STUDIES OF THE DECOMPOSITION OF SODIUM-CARBONATE IN HIGH-TEMPERATURE SODIUM

Carburization is one of the causes of the degradation of the mechanical properties of structural components in a liquid-metal fast breeder reactor. Sodium carbonate is a carbon-bearing impurity species that can decompose in the sodium coolant of the reactor loops and give rise to highly carburizing conditions. While there are conflicting reports about the temperature of the onset of decomposition of sodium carbonate in liquid sodium, there is general agreement about its instability at elevated temperatures. Two methods have been used to monitor the onset and kinetics of decomposition of sodium carbonate in liquid sodium. In the first method, carbon generated by the decomposition of sodium carbonate reacts with hydrogen (dissolved in sodium) to form methane, and the temperature at which the pressure of methane abruptly increases is determined. In the second method, the increase of carbon activity of sodium, resulting from decomposition of the sodium carbonate, is monitored by an electrochemical carbon meter. In both cases, mutually complementary results are obtained with regard to the temperature of decomposition; however, a significant difference in the kinetics of decomposition is observed, the rate being very high in the presence of hydrogen.