This isn't a one-man project, and nuclear engineers are not idiots. The worst-case scenario here is that the sodium coolant gets out somehow and reacts with water, causing a chemical explosion and breaching containment. Sodium-cooled reactors are not a new technology, and there are well-developed methods of making such disaster scenarios vanishingly unlikely, as well as ensuring that the side-effects if they do happen would be minimal.
So yes, you are missing something.