Is the population setting in the game reasonable?

According to the settings of the game, billions of people inhabit the outer solar system. Considering the real situation in the Kuiper belt with the quantity of fission element stored there, it's unreasonable for humanity to produce all its electricity from fission materials from Kuiper belt, which means that there's no way to feed anyone in the outer solar system. The only possibility is transporting the fissionable materials from a number of inner-terrestrial planets. which leads to a low efficiency.