BTW, supercritical fluids really are a lot different. Above 3200 psi, it is actually possible to have an
underwater fire.
Instead we use a noble gas. Helium is the perfect stuff from every angle except long term availability. Helium is co-produced from natural gas wells and is generally released during use. And then floats to the top of the atmosphere and gets blown away by solar wind. However 1% of the atmosphere is argon. It catches too many neutrons to be used directly in a core but otherwise works just fine and we'll never run out. A noble gas means no corrosion or fouling or chemical problems with the salt.