The adoption of mercury as a coolant has a long history. But, the main scientific-technical results in this field were obtained in connection with the development of nuclear power plants with fast reactors when sodium was chosen as the main coolant on the basis of a comprehensive analysis and comparison of the properties of different coolants. Operations with the coolant, including production and loading into the apparatus, up to shutdown of the nuclear power plant were examined (see Fig. I). As a result, a comprehensive approach was formulated for studying the system sodium --impurity --constructional materials --shielding gas. On account of the high chemical activity of sodium, special attention was devoted to safety.The path from the laboratory stands to adoption of sodium as a coolant and the development of experimental setups with fast reactors of the BR-5 type up to the demonstration commercial nuclear power plants with and BN-600 reactors has now been traversed.Analysis of more than a 40-year period of scientific-research, and experimental-constructional works and experience in operating experimental and commercial plants, shows that the sodium coolant technology which has been developed and adopted permitted long term operation of nuclear power plants at nominal parameters and with deviations from the nominal parameters (sodium leakage and sodium burnup, water leaks in the steam generators, anomalous comamination of the coolant and the shielding gas, and others).Impurities in Sodimn. The composition of the impurities in the initial sodium and the impurities appearing during operation of the systems is diverse. The greatest attention is devoted to oxygen, hydrogen, carbon, products of corrosion of the constructional materials, and radionuclides [1, 2]. The main difficulties with the initial sodium were associated with carbon (hydrocarbons and carbon electrodes, employed in production) and calcium. The development of a special apparatus for producing reactor-purity sodium at the Bereznikov soda plant solved this problem. However, this sodium was several times more expensive. It seems that under current conditions the final purification of sodium is best done at the nuclear power plant. Optimization of this process requires additional scientific-research reactors, which will more than pay for themselves.The constant sources of impurities, acting under normal operating conditions, are corrosion hydrogen from the steam generator, tritium from the core, oxygen and hydrogen from the shielding gas and from maintenance work, and products of corrosion of the constructional materials (see Table 1).It should be noted that the corrosion processes occurring in sodium are much less intense than in water or lead-based coolant [3]. However, it has been found that the rate of corrosion at the location of a water leak in the sodium through defects in the constructional material separating the sodium and water is several times higher than the corrosion rate in sodium under normal conditions [4]. For this reason, secondary d...
