Heat pipes are used in CPU coolers and work on the same principle. A small amount of liquid is in the pipe, at reduced pressure, which makes heat transfer very efficient.
I suck at chemistry, but wiki clearly discusses advantages of sodium for heat transfer in nuclear reactors: https://en.wikipedia.org/wiki/Sodium-cooled_fast_reactor (tl;dr: safety margin from the large range of temps at which sodium is liquid; doesn't like to absorb neutrons, and isotopes aren't that problematic when it does).
Yes, but they use water. (Also reduced pressure means reduced density, which should in theory make heat transfer less efficient... except when phase changes are involved !)
That reactor is a radically different design, operating at a much lower temperature, where sodium doesn't evaporate.
But the whole point of a heat pipe is that phase changes -are- involved! So, water’s fine for temperatures in a “normal” range, like a CPU, but perhaps not great for higher temperature ranges.
And then, this is a nuclear reactor. Whatever’s in the heat pipe in a reactor will be bombarded with neutrons. Water is a neutron moderator, so you wind up with heavy water in the heat pipes and not-very-nice fission products in the reactor due to slower neutrons.
I suck at chemistry, but wiki clearly discusses advantages of sodium for heat transfer in nuclear reactors: https://en.wikipedia.org/wiki/Sodium-cooled_fast_reactor (tl;dr: safety margin from the large range of temps at which sodium is liquid; doesn't like to absorb neutrons, and isotopes aren't that problematic when it does).