Comment Chernobyl didn't melt down. (Score 3, Insightful) 204
Chernobyl was and is still the worst nuclear disaster because it didn't melt down, it blew up. Reactor 4 was supposed to be used for an experiment but was shutdown before the experiment could take place. However to try the experiment, the reactor was started up without letting the Xenon-135 decay to the point were the reactor could be started safely.
Nevertheless the reactor was started in a VERY unstable state, it soon "burned through" the Xenon-135 and the reactor power output rose to ten times it's rated limit and the containment vessel exploded, blowing fuel across the countryside. Following that, the moderator, graphite, burned spewing even more fuel into the atmosphere.
Chernobyl was human error, avoidable but human error. http://en.wikipedia.org/wiki/Chernobyl_disaster
Now if there had been serious fires in the spent fuel pools at Fukushima, Chernobyl would have paled in comparison.