I'll give an example of molten salt reactors [1]; other designs have different but comparable safety properties. The nuclear fuel is suspended in a molten salt which is separate. If the reactor is breached, you'll have what's effectively a contained chemical spill that has a very limited spread. Current water reactors trigger a steam explosion that spews out radiation into the atmosphere, which is why it's so catastrophic.

If you get a runaway reaction, it has a fail-safe described in the article that drains the reactor into containment vessels underground.

[1] https://www.technologyreview.com/2015/09/04/166330/meltdown-...