Yes that's true that there was never uncontrolled criticality. But that's because people were in there restoring some way to get water into the cores. Had that not happened, they likely would have melted down. Once all the water boils the heat doesn't have anywhere to go so temperatures go up. If that happened that's a melt-down and the core liquefies and then may or may not have been stopped by the containment shell. http://www.nbcnews.com/science/if-theres-meltdown-then-what-...

In my mind the worst-case scenario is that the core melts, the containment shell is cracked and the whole mess goes through the floor and into the ground. Has that been simulated at all? I'd love to read a paper if it has. I haven't seen anything with a cursory search.