[rb1]

It's not just the design(s), it is the implementation too.

At Chernobyl the powers at be (accountants?) got involved and decided to use flammable bitumen coverings on the roof of reactor 3, one would assume to save money. Unsurprisingly, the roof of reactor 3 caught fire.

[fusiongyro]

This is a good example of one of the failings at Chernobyl, but it took many of them coinciding to make the disaster as bad as it was.

They were required to use another material to build the roof, but they were also required to be finished with construction by a certain time. Soviet central planning led to a shortage of the proper roofing material. I'm sure the decision to use bitumen was either made in ignorance or with the assumption that it wouldn't matter for other reasons. Lots of mistakes were made with the assumption that the rest of the system was safe so it wouldn't matter.