A technique that has been developed to measure the “carbon potential” of heat transport system sodium involves the equilibration of low-carbon Type 304 stainless-steel foils at a standard temperature of 704°C (1300°F). The equilibrium carbon concentration of the foil in parts per million is used as the measurement of the carbon potential of the sodium and is, in fact, a direct function of the carbon activity of the sodium. Relationships between the carbon potential, Cs, and carbon equilibrium values in Types 304 and 316 stainless steel, Ce, at temperatures, T, have been developed by experimentation. These correlations allow the surface carbon levels in system components to be predicted without knowing the transporting species or the thermodynamic relationships between the carbon in the sodium and in the steels. The data have been applied to the fast flux test facility (FFTF) primary system after making certain assumptions concerning the carbon potential of the FFTF primary sodium. The results suggest that there is little or no driving force for carburization, but that mechanical compensation will be needed for decarburization in some regions.