Might be, but probably aren't. The most troublesome fission products all end up chemically bound in the fuel salt, and most designs are passively safe, and inherently pretty stable since the reaction slows down significantly as the fuel heats. There's a good safety margin since the salt is liquid over a wide range.

A "melt down" isn't exactly a thing for them since the fuel is melted already by design, but if the fuel manages to overheat, then a plug melts and either the fuel dumps into cooling tanks, or neutron poisons spread through the fuel.