We have learnt to not put reactors on top of the water tanks, and to not shutdown all safety systems to see if the plant explodes.
Chernobyl had many known design problems ever before it exploded. It's misleading to imply that it was consider a good design before.
There is a comment with more details from pdkl95
>> Chernobyl
> The RBMC design was always known to be an absolutely terrible idea because it's use of graphite as a moderator that required active cooling, creating an inherently unsafe positive void coefficient[1].
Read the complete comment: https://news.ycombinator.com/item?id=10257459