Graphite reactors are not a good idea because they inherently have, well, graphite. That can burn.

The worst possible case for water-moderated reactors is uncontained meltdown. And it's not _terribly_ horrible. You will get contamination with volatiles, mainly cesium. But there's not a lot of it in the reactor, so it'll affect only a small area around the plant. Some fuel might get initially mobilized by steam explosions, but again, only a fraction.

The worst case for a graphite reactor is an uncontained core fire. That can burn for weeks and spread a significant part of the fuel as particulates over large territories (Chernobyl).

Is it likely? Nope. But there are black swan events: earthquakes, mega-hurricanes, meteorite strikes, Godzilla attacks.