Carbon activity measurement by using metal foils equilibration in liquid sodium environment at 550 C. degrees - 15484

Non-metallic elements, such as carbon, oxygen, and nitrogen in sodium have a large effect on the mechanical behavior and corrosion rates of materials in the primary and secondary coolant heat transport system in sodium-cooled fast reactors (SFRs). Austenitic Fe-18%Cr-8%Ni foil and ferritic Fe foil were equilibrated at 550 C. degrees sodium. After the equilibration, the concentration of carbon in the foils was analyzed by carbon analyzer (Leco). Using Pillai's equation, the carbon activity in liquid sodium was calculated. Also the thermodynamic activity of carbon in ferrite iron (Fe-C alloy) was calculated in the temperature range of 682 - 848 Celsius degrees. An equilibrium method using ferrite iron steel has been developed to measure the activity of carbon in liquid sodium. The results obtained are discussed and a new expression is proposed relating carbon activity with the composition of steel. The thermodynamics of carbide equilibrium in the steel are discussed by characterizing the carbide phase. These data have been analyzed using regular solution model for the carbide phase, Fe₃C and based on this observed composition of Fe₃C as well as the variation of the carbon potential of steel with temperature have been explained. (authors)