The EBR-II tests you're referring to were liquid metal sodium, not molten salt. Molten salt is much different (though it also can be used in reactors and may be able to demonstrate passive safety as well)

[1] https://en.wikipedia.org/wiki/Experimental_Breeder_Reactor_I...

[2] https://whatisnuclear.com/blog/2020-11-23-sodium-vs-salt.htm...