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?