Chernobyl is presumably what you're talking about. It was a high pressure steam explosion due a sudden introduction of energy into the system, which of course would cause an explosion and contaminate a wide area [0].

> The engineering and physics is well understood.

as are the safety issues which is why there what research there is for a modern nuclear reactor design would not repeat those mistakes (e.g. molten-salt).

Whether the region around Chernobyl is uninhabitable is debatable, given that animals have returned to it in abundance.

[0] https://en.wikipedia.org/wiki/RBMK