There is a good classroom demo where somebody melts a molten salt in a crucible with a bunsen burner and demonstrates that it becomes transparent, conducts electricity, etc.
A molten salt can be used as a coolant, competitive with sodium in fast reactors. It is not so chemically reactive and you can see through it (imaging repairing a reactor you cant see -- a bad problem in the 1970s, not so bad now with ultrasound). It is less heat conductive than sodium though, which hurts the econonics of fast reactors which need a large amount of nuclear fuel to form a critical mass and can get more value out of that critical mass by getting as much heat out as possible.
You can dissolve uranium, thorium and/or plutonium in that salt and that is proven (see MSRE) but needs development.
Most of the dangerous products of the reaction stay in the salt, it works a lot better than you think it would. (when a sodium reactor melted down in the late 50s they never detected radioactive iodine because it reacted with the sodium and the salt dissolved in the coolant, then it decayed in place.)
You still have some noble gases coming out and fine particles of platinum group metals coming out, but that's less of a problem than finding materiald that can survive 30+ years of that treatment.
There are many solutions for this. Corrosion is absolutely a problem you can find engineering solutions to. And Moltex has very elegant one. Other companions like Terrestrial are simply replacing the reactor core after 7 years.
The MSR experiment in the 60s proved it worked. They had some corrosion issues after quite a while in operation, but still operated if for quite a while, and that was in the 60s.
It basically fixes all the really terrible ideas about sodium reactors and gives you the advantage of molten salt reactors.
The problem is, what we are doing now are all things that could and should have been done in the 70-80s.