Gas entrainment (GE) from cover gas, which is an inert gas to cover Sodium coolant in a reactor vessel, is one of key issue for Sodium-cooled fast reactors (SFRs) design to prevent unexpected effects to core reactivity. In this study, evaluation method has been investigated for surface dimple depth growth of unstable drifting vortex dimples on the liquid surface in the reactor vessel. By using a computational fluid dynamics (CFD) code, analyses have been conducted to estimate the drifting vortexes on water experiments in the circulating water tunnel. The unstable drifting flow vortexes on the water surface were generated as wake vortexes behind a plate obstacle. Downward flow velocity was induced by bottom slit flow pass along the flow channel. In CFD analysis, flat water surface assumption was used with free-slip model for the development of a conventional design method. The gas core lengths from the surface vortexes were evaluated based on existing non-dimensional numbers method by using the stream-viewer code provided from Japan Atomic Energy Agency (JAEA). In this paper, the results of comparison between experiments and analyses were discussed.

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