It is literally just a tube with a fan blowing over it. Most designs just barely solidify it, so any over-temperature events also cause a passive shutdown.

Yeah, I got that (and it seems like a really elegant solution! very cool, pun intended, etc). My question was about how salt (or any other matter, really) can simultaneously be molten and frozen.

It can't. Well, maybe at its triple-point, but that is another story. It is just a quirk of the language due to it being know as molten salt. For example, my drink is full of molten water that has been frozen into cubes.

Part of it is molten (above the plug), part of it is frozen (the plug).

Think pipes in a house in winter, where one little part of the pipe gets frozen while the rest of the pipes have liquid water in them.

And in Spring, when the broken pipe plugged with solid molten ice thaws, your house's molten ice circuit performs an emergency evacuation into the yard, saving you from the convenience of adequate molten ice pressure

Water is actually a very unusual substance in that it expands upon freezing. (Plutonium being another)

Most materials instead contract when freezing.

Hah! Well, we usually think of salt in the solid state (i.e. frozen) and not in the liquid state, so it's not so strange to be explicit in this case. For H2O, we do have pretty common encounters with three of its phase states, so there's less need to be explicit (or, rather, we have separate words for each state: steam, water, ice).

Think of a lake in the winter. Just the top layer of water is frozen, exposed to the cold air above the lake. Some of the lake water is frozen and some is liquid, depending on its position in the lake.