Why do liquid sodium nuclear reactors need 3 circuits?

That is a good question.  I probably has to do with sodium's chemistry you probably don't want that stuff turning a turbine.  

It seems like they are attempting to separate the reactor from the other systems, because of my chemical/radiological issues using sodium as a coolant.  Its can excellent coolant... low melting point, high thermal conductivity, but it's corrosive as fart because it is chemically reactive.

I am doing research on Very High Temperature Reactors... and I think lead cooled reactors have great potential if we can get past it solidifying and jamming everything.