Development of Evaluation Method of Gas Entrainment on the Free Surface in the Reactor Vessel in Pool-Type Sodium-Cooled Fast Reactors -Gas Entrainment Judgment Based on Three-Dimensional Evaluation of Vortex Center Line and Distribution of Pressure Decrease

Abstract In a sodium-cooled fast reactor (SFR), cover gas entrainment (GE) by vortices generated at the free surface in the upper plenum of the reactor vessel is one of the significant issues in the safety design and the development of an evaluation method for GE is required. A GE evaluation method using numerical results of three-dimensional computational fluid dynamics (CFD) analysis has been developed for loop-type SFRs in Japan Atomic Energy Agency, and an evaluation tool using this method, named StreamViewer, has been developed. However, the current evaluation model employed in the StreamViewer gives conservative results to keep safety margins in conventional design basis. In this study, the modification of the evaluation method of the StreamViewer to rationalize the conservativeness in the evaluation results was examined by identifying the vortex center lines and limiting the evaluation area to the vicinity of the suction port, and calculating the three-dimensional distribution of pressure decrease along the vortex center lines. The applicability of the modified method was checked using a water experimental result in a rectangular open channel in which unsteady vortices are generated. As the result, it was indicated that the evaluation results on gas core depth which were excessive in the current method were improved in the modified method, and it is confirmed that the modified method has a potential to discriminate the onset of GE with the appropriate criteria.