In sodium-cooled fast reactors, it is very important to evaluate sodium-water reaction from the viewpoint of safety. If once it happens in the steam generator, it is desirable its scale is small and its reaction is mild. The sodium dispersing nano particles is expected to mitigate the sodium-water reaction. Nano particles is titan particles whose diameter is tens nanometer and dispersing rate in sodium is several percent. In order to find out the effect clearly, it is essential to evaluate the reaction behavior and characteristics of physical properties closely. For this purpose, we tried to measure the surface tension of sodium dispersing nano particles as an important part of foundational physical property. We had employed the pendant drop method for the measurement, with which we can measure surface tension from the shape of pendant drop. This method is suitable for our measurement because nano sample is very limited. The measurement was done in a sealed box keeping inert gas atmosphere, which is required for sodium experiment because of an easy chemical reaction of sodium with oxygen and moisture. At first, we did a series of experiments for pure sodium. We succeeded to make pendant drops between 150°C and 350°C. For the measurement, we selected only the drops in clear mirror surface with axial symmetry. The scatter range of obtained results is slightly larger than that of a recommended equation. We estimated that it was caused due to a weak wet condition on the edge face of nozzle. Then we measured the surface tension of the sodium dispersing nano particles under the same condition. The obtained results almost show values larger than those of pure sodium. The dispersion rate of nano particles has an uncertainty in the present study. However even in giving the worst evaluation, the measured surface tensions for the sodium dispersing nano particles show larger values than those for the pure sodium.

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