The improvements were already mainstream before Chernobyl was built. It's all about making reactors tolerant to operator error, and if something does go wrong,

The improvements were already mainstream before Chernobyl was built. It's all about making reactors tolerant to operator error, and if something does go wrong, make sure it goes wrong slowly, so operators have lots of time to react. The RBMK reactor is quite an elegant design. Simple plug-and-play architecture for adding and removing fuel and other assemblies while the reactor was running, perfect for things like doping silicon for semiconductors and producing plutonium for weapons. But to get this capability you have to either give up affordability or safety, and they chose to give up safety.