LIQUID-SODIUM - THE HEAT-TRANSPORT MEDIUM IN FAST BREEDER-REACTORS

Liquid sodium is used as a coolant in fast breeder reactors on account of its excellent heat transfer properties. It must, however, be in the pure form to be compatible with structural materials. Techniques for its purification to nuclear grade and its characterization had to be developed in our laboratory before we could embark on an R&D programme. It is essential to monitor hydrogen at ppb levels in the sodium circuits of the fast reactor in order to detect water leaks in the steam generator in a timely manner. Similarly it is useful to make on-line measurements of oxygen and carbon at trace levels. Electrochemical sensors have been developed in our laboratory for this purpose. These compact sensors work on the principle of concentration cells and are far superior to devices used elsewhere for this purpose. Corrosion of structural materials in the sodium environment depends on the oxygen content of sodium. In order to understand the mechanism of this corrosion, one must have a good grasp of the thermochemistry of the ternary systems, Na-M-O, where M stands for the alloying constituents of stainless steels. The phase diagrams of most of these systems were established in our laboratory. A specially designed sodium loop is used in the study of corrosion, activity transport and kinetics of sodium-water reaction.