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Consider the following:

We have a city, designed to be a retreat. Everything consumed and needed by the population is created via a machine in the middle of this stronghold.

This machine is able to convert matter into another type of matter and into energy.

It could be simplified: We have a really big and efficent fusion reactor in a fixed position.

My question: What would be the best energy source for this type of setup? What could support the reactor for a really long time?

From my other questions I know that I will need a lot of handwavium, at least for the matter-to-matter conversion.

But I want to keep this at least consistent and plausible. So no energy/matter out of nowhere. But where should it come from?

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    $\begingroup$ My question: What would be the best energy source for this type of setup? People I don't like, obviously. Toss 'em in the matter converter to put them to good use.... waste not, want not, and all that. Point being that "best" is a subject term and depends heavily on what your goals are. $\endgroup$ – HopelessN00b Sep 19 '16 at 16:21
  • $\begingroup$ How long do you mean by "really long"? Years? Centuries? Millenia? $\endgroup$ – Periata Breatta Sep 19 '16 at 18:41
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Water, specifically sea water

Hydrogen fuses quite nicely into helium and provides a considerable amount of power in the process. Assuming exceptional fusing capabilities of this fusion reactor, creating elements from Hydrogen all the way up to iron would be feasible. Elements heavier than iron will require a fancier fusion process since fusion reactions only yield energy up to iron. Heavier than iron, they consume energy, so the hydrogen fusion portion of the reactor will have to provide the power to get the heavier-than-iron elements.

Using the freed oxygen from splitting water will provide a nice jump start on the heavier, sub-iron elements. Sea water has a considerable mix of heavier elements that could be collected and processed.

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  • $\begingroup$ It turned out somebody had already asked somthing similar: worldbuilding.stackexchange.com/questions/11408/… $\endgroup$ – user6415 Sep 19 '16 at 13:13
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    $\begingroup$ Hydrogen fuses quite nicely into helium - is not Proton-proton reaction, you probably meant Deuterium. Hydrogen is a mix of isotopes, or pure isotope $H^1$. $\endgroup$ – MolbOrg Sep 19 '16 at 15:27
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    $\begingroup$ @MolbOrg, agreed. My answer does handwave away the really important details you mentioned. $\endgroup$ – Green Sep 19 '16 at 15:33
  • $\begingroup$ I'm not against hydrogen fusing, it does and probably some kind of Muon magic could help(mean - I hope we can figure out in future how to burn H efficiently). Just not like that quite nicely because if it would be so, all stars would burn out to this moment(my guess), so it is kinda vital difference. And no need to handwave as D is kinda ok too. Theoretically fusing H is possible, so no problems with that. $\endgroup$ – MolbOrg Sep 19 '16 at 19:40
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The waste of civilisation

With the best will in the world, humanity is far from efficient and we generate a vast amount of waste. What currently goes to landfill, slag and spoil heaps could just as well go into feeding the great machine that powers civilisation.

Then top up with water.

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How about rocks? They are denser than water, nobody wants to drink them, and if you build your structure underground now you have something to do with the debris of the drilling efforts to expand the settlement.

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  • $\begingroup$ Could be feasible, but the retreat is designed to be quite static and last for a really long time. This would be cutting the bough you sit on, right? $\endgroup$ – user6415 Sep 19 '16 at 13:17
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    $\begingroup$ @openend I don't know your design. But if it is to stand for a long time, you need to think how to accomodate a growing population. There is no need to go directly below... Besides you have matter to matter conversion, you could build a support layout out of superior materials - something lighter, stronger and more compact than the rocks that were there. How do tunnels work? $\endgroup$ – Theraot Sep 19 '16 at 13:23
  • $\begingroup$ Thanks, you are right. Turns out, that water is a really good answer for reasons within my story, but from a neutral perspective, your answer is feasible. Thanks a bunch! $\endgroup$ – user6415 Sep 19 '16 at 13:26
  • $\begingroup$ You'd need a huge additional energy source to convert your "rocks" to anything. Nuclear fusion of oxygen (to max. iron) only gives a very small amount of energy. $\endgroup$ – Karl Sep 19 '16 at 17:01
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Water. With enough handwavium, you can readily pour this into your fusion reactor. Handily enough, it can also be used for cooling the reactor.

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  • $\begingroup$ I was thinking more about the lines of some really dense and energy rich material. Would this make a difference to water? But water would be very nice, because the stronghold is on an island, and it could explain, why the water level is falling, right? $\endgroup$ – user6415 Sep 19 '16 at 12:34
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    $\begingroup$ You were first Pete, but Greens answer is a little longer and has more usable bits for my creation process. Take my upvote and my thanks, though :) $\endgroup$ – user6415 Sep 19 '16 at 13:09
  • $\begingroup$ No problem... You had my upvote at "handwavium"... :D $\endgroup$ – user10945 Sep 19 '16 at 13:12
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Your physicists have found a way to create short-lived mini-black holes.

