There are 1) known reasons for known catastrophes, so we can protect against them, 2) unknown reasons for known catastrophes, so we need to make a guess, and 3) unknown reasons for unknown catastrophes, which are not happened yet, including state sponsored attack on a nuclear plant. We cannot be prepared for unknown unknowns, so we need to plan for the worst case scenario. The worst-case scenario for nuclear plant is continent scale Red Forest (about 1M Chornobyl's).

Do you know how to reduce continent scale threat to just the size of a nation or a town?

Nuclear fusion or LENR can do that, because of small amount of radioactive materials and no positive coefficient by design, but how you can do that for massive fission?