The explosion at Chernobyl was not nuclear but rather an extremely vigorous chemical one. It was caused by a runaway criticality but one that's nowhere near the like of a nuclear weapon. It was a prompt criticality, I believe, but one that promptly stopped. To be sure, a prompt criticality is something which power plants strive to avoid and weapons strive to happen but they do still vary in magnitude:
Demon Core produced a prompt criticality but not an explosion.
This may be theoretically possible in the context that, were all control and shutdown rods to somehow disappear from the core very bad things could happen but, it would likely need something like the Earth's gravitational field becoming negative to make this happen (this would be
really bad everywhere). But, like you say, it's not plausible.