Earlier quoted context omitted.
Much of the nasty stuff comes from the fact that current (light water) reactors were actually meant to produce military plutonium. Bombs first. Energy was a by-product, and a very nice way to sell nuke-making to the civilians. There is another way of doing fission, that was demonstrated (working prototype and all) in the 60's: molten salt. You basically dissolve the fissile matter in molten salt, provide initial heat…
Molten halogen salts are nasty stuff. I'm sure with some extra R&D people could make those reactors reliable, but if you search the new generation reactors on Wikipedia, you'll see every one of them ended in some kind of failure.
Another possible cause is path dependence. We have a whole industry around solid fuel, light water reactors. Switching to molten salt throws much of that away.
Much research still need to be done. We may want cleaner salts, we want even less radioactive waste, and we definitely want to accurately assess the safety of this stuff ("passive security" is not enough by itself). This means more prototypes reactors, of various sizes, each more expensive than before. We want a few million dollars for the small ones, then a few billions for the big ones.