Sodium makes little sense as the heat transfer fluid for a Concentrated Solar Power (CSP) tower, due to its high cost and high reactivity with water and air. If you are looking for alternative CSP heat transfer fluids, supercritical carbon dioxide (s-CO2), molten salts, and ceramic pellet fluidized beds are the subjects of actual research. These materials are low cost, chemically inert, and could be utilized for thermal energy storage. Supercritical CO2 could also be used as the working fluid in a closed Brayton cycle, eliminating the heat transfer step that would be needed if a steam Rankine cycle was used.
Yes, molten sodium could be used in a fast breeder reactor, but only because few other materials have comparable transparency to neutrons.
Yes, molten sodium could be used in a fast breeder reactor, but only because few other materials have comparable transparency to neutrons.