Moltex has the best chance to succeed. No nuclear salt running through pipes. Just a bath and let convection (with soime help) do it's work.
Oak Rich testing in the 60's was for possible airplane inplementaion, that is why they didn;t cjhose thew bath tub design with convection of the molten coolant salt (because it doesn't work in freefall)
https://www.moltexenergy.com/