In order to estimate inert gas behavior in a circulating system of a sodium-cooled fast reactor (SFR), it is necessary to develop a computational code for a dynamics of the gas in the primary system. In the present study, multi-dimensional analysis in the upper plenum of the SFR is performed using a numerical method for gas bubble transportation developed based on one-way-coupling method. As a result of the analyses, non-dimensional correlation model for gas behavior has been derived. The model is employed in the VIBUL code that is a plant dynamics code based on one point approximation. In order to investigate the usefulness of the present model in the plant dynamics, analyses in the rated condition are carried out. We discuss the difference of the two models in predicting the bubble number density in the upper plenum.