You know, when I mentionned "Molten Salts Reactor", I mentionned it as the PRIMARY Cooling Circuits ... because it's much more efficient (works with Thorium directly molten with the thingy, which allow neutrons to be easily captured by nucleus and so more fissions reactions).
But you know what it means ? That Molten Salt could be used IN ANY THERMAL-BASED GENERATOR! It would just be an intermediate between the thing generating heat and the Steam turbine, which would be easier to transport (because it store even more energy than Water, whom the enthalpy is already extremly high). And due to its high enthalpy, you don't even need to wait your Salts to crystalize again, you can just output out of the Turbine a Liquid like 400°C/K hot less! So it stay Liquid and is easier to use
Unless ... you wanna use the Salts as the Primary and the Secondary Coolant Circuit ? Which mean that Salts would directly cool down your reactor ?
At least you can (and have to ^^) implement Water/Molten Salts external cooling cooling down internal, aka Coolant Cells (so they would be more useful than currently ... ^^)
I'd like it not to be Dirt since it's even renewable, but Clay, or maybe the Dirt-like ressources Greg mentionned once. You know, things he didn't add yet
Sirus, that's not more efficient at all. It would be extremly expensive, and according to Thermodynamic Laws, since entropy can't be <0 and since the thermal enthalpy of helium isn't very high (I think, didn't checked), you wouldn't be able to generate lots of energy out of that. Or you could, but using like 10x the amount of Water needed (but in Helium).
And something can't be "more gaseous". The only thing that may change are Pressure and Volume (and so Temperature, because you know, PV=nRT)
But It could be ... more plasma-ish/ous/dunnohowtocallthat.