it looks like, not MAST. JET was normally run on D-D with results extrapolated to D-T, and it appears there are some errors (extrapolated Q about 1.25, actual measured with D-T about 0.67).
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Old 08-11-2020, 01:55 AM   #8
AlexanderHowl
 
Join Date: Feb 2016
Default Re: Why mine Saturn for He-3

Yes, you are right, it is JET.
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Old 08-11-2020, 09:17 AM   #9
malloyd
 
Join Date: Jun 2006
Default Re: Why mine Saturn for He-3

Quote:
Originally Posted by Fred Brackin View Post
My goodness, that was a long post on the subject that just happened to not use the words "neutron" and "lithium". 80% of that generic-sounding "energy" is actually in the inconvenient form of neutrons and those neutrons have to be absorbed by lithium to recover that energy and make all that He3 and Tritium.
That's actually a separate issue. The original post point is that D-D fusion produces He3 directly and tritium with decays to He3. Which it does, but recovering it from the plasma is not a really a practical source, indeed most of it will get consumed in side reactions, and the half life of tritium is long enough that waiting for it to decay is just hopelessly slow unless you have way more of it that you want He3.

The neutrons that matter for choosing DHe3 as a reaction are the ones produced by the reaction - DHe3 is much less radioactive than DT or DD, which is a big deal if you are trying to build a lightweight reactor to fly a space mission. Exactly why anybody would prefer DHe3 on the ground is unclear, but that's the chosen excuse. Comes from the original papers for why you'd want to mine it on the moon I believe, which is also why maufacturing He3 from neutron bombardment of lithium is overlooked as a production method, the whole point is to provide an excuse for mining it in space, so as to make lunar (and eventually outer system) colonization make some sort of economic sense.

And actually the world's top lithium producer now is Western Australia - China is about 3rd or 4th - but lithium isn't a particularly rare mineral, so it's mostly a matter of economics who produces how much. Anyway the amounts you'd need to breed He3 even if you were using DHe3 for the entire world's energy needs are, well not *negligible* by current lithium consumption standards, but I think still in the single digit percentages. Frankly I wouldn't be surprised if you could recover most of it from the same volume of seawater you were getting the deuterium from.
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Old 08-11-2020, 10:49 AM   #10
AlexanderHowl
 
Join Date: Feb 2016
Default Re: Why mine Saturn for He-3

Neutron bombardment of lithium produces tritium, which decays into helium-3, which is why you have helium-3 in lithium deposits. This is the entire process behind tritium breeding (and the majority of helium-3 production comes from tritium decay). In general though, if you have mastered D-He-3 fusion, it makes more sense to just create He-3 through D-D fusion than to attempt tritium breeding.
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