What makes newer nuclear plants safer?

We've learned to make designs that really take advantage of physics. It's probably misnamed by calling it passive safety, but I would call it better using and understanding the physics. We use gravity, natural convection, those sort of things to help cool the plant after it shuts down. So GE Hitachi's ESBWR has the ability to remove heat passively without any electricity, without any operator actions for well over 7 days. The PRISM reactor can do that for a very, very long time and if not, forever. So that's where reactor vendors like GE Hitachi have learned "what do we need to do to these designs to make them better and to make them safer?"