You can have severe core damage without any nuclear reactions. A meltdown is severe core damage caused by a nuclear reaction that got out of control. Severe core damage here means that radioactive material or fissile material escaped the containment of the core.

There aren’t “steam explosions” in physics. There are chemical reactions that cause an explosion, pressure buildups that cause an explosion, and nuclear explosions.

The steam was a pressure buildup that caused the incident resulting in an initial pressure explosion. The thing that “melted” the core of Chernobyl was the hydrogen exploding, hence a chemical explosion. Had it been a nuclear event that melted that core, neither Kiev nor Moscow would be inhabitable. The capital of Russia would be St. Petersburg, and Ukraine wouldn’t exist, as well as several other Soviet Oblasts in the area.

Fucking with the literal power of atoms, in a mostly controlled environment, is the closest we’ve ever come to the Icarus myth. I’m fairly certain that fusion power, should it ever come out of the theoretical stage, won’t be nearly as dangerous as fucking around with fission the way that we currently do.