SURFACE TENSION OF Pb<sub>44,6</sub>Bi<sub>55,4</sub> EUTECTIC MELT IN AIR ATMOSPHERE
In the 1950s liquid lead and the lead-bismuth eutectic alloy (Pb<sub>45</sub>Bi<sub>55</sub>) were considered candidates for their use as coolants for nuclear power systems in the USSR and the USA. At the same time, the alloy (Pb<sub>45</sub>Bi<sub>55</sub>), first proposed by A.I. Leypunsky, was chosen as a coolant for the nuclear submarine “Alpha”, which was ahead of its time in terms of its tactical and technical data. However, in 1968, one of them suffered a severe radiation accident caused by the melting of fuel elements in the core of an onboard nuclear reactor due to the precipitation of oxides from the coolant and their accumulation, which blocked the pipeline cross-section and sharply worsened the cooling of the reactor, which led to its failure. In fact, the primary cause of the accident was a lack of knowledge about the physicochemical and technological properties of the lead-bismuth coolant. Thus, the main disadvantage of the Pb<sub>45</sub>Bi<sub>55</sub> coolant is its corrosiveness to structural materials used in the nuclear power plant. But it has been found that corrosion by liquid lead alloys can be reduced by adjusting the oxygen level in the coolant. For example, the corrosion rate of martensitic steel at 770 K in a Pb<sub>45</sub>Bi<sub>55</sub> flowing coolant without oxygen is about 1 mm per year, but it can be reduced to 0.01 mm per year, i.e. 100 times, if oxygen is dissolved in Pb<sub>45</sub>Bi<sub>55</sub> coolant and its mass concentration is maintained at the level of 0.01 ppm. The observed effect is explained by the protection provided by the oxide layer formed on the steel surface of the pipeline. Thus, for a deeper understanding of the phenomena occurring at the boundaries of the liquid metal “coolants - gases” section, it remains relevant to study the processes of formation and destruction of the protective oxide layer and its behavior in coolant fluids, especially from the point of view of long-term operation of nuclear power plants. In this regard, data on the surface tension of the “coolant - gas” interphase boundaries are of great scientific and practical importance. In this connection, the present work sets the task of experimentally studying the influence of atmospheric air on the surface tension of Pb<sub>45</sub>Bi<sub>55</sub> eutectic melt. Measurements of the surface tension of the eutectic melt Pb<sub>44,6</sub>Bi<sub>55,4</sub> prepared by the authors were carried out in a non-stop mode sequentially, under static vacuum and atmospheric air, on the same surface. About three hundred experimental points obtained in this work made it possible to describe the dynamics of the surface tension changing process depending on the time of exposure of the coolant surface in vacuum and atmospheric air. It is shown that in comparison with the results obtained by the authors by the large lying drop method in a static vacuum, in the first 10 minutes from the beginning of the exposure of the eutectic melt in atmospheric air at a pressure of about 300 mmHg), the surface tension of the eutectic Pb<sub>44.6</sub>Bi<sub>55.4</sub> decreases by 55 mN/m and more, which is an order of magnitude higher than the total error (2 %) of our measurements.