This is repeating points from other replies, but it has to be said again that much of the passive-safe design in a BWR/PWR has to do with the fact that it will

This is repeating points from other replies, but it has to be said again that much of the passive-safe design in a BWR/PWR has to do with the fact that it will melt down (rather than go prompt-critical and explode) even when every cooling system fails, since the coolant itself is what allows the reaction to continue.