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. 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.
Gen III+ reactors are pretty fucking close to meltdown proof.
Any reactor in use today will have a negative void coefficient. Which means, if you don't put power into it, the reaction naturally stops. Then you've got control rods and neutron moderators that will fall back upon the core if there's no power. Then Gen3+ includes a core catcher in which, should it breach its reactor, it just falls in there and cools down.
All five large reactor disasters occurred with reactors that also had fail-safes that rendered them theoretically safe. In the case of Three-Mile-Island, unlucky technicians had to work very hard against the reactor's system to sustain the failure.
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-...