The moon? Asteroids? Or even just ship up unrefined ore such that a catastrophic failure can’t threaten the population.

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?

What risks make that extraordinary cost worthwhile?

The moon's dust contains huge amounts of helium-3.

Which apparently is an amazing power source.

Also, why fission? We do have working fusion reactors. They are called hydrogen bombs. (The outer part, at least.) As long as you can keep the G forces low ...

> Which apparently is an amazing power source

Theoretically, for reactors we don't have.