The reactor was never in danger of turning into a nuke or rendering huge swathes of Europe uninhabitable. Nuclear explosions only happen under tightly engineered conditions. A big pile of molten reactor slag, while certainly dangerous, can’t turn into a bomb.

The danger wasn’t that it would cause a nuclear explosion, it was that it would melt its way into a large reservoir of water underneath the reactor, instantly turning it all into steam, causing a massive explosion that would fling radioactive material over a much wider area

I don’t know if there was a risk of that happening in reality, but that’s how it was portrayed and explained in the show