https://en.wikipedia.org/wiki/SNR-300
Another related timing coincidence is that a smaller sodium-cooled reactor in the USA (the EBR-2) demonstrated completely passive shutdown in loss of flow and loss of heat sink accidents without any control rods going in just 2 weeks before Chernobyl happened. Of course today almost no one has heard of it.
https://en.wikipedia.org/wiki/Experimental_Breeder_Reactor_I...