Fermi wasn’t the same type. It was cooled by salt (sodium), but it was a fast breeder. The fuel wasn’t mixed into the salt . This design is very different.
Actually Fermi was liquid sodium metal.
Few elements make great moderators (slowing neutrons), are liquid or gases at fairly low temp and do NOT absorb neutrons.
Typical Moderators
Hydrogen
Helium
Carbon
Sodium Metal
Heavy Water (presence of Hydrogen)
Distilled Water (presence of Hydrogen)
Sodium is difficult to work with as refueling is done blind.
No way to see where the fuel bundles are.
You need a minimal amount of heat to get pumps flowing.
Also goes BadaBOOM in presence of any water.