Hey!
To start off, I'm very new to this discussion board, correct me on any breaches of etiquette. I was actually also interested in some foundation to futuristic fantasy stories, and wandered my way onto the site. I was taking a look at some of the previous post dates as I write this and realize I might be a little late to the punch; maybe an excellent sci-fi book has already been published(?). If not, or if the information is still relevant (I don't see why it wouldn't be), I wanted to address/ add onto the original Uranium dilemma. I was interested in NTR propulsion tech, as the Spec. Impul. in these systems and their very solid/tested design parameters makes them a good-plausible candidate for future long haul manned missions, especially intra-solar system missions. Uranium pellet reactors have been most intensively studied, but Uranium is just so damn rare (expensive) at current market values, and the cost of shuttling the super-heavy metal into earth orbit is a huge hydrocarbon fuel drag (assuming NTR propulsion wouldn't be used on terrestrial earth launches with risk of fallout). But ask any geologist and they'd tell you Uranium is a super plentiful metal, especially in seawater. The problem is not a shortage of Uranium, but a shortage of U-235 that drives cost. Only odd numbered Uranium isotopes are fissile, and U-235 makes up only ~.07% of crust layer Uranium on earth, U-238 making up most of the remaining 99% occurrence. My epiphany, however, was that this future society might have cracked tokamak fusion, and formed a heavy reliance on fusion energy to meet their energy needs. This fusion energy still would not be capable of being made compact enough to use on spacecraft, but it could be used to produce fissile fuel for spacecraft. These types of fusion reactors produce grossly high energy neutron radiation during plasma ignition, which is a major source of inefficiency in these reactors. This energy could be recycled, however, if say a depleted uranium shield (composed entirely of U-238) were used at the interface of the tokamak. This uranium is readily available (as is Thorium-232), and can absorb high energy neutrons to quickly produce Pu-239. This Pu can be oxidised and mixed with plain old U-238 dioxide to make an MOX pellet with essentially the same characteristic as enriched U-235 used in current NTR pellets. Pu MOX would have greater energy density and would be more fissionable than the U-233 produced for Thorium, and could be produced pretty limitlessly on Earth, maybe even cheaply enough to compensate orbital shuttling. Additionally, some of the waste energy of the fusion reactor could be recovered to produce the fissile fuel. Sorry this is way longer than I wanted it to be, but hope its thought provoking/useful. Thanks for the opportunity.
Roanoak.
