There are elements with far more favorable decay paths. Short decay + using that decay too = pretty much a clean nuclear reactor.
And more importantly RTGs don't put out nearly enough heat to make a usable nuclear thermal rocket. The important thing is being able to turn them on when you're doing a burn but then turn them off when you're coasting to your destination then turn them on again to stop there. RTGs can't do that.
"Mined" from where? How?
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?
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 ...
Theoretically, for reactors we don't have.
Thankfully nuclear reactors aren't particularly radioactive until you turn them on, which is a big improvement on the radiothermal generators, RTGs, that we sometimes use in probes headed for the outer solar system where solar panels don't work. It's during launch, before this part gets turned on, that you have a risk of crashing and losing the reactor somewhere on Earth.