[mitchty]

Neither of the first two are options... at all realistically, nither uranium nor thorium exist in anything below a large planetary size body in any quantity: https://www.quora.com/Do-uranium-and-thorium-exist-in-signif...

And shipping up unrefined ore is also a bit of a ludicrous idea for mass reasons and the rocket equation alone. You do realize you can isolate a nuclear reactor core from explosions on rockets right? What catastrophic failures are you attempting to design your solution of avoiding a nuclear reactor around?