This will turn any kind of matter into radiation. This will turn the full energy content of the mass (according to the famous formula $E=mc^2$) into usable energy. You only need a way to cool your energy creating apparatus.

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Like @Theraot suggests, rocks are a good idea. Depending on the kind of rock your retreat is built on, you might have up to 60ppm lithium in those rocks. Out of the millions of tonnes of rock that you could easily have access to (particularly if your retreat is built at least partially underground, or especially if that part is still being built), you might therefor have a few hundred tonnes of lithium available to you. Of that quantity just under 8% is Lithium-6.

Now, you're going to also need a few other things. Particularly, you're going to need a source of protons, and some Helium-3. These are a little harder to get hold of, but don't worry because the reaction you're going to perform produces as much of them as it consumes. You're only going to consume the lithium-6.

The key is to fuse a proton with an atom of lithium-6. This will release a helium 3 (which will need extracting during reprocessing later on), a helium 4 (which is waste, although there are some fun party tricks you can perform with it), and 4 million electron-volts of energy.

At the same time, you will also fuse a helium 3 atom with another lithium-6 atom. This will release a proton (you see where this is going) and 2 more helium 4 atoms, and 16.9 million electron-volts.

So, you've used 2 atoms of lithium-6 and got 20.9 million electron-volts and some waste helium (and some other products that need to be reprocessed to feed back into the reaction again in order to sustain it).

Now, to relate this to useful units. A mole of lithium-6 weighs 6 grams and contains roughly 6x10^23 atoms. This means that 6 grams can run 3x10^23 of our reactions and thus generate a little over 6x10^30 electron volts, which is to say about 267 megawatt hours, or enough power to keep the average person going for 12 years (or just under 3 years if they're American).

This means that the rocks under your citadel ought to be able to keep a population of, say, 50,000 people going for somewhere between 60 and 240 years, depending on how frugal they are with power. Obviously shorter for a larger population and longer for a smaller population.

Other than potential availability (depending on your city's location), there are other advantages to this over fusing deuterium from (heavy) water:

  • The relative abundance of lithium-6 versus lithium-7 is far higher than deuterium versus hydrogen. This means that you have to process less of it to get a useful quantity out.
  • The process does not release significant numbers of neutrons. Neutrons have a habit of being difficult to capture and turning many materials they may strike into radioactive sources. Neutrons are dangerous and difficult to work with. You don't want neutrons being produced in your reactor if you can possibly help it.
  • It's less cliche than deuterium-based fusion.
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If we allow for a great deal of hand-waving, Then I see three machines working in concert. @Green's water into everything and energy reactor is the base.

Then a matter reclamer, that uses less energy to turn matter into the same matter. For example iron into iron. This would take less energy, and could work like a futuristic smelter. Paper shreds go in and carbon, and other base materials come out.

Finally a matter exploder. It's a "Green Reactor" but backwards. Or you know, a normal-ish fusion reactor. The main idea being that you take something like gold and "reacts" it down to helium.

To keep it realistic, the Green reactor should produce the most energy, the Reclaimer should consume the most energy. The fusion reactor should be little better then break even. (even that's not realistic but just get those hands a waving)

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  • $\begingroup$ Don't forget you can poof matter into nothing, so all those splits from Au to He are going to make more things that need to be reacted down. $\endgroup$ – coteyr Sep 19 '16 at 18:47
  • $\begingroup$ I believe (but don't quote me on this) that you'd have to add energy to convert gold to helium. There's a peak at iron, and you only gain energy while you're moving towards it, AIUI. $\endgroup$ – Periata Breatta Sep 19 '16 at 19:30
  • $\begingroup$ Yes, yes, but hand waving. It's easy to believe that a Au to He conversion should release energy, or at least be close to energy neutral, but that going up takes energy. $\endgroup$ – coteyr Sep 19 '16 at 19:40
  • $\begingroup$ easy to believe for anyone who hasn't looked at the binding energy curve, but now I've seen it I'd never believe any such device would be practical -- it would use so much power that nobody would ever consider using it. $\endgroup$ – Periata Breatta Sep 19 '16 at 19:44
  • $\begingroup$ Star Trek fans accept a warp drive, and all we know about it is that it uses anti-matter and some kind of crystal thingy to go woosh, and it can produce a nearly unlimited supply of energy. Don't let science ruin sci-fi. $\endgroup$ – coteyr Sep 19 '16 at 19:47

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