The simple skinny is that the contents of the cells must be maintained at a constant temperature of roughly 200C (400F).
Where does the heat come from and how is it maintained? Maybe nuclear?
Good point. They claim renewables but that may or may not work. Nuclear would be cheaper but still an awful lot of energy to maintain 200C. Plus the energy needed to actually produce the Sodium metal in the first place.
Simple. Go fast enough to have sufficient heating by aerodynamic friction.