It not about predicting huge accidents, it's about designing reactors so they don't have catastrophic failure modes. Like, for example. pebble bed reactors (there are other designs as well). The main insight is the need for the reactors to be passively safe, which means that if the operation is interrupted at any time the reactor just shuts down by itself and remains contained.
But, for various (political and other) reasons, we have lost precious decades we could have used to perfect those types of reactors, and here we are, at the time when they would really help in decarbonising our economies.