Fission vs. fusion as a heavily-restricted sci-fi power source

Question

In a sci-fi setting I'm working on, the power dynamics between two civilizations are maintained by one civilization ("captives") being completely dependent on the other civilization ("captors") for electrical power, in an environment (artificial colonies on an airless planet) where electricity is a life-and-death matter.

The primary source of power is a huge solar farm owned by the captors, but some of the captors' colonies are powered by their own internal reactors, and are separate from the main power grid. Both civilizations originally arrived on the planet via a generation ship, and the reactors' fuel should have been brought as the ship's power source; whatever this fuel is, it should be impossible or impractical for the captives to mine or produce.

This leads to the captives attempting to steal reactor fuel from the captors' colonies, the rationale being that if the captives can establish their own reactor, and become energy-independent, they can stage a revolution without fear of having their power shut off.

My question is: what reactor power source would work best to set up this scenario, and which technologies would need to be impossible/impractical in the setting for this scenario to make sense?

Fusion seems like the most obvious power source for a sci-fi setting, but the most likely fuel would be deuterium, which is simple enough that it's probably not particularly hard to find or synthesize.

Fission fits the bill a little better--it depends on hard-to-find radioactive fuel, like uranium or plutonium, and it could be established that the planet doesn't contain any fissionable elements--but, for the scenario to work, fusion would have to be completely impossible or impractical. Given that the story is set several thousand years in the future, is it reasonable to assume that scientists eventually discovered that efficient fusion just wasn't possible? Is that even a possible outcome, given enough time and research?

Or is there another, better reactor option I might be ignoring? Something with antimatter, or another exotic material?

Answer 1

Fission, in the form of a pebble bed reactor is the way to go.

Fuel is portible and as safe as any industrial supply: billiard-ball sized spheres, each a self-contained sealed unit. Each can be engineered for available fuel and concentrations, layering fuel isotopes and neutron absorbers in a precise manner. Spent fuel and waste can make different pebbles with suitable formulation.

Stack a million of them together and it gets hot. But, not so hot as to wreck the contents or the vessel! Each pebble is a solid-state engineered feedback system that shuts down the reaction as it gets warm. So, it reaches a pre-designed temperature and automatically levels off the output.

To extract power, pump coolant through the pile, which nicely flows between the balls. The more agressive you cool it, the more power it puts out.

So, someone stealing a shipment of pebbles can have a useful radiothermal generator with zero engineering effort, and a modest amount of technology (and a large enough supply of pebbles) can give you a safe full-power fission reactor.

The size of a million pebbles might be tough. Perhaps small loads are not tightly locked down because it is too small to be useful...so they think.

This can be part of your plot: a single load or small supply of pebbles is not enough to run their reactor as designed so they don’t think it is enabling the thiefs to build a power supply, even if they know it’s missing.

Your protagonists might use external neutron sources to work with a small pile. Or maybe the hero figures out how to alter the pebbles; say, file down the outer layers and coat with something different, or spaces the pebbles apart inside a fluid that itself has neutron modulating features. It can be something novel and unexpected by the bad guys, and/or rely on something unique to the good guys.

You can adjust the sizes to suit the logistics of the story: You can specify a different nominal size, say 100K for a "small" power supply. Then a single pile of 5000 or 10000 might be used for the covert reactor. You could make the pebbles smaller too in your story, but ought to mention that for beleivability when explaining the reactor in exposition.

Answer 2

I would think that if we get fusion to work, most expected designs are going to be big, and require lots of energy to the reactor and a lot to startup/contain the reaction. You have a small sun going on in there.

So while fusion fuel might be 'easy' to come by, the reactor is not going to be easy to build or hide from the oppressors. Fission reactors will likely be much easier to hide and create (even though it isn't easy). Solar would be much easier power source.

Answer 3

Polywell fusion is credible, clean and cool-looking

You say...

but the most likely fuel would be deuterium

There are many more fusion cycles that can be used, and far from all use deuterium.

