I'm not an expert in this at all, but a friend of mine from college is; he's working at Argonne National Lab doing liquid sodium research for nuclear reactors--so I know enough as you would talking to a guy over beers about his job.
As said above, the chance of a meltdown is slim as sodium is a solid at room temp, therefore you don't have to worry about it ever boiling off. It would have prevented a Fukushima type meltdown, but sodium is extremely reactive and had the sodium touched water, you would have had huge explosions. Their test loop in Argonne had to go through extreme welding checks to prevent possible humidity from getting in. Salt also has better heat transfer properties than water. Interestingly, research in this area really stagnated, and its only recently gotten attention. The US went all in on water due to the nuclear navy.
So is sodium but it'll burn rapidly if you drop it in water. Salt won't even if it's molten (though it can cause a steam explosion just from the heat).
That depends on the salt and the metal. I saw a presentation by someone from Elysium, who said that chloride salt (which they're using) corrodes stainless steel less than water does. A mix of salt and water is more corrosive, but molten salt by itself is fine.
FLiBe is more corrosive to steel but there are advanced alloys that withstand it fairly well.
Another approach is to use a modular design that makes it easy to replace parts before they corrode too much. Thorcon and Terrestrial Energy use small reactor cores that get replaced every few years.
As said above, the chance of a meltdown is slim as sodium is a solid at room temp, therefore you don't have to worry about it ever boiling off. It would have prevented a Fukushima type meltdown, but sodium is extremely reactive and had the sodium touched water, you would have had huge explosions. Their test loop in Argonne had to go through extreme welding checks to prevent possible humidity from getting in. Salt also has better heat transfer properties than water. Interestingly, research in this area really stagnated, and its only recently gotten attention. The US went all in on water due to the nuclear navy.