Windscale and Chernobyl pages on wiki are well worth a read. Both show just how poor our understanding of the safety of these things was back then.
Worth noting Fukushima was a very, very old reactor design. More recent designs are very much informed by the handful of major disasters (and some very-nearly-disasters).
The first nuclear reactors are the proper scary stuff. There was a reason they were called “piles”, it was literally a big pile of stuff, assembled pretty much by hand, just made large enough to start reacting and producing heat. Some of the accidents with early cores (see “demon core”) were pretty diabolical too. The US had a few small military ones go pop, again wiki has a great summary of what has gone wrong and why.