The various Polywell projects are looking to use plain ol' Boron as fusion fuel. Not only is this an aneutronic fusion reaction(*), but it also makes Helium as its "waste", plus it is sci-fi looking to such a degree that it puts every Star Trek "warp core" special effect to shame.

EMC2 Fusion Development Corporation's "Wiffle-Ball 8" reactor in operation

_{(*) Which means you do not have the pesky problem of making all your machinery radioactive}

Answer 4

Since the key element to this story is being able to "steal" the fuel, then nuclear fusion would seem to be the way to go. Fusion is very efficient in terms of extracting energy from the mass of the material undergoing fusion, so a small amount of fusion fuel can go a long way.

There are several issues, however.

The sort of fusion we have been 20 years away from achieving for the last 50 years involves using D2 as the fuel, and most machines designed to extract fusion energy from Deuterium are quite large and complicated. The ITER (International Thermonuclear Experimental Reactor) being built has the mass and capital cost of an aircraft carrier, hardly something people under the constraint of having their energy use monitored and controlled will be able to pull off in secret. As well, most machines using Deuterium fusion as their energy source emit up to 80% of the energy in the form of neutrons, which requires elaborate measures to mitigate (the entire colony will glow in the dark otherwise) and to extract useful energy. Once again, the giant heat exchangers and steam turbines needed will be a bit difficult to fit under a mattress when the conquerors arrive to see just what is going on in their colony.

Aneutronic fusion is theoretically much better. The reactors can be quite a bit smaller since they don't need as much shielding and can extract a high percentage of their energy from the charged particles being emitted from the reactor. The trouble here is aneutronic fusion is an order of magnitude more difficult than D2 (for various reasons), so your aneutronic reactor will need a supply of genius designers and builders and probably some pretty elaborate materials to work (superconducting magnets or other exotica). There are a lot of different theoretical approaches to this sort of fusion reaction (https://en.wikipedia.org/wiki/Aneutronic_fusion), and many different possible fuel combinations, but probably the most "practical" would be Boron for the B+p reaction, since carrying a box full of boron seems to be more practical than carrying around a cryogenic dewar of 3He.

Answer 5

• If the captives want to steal the fuel for their power system, it has to be reasonably portable, unless they want to steal and hide an entire train. That might be a point for antimatter, if there are portable containment devices. If the containment fails, you get a big bang. That could help with the heist, too -- the captors won't dare to shoot at escaping captives if they carry enough AM.
• I think you're underestimating the difficulty of building the reactor, as well as the problem of hiding it. Fuel isn't the only thing. Historically speaking, it seems that fission is much easier than fusion. Even North Korea has a fission reactor, while fusion is still a pie in the sky.
• Both colony expeditions would have planned to build redundant power systems after their arrival. What happened to the captives' stockpile? Perhaps the captors have some crates with disassembled reactors from the captives somewhere. Along with a Helium-3 mine or an enrichment plant.

Answer 6

Perhaps you could have a conflict involving a passing Comet? Earth's Water is known to contain small amounts of Deuterium (read: 1 Deuterium atom to 6420 Protium atoms of Hydrogen.), and the comet 67P/Churyumov-Gerasimenko apparently has three times this amount, comparably to Earth's water supply.

In the vastness of space, there is probably a comet or two thousand that have excessive amounts of this substance.

You would use this as the "first spark" to initiate a Fusion reactor. Only the Captors would initially know about it, however, one sneaky Captive would overhear about it and plan to steal some/all of it. This would create both plot, tension, and motive for the Captives to risk their all for this one chance to be rid of their overlords.

Perhaps they've already built a fusion reactor, they just don't have the necessary pieces to finish it, or the motivation because they can't finish it without the final pieces.

Answer 7

I find the majority of these answers are not accurate as they discount the fact that every element is present everywhere but only in small quantities....you can process the dirt outside your own house and obtain uranium at a percentage of about 1part per millon. the enrichment process is long and requires substantial research into the process but can be done...