Watch the "A is for Atom" episode of Adam Curtis' Pandora's Box series. It's linked in one of my other comments. Basically industrialists and sales people got involved, brushing off scientists' concerns. And irresponsible politically connected engineers in the USSR. That's why we have Chernobyl and Fukushima.

However, these are lessons learned. Today it's quite hard to come up with a reactor design without many safeguards and passive safety features, like it's quite hard to come up with a car with no seatbelts or airbags.