In this case, leaking molten sodium merely causes a nontoxic metal fire. Definitely in the category of "not fun", but not the end of the world either. Just dump some sand on it and wait for it to cool down.

Leaks of molten salt nuclear fuels are an entirely different category of industrial disaster. A nuclear accident at best, and a large-scale disaster at worst.

It's like the difference between a water truck having an accident and spilling water on the road and a damn bursting and flooding a city.

One makes for a funny picture on Reddit, the other gets the national guard called up to deal with the emergency. Both involve spilled water.

There are systems that use molten salt to transport heat. Several types of power generation systems are more efficient above the boiling point of water. The most famous of these would probably be solar collectors. The efficiency gains are higher at increased operating temperatures, so these systems are usually pushed to their limits. These can leak without causing an international incident.

Molten salt reactors, MSRs, are nuclear reactors that use a liquid fuel in place of a solid one. As far as I know, only a handful have ever been built and reached criticality. These were all done in the 50's and 60's. These systems have secondary and probably tertiary containment vessels in the case of leaks.

These are two very different, but often conflated, systems. Part of the reason for this is that many MSR designs use a secondary molten salt loop as a temperature step-down.

Here is the Wikipedia diagram for a MSR. Note the fuel salt loop, the molten salt coolant loop, and the steam turbine.

https://en.wikipedia.org/wiki/Molten_salt_reactor#/media/Fil...