You still have to haul around huge amounts of radioactive materials with whatever technology. That alone is a big factor in the impossible risk calculations.

So far there has never been a dirty bomb. What if the pro-nuclear fanboys get their wish and we solve the energy problems of the developing world using nuclear power? What if a modestly developed country gets overrun or overturned? I don't like the odds. Most risk calculations don't take into account that people seem to make mistakes or might eventually even blow up a reactor intentionally. It's unthinkable until it happens.