Depends on the salt used.

If you check here on table 5 you'll see that common table salt (NaCl) melts at 801º C.

As for what's used, in Chapter 2 of that paper they say "Molten salts consist of alkali metal or alkali metal halides and oxygen-containing salts", so it's not actually table salt that's used in Generation 2 of those kind of power generators.