[en] An innovative sodium-cooled fast reactor (JSFR) has been investigated in a frame work of the Fast Reactor Cycle Technology Development Project. A feature of this reactor is the compact reactor vessel, which can reduce the construction cost. This compact sizing of R/V may cause the cover gas entrainment (GE) at the free surface in the upper plenum of reactor vessel due to the increase of the sodium coolant velocity. The entrained gas bubbles disturb the stable operation of reactor. The prevention of gas entrainment is one of the significant thermal hydraulic issues. Therefore, in the current design of the reactor, horizontal plates called dipped plates (D/Ps) are set bellow the free surface to block the vertical flow toward the free surface. In the previous study, a water experiment using the 1/1.8 scale partial model of the reactor upper plenum was performed to see the adequacy of present design and obtain the onset condition of GE. From the result, it was assured that GE did not occur in the rated condition of reactor. However, there are the specific operating conditions with the low sodium temperature concerning the cold startup or shutdown of reactor. In those operating conditions, the sodium level becomes low due to the thermal contraction of sodium. The objective of this study is to evaluate the onset condition of gas entrainment in the low liquid level condition concerning the startup or shutdown operation of the reactor. The onset condition of gas entrainment in the low liquid level operation was obtained by using the 1/1.8 scale partial model. From the onset condition of gas entrainment, it was found that gas entrainment did not occur with the doubled D/P geometry under the start up or shut down operation. (author)