There is a possibility that argon (Ar) cover gas in the upper part of the reactor vessel (RV) could enter the sodium coolant by vortices, causing output disturbance. Hence, it is necessary to evaluate this gas entrainment phenomenon. To predict the flow pattern in the upper part of the RV using computational fluid dynamics analysis, there is a need to establish an appropriate mesh arrangement.

In this study, the applicability of the adaptive mesh refinement (AMR) method to predict gas entrainment vortices accurately was examined. An initial coarse mesh (20 mm) that simulate the test section of the experimental apparatus in the circulating water loop was created. The initial mesh was refined with two indices: the first index (index 1) is when the second invariant, Q, of the velocity gradient tensor is negative, and the second one (index 2) is the pressure gradient index added to index 1. Transient calculations were then performed on the refined meshes under each condition, and the results were compared with a reference mesh with cubic cells of a 5-mm width.

As a result, comparing the pressure distribution of the reference mesh with the other meshes refined with the two indices, index 2 was found to be more similar to that of the reference mesh. In conclusion, the applicability of the AMR method with the condition of index 2 was confirmed for this experimental system in which unsteady wake vortices are generated.