Behaviour and monitoring of non-metallic impurities in liquid sodium

Full text: Liquid sodium is the coolant of choice for fast breeder reactors. Liquid sodium is highly compatible with structural steels when the concentration of dissolved non- metallic impurities such as oxygen and carbon are low. However, when their concentrations are above certain threshold limits, enhanced corrosion and mass transfer and carburization of the steels would occur. The threshold concentration levels of oxygen in sodium are determined by thermochemical stability of various ternary oxides of Na-M-O systems (M = alloying elements in steels) which take part in corrosion and mass transfer. Dissolved carbon also influences these threshold levels by establishing relevant carbide equilibria. An event of steam leak into sodium at the steam generator, if undetected at its inception itself, can lead to extensive wastage of the tubes of the steam generator and prolonged shutdown. Air ingress into the argon cover gas would lead to an increase of oxygen in sodium and a steam leak at the steam generator would lead to an increase in oxygen and hydrogen concentrations at the secondary sodium circuit. A leak of hydrocarbon oil used as cooling fluids of the shafts of the centrifugal pumps of sodium systems are the sources of carbon and hydrogen impurities in sodium. Continuous monitoring of the concentration of these non-metallic impurities in sodium coolant would help identifying these ingresses at their inception. Yttria Doped Thoria (YDT) electrolyte based oxygen sensor is under development for monitoring dissolved oxygen levels in sodium. An electrochemical hydrogen sensor based on CaHBr-CaBr2 hydride ion conducting solid electrolyte has been developed for detecting the steam leak during normal operating conditions of the reactor. A nickel diffuser based hydrogen sensor system using thermal conductivity detector (TCD) and Pd-doped tin oxide thin film sensor has been developed for use during start up or low power operations of the reactor. For monitoring carbon in sodium, an electrochemical sensor with molten Na2CO3-LiCO3 as the electrolyte and pure graphite as reference electrode has been developed. Fabrication, assembly, testing and performance of these sensors both in laboratory and large sodium test rigs will be presented in this paper. (author)